[advances in marine biology] the biology of the penaeidae volume 27 || 4. zoogeography and evolution

31
4. Zoogeography and Evolution 1. Introduction The shallow, warmwater marine faunas of the world are traditionally divided into the Indo-West Pacific, eastern Pacific, western and eastern Atlantic regions (Ekman, 1953; Briggs, 1974; Abele, 1982). The Americas and Afro-European landmasses clearly separate the Atlantic from other seas, while the oceanic deeps tend to divide the eastern and western faunas of this ocean. The present Isthmus of Panama was finally established only in the Pleistocene, so faunas on the east and west sides are closely related and “twin” species are common. The eastern Pacific is divided from the rest of the Pacific by a wide expanse of deep ocean, with very few islands, coupled with cold water masses flowing along the west coasts of both North and South America towards the equator. Scheltema (1988) has produced evidence that it is not a complete barrier to shallow- water invertebrate dispersal, but rather is a filter, allowing only those larvae with an exceptionally long larval life to be transported from the central tropical Pacific. However, as the larval life of the Penaeidae is short, the eastern Pacific deep ocean is an effective barrier to their dispersal. Springer (1982) argued on the basis of the shallow-water fishes, that the Pacific Plate was a separate zoogeographical region, but this is not supported by the distribution of corals and echinoderms (Ekman, 1953). No marked boundaries to dispersal of shallow-water tropical faunas exist between the Indian and Pacific Oceans and thus the whole Indo-West Pacific is regarded as a single, complex, region. The Penaeidae in general conform to Ekman’s (1953) regional classification of tropical shelf faunas. Each of these regions includes extensive shallow seas, often along lengthy coastlines, and barriers to the distribution of groups such as the Penaeidae are often not obvious. Possible barriers to dispersion are: 1. Temperature. The Penaeidae are predominantly tropical steno- 127

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Page 1: [Advances in Marine Biology] The Biology of the Penaeidae Volume 27 || 4. Zoogeography and Evolution

4. Zoogeography and Evolution

1. Introduction

The shallow, warmwater marine faunas of the world are traditionally divided into the Indo-West Pacific, eastern Pacific, western and eastern Atlantic regions (Ekman, 1953; Briggs, 1974; Abele, 1982). The Americas and Afro-European landmasses clearly separate the Atlantic from other seas, while the oceanic deeps tend to divide the eastern and western faunas of this ocean. The present Isthmus of Panama was finally established only in the Pleistocene, so faunas on the east and west sides are closely related and “twin” species are common. The eastern Pacific is divided from the rest of the Pacific by a wide expanse of deep ocean, with very few islands, coupled with cold water masses flowing along the west coasts of both North and South America towards the equator. Scheltema (1988) has produced evidence that it is not a complete barrier to shallow- water invertebrate dispersal, but rather is a filter, allowing only those larvae with an exceptionally long larval life to be transported from the central tropical Pacific. However, as the larval life of the Penaeidae is short, the eastern Pacific deep ocean is an effective barrier to their dispersal. Springer (1982) argued on the basis of the shallow-water fishes, that the Pacific Plate was a separate zoogeographical region, but this is not supported by the distribution of corals and echinoderms (Ekman, 1953). No marked boundaries to dispersal of shallow-water tropical faunas exist between the Indian and Pacific Oceans and thus the whole Indo-West Pacific is regarded as a single, complex, region. The Penaeidae in general conform to Ekman’s (1953) regional classification of tropical shelf faunas.

Each of these regions includes extensive shallow seas, often along lengthy coastlines, and barriers to the distribution of groups such as the Penaeidae are often not obvious. Possible barriers to dispersion are:

1. Temperature. The Penaeidae are predominantly tropical steno-

127

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1

FIG. 4.1. World minimum winter 15°C and 20°C isotherms. Very few species of Penaeidae occur outside the 15°C isotherms, and the majority are within the 20°C isotherms.

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Z O O G E O G R A P H Y A N D EVOLUTION 129

therms, with few species thriving below a minimum of 15°C. Temperatures below this, caused by an increase in latitude, cold from continental land-masses or cold currents, are probably the main barriers to distribution. Examination of world oceanic annual minimum temperatures (Sverdrup et al. 1942) shows that the 15°C winter isotherm marks the outermost latitudinal boundary of distribution for nearly all penaeid species (Fig. 4.1; also defined in Table 4.1).

2. Ocean currents. The pelagic larval life makes most species susceptible to the influences of currents flowing in unfavourable directions.

3. Ocean deeps. These constitute a barrier for shallow-water species, especially in conjunction with unfavourable currents.

4. Coastal geography. Most species are shallow-water inhabitants, particularly at the post-larval and early juvenile stages. Thus a desert coastline with high inshore salinities, or a very rocky coast with deep water inshore, may hinder the dispersal of certain species.

Some penaeid species are widely distributed within each region, while others are quite restricted, but in general, their distribution conforms to many of the sub-regions or provinces defined by other invertebrate groups (Ekman, 1953) and by Crustacea (Abele, 1982). These are listed in Table 4.1, and shown with arbitrary code numbers in Figs 4.2 and 4.3. It is apparent from Table 4.1 that there about five times more penaeid species in the Indo-Pacific than in the Atlantic. This has been noted for other faunistic groups. Briggs (1974) suggests that the number of species of tropical shallow-water marine animals is directly correlated with shelf area. He cites Newel1 (1971), who estimated that the coral-reef habitats of the Indo-Pacific were about five times that of the Caribbean and that the number of invertebrate species (corals, shelled molluscs, cidarid echinoids) was roughly proportional to shelf area. This hypothesis does not take into account changes in shelf area, such as its reduction in the Indo-West Pacific during the Quaternary Glacial Epoch (Cline, 1981; see below). Abele (1982) shows that species diversity in shrimps correlates best with shoreline length, and this is also likely to apply to the Penaeidae, since most species inhabit a relatively narrow coastal fringe.

While the boundaries of the major regions are well defined, those between the sub-regions defined here are often arbitrary. They are based on the presence of endemic species and usually accompanied by a marked change in species composition. A boundary between adjacent sub-regions does not imply that there is no interchange between them, but rather that there is some restriction on dispersal of certain species. The various

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130 BIOLOGY OF PENAEIDAE

TABLE 4.1. Definitions of penaeid zoogeographical regions and sub-regions with total number of species and endemic species found only in that sub-region

(see Figs 4.2, 4.3).

