[advances in marine biology] the biology of the penaeidae volume 27 || 9. food and feeding

9. Food and Feeding

This chapter describes the types of food eaten by penaeids, feeding periodicity and variation. Biochemical aspects of ingestion and assimilation are dealt with in Chapter 5 , Physiology.

1. Food

The diet of penaeids was first described by Williams (1955). He stated that the material in the stomachs of prawns from estuaries in the eastern United States was finely triturated and difficult to identify but was probably a mixture of partly digested tissue and organic deposits from the bottom together with fragments of crustaceans, annelids and plants. The macerated condition of the contents of prawn guts makes food items of penaeids difficult to identify under a microscope: nevertheless most of our knowledge of penaeid diets has come from this method. More recently, stable carbon isotope ratios (Hughes and Sherr, 1983) and immunological techniques (Hunter and Feller, 1987) have been used in diet analysis, and are especially valuable when visual identification fails.

The condition of the gut contents prevents the use of most methods of quantifying diet. Techniques such as weighing the food, measuring the size of prey or reconstituting prey cannot be used. Consequently most workers have restricted their analysis to frequency of occurrence, although this gives little information about the relative importance of the items (Hyslop, 1980). The relative amounts of each food category are occasionally estimated. Where the food can be identified into even broad taxonomic categories such as crustaceans or molluscs, numerical compo- sition can be used in statistical tests of comparisons.

There is no evidence that food is a density-dependent limiting factor for penaeids and there are no integrated studies of penaeid food requirements in relation to food availability. Stephenson (1980), however, found a negative correlation between the quantity of benthos in Moreton Bay,

315

316 BIOLOGY OF PENAEIDAE

Australia, and the abundance of prawns and suggested that large prawn populations could deplete the benthos. There is also no evidence for resource partitioning among penaeids and, as shown below, there are considerable overlaps between the diets of different species from the same area (e.g. Wassenberg and Hill, 1987a; Stoner and Zimmerman, 1988).

There are published descriptions of the diet of nearly 40 species of penaeid (Table 9.1). Their food falls into three major categories: microbial and detrital material, plants and animals.

A. Microbial and Detrital Material

Dall (1968) suggested that prawns feed on bacterial colonies and filamentous blue-green algae together with an associated fauna of protozoans, harpacticoid copepods and nematodes. Subsequent studies, however, have shown that bacteria contribute less than 2% of the organic matter in the food of adult penaeids (Moriarty and Barclay, 1981). Many authors have claimed that penaeids are scavengers that feed mainly on organic detritus (e.g. Hindley, 1975b). This conclusion was based largely on studies that suggested that prawn diets were composed of unidentifiable material that was probably of detrital origin (e.g. Odum and Heald, 1972). Detritus was defined by Darnel1 (1967) as “all types of biogenic material, in various stages of microbial decomposition, which represents potential energy sources for consumer species”. Velimirov et al. (1981) described detritus as dead material with an unrecognizable origin. Nearly all food items ingested by prawns have been macerated by the mouthparts and gastric mill so they are dead and, because they are broken and damaged it may be difficult to decide their origin. This, however, does not mean that they should be classified as detritus. The problem lies with identification and with care most of the material can be identified. Semi- digested molluscs, crustaceans and polychaetes should not be described as debris or detritus merely because they are not intact.

Some material in penaeid guts does appear to be detritus and this appears to be almost exclusively derived from plants. Extremely large amounts of plant detritus are found in some coastal areas. The extensive Zostera beds in North Carolina estuaries, for example, contain at least four times as much dead plant material as living (Adams and Angelovic, 1970). Along the coasts of the Gulf of Mexico, Spartina salt marshes produce massive quantities of plant detritus. Mangrove estuaries also may have large amounts of detritus. The postlarvae and juveniles of many penaeids are found in shallow inshore areas that are also often the sites of

FOOD AND FEEDING 317

TABLE 9.1. Genera and species of penaeids for which the diet has been described. Only cases where at least 10 animals from the wild were examined have been

included.

Species Reference

Macropetasma africanus

Metapenaeopsis barbata goodei stridulans

Cockroft and McLachlan, 1986

Hall, 1962 Huff and Cobb, 1979 Hall, 1962

Metapenaeus ajjinis brevicornis dobsoni endeavouri ensis

macleayi monoceros muratk

Parapenaeopsis coromandelica hardwickii hungerfordi sculptilis

tenella venusta

(= ajjinis)

Parapenaeus fissurus longirostris

Penaeus aztecus brasiliensis canaliculatus duorarum

esculentus indicus japonicus kerathurus latisulcatus

Kuttyamma, 1974; Su and Liao, 1984 Subrahmanyam, 1974 Menon, 1952; Kuttyamma, 1974; Subrahmanyam, 1974 Moriarty and Barclay, 1981 Moriarty and Barclay, 1981; Su and Liao, 1984; Tiews et al., 1972 Ruello, 1973a George, 1974; Kuttyamma, 1974; Subrahmanyam, 1974 Hall, 1962

