I

THE INTERTIDAL LIFE OF RUAMAHUA-ITI AND MIDDLE ISLAND (ALDERMEN ISLANDS)

by Anne B. Saies*

S U M M A R Y

The distribution of intertidal organisms of Ruamahua-iti and Middle Island, two islands of the Aldermen Islands group, is investigated and results compared with those from similar studies of the shores of New Zealand's mainland and off-shore islands.

I N T R O D U C T I O N

Environmental factors, both physical and biological, act upon organisms so that the shore range occupied by the main population of any plant or animal species is restricted to certain zones, usually in a vertical succession. A study of the pattern of zonation on any shore is a convenient way to ascertain the degree of exposure to the sun and sea with which intertidal organisms must contend, and also reflects the nature of the substrate on which the organisms live.

The survey made on the Auckland University Field Club scientific trip in May, 1972 provides a comparison for patterns of zonation on shores of varying substrate and exposure, and also establishes the distributional relationships of intertidal organisms on Ruamahua-iti, Middle Island, the mainland, and other off-shore islands.

M E T H O D S

Transects of the shore were made at low tide on Ruamahua-iti (six transects) and Middle Island (two transects).

Quantitative sampling (for sessile organisms) and qualitative sampling (for mobile organisms) was carried out at convenient intervals down the shore, along the transects. Seaward-facing rock surfaces were sampled on solid rock shores. Each station (labelled A , B, C , . . . etc. in Figs. 3 to 7) consisted of one square metre of surface substrate for solid rock shores, and one square metre of shore surface excavated to a practicable depth on boulder, pebble, or sand beaches.

Snorkelling was used in the vicinity of Transect 3 to ascertain the species of fish which invade the intertidal area when the tide is rising.

Distributions of sessile animals and plants are presented as plain kites in Figs. 3 to 7, the width of each kite representing the percentage cover of the substrate by the species indicated. Stippled bars in Figs. 3 to 7 represent the range of the shore occupied by individuals o f the vagrant species.

* Department of Geology, University of Auckland.

2

RUAMAHUA-ITI

METRES 250

Fig I. Location of transects around Ruamahua-iti, and nature of shore substrate.

MIDDLE ISLAND

N

0 KEY

• a Pebbles,gravel o r sand solid rock

SCALE

Fig 2. Location of transects on Middle Island, and 0 METRES 5C0

nature of shore substrate.

3

R E S U L T S

Transect 1. (see Fig. 3) On the south-western side of Ruamahua-iti (Fig. 1), the shore was exposed to

waves refracted around the islet on which the transect was made, and also to storm waves. The shore was sheltered from the direct effects of the prevailing north-easterly swells. The transect was made over a solid breccia substrate, the vesicles providing micro-habitats for small organisms such as juvenile Melarapha oliveri.

Key to Figs. 3 -8 : * Characteristic zoning organisms at each station.

< ; S>- Vagrant organisms - range of shore. Attached organisms - vertical scale represents % cover of substrate.

Key to Fig. 3. Kites

1. Xanthoria sp. (yellow lichen) 2. Lecanora sp. (white lichen) 3. Verrucaria sp. (black lichen) 4. Chamaesipho brunnea (surf acorn barnacle) 5. Arthopyrenia foveolata (black marine lichen) 6. Apophloea sinclairii (encrusting red alga) 7. Chamaesipho columna (small acorn barnacle) 8. Coralline paint 9. Laurencia sp. (small red alga)

10. Xiphophora chondrophylla var. minor (brown alga) 11. Carpophyllum angustifolium (large brown alga) 12. Melarapha oliveri (periwinkle) 13. Cellana radians (large limpet) 14. Lepsiella scobina (small barnacle borer) 15. Notoacmea parviconoidea (small limpet) 16. Thais orbita (large barnacle borer, formerly Neothais scalaris) 17. Notoacmea pileopsis (open shore limpet) 18. Cellana ornata (conical limpet) 19. Patelloida corticata (stellate limpet)

4

Transect 2. (see Fig. 4) The transect was located on the southern side of Ruamahua-iti (see Fig. 1)

and was relatively protected from north-easterly swells, although exposed to storm waves. The shore consisted of large boulders (average diameter about 2 metres) which would not be moved by storm waves.

Key to Fig. 4.