No. of Area species

Indo- West Pacific (IWP) Malaysia, Philippines, Indonesia, New Guinea, Australia, through Indian waters to the Arabian Gulf to the Red Sea, to east and South Africa, China, Japan, Oceania and central Pacific islands, including Hawaii 125 Su 6-regions 1. Indo-Malaysian. Sri Lanka-Bay of Bengal-Malaysia-

Philippines-Gulf of Tonkin-south Taiwan-Indonesia including northern West Irian, northern Papua New Guinea, Solomon Islands (not including Aru Is. and southwestern Irian) 85

2. Tropical Australia 54 2a. Northwestern-Torres Strait to Shark Bay

2b. Northeastern-Gulf of Papua through Torres (Dampierian Province)

Strait to Wide Bay, Queensland (Solanderian Province)

3. Sino-Japanese. Gulf of Tonkin to Yellow Sea and the

4. Arabian Sea. West Coast of India, through Arabian Inland Sea of Japan 38

Sea, Gulf of Iran and Red Sea 39 22 16

5. East African coast. Cape Guardafui to Durban 6. South Africa. Durban to Swakopmund 7. Southeastern Australia. Wide Bay to Bass Strait

8. Southwestern Australia. Shark Bay to Vincents Gulf 9. Oceania. South and central Pacific islands, including

(Peronian Province) 9 8

the Hawaiian Islands 20

Endemic species

124

25 12

10

3 1 1

3 2

2

Eastern Pacific (EP) San Francisco through Gulf of California to Punta Aguja, Peru 16 16

Su 6- regions 1. Panamanian. El Salvador to Punta Aguja, Peru 12 5 2. Mexican. El Salvador to San Francisco Bay 11 3 3. Galapagos and Revilla Gigedo Islands 2 0

Martha's Vineyard, USA to Puerto de Rawson, 21 18 Western Atlantic (WA)

(43"30'S), Argentina

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ZOOGEOGRAPHY AND EVOLUTION 131

Area No. of Endemic species species

Sub-regions 1. Caribbean. Bahamas, Cuba, Puerto Rico, Lesser

2 . Eastern Brazil. Sao Luis to Cab0 Frio, Brazil 3. Gulf of Mexico. 4. Eastern USA. Martha’s Vineyard to southeast Florida 5. Southeastern South America Cab0 Frio, Brazil to

Puerto de Rawson (43”30’S), Argentina

Antilles, coast from Cape Catoche to Sao Luis, Brazil

Eastern Atlantic (EA) Mediterranean Sea, Lisbon, Portugal to Porto Alexandre, south Angola