Hall, 1962 Hall, 1962 Hall, 1962 Subrahmanyam, 1974

Hall, 1962 Hall. 1962

Hall, 1962 LagardCre, 1972

Burukovsky, 1975; Hunter and Feller, 1987 Burukovsky, 1975 Tiews et al, 1972 Eldred et al, 1961; Sastrakusumah, 1971; Burukovsky, 1975; Huff and Cobb, 1979 Moriarty and Barclay, 1981; Wassenberg and Hill, 1987a Gopalakrishnan, 1952; Rao, 1967; Subrahmanyam, 1974 Piscitelli and Liaci, 1985 Burukovsky, 1975 Moriarty and Barclay, 1981; Rao, 1967; Thomas, 1973; Subrahmanyam, 1974


TABLE 9.1. continued

Species Reference

merguiensis Tiews et al. 1972; Subrahmanyam, 1974; Chong and Sasekumar, 1981; Moriarty and Barclay, 1981; Robertson, 1988 Thomas, 1973; Kuttyamma, 1974; Subrahmanyam, 1974; Rao, 1967; Mohanty, 1975; Marte, 1980; Moriarty and Barclay, 1981; Luna-Marte, 1982; El Hag, 1984; Su and Liao, 1984

monodon

notialis Formoso and Anderes, 1982 pencillatus Hall, 1962 schmitti Formoso and Anderes, 1982 semisulcatus

setiferus Hunter and Feller, 1987 vannamei Juarez, 1976

Moriarty and Barclay, 1981; Su and Liao, 1984; Thomas, 1980; Tiews et al, 1972; Wassenberg and Hill, 1987a

Trachypenaeus curvirostris Hall, 1962 fulvus Hall, 1962

large amounts of detritus and significant quantities of plant detritus have been found in the guts of juveniles and adults of many species of prawns. Detritus makes up almost 50% of the gut content of juvenile Mefupenaeus mucleuyi from estuarine waters in Australia (Ruello, 1973a). Ruello identified this detritus as coming from mangroves (Avicennia) and reeds (Phragrnites). Chilka Lake in India also has a lot of plant detritus on the bottom and P. monodon from the lake have large amounts in their foreguts (Rao, 1967; Mohanty, 1975). Robertson (1988) found large amounts (74% by volume of gut content) of detrital floc in small (7-10 mm CL) P. rnerguiensis from mangrove creeks in northern Australia.

Because detritus is frequently found in the foreguts of prawns, many workers have concluded that detritus is an important food source for penaeids (e.g. Flint and Rabalais, 1981). Robertson (1988) points out, however, that the nutritional role of detritus is difficult to assess. It is a complex of particulate matter derived from the breakdown of plants bound up with small organic particles derived from the breakdown of faecal pellets of fish and crustaceans. The organic particles are often bound together in a matrix with diatom particles and bacteria.

Several authors have questioned whether detritus is used directly by penaeids. Penaeids may in fact obtain nourishment from the micro- organisms associated with the detritus rather than directly from detritus.


Condrey et al. (1972) suggested that benthic algal and microbial communities on dead Spartina were the most likely food sources for penaeids. Hughes and Sherr (1983) measured I3C values of organisms in a Georgia estuary and concluded that P . duorarum in tidal creeks are more dependent on carbon from planktonic and benthic algae than from Spartina detritus. Using stable carbon isotopes as tracers, Gleason (1986) found that, although P. aztecus eat Spartina detritus, the prawns do not incorporate carbon from the detritus into their tissues. They do, however, take up carbon from the diatom Skeletonema living on Spartina. Postlarval P . aztecus fed on a diet of Spartina detritus that was free of associated macro and meio-fauna and flora did not grow over a 16-day period, whereas those fed on diatoms (Skeletonema) and on detritus carrying epiphytes showed significant growth (Gleason and Zimmerman, 1984). Gleason and Wellington (1988) measured growth rates of postlarval P. aztecus in field and laboratory experiments and found that neither detritus of Spartina and Hernifiora nor their associated epiphytes contributed much to nutrition in this species. Stoner and Zimmerman (1988) similarly have shown that even the smallest P . notialis, P . subtilis and P . brasiliensis from mangrove creeks in Puerto Rico are predators of benthic animals. Although mangrove detritus made up 20-25% of the diet, carbon isotope analysis indicated that the prawns were not deriving carbon from mangroves but from algae.

These studies show that plant detritus itself is not a major food source for prawns. Detritus is never the sole item found in the foregut. Chong and Sasekumar (1981) suggested that P. merguiensis ate detritus only when animal food was unavailable. Penaeus indicus and P . monodon in Chilka Lake feed on crustaceans, plants and bivalves as well as detritus (Rao, 1967). Similar results have been reported for other species of penaeids by many authors (e.g. Marte, 1980; Thomas, 1980; Wassenberg and Hill, 1987a; Stoner and Zimmerman, 1988).