Kites

1. Xanthoria sp. (yellow lichen) 2. Lecanora sp. (white lichen) 3. Verrucaria sp. (black lichen) 4. Lichina sp. (black stubbly lichen) 5. Viva lactuca (sea lettuce) 6. Apophloea sinclairii (encrusting red alga) 7. Codium adhaerens (massive green alga) 8. Coralline Paint 9. Gelidium pusillum (red alga)

10. Nerita melanotragus (black grazing gastropod) 11. Chamaesipho columna (small acorn barnacle) 12. Melarapha oliveri (periwinkle) 13. Cellana radians (large limpet) 14. Cellana ornata (conical limpet) 15. Siphonaria zelandica and Patelloida corticata (stellate limpets) 16. Notoacmea pileopsis (open shore limpet)

14 12 "j a 6 4 2 0 M E T R E S

Transect 3. (see Fig. 5) The site chosen was located on the eastern side of Ruamahua-iti, and was

consequently exposed to swells from the north through to the east, and also to pounding from storm waves arriving from these directions.

The shore was solid rock, terraced at its lower levels (stations H to N ) .

Key to Fig. 5. Kites

1. Chamaesipho brunnea (surf acorn barnacle) 2. Arthopyrenia foveolata (black marine lichen) 3. Ulva lactuca (sea lettuce) 4. Coralline paint 5. Splachnidium rugosum ('gummy weed') 6. Nemastoma oligarthra (red alga) 7. Chamaesipho columna (small acorn barnacle) 8. Novastoa lamellosa (vermetid gastropod) 9. Laurencia sp. (small red alga)

10. Xiphophora chondrophylla var. minor (brown alga) 11. Gigartina alveata (tufted alga) 12. Carpophyllum angustifolium (large brown alga) 13. Melanthalia abscissa (red alga) 14. Melarapha oliveri (periwinkle) 15. Nerita melanotragus (black grazing gastropod) 16. Apophloea sinclairii (encrusting red alga) 17. Lepsiella scobina (small barnacle borer) 18. Notoacmea pileopsis (open shore limpet) 19. Thais orbita (large barnacle borer) 20. Notoacmea parviconoidea (small limpet) 21. Cellana ornata (conical limpet) 22. Sypharochiton pelliserpentis (chiton) 23. Cellana radians (large limpet) 24. Patelloida corticata (stellate limpet) 25. Evechinus chloroticus (sea urchin)

6 Transect 4. (see Fig. 6) The transect was made down a boulder beach on the north-eastern side of

Ruamahua-iti (see Fig . 1). The beach was sheltered at either end by rocks which continued out into the sea. Hence the waves reaching the shore were those refracted round the rocks and those from the swell which entered directly into the bay.

A l l boulders on the beach were easily movable, ranging in size from about 30 cm to 60 cm diameter. The larger boulders tended to be distributed over the upper and lower shore.

Key to Fig. 6.

Kites

1. Ligia novaezelandiae (shore isopod) 2. Melarapha oliveri (periwinkle) 3. Anisodiloma lugubris (beaded top shell) 4. Marinula filholi (ear shell) 5. Cyclograpsus insularum (orange-pink crab) 6. Nerita melanotragus (black grazing gastropod) 7. Apophloea sinclairii (encrusting red alga) 8. Zediloma arida (spotted top shell) 9. Perinereis sp. (polychaete worm)

10. Coralline paint 11. Trachelochismus pinnulatus (lumpfish) 12. Codium adhaerens (massive green alga) 13. Atalacmea fragilis (flat limpet) 14. Cellana radians (large limpet)

7

Transect 5. (see Fig. 7) The transect was located on a north-facing pebble beach on Ruamahua-iti

(Fig . 1). Pebbles were about 4cm in diameter and were considerably moved about, causing the form o f the beach to alter with every tide.

Key to Fig. 7.

Kites

1. Centipede 2. Ligia novaezelandiae (shore isopod) 3. Orchestia sp. (amphipod)

I **T i i i i i i i i i ..

7

2 0 18 16 14 12 10 8 6 4 2

8

Transect 6. (see Fig. 8) The transect was located on Middle Island (Fig. 2), on the north-western

shore. The transect was made over solid rock and was open to the prevailing swells.

Key to Fig. 8.