Sub -regions 1 . Eastern Atlantic. Lisbon, Portugal, 40”N,

to Porto Alexandre, south Angola, 16”s 2 . Mediterranean Sea

16 10 7 7

4

*6

6 *2

2

4

2 0

~~~ ~ ~~ -~ ~

‘Plus at least five migrants through the Suez Canal.

regions and sub-regions are defined below. (Unless otherwise stated, and except for a few unpublished observations, records of penaeid distribution are taken from the taxonomic literature cited in Chapter 3).

II. Indo-West Pacific Region

An Indo-Polynesian sub-region has been proposed as a principal province of the Indo-West Pacific (Briggs, 1974). It extends from the mouth of the Gulf of Iran eastwards to Taiwan and southwards to Fraser Island on the east Australian coast, including the waters of Indonesia and New Guinea, but not including northwestern Australia. Brigg’s rationale was that areas peripheral to Ekman’s (1953) Indo-Malayan sub-region “do not as a rule possess species that are not also found in the central triangle”. This may be true of corals and associated faunas, but the tropical Australian region, including southern New Guinea, contains 13 endemic penaeid species and should be considered as a separate sub-region. The placing of the western boundary of the central tropical sub-region is more arbitrary. Briggs (1974) puts it at the mouth of the Gulf of Iran, but there are a few endemic penaeid species in the Arabian Sea and Bay of Bengal. Hall (1962) suggests that the boundary should be at the northern end of the Straits of Malacca, but De Bruin (1965) shows that there are strong

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Indian Ocean

I

0

FIG. 4.2. Sub-regions of the Indo-West Pacific Region as defined in Table 4.1.

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ZOOGEOGRAPHY AND EVOLUTION 133

affinities between the penaeid fauna of Ceylon and that of eastern Malaya. Existing records of penaeid distribution make it difficult to define a precise boundary, if such exists, and it has therefore been arbitrarily set as the southern tip of the Indian Peninsula. The following sub-regions are defined:

A. Indo- Malaysian

A well-defined boundary to penaeid distribution in the south runs along the oceanic deeps off southern Indonesia. Deep water also extends through the Timor Sea, to the west of the Aru Islands, coming close inshore at the neck of West Irian. Similarly, in the east, the ocean deeps and lack of islands of the western Pacific provide an extensive boundary, while to the north falling temperatures close off the sub-region. The northern coast of New Guinea appears to be at least a partial barrier, since there is a marked drop in species diversity from west to east. This area has several geographic features which appear to be barriers to penaeid distribution: lack of inshore juvenile habitats; the deep water of the New Guinea Trench, which comes close inshore; and the westward current, which persists inshore almost to the surface throughout much of the year (Lindstrom et al . , 1987). As noted above, no well-defined barrier exists to the west of the Indo-Malayan sub-region. Ekman (1953) showed that there is a steady decrease in the number of species from Malaysia through the Bay of Bengal and the Arabian Sea. This is also seen in the Penaeidae (Fig. 4.4). Again, unfavourable currents may be the cause. In the Bay of Bengal there is a prevailing clockwise circulation, which persists at 100 m even at the height of the monsoon, and a similar circulation in the Arabian Sea, but with a surface eastward current all year along the Arabian coast (Wyrtki et al . , 1971). Additionally, the marked monsoonal climate of India, with a long, extremely dry season, may limit the westward movement of some species.

B. Tropical Australia

This sub-region includes the southern coast of New Guinea. The reasons for a barrier between the penaeid faunas of northwestern tropical Australia and Indonesia are not obvious at first sight, but are, at least partly, explained by Fleminger (1986). Very deep water extends from the Timor Sea almost to the coast at the southwest of the “neck” of New Guinea. During October-March offshore northeasterly currents cause

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134 BIOLOGY OF PENAEIDAE

upwelling, which deflects to the west, and in April-September there is a northwesterly flow due to trade winds. There is also evidence that during the Pleistocene glacial stages, water temperatures in Wallacia were unusually cool and may have acted as a barrier to tropical stenotherms (Fleminger, 1986). (Wallacia is the area of Indonesia between Wallace’s original line and Lydekker’s Line, just to the west of New Guinea.) Like the north coast, the southeast of New Guinea is steep and, together with the prevailing trade winds, acts as a further barrier to the distribution of Indo-Malaysian penaeids. In addition, the fauna of tropical Australia is divided into distinct eastern and northwestern parts, because Torres Strait and much of the Arafura Sea are less than 100 m deep and were exposed during a large part of the last glacial epoch (Cline, 1981). In the east, the 20°C isotherm separates the tropical fauna (Solanderian Province) from the warm-temperate fauna (Peronian Province). Along the arid north- western coast, the inshore waters are hypersaline, particularly in shallow bays, and there is an attenuation of species from east to west. The 20°C isotherm meets the coast just south of Shark Bay, which thus marks the southern boundary of fully tropical species with a fall-off in species diversity.

C. Sino-Japanese

In the Gulf of Tonkin and southern China there is a large diversity of species, but this rapidly decreases to the north due to cold continental influences. Penaeus chinensis has, however, adapted to the cold waters of the Yellow Sea by overwintering in the deeper water in the south and migrating northward to spawn in the Pohai Sea in spring (Chang Cheng, 1984). Offshore, the Equatorial Current flows to the north, raising the minimum winter temperature and hence penaeid species diversity, so that the Sea of Japan has a large and diverse penaeid fauna.

D. Arabian Sea

Apart from India, this sub-region is characterized by arid coastlines; the hypersaline inshore waters probably act as a barrier to less adaptable species. Cold continental winds from the north may also enhance this effect in the northern part of the Arabian Sea. The Red Sea is often treated as a separate sub-region because of its distinctive fauna (Ekman, 1953; Briggs, 1974), but there appears to be no published evidence that the penaeid fauna differs appreciably from that of adjoining regions.

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135 ZOOGEOGRAPHY AND EVOLUTION

Briggs (1974) regards the coast from the Gulf of Iran to the southern tip of Africa as one sub-region, but in view of the large distances and the geographical diversity, it is unlikely that there is not some faunistic division of the west Indian Ocean. Cape Guardafui, the southern limit of the Red Sea, has been arbitrarily selected as the southern boundary of this sub-region.

E. East African

With higher rainfall and some large rivers, this sub-region supports a more diverse and possibly larger penaeid population than the northwestern Indian Ocean (Crosnier, 1965). No endemic species of Penaeidae have yet been described from this sub-region, although Metapenaeopsis scotti is so far restricted to this sub-region and South Africa. The southern boundary is not well defined, but there is a drop in species diversity around southern Madagascar.

F. South Africa

In the eastern part of this sub-region the number of penaeid species attenuates from Durban to Algoa Bay and could be included with the East African Coast, except for the appearance of the unique cool-water Macropetasma africanus at Durban. This species becomes common west of Algoa Bay, where other penaeid species are rare, and also extends into the cool waters of the South African west coast north from Cape Town to as far north as Swakopmund, which marks the northwestern boundary of this sub-region. Durban has been arbitrarily selected as the northeastern boundary of this sub-region, but East London could equally well be chosen.

G. Southeastern Australia (Peronian Province)

This sub-region lies within the 20°C and 15°C isotherms. Of the three endemic species, two appear to be siblings of widely distributed species, but the third, Metapenaeus macleayi, is distinctive within the genus and does not have obvious morphological affinities with any other species.

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136 BIOLOGY OF PENAEIDAE