B. Plant Food

The amount of plant material reported in the diets of penaeids varies widely even within one species. In P. monodon, for example, it ranges from 0 to 100% of the gut content (Table 9.2). The type of plant is frequently not stated; typically it is identified merely as “plant remains” (Kuttyamma, 1974; Gopalakrishnan, 1952). There are two categories of plants that could potentially be eaten by penaeids: firstly emergents (e.g. terrestrial plants, mangroves, Spartina and Phragmites) together with submerged macrophytes (seagrasses) ; and secondly algae, including free-


TABLE 9.2. Percentage frequency of occurrence of three categories of food in the diet of P. monodon as recorded by various authors.

Food Stage Detritus Animal Plant Authority

Juvenile Adult Juv. + adult Juv. + adult Adult Juv. + adults

(not stated) Adult “Estuarine” “Sea” “Estuarine”

0 8-20 0 0 0

44 0

72 78 96 0

0 5-26

18 20-60 78 30 41

78 88 98

22-78

100 6-24 8 5-20

<26 17 12 6

39 0 2

El Hag, 1984 El Hag, 1984 Hall, 1962 Kuttyamma, 1974 Luna-Marte, 1982 Mohanty, 1975 Rao, 1967 Su and Liao, 1984 Subrahmanyam, 1974 Subrahmanyam, 1974 Thomas, 1973

living forms and epiphytes. Penaeids rarely eat emergent plants. Analysis of 13C/12C ratios in P. aztecus and P. setiferus led Fry and Parker (1979) to suggest that terrestrial plant material is not an important carbon source for prawns. Stoner and Zimmerman (1988) demonstrated that mangrove tissue is not a food source for penaeids. The “vegetable matter”, “plant material” and “higher plant material” reported by some authors are probably the remains of aquatic macrophytes. Even if these plants are ingested, mere occurrence in the gut does not prove they are digested. Kitting et al. (1984) investigated whether penaeids really assimilate this material; their measurements of 13C ratios show that juvenile P. aztecus and P. duorarum from seagrass beds do not obtain organic carbon from seagrass. Wassenberg and Hill (1987a) found that over 90% of juvenile P. esculentus from seagrass beds in Moreton Bay, Australia, had seeds of Zostera capricorni in their foreguts in summer suggesting selective feeding on the seeds.

There is a clear trend for juvenile rather than adult penaeids to eat plant material. Species in which plants have been found either exclusively or to a far greater extent in the juveniles include Metapenaeus monoceros (George, 1974; Subramanyam, 1974), P . esculentus (Wassenberg and Hill, 1987a), P. indicus (Gopalakrishnan, 1952; Subramanyam, 1974), P. merguiensis (Chong and Sasekumar, 1981; Robertson, 1988), P. monodon (Mohanty, 1975) and P. semisulcatus (Thomas, 1980; Wassenberg and Hill, 1987a). The most herbivorous penaeid is P. monodon. Primavera and Gacutan (1989) maintained juveniles for 30 days on diets of either live or decaying aquatic macrophytes. Survival of


prawns fed live Najas grarninea was better (100%) than those fed on live (59%) or decaying (65%) Ruppia maritima.

Algae, and especially epiphytic algae, are commonly eaten by juvenile penaeids. Some juvenile M . dobsoni examined by Menon (1952) were practically full of Cladophora. Kitting et al. (1984) concluded from a study of 13C ratios in animals and plants in seagrass meadows that epiphytic algae are the main source of carbon for juvenile P. aztecus and P. duorarurn. Direct observations at night showed that the prawns feed preferentially on epiphytes near the tops of seagrass fronds. They found that production by epiphytic algae exceeds that of seagrass and is capable of sustaining heavy grazing. Von Prahl (1980) found that the first postlarvae of P. stylirostris are capable of digesting wax from the leaves of mangroves by means of wax degrading enzymes. After removal of the wax, mangrove leaves are colonized by epiphytes that can be eaten by postlarval prawns. Despite these observations on feeding by juvenile penaeids on epiphytes, epiphytic algae appear to be only a minor nutritional source for penaeids (Gleason and Wellington, 1988). Never- theless, algae are probably a major source of carbon for penaeids even though this may be via intermediate prey (Stoner and Zimmerman, 1988).

Diatoms are not eaten in large amounts by adult prawns. Chong and Sasekumar (1981) noted the near absence of diatoms and blue-green algae in the diets of P. rnerguiensis, although these were abundant on the substratum.

C . Animal Food

The foreguts of 765 individuals from 31 species of penaeids were found by Hall (1962) to contain mainly small crustaceans (49% by frequency of occurrence) followed by vegetable material (39%), large crustaceans (27%), polychaetes (26%) and molluscs (18%). Hall found large differences between genera with respect to the most abundant food categories. Polychaetes were the most common food item in Trachy- penaeus, whereas crustaceans were the most common in Penaeus, Parapenaeopsis and Metapenaeopsis. Subrahmanyam (1974) found crustaceans, molluscs (bivalves and gastropods), fish and foraminiferans to be the main dietary items of seven species of penaeid from the Bay of Bengal. Moriarty and Barclay (1981) reported that the main components of the diet of seven species of penaeid from the Gulf of Carpentaria were foraminiferans, small crustaceans, molluscs and polychaetes.