1. Chamaesipho brunnea (surf acorn barnacle) 2. Chamaesipho columna (small acorn barnacle) 3. Coralline paint 4. Lichina sp. (black marine lichen) 5. Apophloea sinclairii (encrusting red alga) 6. Novastoa lamellosa (vermetid gastropod) 7. Xiphophora chondrophylla var. minor (brown alga) 8. Carpophyllum angustifolium (large brown alga) 9. Melarapha oliveri (periwinkle)

10. Siphonaria zelandica (stellate limpet - large body) 11. Cellana ornata (conical limpet) 12. Cellana radians (large limpet) 13. Lepsiella scobina (small barnacle borer) 14. Notoacmea pileopsis (open shore limpet) 15. Patelloida corticata (stellate limpet)

Kites

::*> 9

13 12

O

12 10 B 6 A O M E T R E S

9

Transect 7. A transect on the sand beach on Middle Island revealed no life beneath the

surface.

Other observations. The sheltered shore bivalve Crassostrea glomerata (rock oyster) was noted on

Middle Island at Severn Cove. Snorkelling over the area of Transect 3 revealed the following fish species in

the intertidal area:

Aplodactylus arctidens (marble fish) Parma microlepis (black angelfish) Chromis dispilus (demoiselle) Scorpis aequipinnis (blue maomao) Coridodax pullus (butterfish) Navodon convexirostris (leatherjacket) Pseudolabrus celidotus (paketi) Tripterygion varium (blenny)

D I S C U S S I O N A N D C O N C L U S I O N S

From results of transects made over solid rock (Transects 1, 2, 3, and 6) a general pattern of zonation was apparent. Characteristic zone-forming organisms (on solid rock shores) for the Aldermen Islands are: Xanthoria sp. and Lecanora sp. at the top of the shore; Verrucaria sp., Melarapha oliveri, Chamaesipho brunnea, Chamaesipho columna, Apophloea sinclairii and/or Novastoa lamellosa, coralline paint, small red algae, Xiphophora chondrophylla; and at low tide level Carpophyllum angustifolium.

As Transect 4 was the only boulder beach survey recorded for the Aldermen Islands, difficulty was encountered in constructing a general pattern of zonation of dominant organisms. The boulder beach between Transects 5 and 1 on Ruamahua-iti (see Fig. 1) provided a further opportunity to study the boulder beaches of the island, but results were not recorded. The following dominant zoning organisms were recognised on the boulder beaches of Ruamahua-i t i : Ligia novaezelandiae, below the surface layers of rock at the top of the shore; Melarapha oliveri, on the upper surfaces of the top layer of boulders; Marinula filholi; Cyclograpsus insularum, Atalacmea fragilis (below the surface) and Apophloea sinclairii (on the surface); Zediloma arida and Trachelochismus pinnulatus; Codium adhaerens; and coralline paint at sea level.

From the patterns of zonation for solid rock, boulder beach, pebble beach, and sandy beach transects, the effects of shore mobili ty are clearly demon­strated. The diversity of organisms attached to the substrate decreases as shore mobili ty (a function of particle size) increases. A marked increase in mobile organisms is noted on mobile boulder beaches. Accompanying the enhanced numbers of mobile organisms on boulder beaches, diversity of form in these species is obvious.

On solid rock transects, mobile organisms, e.g. limpets and chitons, are

10

restricted to flattened forms which offer less resistance to oncoming waves, or show behavioural adaptations, e.g. barnacle borers sheltering in crevices. In contrast, boulder beach animals are adapted to move rapidly to sheltered depths below the upper layers o f boulders which move considerably during the storms.

On more mobile shores, few animals are able to survive the almost continual movements of the substrate. Hence, though species are limited on the pebble beach, a shifting sandy beach is even more azoic.

The exposure range for the two islands studied was narrow, compared to that of the mainland, but slight differences in patterns of zonation around Ruamahua-iti indicated that the more exposed snores were on the north-eastern side of the island, and relatively sheltered shores were those on the southern side. This result is expected, as the predominant swells are generated to the north-east. On the eastern transect (Transect 3) open shore algae e.g. Splachnidium rugosum, Nemastoma oligarthra, and Melanthalia abscissa, form definite bands on the shore. In contrast, Transect 2, on the southern side of Ruamahua-iti, has none o f these algae, but larger zones of Apophloea sinclairii and Codium adhaerens are apparent. The proportion at Chamaesipho brunnea is reduced on this transect, indicating the less exposed nature of the shore.