H. Southwestern Australia

The warm southerly Leeuwin Current travels down the west Australian coast and across southern Australia to the sheltered, warmer waters of Spencer and St Vincent’s Gulfs (Rochford, 1986). It thus makes this part of the continent, which would otherwise be too cold, habitable for penaeids. Two endemic Metapenaeopsis species have recently been described (Manning, 1988).

I. Oceania

Most of the penaeids recorded for this very large sub-region are those with an extensive distribution throughout the Indo West Pacific, including some deep-water and pelagic species. Lack of suitable habitats in many tropical oceanic islands and wide expanses of deep oceans has probably prevented dispersal of many tropical species through this area. The South Pacific has the greatest diversity and appears to represent a progressive attenuation of the Indo-Malaysian fauna as far as Tonga (Braley, 1979; Choy, 1983). It is possible that coral-inhabiting species, such as Heteropenaeus and some Metapenaeopsis spp., occur throughout this sub- region. Only two endemic species have been described for this sub- region. Metapenaeopsis commensalis is a commensal with corals and may have a wider distribution than its type locality in the Pacific.

111. Eastern Pacific

There is a substantial change in species composition from north to south through this region, but boundaries are not well defined. Mainly on ichthyological evidence, Briggs (1974) places a boundary in the Gulf of Tehuantepec to separate Mexican and Panamanian sub-regions. For penaeids, the greatest change appears in the El Salvador region which has been adopted as the boundary. Briggs (1974) considers the Revilla Gigedo Islands as part of the Mexican sub-region, but they include one penaeid species, not yet found elsewhere. The Galapagos Islands are always treated as a separate faunistic sub-region (Briggs, 1974); only one penaeid species, not endemic, has been recorded from this area. Clipperton Island, midway between, is a coral atoll and probably does not support any penaeids. Because of a lack of faunistic records and a probably sparse penaeid fauna, these offshore islands are therefore grouped tentatively as a separate sub-region. The following sub-regions are defined:

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C

Z O O G E O G R A P H Y AND EVOLUTION 137

0

~~

FIG. 4.3. Sub-regions of the eastern Pacific, western Atlantic and eastern Atlantic regions as defined in Table 4.1.

A. Panamanian

As noted above, the placing of the northern boundary is debatable, but the southern boundary is clearly defined by the cold Peruvian Current diverting offshore around Punta Aguja, Peru.

B. Mexican

Briggs (1974) sets the northern boundary of this sub-region in the Gulf of California and on Baja California at about 23"N. However, penaeids have been recorded from San Francisco Bay, so this is defined here as the northern boundary. The penaeid population between here and 23"s is probably attenuated; penaeids only become abundant in the lower Gulf of California.

C. Offshore Islands

Revilla Gigedo, Clipperton and Galapagos Islands.

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138 BIOLOGY OF PENAEIDAE

IV. Western Atlantic

Briggs (1974) defines this region as extending from Bermuda and southern Florida through the Gulf of Mexico and the West Indies to Cab0 Frio, Brazil. Penaeidae have, however, been found as far north as Martha's Vineyard, 43"N, and a commercial fishery exists in Argentina down to Puerto de Rawson, 43"s. Briggs (1974) defines a West Indian sub-region; a coastal Caribbean sub-region, extending from southern Florida across the Gulf of Mexico along the Tropic and thence along the coast to the north of the Orinoco delta; and a Brazilian sub-region down to Cab0 Frio, Brazil. Briggs also argues against the northern part of the Gulf of Mexico being included in the tropical region, but penaeids are abundant in this area, which is included here in the Gulf of Mexico sub- region. The western Atlantic sub-regions are defined as follow:

A. Caribbean

Includes the coast from Cape Catoche, Mexico to Sao Luis, Brazil. This corresponds to sub-region 1 in the Indo-West Pacific and has the greatest penaeid species diversity (16) within the Atlantic Ocean, with three endemic species. The mouth of the Amazon, with its huge freshwater outflow does not seem to be a significant barrier to dispersal (this is also true for the mouths of the major rivers of the Bay of Bengal). Instead the boundary appears to lie around Sao Luis to the east. The strong westward current, plus a more monsoonal climate may be the cause of the sudden drop in species east of this point.

B. Sao Luis to Cab0 Frio

At Cab0 Frio the warm southerly Brazilian current moves offshore, to be replaced by a cooler-water northerly current. The resultant drop in water temperature causes a corresponding drop in species numbers to the south of Cab0 Frio.

C. Gulf of Mexico

It is difficult to see why there should be a zoogeographical boundary in the east of the Gulf, but the distribution of three Penaeus spp. stops at this point. The identity of these three forms as valid species perhaps

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ZOOGEOGRAPHY AND EVOLUTION 139

needs re-examination. Nevertheless, the low winter temperatures of the northern Gulf affect the species diversity of Penaeidae in this sub-region; there are only seven species, compared with 16 in the adjacent Caribbean.

D. Eastern USA

This extends from Martha’s Vineyard to southeast Florida. Penaeus aztecus has been recorded from Martha’s Vineyard and P. duorarum and P. setiferus from New Jersey to Virginia, but Cape Hatteras is the northern limit of abundant penaeid distribution in the western Atlantic. As this sub-region includes two species not found in the Gulf of Mexico and three not found in the Caribbean, it has been separated as a sub- region, with southeast Florida as the southern boundary.

E. Southeastern South America

This cooler-water sub-region extends from Cab0 Frio, Brazil to Puerto de Rawson, Argentina and has two endemic species: Penaeus paulensis and Artemesia longinaris. The latter is analogous in its range of temperature tolerance to Macropetasma in South Africa, but the two species are morphologically very different. Penaeus paulensis, on the other hand, is a “grooved” species and thus an analogue of P. plebejus in southeastern Australia (IWP sub-region 7).

V. Eastern Atlantic

The fully tropical region extends only from approximately Cape Verde, 15”N, to Cape Santa Marta, Angola, where the cold Benguela Current is diverted offshore (Briggs, 1974), but penaeids are found down to about Porto Alexandre, 16”S, The normal northern boundary for penaeid distribution is about the mid-coast of Portugal, although Penaeus kerathurus has been recorded in the southern English Channel. The southern Mediterranean contains an abundant penaeid fauna. It is traditionally regarded as a separate sub-region because it has little interchange with the Atlantic, but the situation is complicated by the invasion of the eastern Mediterranean by five species, via the Suez Canal, from the Indian Ocean. This has been called the “Lessepsian migration” and is fully documented by Por (1978). The penaeids are all hardy,

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140

100 -

80 -

- 8 60-

v)

$ 40-

0

8

B I O L O G Y OF P E N A E I D A E

I

S Afr EC Afr GI-RS W C I W BB E BB M-lnd

Region w 4 E

FIG. 4.4. Fall in species diversity, from east to west through the Indian Ocean, as percentage of species in the Indonesian-Malaysian sub-region. S Afr, South Africa; E D Afr, east coast of Africa; GI-RS, Gulf of Iran to Red Sea; WCI, west coast of India; W BB, west coast of the Bay of Bengal; E BB, east coast of the Bay of Bengal.

adaptable species and will, presumably, eventually reach the eastern Atlantic. Thus the two sub-regions are:

A. Eastern Atlantic (Lisbon, Portugal to Porto Alexandre, Angola, 16"s.)

B. Mediterranean Sea

VI. Distribution of Species by Region and Sub-region

The species included in the keys in Chapter 3 are listed alphabetically by region in Table 4.2. Within each region the sub-regions have been given arbitrary numbers, which are shown in Figs 4.2 and 4.3 and defined in Table 4.1. Many species have a wide distribution, some extending

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ZOOGEOGRAPHY AND EVOLUTION 141