These and other studies of individual species identify the same prey


groups: crustaceans, molluscs, polychaetes and foraminiferans (Gopala- krishnan, 1952; Rao, 1967; Dall, 1968; Sastrakusumah, 1971; Ruello, 1973a; George, 1974; Kuttyamma, 1974; Mohanty, 1975; Marte, 1980; Thomas, 1980; Chong and Sasekumar, 1981; Luna-Marte, 1982; El Hag, 1984; Wassenberg and Hill, 1987). The crustaceans are chiefly copepods, amphipods, tanaids, mysids, and carid and penaeid shrimps. Penaeus aztecus and P. setiferus also feed on crabs (Hunter and Feller, 1987). The molluscs eaten by penaeids are mainly bivalves, but in shallow waters gastropods are also important. The only echinoderms eaten in any quantity are ophiuroids; prawns swallow the arms but not the discs. Coelenterates are rarely eaten although Gopalakrishnan (1952) found remains of hydroids in P . indicus and observed the prawns attacking and eating small ctenophores and medusae in aquaria. Foraminiferans are frequently found in penaeid guts. They are the most common food of P . semisulcatus, P. cunaliculatus, Metapenaeus monoceros and P. merguiensis caught offshore in the Philippines (Tiews et al., 1972). A variety of foraminiferans are eaten but as they occur sporadically in diets, this probably reflects their distribution. Carids feed on meiofauna (animals that can pass through a 500 ym mesh) (Bell and Coull, 1978), but there is no proof that, under natural conditions, juvenile or adult penaeids feed directly on meiofauna other than copepods. Insects, especially dipteran larvae and ants that are common on mangrove sediments, made up to 9% by volume of the gut content of small (11-15 mm CL) P. rnerguiensis from mangrove creeks in Australia (Robertson, 1988).

Few penaeids feed on plankton. Macropetasma africanus, which lives in the surf zone in southern Africa, and P. merguiensis in the Philippines, are exceptions (Cockroft and McLachlan, 1986 and Tiews et al., 1972 respectively).

Penaeids can catch mobile free-swimming prey; the remains of fish and squid form a major part of the diet of several species of penaeid, e.g. P . indicus and Metapenueus ensis (Hall 1962), P. semisulcatus (Thomas, 1980), P. merguiensis (Chong and Sasekumar, 1981) and P. monodon (Luna-Marte, 1982) and, on a few occasions, also P. duorarum (Sastra- kusumah, 1971). The foregut contents of a sample of P. plebejus from Sydney Harbour consisted almost entirely of the mysid Rhopalophthalmus dakini (Suthers, 1984). Juvenile and adult P . merguiensis in Malaysia feed on sergestids, mysids and fish (Chong and Sasekumar, 1981). Weymouth et ul. (1933) found that, in aquaria, P. setiferus attacked and ate fish and other shrimp. Racek (1959) reported almost identical behaviour for P . plebejus and P. esculentus.

Flint and Rabalais (1981) suggested that an additional food source for prawns could be material discarded from trawlers. Sheridan et al. (1981)

FOOD A N D FEEDING 323

thought that penaeids are the main scavengers of this material in the Gulf of Mexico. Direct observations in Australian waters, however, found no evidence of penaeids feeding on trawler discards (Wassenberg and Hill 1987b).

II. Feeding Behaviour

A. Locating and Handling of Food

Penaeid prawns search the bottom for food by probing with their pereopods; any food found is picked up and manipulated by their pereopods and mouthparts (see Chapter 2). Hindley and Alexander (1978) described the feeding behaviour of the banana prawn P. merguiensis. The prawn holds its first three pereopods in line at right angles to the long axis of the body and uses their distal segments to make rapid probes in the substratum as it walks over the bottom. The width of the resulting search path is equivalent to about twice the length of the carapace. Large prawns walk faster than do small ones (Hill, 1985), and so the area searched is related to the size of the prawn. Adult (30 mm CL) P. esculentus walk about 190 mlnight (Hill, 1985). Assuming a search path width of 60 mm (twice carapace length), these prawns could search an area of about 11 m2 in a night. Penaeus merguiensis can use a strong beat of the pleopods to blow away the substratum to form a small excavation about 1 cm deep. It then probes at the collapsing sides and picks up and eats food it finds (CSIRO, unpublished observations). When offered a large piece of food, Macropetasma africanus seizes it, holds it against the mouthparts with the maxillipedes and swims away (Cockroft and McLachlan, 1986). Penaeus merguiensis also swim up off the bottom when they find a large piece of food but P. esculentus remain on the bottom.

