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Populations of Aldabra Atoll, Indian Ooean

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Populations of Aldabra Atoll, Indian Ooean Offprint from ‘"Marine Biology" Intemalional Journal on Life in Oceans and Coastal Waters, Fol. 9, JVo. 4, June 1 9 7 Pages 290— 299 ' © by Springer-Verlag 1971 - PriTited in Germany Notes on the Nerita (Archaeogastropoda) populations of Aldabra Atoll, Indian Ooean / R . N. H ug h es Department of Biology, Dalhousie University; Halifax, Nova Scotia, Canada The Royal Society ALDABRARESEARCH STATION CONTRIBUTION ^ Abstract energetics of the latter species have been studied Nerita undaki, N. piioala, N. poliia. N. albicilla and N. bj'^ HtTGHTES (in press). Of the Pacific species, tej-Hlis are common intertidal gasti'opods on Aldabra Atoll. SuzcTKi (1935) investigated the effects of water level Each species prefers a different level on the shore and/or on the movement, geotaxis, phototaxis and rheotaxis different degrees of exposure to wave action. Patterns of of N. japonica D u n k e e , and F i s c h e e (1958) briefly zonation, population size-frequency structure and biomass are given. N. und-ata exhibits 2 distinct ecophenotypes. Normal described zonal heights on the shore in 8 species of types occur on exposed to moderately sheltered shores, but Indo-Pacific Nerita. F i s c h e r and B b u n e l (1953) are replaced by ‘type B’ in extremely sheltered conditions. looked at nitrogenous excretion in N. albicilla L,, Difference.? in shell morphologj^ and population structure are N. undata L., and N. costata (C hem m .). Recently, also noted. Foraging b}f all species was confined to houi-s of darkness, generally at low tide. Activit}!- of N. polita was F r a n k : (1969) studied growth, longevity and mortal- induced slightly on overcast days. N. undcUa and N. texlilis itj^ of N. albicilla from Heron Island. Notes on the are cryptically coloured and often visible during the day. habitats of N. debilis Dupo, N. undata, N. plicata L., N. plicala is conspicuous, but is protected by a strong shell. N. textilis DillwyjS, N. polita L., and N. albicilla N. polita and N. albicilla are often brightl}^ and variably coloured, but both are concealed during daylight. No Nerita around Mahe, Seychelles, are given by T a y l o e (1968). were found infected with treraatodes, suggesting that the As part of the Royal Society Expedition to degree of infection was extremely low. Habitats of the Alda- Aldabra AtoU (April/July, 1968), the habits, popula­ bran Nerita are compared with 3 species on Barbados, West tion stiucture, density, zonation and distribution of Indies. Nerita undata, N. plicata, N . poliia, N. albicilla and Introduction N. textilis were examined. Observations were also Nerita species are important contributors to the made on trematode infection in Nerita and several biomass of rocky shores in the tropics and sub-tropics. other littoral gastropods. They are primitive prosobranch gastropods, which graze upon thin films of blue-green algae, diatoms and Methods detritus covering rocks in the intertidal zone. Copious quantities of faeces, composed largely of fine rock Populations were sampled at 9 stations ranging particles rasped off by the radula, are produced daily, from sheltered inland tide pools to wave-battered making Nerita an important contributor to the bio­ cliffs on the south and south-east shores (Fig. 1). At logical erosion of rocky shores. each station, a 0.25 m wide strip was taken through Few studies exist on the ecology of Nerita, although the middle of efich species’ zone and divided into there are a number of early works on their taxonomy 0.25 m segments. Thirty segments were chosen using and morphology. Only the 4 species from the Atlantic random number tables, and all animals were removed. coast of North and Central America are well known. Collections were made at night, since many snails K o lipin ski (1964) examined the life history, growth retired into deep cracks in the rock during the day. and ecology of Nerita tessdlata G m elen, N. versicolor Shell aperture widths were measured to the nearest G m e u n , N. Pdwonla L. and N. julgurans G m e l in . 