throughout most of the Indo-West Pacific, while others appear to be restricted to a relatively small area. Some of these differences are probably artefacts arising from lack of systematic collection in some areas, compared with others (e.g. Malaysian Archipelago versus Australia or Japan). Also some species prefer habitats where prawn fishing does not normally occur, such as coral rubble, and so tend not be collected in commercial catches. This, combined with small size, may easily result in a species being overlooked or considered rare. Heteropenaeus, many Metapenaeopsis spp. and some smaller Parapenaeopsis fall into this category. Nevertheless, after taking the above factors into account, there appear to be substantial differences in distribution, both between and within genera.

The widest recorded distribution is for the pelagic genus Funchalia, some species of which have been recorded in both the Indo-Pacific and Atlantic Oceans. (If Trachypenaeopsis is monospecific, this genus would have an equally wide distribution, but at present two species, one in the Pacific and one in the Atlantic, are assigned to this genus (see Chapter 3)). Parapenaeus and Penaeopsis are deep-water genera and most Parapenaeus are widely distributed within each major region. Penaeopsis appears, however, to be less widely distributed, but it is a deeper-water species than Parapenaeus and its apparent distribution may be a function of the sampling frequency of deep-sea expeditions.

Of the inshore genera, apart from Trachypenaeopsis, Penaeus has the widest distribution, with only four out of a world total of 27 species restricted to one sub-region. High fecundity, active swimming in many cases and its position as the oldest extant penaeid genus in the fossil record (see below), may all contribute to this extensive distribution.

In contrast, the species of Metapenaeus, also an important genus commercially, are generally smaller than those of Penaeus and are mostly restricted to one or two sub-regions.

Metapenaeopsis is the largest genus and some species are quite abundant, but many prefer hard substrata (coral rubble, reef flats) and none are major target species in prawn trawl fisheries. Like Metapenaeus, the majority of the species appear to be restricted to one or two sub- regions. A number of Parapenaeopsis spp. also prefer rough substrata and are similarly restricted in distribution, but most are not abundant and only a few are target species.

Trachypenaeus is largely an Indo-Pacific genus (two Atlantic species), with varied distribution patterns. T. curvirosfris is very widely distributed, while others occur in only one or two sub-regions. The species are generally confined to the warmer waters of the penaeid range.

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142 BIOLOGY OF PENAEIDAE

TABLE 4.2. Distribution of species of the Penaeidae by region and sub-region (excluding the cosmopolitan genera Funchalia and Pelagopenaeus). Abundance (this often differs throughout the range of a species): Cl, commercially abundant; mC1, of minor commercial abundance; C, common; U, uncommon. For definitions of the sub-region code, see Table 4.1. (Species endemic to a sub-region

have only one sub-region code).

Region Abundance Sub-region

Indo-West Pacific Region (IWP) (including S . Africa)

Atypopenaeus A. bicornis Racek & Dall, 1965 U 2 A. dearmatus De Man, 1907 U 1 A. formosus Dall, 1957 mC1 2,9 A. stenodactylus (Stimpson, 1860) mC1 1 ~ 3

Heteropenaeus H. longimanus De Man 1896

Macropetasma M . africanus (Balss, 1913)

Metapenaeopsis M. acclivis (M.J. Rathbun, 1902) M. andamensis (Wood-Mason, 1891) M. angusta Crosnier, 1987 M. assimilis (De Man, 1920) M. barbata (De Haan, 1844) M. borradailei (De Man, 1911) M . commensalis (Borradaile, 1898) M. coniger (Wood-Mason, 1891) M. crassissima Racek & Dall, 1965 M. dalei (M.J. Rathbun, 1902) M. disfincta (De Man, 1907) M. dura Kubo, 1949 M . erythraea Crosnier, 1987 M. evermanni (Rathbun, 1902) M . faouzii (Ramadan, 1938) M. fusca Manning, 1987 M. hilarula (De Man, 1911) M. incompta Kubo, 1949 M. insona Racek & Dall, 1965 M. kyushuensis (Yokoya, 1933) M . liui Crosnier, 1987 M . lamellata (De Haan, 1844) M. luta Kubo, 1949 M . lindae Manning, 1987 M . mannarensis De Bruin, 1965 M . mogiensis (M.J. Rathbun, 1902)

c1 6

mCI mC1 U U c1 U U U mC1 U U U U U U U *

C U U U U mC1 C U U C

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ZOOGEOGRAPHY AND EVOLUTION 143

Region Abundance Sub-region

M. novaeguineae (Haswell, 1879) M. palmensis (Haswell, 1879) M . philippii (Bate, 1881) M. provocatoria Racek & Dall, 1965 M . quinquedentata (De Man, 1907) M. rosea Racek & Dall, 1965 M. scotti Champion, 1973 M . sibogae (De Man, 1911) M. sinuosa Dall, 1957 M . stridulans (Alcock, 1905) M. tarawensis Racek & Dall, 1965 M . toloensis Hall, 1962 M . velutina (Dana, 1852) M . wellsi Racek 1968

mC1 mC1 mC1 U U mC1 U U U mC1 U mC1 U C

Metapenaeus M. affinis (H. Milne Edwards, 1837) M . anchistus (De Man, 1920) M. bennettae Racek & Dall, 1965 M. brevicornis (H. Milne Edwards, 1837) M. conjunctus Racek & Dall, 1965 M . demani Roux, 1921 C M . dobsoni (Miers, 1878) C1 M . eboracensis Racek & Dall, 1965 c1 M. elegans De Man, 1907 mC1 M . endeavouri (Schmitt, 1926) c1 M. ensis (De Haan, 1844) c1 M . insolitus Racek & Dall, 1965 mC1 M. intermedius Kishinouye, 1900 c1 M. joyneri (Miers, 1880) Cl M . krishnatrii Silas & Muthu, 1976 M . kutchensis George, George & Rao M. lysianassa (De Man, 1888) M . macleayi (Haswell, 1879) c1 M. monoceros (Fabricius, 1798) c1 M. moyebi (Kishinouye, 1896) c1 M. papuensis Racek & Dall, 1965 M . philippinensis Motoh & Muthu, 1979 M. stebbingi Nobili, 1904 c1 M. suluensis Racek & Dall, 1965 M . tenuipes Kubo, 1949 c1

C1 c1 c1 C1 mC1

C U C1

C C

R

Parapenaeopsis P. acclivirostris Alcock, 1905 mC1 P. arafurica Racek & Dall, 1965 U P. aroaensis Hall, 1962 U P . cornuta (Kishinouye, 1900) mC1 P. gracillima Nobilii, 1903 U

2

1,2,6,7,9 1,43596 2 5,6 1 2 1,4 9 (Gilbert I.) 1 1 2

1,4 1,9 7

132 2

2 1,2,9 2 1,2,3,9 2b 13 3 Andaman Is. 4 (G. of Kutch) 1 7 1,4,5,6 1 , 2 3 3 1,2b Philippines 45,6 1,2b 1

1,4

1,4

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144 BIOLOGY OF PENAEIDAE

TABLE 4.2. continued

Region Abundance Sub-region

P. hardwickii (Miers, 1878) P. hungerfordii Alcock, 1905 P. indica Muthu, 1969 P. nana Alcock, 1905 P. sculptilis (Heller, 1862) P. stylifera (H. Milne Edwards, 1837) P. tenella (Bate, 1888) P. uncta Alcock, 1905 P. venusta De Man. 1907

Parapenaeus P. australiensis Dall, 1957 P. fissuroides Crosnier, 1985 P. fissurus (Bate, 1881) P. investigatoris Alcock & Anderson, 1899 P. lanceolatus Kubo, 1949 P. longipes Alcock, 1905 P. murrayi Ramadan, 1938 P. perezfarfante Crosnier P. ruberoculatus Hall, 1962 P. sextuberculatus Kubo, 1949

Penaeopsis P. balssi Ivanov & Hassan, 1976 P. challengeri De Man, 1911 P. eduardoi Perez Farfante, 1977 P. jerryi Perez Farfante, 1979 P. rectacuta (Bate, 1888)

Penaeus P. canaliculatus Olivier, 1811 P. chinensis (Osbeck, 1765) P. esculentus Haswell, 1879 P. indicus H. Milne Edwards, 1837 P. japonicus Bate, 1888 P. latisulcatus Kishinouye, 1896 P. longistylus Kubo, 1943 P. marginatus Randall, 1840 P. merguiensis De Man, 1888 P. monodon Fabricius, 1798 P. penicillatus Alcock, 1905 P. plebejus Hess, 1865 P. semisulcatus De Haan, 1844 P. silasi Muthu & Motoh, 1979

Trachypenaeopsis T. richtersii (Miers, 1884)

mC1 mC1 C C C1 c1 C C U

C C C C

U U C U C

mC1 c1 c1 c1 c1 c1 Cl mC1 c1 C1 c1 c1 c1 c1

U

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ZOOGEOGRAPHY AND EVOLUTION 145

Region Abundance Sub-region

Trachypenaew T. albicomw Hayashi & Toriyama, 1980 U T. anchoralis (Bate, 1881) ~

T. curvirostris (Stimpson, 1860) T. fulvus Dall, 1957 T. gonospinifer Racek & Dall, 1965 T. granulosus (Haswell, 1879) T. longipes (Paulson, 1875) T. pescadoreensis Schmitt, 1931 T. sedili Hall, 1961 T. villaluzi Muthu & Motoh, 1979

Eastern Pacific Region (EP)

Metapenaeopsis M. beebei (Burkenroad, 1938) M. kishinouyei (Rathbun, 1902) M. mineri (Burkenroad, 1934)

Parapenaeopsis P. balli Burkenroad, 1934

Penaeus P. brevirostris Kingsley, 1878 P. californiensis Holmes, 1900 P. occidentalis Streets, 1871 P. stylirostris Stimpson, 1874 P. vannamei Boone, 1931

Protrachypene P. precipua Burkenroad, 1934

Trachypenaeus T. brevisuturae Burkenroad, 1934 T. byrdi Burkenroad, 1934 T. faoe Obarrio, 1954 T. fuscina Perez Farfante, 1971 T. paciJicus Burkenroad, 1934

Xiphopenaeus X . riveti Bouvier, 1907

Western Atlantic Region (WA)

A. longinaris Bate, 1888 Artemesia

Metapenaeopsis M. gerardoi PCrez Farfante, 1971

C mC1 mC1 U U U U U U

U U C

C

C1 c1 C1 CI C1

C

mC1 mC1 mC1 mC1 mC1

C1

C1

C

2 1 2 4 1,394 1 1,45 Philippines

2 2 2,3

1

1

2 1 1 1 12

1.2

5