As well as feeding on small isolated particles or organisms, penaeids can cope with large food items such as algal mats or large live prey. Penaeus setiferus and P. aztecus tear off a portion of algal-microbial mats and pass it to the mouthparts (Condrey et al . , 1972). They then hold it in the exhalent respiratory current and rotate it to wash most of the silt from the mat. Once it is clean, they tear off the peripheral area and swallow it. Racek (1959) reported that P. plebejus and P. esculentus that had been starved in aquaria for a few days would attack smaller prawns or newly moulted individuals of nearly their own size. Two or three would force a newly moulted prawn out of the sand and chase it. When they caught it, they invariably first attacked and ate the eyes.


Prawns respond to chemical stimuli in the water. The initial response to a food odour is an upstream rheotactic movement (Hindley, 1975b). Low concentrations ( 10-5-10-6~) of amino acids elicit a feeding response in P. merguiensis: they walk faster (up to 3 c d s ) , probe deeper into the substratum and spread their limbs wider (Hindley and Alexander, 1978). L-isoleucine appears to be important in inducing a feeding response in penaeids (Dos Santos Filho, 1982). The first three pereopods carry chemosensory setae that could be used to detect food particles; these are then picked up by the chelae and passed to the mouthparts (Hindley, 1975b). The mouthparts are more discriminatory than are the chelae and occasionally reject material passed to them by the chelae (Ruello, 1973a). Masticatory movements of the mouthparts are induced by exposure to amino acid concentrations of around 10-'-10-*~; this is far higher than that required to induce the initial feeding response (Hindley, 1975b). Thus P. merguiensis has two levels of sensitivity to food stimuli: a low concentration chemoreception that allows food to be detected at a distance, and a high concentration or contact chemoreception that enables the prawn to distinguish food from other material before swallowing it (Hindley, 1975b).

B. Ingestion and Foregut Volume

Although penaeids shred and crush most of their food, they can swallow small food items whole. Wassenberg and Hill (1987a) found intact gastropods and bivalves 1.3-2.0 mm in diameter and scaphopods 2.0- 3.0 mm long in the foregut of adult P. esculentus. Large items are macerated by the mouthparts. Sastrakusumah (1971) often found only parts of animals in the foregut of P. duorarum, possibly because the prey animals were too large to swallow whole.

The foregut of penaeids is small; Hill and Wassenberg (1987) found that the volume (V in pl) of the foregut of P. esculentus and P. semisulcatus could be expressed as a power function of the carapace length (CL in mm) as follows:

V = 0.0093 CL2.818 ( r = 0.96, n = 60).

The volume of the foregut of a 30 mm CL adult is about 140 p1. This is the maximum expanded volume of the foregut when removed from the animal and filled with water. The foregut cannot expand to this volume in the animal, however, because of the muscles surrounding the gastric mill and the limited space in the cephalothorax. Even when P. esculentus is given unlimited access to food, the volume of food in its foregut is only


about 60% of the maximum expanded volume (Hill and Wassenberg, 1987).

According to Alexander and Hindley (1985), juvenile P. merguiensis can ingest a piece of food in less than 20 s. At this rate, the foregut is filled rapidly. Dall (1967a) found that the anterior chamber of the foregut of M. bennettae filled to capacity within 1 min when a starved prawn was given food. Penaeus esculentus takes about 10 min to fill its foregut (Hill and Wassenberg, 1987). As feeding proceeds the digestive gland fills and ingestion slows. According to Balasubramanian et al. (1979), when M . dobsoni are offered excess food (tanaid crustaceans), they feed continuously for at least 2 h but there is a rapid fall off in the number of tanaids eaten in successive 15-min periods: from 7.1 tanaids in the first period down to 1.7 after 1 h and 0.9 after 2 h.

C . Feeding Periodicity and Foregut Clearance

Most penaeids spend the day buried in the substratum and emerge and feed at night. Examination of the foregut contents of P. esculentus caught in the wild showed that the quantity of food in the foregut increased after sunset (Wassenberg and Hill, 1987a). Penaeus esculentus do not start feeding immediately after emerging from the substratum; on average, they begin feeding 40 min later and their feeding activity peaks 2 h after dark (Hill and Wassenberg, 1987). Reymond and Lagardkre (1988) found that in P. monodon, feeding by 26-27 day-old animals (mean weight 0.5 g) showed no die1 periodicity, at 43-44 days (mean weight 3.2 g) slightly more feeding took place at night than during the day and at 62-63 days (mean weight 7.0 g) feeding was almost entirely at night beginning at sunset and decreasing after dawn. McTigue and Feller (1989) found little evidence of any periodicity in feeding by juvenile P. setiferus apart from a slight increase after dawn.