0.5 mm nsLng vernier caUipers. Samples of each L ew is et al. (1969) studied the population structures species were preserved in 4 % neutral saline formalin of N. tessdlata, N. versicolor and N . peloronta in and used to obtain regressions of logj^o dry body Barbados, and compared their growth rates with weight (excluding the operculum) -on logjp shell those of the same species in Jamaica. L e w is (1963) aperture width. Dry weights were obtained by heating demonstrated the effect of evaporative cooling in animals for 48 h at 60 °C in a vacuum oven. Samples N . tessdlata and has recently studied the effects of of Nerita and several other gastropods were examined tidal emersion on the respiration of N. tessellata, N. for trematode infection by searching the teased-out versicolor and N. peloronta (Le w is , in press). Ecological visceral masses imder a binocular microscope. Results by Nerita p}icala and Littorina uiidulatn, although HabitMs and zonation onlj7 the latter oecui-s 1 to 3 cm above the level of the sand. Probably, L. undulaia can withstand more The general ecology of Aldabra Atoll is described abrasion than N. plicala. N. albiciUa. occurs at the by S t o d d a e t (1967). Most of Aldabra is fringed by lowest levels on the shore, among pools and wat-er- limestone cUffs t«rmed “champigBon”. The cliffs fUled channels in the beachrock. range from 2 to 7 m in height, and are occasionally interrupted by small sand pocket-beaches. Extending seaward from the base of the chffs is a platform varj-- Moderately exposed shores ing in width up to 100 m ; it is narrowest on the most The rest of Aldabra, except along the south and exposed shores. The platform is completely uncovered east coasts, is fringed by taller champignon cliffs only during spring tides. The lagoon, which has an about 7 m high (Pig. 2 b). Examples studied were average depth of about 3 to 4 m, is fiinged by man­ Anse Cedres and Anse Malabar. The splash zone has a groves and low champignon clififs 2 to 4 m high. On vertical range of about 2 m, but is greatlj' extended West Island (Pig. 1) there are tidal solution pools horizontally ii\to a gradual seaward slope of deeply 2 to 100 m in diameter, which connect underground dissected champignon. The upper metre of this zone is w th the lagoon. occupied bj^ LiMorina, undulaia and Nerita plicata. In Anse Malabar . solulion pools with N feXf/7/s Wesi Islandj^ < r m a 7 7 7 7 T T - n —;------ Channel L.undulata Bosin Cabri—^ ■ N. plicata Hawksbill P o o l-^ ;^ - ] N. plicata Passe Femme N undata N. undata N albicilla or N polita S.E. trades April-Dec) _ } L undulata Fig. 1. Aldabra Atoll showing the positions of sampling N undato-3 stations Nalbictlfa or N.polita '^detnrus d sand Moderately sheltered shores N. plicata • L. undulata N. textilis Along the north-west outer shores and at the entrances of the channels are sHghtly undercut cham­ Fig. 2. Profiles of shores under different conditions of exposure pignon cliffs about 3 m high (Fig. 2a). Such areas to wave action, showing the zones occupied by Nerita spp. and Liltorina vmdulala. (a) Moderately sheltered shore; (b) mo­ were sampled on West Island around the entrance of derately e.xposed shore; (c) extreme shelter, inland tidal solu­ Passe Femme, and at the entrance of West Channel tion pool; (d) extremely exposed shore on Middle Island (Pig. 1). The top metre of cliff consists of very jagged champignon in the splash zone. many places, the lower half consists of pinnacles Littorina undulaia G r a y is abundant, \vhile Nerita 0.1 to 1.0 m high, separated by tide pools 0.5 to 1.0 m plicata occupies the lower half of the zone. Immediate­ in diameter. N . textilis lives among bases of the pinna­ ly below the splash zone, the cliffs are undercut cles and in the pools, while N . plicataiB confined to the (solution notch) at high spiing-tide level and slope upper regions of the pinnacles. Immediately below inwardty doT\Ti to the beach platform (Fig. 2a). At the the splash zone, the chffs are deeply undercut and undercut, N. plicata gradually gives way to a wider recede for 1 to 3 m to a vertical wall 4 to 5 m high, zone of N. undata which, although most dense at which is usually worn smooth by wave action. N. about high spring-tide level, maj' extend do'vra to the undata is found on the undercut, but is completely beach platform. The beach platform may be hard absent from the vertical cliff face (Fig. 2 b). At the rock strewn with boulders and pebbles, or a varying base of the cliffs and among stones on the beach cover of sand between boulders. In the former situa­ platform, is a zone of N'. albicilla. N. polita is found tion, N . albicilla occupies a band about 2 m wide from where sand patches occur. the base of the cHfFs. N. polita replaces N. albiciUa where sand predominates. On West Island is a long stretch of steeply shelving Very exposed shores sand beach with 0.25 to 2.0 m high slabs of beachrock Along the south and cast shores exposed to the smoothed by sand abrasion. The beachrock is colonised south-east trade winds from April/December, the Ixsach platform is very narrow and the cliffs become N .
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