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146 BIOLOGY OF PENAEIDAE

TABLE 4.2. continued

Region Abundance Sub-region

M . goodei (Smith, 1885) M . hobbsi PCrez Farfante, 1971 M. martinella Perez Farfante, 1971 M . smithi Schmitt, 1924

Parapenaeus P. politus Smith, 1881

Penaeopsis P. serrata Bate, 1881

Penaeus P. aztecus Ives, 1891 P. brasiliensis Latreille, 1817 P. duorarum Burkenroad, 1939 P. notialis PCrez Farfante, 1967 P. paulensis PCrez Farfante, 1967 P. schmitti Burkenroad, 1936 P. setiferus (Linnaeus, 1767) P. subtilis PCrez Farfante, 1967

Tanypenaeus T. caribeus PCrez Farfante, 1972

Trach ypenaeopsis T. mobilispinis (Rathbun, 1919)

Trachypenaeus T. constrictus (Stimpson, 1874) T . sirnilis (Smith, 1885)

Xiphopenaeus X . kroyeri (Heller, 1862)

Eastern Atlantic Region (EA)

Metapenaeus M. monoceros (Fabricius, 1798) M . stebbingi Nobili, 1904

Metapenaeopsis M . miersi Holthuis, 1952

mC1 C C C

mC1

C

c1 C1 c1 c1 Cl c1 c1 c1

U

U

mC1 mCI

c1

mCI mC1

C

L4,5

1

1

2(via Suez) 2(via Suez)

1

Parapenaeopsis P . atlantica Balss, 1914 c1 1

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ZOOGEOGRAPHY AND EVOLUTION 147

Region Abundance Sub-region

Parapenaeus P . longirostris (Lucas, 1846)

Penaeopsis P. serratu Bate, 1881

Cl 1,2

C 1

Penaeus P. japonicus Bate, 1888 mC1 2(via Suez) P. keruthurw (Forskal, 1775) c1 132 P. notialis PCrez Farfante, 1967 c1 1 P. semisulcatus De Haan, 1844 mCI 2(via Suez)

Trachypenaeus 7'. curvirosfris (Stimpson, 1860) C 2(via Suez)

The remaining genera are small and some are monospecific and restricted to one sub-region: Macropetasma africanus to South Africa; Artemesia longinaris to southeastern South America; Tanypenaeus caribeus to the Caribbean; Protrachypene precipua to the northwest of South America. The two Xiphopenaeus spp. provide an example of twin species (Ekman, 1953), X . riveti being on the western side of the Isthmus of Panama and the very closely similar X . kroyeri on the east side. (Trachypenaeus pacijicus and T. similis are other examples of twin species.) Atypopenaeus is an Indo-West Pacific genus with two of its four species restricted to northern Australia, one rare, and the fourth fairly widely distributed.

VII. Penaeid Zoogeography, the Fossil Record and Paleogeograp hy

While the overall distribution of penaeids conforms to that of other tropical and subtropical shallow-water marine animals (Ekman, 1953), there are some unusual features. The temperate sub-regions of South America, South Africa and Australia, all include unique species (Table 4.3). Of these, each sub-region has at least one that may be a sibling of a more tropical species in an adjacent sub-region (South America: Penaeus paulensislP. aztecus ; South Africa: Metapenaeopsis scottilM. philippii ; southwestern Australia: M . fuscalM. barbata; M . lindaelM. acclivis ; southeastern Australia: Penaeus plebejuslP. latisulcatus; Metapenaeus

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148 BIOLOGY OF PENAEIDAE

bennettaelM. moyebi). However, Artemesia in South America and Macropetusma in South Africa are quite unlike other penaeid genera and are also quite unlike one another. Australia does not have any unique genera, but Metapenaeus macleayi is not an obvious sibling of any other Metapenaeus sp. Africa, South America and Australia were all part of Gondwana and the unique penaeid fauna in the south of each of these continents suggests that they could be relicts of this former land-mass. South America and Africa separated first and thus have two genera not found elsewhere; Australia separated last (about 80 million years ago) and has only one unique species and three sibling species.

A. Continental Drift and Warmwater Faunas

In the upper Cretaceous (about 80 million years BP) the temperature of the ocean adjacent to Gondwana was above 20°C (Shackleton and Kennett, 1975). There is good evidence that Australasia was then part of Antarctica, but South America and Africa had been separated for some time and India was just moving away (Zinsmeister, 1979, 1982). Warmwater decapod crustacean fossils from the early Tertiary have been recorded from Antarctica (Feldmann and Zinsmeister, 1984a, b). This and temperature data indicate that a penaeid fauna might have existed in these waters, at least in the late Cretaceous, but later molluscan fossil evidence suggests that such a fauna could not have persisted (Zinsmeister, 1982). As Australasia moved northward, there were extreme environ- mental changes. Firstly, the temperature fell to less than 10°C as the ocean became continuous around Antarctica about 40 million years BP, and continued to fall through the remaining Tertiary. Meanwhile, as Australasia moved northwards, the temperature of its waters rose to over 20°C about 30 million years BP and the endemic fauna was replaced by warmwater invading species from the Indo-Pacific. The temperature fell

TABLE 4.3. Species unique to sub-regions of the southern continents.

S.E. S . America South Africa S.W. Australia S.E. Australia (Sub-region WA 5) (Sub-region IWP 6) (Sub-region IWP 8) (Sub-region IWP 7)

Artemesia Macropetasma Metapenaeopsis Penaeus plebejus

Penaeus paulensis M . lindae Metapenaeus bennettae

M . macleayi

longinaris africanus fusca

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ZOOGEOGRAPHY AND EVOLUTION 149 again during the Miocene, approaching 10°C in the Pliocene-Pleistocene. Thus it is most unlikely that any penaeid species of southeastern Australia are relicts from Gondwana, but as South Africa and South America separated earlier, Macropetasma and Artemesia could be such relicts. The rate of evolution of the Penaeidae, as seen from their fossil record, and their genetics, could resolve this question.

B. The Fossil Record

The Penaeoidea are very abundant in many warm seas today and it seems likely that they would have been similarly abundant in previous geological epochs. Their fossil record is, however, very incomplete, because Crustacea are poor candidates for fossilization, as discussed by Bishop (1986). Plotnick (1986) has demonstrated that shrimp are unlikely to be fossilized under their normal environmental conditions. Because of this, it is not surprising that many of the fossil Penaeoidea appear to have been more heavily armoured than recent species, and thus represent species more likely to be fossilized. Most have been found in various shales of central Europe, Great Britain and the eastern Mediterranean (Woods, 1925; Glaessner, 1969), with a few in North America (Herrick and Schram, 1978).

Formerly, the Penaeoidea were regarded as the most primitive group of the Decapoda, because of the nauplius larval stage, some morphological features and the fossil record; the earliest occur in the lower Triassic, whereas the Caridea did not appear until later (Calman, 1909). The concept that the Dendrobranchiata, which includes the Penaeoidea, diverged from the parent stock before the other major extant decapod groups has been supported recently by Felgenhauer and Abele (1983). In contrast, Schram (1982) proposes a much earlier origin for all the major decapod groups, rather than the classical “phyletic tree” concept. Schram et al. (1978) redescribed Palaepalaemon newberryi, noting that it possessed characters of both palinurans and astacurans and placing it in the Pleocyemata. Felgenhauer and Abele (1983) suggest that the large scaphocerite also indicates a natantian affinity. Schram et al. (1978) assigned the genus to the upper Devonian, making it by far the oldest indubitable decapod so far described. Schram and Mapes (1984) described Zmocaris, a dromiacean from the lower Carboniferous, citing it as further evidence that there was considerable diversity and radiation in the Malacostraca in the late Paleozoic (Schram, 1977; Schram and Horner, 1978; Schram, 1981). No Penaeoidea have so far been described from the Palaeozoic, but Felgenhauer and Abele (1983) point out that the

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150 BIOLOGY OF PENAEIDAE

Penaeidae are advanced Dendrobranchiata and were well established by the Permo-Triassic, so the Dendrobranchiata must have been established earlier than this period. Schram (1977; 1982) tentatively places the origins of this group in the Carboniferous.