Because of the small volume of the foregut of penaeids, they have to feed several times each night in order to obtain sufficient food. Under natural conditions, where food items are small and dispersed, searching and ingestion probably continue through the night and rates of ingestion and egestion are probably similar (Dall, 1968). This allows the foregut to be filled repeatedly and enables penaeids to take in considerably more food than if they fed only once per night. Sedgwick (1979b) found experimentally that P. merguiensis increases weight more rapidly and utilizes food more efficiently when fed four times daily than when fed the same total ration only once a day. Direct observations showed that P. esculentus feed about six times each night (Hill and Wassenberg,


1987). These feeding bouts are separated by non-feeding periods of about 40 min in which the food is digested and the foregut partially cleared. About 75% of the food is cleared in 1 h in P. esculentus (Hill and Wassenberg, 1987). In P. monodon about 50% is cleared in the same period (Marte, 1980). Defecation in M . bennettae begins 1 h after feeding, reaches a peak in 4-6 h and ceases after 8 h (Dall, 1968). Similar periods have been reported for Mucropetasma africanus (Cockroft and McLachlan, 1986), P. stylirostris and P. californiensis (Arosamena, 1976), and P. vannamei (Juarez, 1976). Because penaeids clear their guts rapidly, samples for gut analysis should be collected shortly after feeding.

Cuzon et al. (1982) found that delaying the time of introduction of food for 3 h after dark increased the growth rate of P. japonicus by 2349%. They explained this by pointing out that production of digestive enzymes in penaeids peaks 3-4 h after the onset of darkness. In addition, proteins, carbohydrates and water-soluble vitamins leach rapidly from pelleted formulated foods. Delaying the introduction of the feed ensures it becomes available only when the prawns are ready to feed and minimises losses due to leaching.

D. Selectivity in Feeding

Penaeids have been described as “opportunistic omnivores”. Ruello (1973a) compared the diet of M . macleayi captured in estuarine and coastal oceanic waters with the availability of food items and concluded that they do not show any food preferences. Dall (1968) did not find any important differences in food preferences among P. esculentus, P. merguiensis, P. plebejus, M . bennettae and M . macleayi in the laboratory. Tiews et al. (1972) identified 37 genera of foraminiferans in the gut of M . monoceros and 34 in P. merguiensis and concluded that penaeids do not prefer any particular genus. Other authors, however, suggest that prawns are selective feeders. In Chilka Lake the diet of P. indicus contains less animal tissue (22% frequency of occurrence) and more higher plant material (21%) than does the diet of P. monodon (41% and 11% respectively) (Rao, 1967). Unfortunately it is not clear whether the prawns were all collected from the same site at the same time. The diets of juvenile P. esculentus and P. semisulcatus collected in the same trawls showed significant differences (Wassenberg and Hill, 1987a); Penueus semisulcatus ate more bivalves, crustaceans and foraminiferans and fewer gastropods than did P. esculentus. These results indicate that these two species select slightly different food under natural conditions.


Under experimental conditions, penaeids show preferences when given a choice. Postlarval P. aztecus and P. setiferus both preferred Artemia to artificially formulated foods (Karim and Aldrich, 1976). Penaeus setiferus was more selective than P. aztecus and the authors suggested that this could be a factor in the wider geographic distribution of the latter species. P. monodon in a Y-maze preferred fish to the algae Cludophora (El Hag, 1984). In this experiment, however, choice could have been affected by the fish providing stronger waterborne stimuli than the algae. Liao (1969) found that P. japonicus clearly preferred short-necked clams and polychaete worms to fish flesh. Metapenaeus dobsoni show a clear preference for one species of tanaid crustacean (Apseudes chilkensis) over another (Apseudes gymnophobia), regardless of whether the prey are offered live or dead (Balasubramanian et al., 1979). Metapenaeus dobsoni prefer A. chilkensis to the amphipod Eriopisa chilkensis. When penaeids and bivalves are simultaneously available to P. esculentus, it feeds on both but prefers one. The choice is not modified by a previous history of feeding on only one of the foods offered (Hill and Wassenberg, 1987). Penaeus esculentus prefers the penaeid M. bennettae to the mussel Perna canaliculatus and prefers this mussel to the penaeid P. longistylus. It can distinguish between two species of bivalves, and prefers the clam Donax deltoides to the mussel Perna canaliculatus. In field caging experiments, P. duorarum showed a statistically significant preference for polychaetes, decapod and tanaid crustaceans over other crustaceans and molluscs (Nelson, 1981).

Although prawns have been regarded as scavengers, they do not prefer dead food. Metapenaeus dobsoni prefer live and freshly killed organisms to dead and decaying animal matter (Balasubramanian et al., 1979). When given fresh prawn and bivalve tissue at dusk, P. esculentus initially feed at a high rate but this declines after the food has soaked for a few hours and they spend nearly three times more time feeding before than after midnight (Hill and Wassenberg, 1987). If additional fresh food is offered after midnight, the amount of time spent feeding before and after midnight is the same. After midnight they spent about 5 min feeding on the old food compared to 32 min on the fresh food, this shows a clear preference for the fresher food. Postlarval and juvenile P. duorurum feed less frequently the longer the food has been in the tank (Sick and Baptist, 1973; Sick et al . , 1973).