In comparison to studies on the mainland of New Zealand, made by Morton and Miller (1968), the shores of the Aldermen Islands are highly exposed. Zonation patterns for the Aldermen Islands are markedly similar to those described by Morton and Miller for Ocean Beach, Whangarei Heads (zonation of algae), and for Pacific Coast, Whangarei Heads.

Distribution of dominant organisms on solid rock transects are closely similar at the Poor Knights Islands (Morton and Miller , 1968) and the Aldermen Islands. The presence o f zones of Novastoa lamellosa on these islands indicates that the Aldermen Islands are bathed by a subtropical current, as is the case of the Poor Knights Islands. Further evidence for the subtropical current passing the Aldermen Islands is the occurrence of fish species with subtropical affinities (see Grace, 1973) and the exceptional clarity of the water.

Zonation patterns on the Aldermen Islands indicate a slightly greater degree of exposure than Red Mercury Island (Saies et al, 1972). The presence of Novastoa lamellosa and clearer water o f the Aldermen Islands contrasts with the situation at Red Mercury Island, and it is assumed that the difference between the two localities is due to differences in hydrology of the areas.

Boulder beach transects made on the Aldermen Islands reveal zonation patterns of intertidal organisms which are basically similar to those recorded by Morton and Miller (1968) for Smuggler's Bay, Whangarei Heads. A notable difference is the absence of Cyclograpsus lavauxi at the Aldermen Islands (also noted by Bacon, 1971) and its replacement by Cyclograpsus insularum. Small colonies o f Cyclograpsus insularum have been recorded by Bacon (1971) at the northern end of Smuggler's Bay, Whangarei Heads. The presence of C. insularum only on the Aldermen Islands is contrasted by the occurrence of both species of Cyclograpsus on Red Mercury Island (Saies et al., 1972). This is probably due to the reduced quantities o f sand and silt beneath boulders on the Aldermen Islands (see Bacon, 1971).

Boulder beach zonation patterns have also been described for Little Barrier Island by Trevarthen (1953), where zonation patterns are similar to those of the

11

Aldermen Islands. It is l ikely that Trevarthen identified both species of Cyclograpsus as C. lavauxi in his study, as both species have been recorded on Litt le Barrier Island by Bacon (1971) and Doherty (1972).

Distribution of boulder beach organisms on the Aldermen Islands are also consistent with those recorded for Otata Island (Traverse 1) by Wood (1963), with the exception of C. insularum, which is absent at this locality.

The Aldermen Islands exhibit patterns of zonation of intertidal organisms basically the same as those observed on the shores of New Zealand's mainland and off-shore islands, but modifications caused by special conditions, e.g. differences in hydrology and exposure, give the shores of the islands their own characteristic distributional patterns.

A C K N O W L E D G E M E N T S

I should like to thank Dr R . V . Grace and Prof. J .E . Morton for advice and helpful comments on the manuscript. Thanks are also due to Mr B.W. Hayward for supplying drawing equipment.

REFERENCES

Bacon, M.R. 1971: Distribution and ecology of the crabs Cyclograpsus lavauxi and C. insularum in northern New Zealand. N.Z. Jl. mar. Freshwat. Res. 5 (3 and 4): 415-26.

Doherty, P.J. 1972: The physical and biological characteristics of boulder beaches. Unpublished Zoology IIIB project, University of Auckland.

Grace, R.V. 1973: A checklist of fishes of the Aldermen Islands, north-eastern New Zealand, with additions to the fishes of Red Mercury Island. Tane 19 (this volume).

Morton, J.E., Miller, M.C. 1968: "The New Zealand Sea Shore." Collins, London-Auckland, 638pp.

Saies, A.B., Beever, S.E., Hay, J.R., Woods, R.G. 1972: The intertidal life of Red Mercury Island. Tane 18: 43-56.

Trevarthen, C.B. 1953: Features of the marine ecology of Little Barrier, Mayor, and Hen Islands. Tane 6: 34-60.

Wood, D.H. 1963: The boulder beach fauna of Otata Island. Tane 9: 41-55.

12