A number of genera of Penaeidae have been described from the Mesozoic. Aeger first appears in the upper Triassic, while Acanthochirana, Dusa and Antrimpos (Fig. 4.5) have been found in Jurassic deposits (Glaessner, 1969; Herrick and Schram, 1978). The resemblance of Antrimpos speciosus (Fig. 4.5) to recent Penaeus spp. is obvious and some authors have included Antrimpos with Penaeus (Glaessner, 1969). Undoubted Penaeus spp. have also been found in Jurassic shales (Woods, 1925) and become more common in the Cretaceous, with a record from India in the lower Tertiary (Glaessner, 1969). Thus, on fossil evidence, Penaeus is the oldest extant genus of the Penaeidae. Unfortunately, no penaeids have yet been found in more recent deposits, so there is no indication from the fossil record when other existing penaeid genera may have arisen.

In such a situation, estimation of the times at which divergences of the various taxa occurred has to be made by other means. The classical method is by comparison of morphological features, but calculation of genetic distances of the various taxa may be used where appropriate data are available, or in other cases, palaeogeography. Burkenroad (1983), summarizing his earlier taxonomic work, divided the Penaeidae into three tribes based on morphological features that he believed were basic: the Peneini (Penaeus , Heteropenaeus, Funchalia, Pelagopenaeus) , the Para- peneini (Parapenaeus, Artemesia, Penaeopsis, Metapenaeopsis) and the Trachypeneini (Metapenaeus, Macropetasma, Trachypenaeopsis, Atypo- penaeus, Protrachypene, Xiphopenaeus, Parapenaeopsis, Trachypenaeus). However, the phylogenetic importance of external morphological features, when considered in isolation, tends to be very subjective. For example, Burkenroad regarded fixed spines on the telson as a basic distinction between his Parapeneini and Trachypeneini, but Heldt (1938) showed clearly that the spines of Parapenaeus become fixed only later in larval development. Also, there are anomalous species with fixed spines in genera that otherwise have movable spines (Chapter 3). Kubo (1949) constructed a phylogenetic tree for the Penaeoidea, based on a complex classification of morphological features. He distinguished five groups within the Penaeidae: Penaeus ; Penaeopsis ; Atypopenaeus, Trachy- penaeopsis, Metapenaeus ; Parapenaeus, Parapenaeopsis, Trachypenaeus ;

FIG. 4.5. Reconstructions of fossil Penaeidae from the Triassac and Jurassic. Scale represents 1 cm in all cases. (Redrawn after Glaessner, 1969)

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ZOOGEOGRAPHY A N D EVOLUTION 151

L(

Antrimpos kiliani

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152 BIOLOGY OF PENAEIDAE

Metapenaeopsis. The Parapenaeus group diverged a little before the others, but the remaining four groups diverged together. Although he attempted a more sophisticated treatment than Burkenroad (1983), Kubo’s (1949) conclusions were still necessarily subjective. Such analyses lend themselves to the use of computer similarity matrices, but this has yet to be done, and at present the only way of estimating divergence times is from recent research on biochemical genetics of the Penaeidae and from palaeogeography.

C. Genetic Diversity and Evolution within the Penaeidae

Redfield et al.(1980), using electrophoresis of a range of tissue proteins, found that the heterozygosity of a number of tropical decapod Crustacea was very low, the lowest being Penaeus merguiensis. Mulley and Latter (1980) analysed 37 genetic loci in 13 Australian penaeid species from two genera and confirmed that their genetic diversity was comparable with that of Penaeus merguiensis. A similar conclusion was reached by KO (1984) for 14 species from different geographic localities, while Salini and Moore (1985) found even lower levels of heterozygosity in Metapenaeus bennettae. This is a largely estuarine species, and only in populations separated by 900 km was there any evidence of significant genetic divergence (Salini, 1987). Mulley and Latter (1981a, b) found that significant genetic divergence between populations of Penaeus latisulcatus did not appear until the distances separating them were even greater. It appears that genetic diversity in the Penaeidae is amongst the lowest recorded for any animals.

The low heterozygosity of the Penaeidae presumably makes them more vulnerable to selection pressures than animals with high heterozygosities. Reduction in the size of the breeding population would have the effect of lowering the genetic diversity even further. This was demonstrated by Sbordoni et al. (1986), who found a reduction of about 60% average heterozygosity in artificial populations with small numbers of breeding animals. Thus a serious bottleneck in a breeding population could render the population more vulnerable to selection pressures, but Mulley and Latter (1980) suggest that this is extremely unlikely for marine species such as the Penaeidae. Selective elimination of mutational variation at the majority of genetic loci appears to be the most likely explanation for the low levels of heterozygosity that have been recorded. Nevertheless, dramatic environmental changes, such as the Quaternary Glacial Epoch, could have induced bottlenecks in breeding populations, with resulting breaks in population structures. In contrast, the high fecundity of

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ZOOGEOGRAPHY A N D EVOLUTION 153

penaeids would result in non-lethal mutations becoming rapidly distributed through the population. Although heterozygosity is low, this does not necessarily mean that the rate of mutation is low and there is indirect evidence that considerable divergence has occurred in the Penaeidae in recent geological epochs.

Nei (1972, 1975) has demonstrated that genetic distance (D) between populations can be simply related to genetic similarity or identity ( I ) :

D = - logel

and that if a number of assumptions are made,

t = DI2a

where t = time after divergence of two populations and a = rate of gene substitution per year.

While I may be determined with some precision from electrophoresis of tissue proteins, Nei (1975) points out that our current estimate of the constant a is very crude. An average rate of lo-' is suggested. From the data of Mulley and Latter (1980) genetic identity, I , for the genera Penaeus:Metapenaeus is 0.39 giving an estimate of t = 4.7 X lo6 years, that is, the two genera separated in the late Pliocene. Decreasing a to lo-' places the time of separation at the beginning of the Tertiary, while an estimate of places it in the middle Pleistocene. At present, there is no reliable way of calibrating a, but since Penaeus is the only existing genus so far found in the early Tertiary, is a reasonable lower limit for rate of gene substitution in the Penaeidae. Values of D within Penaeus and Metapenaeus range from 0.18 to 0.55 (Mulley and Latter, 1981a, b), giving an estimate of separation of species ranging from the middle of the Tertiary to the late Pleistocene, again a plausible estimate.

It is, however, possible to estimate the time of separation of penaeid genera from palaeography. Contintental drift data show that the North Atlantic opened up and the Tethys Sea became circum-equatorial during the Jurassic; this situation persisted into the Tertiary (Smith et al., 1981). This is supported by evidence from warmwater fossils (Ekman, 1953). The first fossil Penaeidae were found in the Jurassic, and presumably also became circum-equatorial in distribution. This distribution was interrupted with the closing of the Mediterranean in the Tertiary, and finally the Atlantic was separated from the Indo-West Pacific in the Miocene about 20 million years BP (Sclater and Tapscott, 1979; Smith et al., 1981). The land bridge between North and South America was interrupted several times in the Tertiary, but fossil evidence suggests that the eastern Pacific was separated from the western Pacific also in the Miocene period, so any subsequent exchange of tropical species would have been with the

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154 BIOLOGY OF PENAEIDAE

western Atlantic (Ekman, 1953). It may be assumed, therefore, that genera that are now well

represented in both Atlantic and Indo-West Pacific regions were in existence before the tropical Atlantic was closed off. Only four can be said with certainty to be common: the ancient Mesozoic genus Penaeus and the presumably more recent Metapenaeopsis, Parapenaeopsis and Trachypenaeopsis. However, only one species of Parapenaeopsis occurs in the Atlantic and this is confined to the eastern part and it may be a post- Miocene arrival. There are five species of Trachypenaeus in the eastern Pacific, but only two in the western Atlantic, of which one, T. similis, appears to be a twin species with T. paciJicus. Thus this genus may have entered the western Atlantic from the eastern Pacific relatively recently. The pelagic genus Funchalia and the deep-water Parapenaeus and Penaeopsis cannot be included because of the possibility of a more recent exchange via the open sea. Of the remaining genera, three (Atypopenaeus, Macropetusma, Metapenaeus) are endemic to the Indo-West Pacific; three (Artemesia, Tanypenaeus, Xiphopenaeus) are endemic to the Atlantic; and one (Protrachypene) to the eastern Pacific, with Xiphopenaeus (another example of twin species) shared between this area and the western Atlantic. These distributions suggest that at least half of the present penaeid genera originated in the late Tertiary, that is, less than 20 million years BP.