Penaeus esculentus has preferences when offered a choice between two types of food, but they also eat some of the less preferred food. Thus even when a preferred type of food is available, penaeids have a mixed diet. Chamberlain and Lawrence (1981) found that P. vannamei grow more rapidly when fed a composite diet of squid, penaeid, polychaete and


clam flesh than when fed on only one of these foods. Penaeus juponicus grow faster on a diet of anchovy and clam than on a diet of only one or the other (Choe, 1971).

E. Effects of Starvation

Starvation for six days results in a progressive increase in activity in P. esculentus. This includes spending a longer time emerged each night, a greater frequency of walking and swimming and a tendency to emerge during the day (Hill and Wassenberg, 1987). Racek (1959) noted that, whereas starved M . mucleayi become considerably more active when starved, M . bennettae become less active and eventually remain buried until food is again offered.

111. Variation in Diet

Several factors can cause penaeids to change their rate of feeding: physiological factors such as age, sex and moult stage; spatial factors such as habitat and availability of food; and factors with a temporal component including time of day or night, season and tide. Some of these factors are linked; for example, as prawns grow they generally change habitat. Consequently the prime source of variation is often not easily identified.

A, IntraspeciJic Variation

Reports of the diet of individual species of penaeids are conflicting. In P. monodon for example, the reported frequency of occurrence of the three categories of food varies between zero and nearly loo%, and there is no discernible trend in the data (Table 9.2). For example, Penaeus monodon caught in estuaries by Subrahmanyam (1974) had been feeding heavily on detritus whereas estuarine individuals caught by Thomas (1973) had not eaten any (Table 9.2). Reports differ for nearly all species that have been studied by more than one worker. Some of the differences can be explained, for example differences in the diet of P. semisulcatus from the Philippines. Tiews et al. (1972) found foraminiferans to be the major food but Marte (1980) found none. This apparent discrepancy is probably due to the source of their material. Marte (1980) obtained her prawns from an estuary whereas Tiews et al. (1972) collected their prawns from offshore and noted that the incidence of foraminiferans in diets was

FOOD A N D FEEDING 329

greater in offshore than in inshore samples. Thus one of the sources of the disparity seen in Table 9.2 is almost certainly the different types of food available to prawns in different areas.

Wassenberg and Hill (1987a) found significant differences in the diet of P. esculentus from three widely separated regions in Australia; this was more marked in juveniles. The percentage frequency of occurrence of bivalves varied between 1 and 15% (i.e. a factor of 15X) for juveniles but only between 34 and 49% (a factor of 1.5X) for adults. Ophiuroids ranged between 0.1 and 5% for juveniles and between 14 and 18% for adults. Such large differences seen in the same species from different areas invalidate comparisons between species unless all the material was collected at the same time and from the same site or unless persistent trends are seen in large samples from several different regions.

B . Physiological Causes of Variation

The diet of many species of prawns changes significantly with size or age. There appear to be two main reasons for this, first a change in habitat as they grow, and second a switch away from a juvenile diet incorporating plants to an adult diet that may be exclusively carnivorous. Examples of the latter include juvenile P. monodon in the Red Sea that are exclusively herbivorous and feed on green algae and unidentified plant matter, whereas adults are omnivorous with a diet of crustaceans, annelids and algae (El Hag, 1984). Juvenile M. afJinis similarly feed to a large extent on vegetable matter, whereas adults eat mainly polychaetes (Kuttyamma, 1974). Examples of a change in diet with habitat include that of M . macleayi in which Ruello (1973a) found that the change in diet with age reflected the difference in the type of food available to the estuarine juveniles and offshore adults. Chong and Sasekumar (1981) reported that the main food of pelagic postlarvae of P. merguiensis was calanoid copepods; in the nursery population (juveniles of 9-21 mm CL), the most common identifiable organisms were plants, calanoid copepods and for- aminifera; sub-adults (15-33 mm CL) captured in inshore waters mainly ate sergestids and mysids; adults (23-45 mm CL) from offshore waters had eaten considerable amounts of polychaetes and bivalves. Chong and Sasekumar (1981) suggested that the diet of P. merguiensis is related to availability, but that there were indications of food selection, and the low occurrence of crustaceans in young P. merguiensis could reflect an inability to handle active animals such as sergestids and mysids.