The estimate of 4.7 X lo6 years for the separation of Metapenaeus and Penaeus is therefore plausible. With cx = the estimate would be about 15 million years. Alternatively, Metapenaeus may have separated more recently than other endemic Indo-West Pacific genera, but there are no electrophoresis data to support this. Some of the data on speciation do, however, support Nei’s (1975) estimate of a = lop7.

Penaeus latisulcatus is a very wide-ranging species, while the sibling species, P. plebejus, is restricted to east-southeastern Australia. The very close similarity to the parent species suggests that it separated very recently, most probably during the Quaternary Ice Age, when falling sea levels created a land bridge across Torres Strait between Australia and New Guinea. Present depths are only 15-20 m in the shallowest part, so the separation would have begun at the beginning of the glacial period, roughly 1.8 million years BP (Holmes and Holmes, 1964). Southeastern New Guinea, with deep water close inshore, appears to be a barrier to penaeid distribution, which would probably have been enhanced by falling sea levels. Mulley and Latter (1980) estimate that the genetic distance between P. latisulcatus and P. plebejus is 0.35 f 0.11, giving t = 1.8 x lo6 years. Similarly, Metapenaeus bennettae was probably separated from the parent M. moyebi (= M. dalli) in the same manner as

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ZOOGEOGRAPHY AND EVOLUTION 155 P. plebejus. Salini and Moore (1985) estimate the genetic identity between the two species as 0.83, giving t = 0.9 X lo6 years, later than P. plebejus, but still well within the last glacial epoch. Genetic distances for other Penaeus spp. and Metapenaeus spp. are comparable (Mulley and Latter, 1981a, b). If the above assumptions are correct, a large proportion of the present-day penaeid species could have arisen during the last glacial epoch.

The data compiled by Cline (1981) show that, during the last glacial maximum, the minimum winter surface isotherms of 15°C and 20°C differed very little in position from those shown in Fig. 4.1, but there was a much steeper gradient at higher latitudes. Thus the tropical penaeid populations were not subjected to markedly lower temperatures. The lowered sea levels, however, must have had a significant effect on the shallow-water penaeids. On most existing continental shelves the area between 150 m and the outer edge around 200 m is relatively much smaller than that between 0-150 m. Since the maximum fall in sea level is estimated at 135 m (Holmes and Holmes, 1964), the area of present continental shelves was greatly reduced during this period. This may not have significantly affected the population density of the Penaeidae, since most species inhabit a narrow shallow-water fringe along the coast, but it probably increased the likelihood of separation of populations, with deep water close inshore along most tropical coastlines.

A more decided effect was the creation of land barriers critical to penaeid distribution. The most extensive was around the Malaysian Archipelago (Fig 4.6). Deep water south of Sumatra, probably unfavour- able currents, and the land barrier between Malaysia, Sumatra, Borneo and Java, effectively cut off the penaeid populations to the east from those of the Indian Ocean. The present extensive tropical shallow waters were reduced to narrow fringes around the deep Banda and Celebes Seas and a small area in the South China Sea, while the present Timor and Arafura Seas were largely dry land. As shown in Fig. 4.4 there is a constant decrease in species abundance from Malaysia through the Bay of Bengal to the Arabian Sea. The separation of these two areas in the Quaternary Glacial Period by the Malaysian land mass (Fig. 4.6) may be at least partly responsible for this effect, although it is necessary to postulate that there was either a low species diversity pre-glacially in the western region, or a reduction in diversity during the glacial period. Thus there are 25 endemic species in the Indo-Malaysian sub-region, but only three in the Arabian Sea (Table 4.1). As the land barrier disappeared with rising sea levels, migration of species to the west began, giving the species gradient seen today.

Other significant Pacific Ocean land barriers were the separation of the

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156 BIOLOGY OF PENAEIDAE

south China Sea from the eastern tropical seas (Fig 4.6) and the Torres Strait-Arafura Sea land bridge, discussed above. In the Indian Ocean, Sri Lanka was connected to the mainland and the Gulf of Iran and the Red Sea were closed, but otherwise there were no significant changes, except that the present narrow continental shelf of the east African coast would have been even narrower. No great changes occurred in the eastern Pacific or western Atlantic, but in the eastern Atlantic, southerly cool currents would have moved the northern limits of the penaeid population down to the westerly tip of Africa (Cline, 1981).

Another possible source of "new" penaeid genera is the deep sea. Beurlen (1930) presents evidence that a number of decapod genera, including the Penaeoidea, retreated into deeper waters at the end of the Mesozoic. These genera could well readapt to shallower water, particularly in higher latitudes. As noted in Chapter 3, Macropetusma is unique in the Penaeidae in possessing abdominal photophores, a feature found in only two other penaeoid species, both deep sea. This suggests Macropetusma originated in deep water. Also, Artemesiu superficially resembles two

FIG. 4.6. Maximum extent of shorelines in the Indo-West Pacific during the Quaternary Ice Age, with sea levels 135 m below present levels (dotted lines ).

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ZOOGEOGRAPHY AND EVOLUTION 157

deep-water penaeoid genera (Bate, 1888), again suggesting a deep-water origin. Unfortunately for this hypothesis, these, or closely similar penaeid genera, have not been recorded in deeper waters.

There is no valid reason at present for suggesting that Macropetasma and Artemesia are relict genera from Gondwana. The use of electro- phoretic techniques could help to decide this, by determination of their specific identities with the mesozoic genus Penaeus, but it is more likely that the two genera arose in the Tertiary, together with other extant penaeid genera. Adaptation to cooler water by these genera is not exceptional. Penaeus chinensis, morphologically very close to the tropical P. merguiensis-P. indicus group, has adapted to the cold waters of the Yellow Sea. This adaptation must have been post-glacial, since the whole area was dry land during the Quaternary Glacial Epoch (Fig. 4.6). P. plebejus and P. paulensis have both adapted to waters down to lWC, while P. aztecus has been recorded from Martha's Vineyard in eastern USA. Determination of the specific identity of P. paulensis with the closely related western Atlantic P. aztecus would provide interesting evidence of time of separation. Unlike P. latisulcatus and P. plebejus populations, there was no obvious land bridge during the Quaternary Glacial Epoch to divide them, but from their close morphological similarity, the separation of P. paulensis and P. aztecus (and probably other western Atlantic grooved Penaeus spp.) may have been recent.

In summary, the Penaeidae probably arose in the Palaeozoic, were well established in the Mesozoic and diverged into a number of genera, which were distributed around the tropics in the Tethys Sea. Penaeus is the only genus that has survived through the Cainozoic, but a number of new genera arose throughout the Tertiary, about half in the latter part of this era. Conditions in the late Tertiary and Quaternary, such as lowering of sea levels and the formation of land-bridges during the Quaternary Glacial Epoch, enhanced speciation. Rising post-glacial sea levels permitted the spread of these populations, giving the distribution of species seen today.