In addition to the change in the amount of plant material eaten with size, there are other variations in diet with size that are not related to


differences in habitat. The diet of P. esculentus for example changes with prawn size even in samples collected at the same time from one site (Wassenberg and Hill, 1987a). The frequency of bivalves, gastropods and ophiuroids in the diet of P. esculentus rises with increasing carapace length whereas the frequency of copepods declines. The incidence of bivalves, gastropods and ophiuroids, however, does not change with size and is relatively high even in small individuals (12 mm CL) (Wassenberg and Hill, 1987a). Stoner and Zimmerman (1988) reported similar changes in diet with size in three species of penaeid from a mangrove estuary in Puerto Rico. This change with size is not found in all species. Hunter and Feller (1987) noted that although the larger P. azrecus and P. setiferus tended to eat larger prey, there was little indication of a change in diet with size. Kuttyamma (1974) found no size-related changes in the diet of P. indicus over the range 29-132 mm TL, confirming earlier observations by Gopalakrishnan (1952) on this species. Sastrakusumah (1971) found no change in diet with size of P. duorarurn collected from estuarine areas.

The only record of a marked difference in diets between sexes is that for P. sernisulcatus from Philippine waters (Tiews et al., 1972). Foraminiferans make up 75% of the diet of males but only 37% of the diet of females. Females eat far more phytoplankton (43% of diet) than do males (6% of diet). There were only minor differences in diet between male and female P. merguiensis, P. canaliculatus and M . rnonoceros (Tiews et al., 1972). There were no significant differences in diet between male and female P. esculentus and P. sernisulcatus from Australian waters (Wassenberg and Hill, 1987a).

There appears to be no information on any changes in diet over the moult cycle, but Dall (1986) has shown that food consumption of P. esculentus falls in the two nights preceding ecdysis. Huner and Colvin (1979) found P. californiensis and P. stylirostris ate less for 12-24 h before and 6-12 h after ecdysis. They noted that the period of reduced feeding is longer for large individuals than for small ones. Bursey and Lane (1971b) found that P. duorarum stopped feeding 36 h before ecdysis and began again about 36 h after ecdysis.

C. Environmental Causes of Variation in Diet and Quantity of Food Eaten

The time of day or night when prawns are collected affects the quantity of food in the foregut. Penaeus esculentus emerge and feed at night so specimens caught immediately after emergence in the evening have empty guts (Wassenberg and Hill, 1987a). Penaeus duorarum also feed mainly at night but feed during the day under turbid conditions (Sastrakusumah,

FOOD AND FEEDING 33 1

1971). It is not known whether species that do not bury during the day, such as P. merguiensis, have any die1 variation in feeding behaviour. Sick and Baptist (1973) found that light intensity does not affect the food uptake of postlarval P. duorurum; this stage does not bury and may not be very sensitive to light. The feeding rate of the juvenile stage of P. duorurum is higher in bright than in dim light, apparently because unlike most penaeids, this species is more active during the day than at night (Sick et u f . , 1973).

Diet may vary seasonally but it is not clear whether this is because of changes in availability, or because the prawns’ preferences change with season. For example, polychaetes, molluscs and crustaceans are the main components of the diet of P. semisufcutus in some months, but they are rarely eaten in others (Thomas, 1980). There is more algal material in the diets of P. indicus, M . dobsoni, M . ufjinis and M . monoceros in monsoon than in dry months (Kuttyamma, 1974). Animal material also varies seasonally; crustaceans are more common in M. dobsoni foreguts in the monsoon period, and in M . ufjinis and P. indicus in the post-monsoon period. Molluscs and fish vary more seasonally in the diet of P. monodon than do crustaceans, and the quantity of food varies seasonally with three peaks in the year (Luna-Marte, 1982). Although these peaks and spawning activity do not coincide, Luna-Marte (1982) suggested that changes in diet may be related to gonad development. Nearly all dietary items of P. merguiensis varied every month over 15 months, but peaks in particular food items did not occur at the same time each year (Chong and Sasekumar, 1981). Seasonal variation was also found in the proportion of P. duorurum with food in their guts (Sastrakusumah, 1971). In the cold winter months, most P. duorurum sampled had empty guts; the amount of food in foreguts rose briefly in spring but fell rapidly in early summer to the winter levels. In the second half of summer feeding was at a high level. Liao (1969) found that food consumption by P. juponicus at 15°C is only 20% of that at 25°C. Brisson (1977) found seasonal variations in the quantity of food eaten by P. brusiliensis and P. puufensis were caused by the effects of temperature and moonlight. There was a positive relationship between food uptake and temperature in the range 20-27°C. Food consumption was lowest at full moon; it then rose through the last quarter and new moon to peak around the first quarter, before falling again around full moon. This resulted in an overall reduction in food consumption in winter relative to summer but with monthly peaks and troughs that tended to obscure the seasonal trend. Stoner and Zimmerman (1988) found that seasonal changes in the diet of three species of penaeids could be related to changes in availability of prey.


Many penaeids have strong tidally based patterns of behaviour. Feeding by adult P. monodon in estuaries, reaches a peak on the ebb tide and drops to the lowest level on the flood tide (Marte, 1980). Penaeus duroarum show the opposite response. According to Sastrakusumah (1971), considerably more prawns caught on rising tides had food in their stomachs than those caught on falling tides.

[advances in marine biology] the biology of the penaeidae volume 27 || 9. food and feeding

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