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 . alhicilla occur among the pebbles, N . undaia is an extremely dissected, ramp-like band of champignon scarce, and confined to the higher parts; N. poUta and 6 to 10 m wide. The ramp has a vertical range of N. alhicilla are plentiful over most of ridge. The qlfout 2 to 3 m, and slopes up to the wind-blown sand shingle ridge is an atypical habitat, and all the Nerita oeaehes (Fig. 2d). These exposed blocks of champignon species are represented by very small individuals, have manv pinnacles and tide pools, the middle able to shelter among the interstices of the pebbles. xone being the typical habitat of Neriia textUis. Examples studied were Dune Jean Louis and Dune Density, size-jrequency and biomass (I’Messe (Fig. 1). Above the N. textilis zone is a WTlde band of N. plicata and Littorina undulata extending Density np t« the sand beach (Fig. 2d). Occasional large Population densities, estimated from random specimens of N. undata were found on the pixmacles in samples taken with 0.25 m^ quadrats from the middle the N. textilis zone. Where champignon is interrupted of the zone of each species at several localities, are by small pocket-beaches, the surrounding cliffs expressed per m^ in Table 1. These density figures re.semble those of the moderately exposed shores, and were multiplied by appropriate factors accounting for N. undala occupies the undercut zone while N. textilis the average width of the zone of each species, to give is absent. Due to constant heavy seas, the platform at densitj^m longitudinal strip of beach (Table 1). the base of the champignon was hardJy ever exposed, but on a few occasions when collecting was possible no N. folita or N. alhicilla were found. Size-frequency The populations examined form polymodal size- Very sheltered shores frequency curves, which are tj'pical of invertebrates Low (0.5 to 2.0 m) champignon cliffs (especially hving more than 1 year and Avith extended breeding among mangroves) around the edges of the lagoon periods. Size-frequency was plotted on arithmetic afford a very sheltered habitat. The narrow splash probabihty paper to enable the means and variances zone is colonised by Littorina undulata.) L. scabra L. of peaks in the polymodal curves to be estimated is commonly found on mangrove stems. The solution quickly (C a s s ie , 1954). Table 2 lists means and notch is often occupied by an ecophenotype of variances of component peaks of size-frequency Nerita undata, which is typical of extremely sheltered curves of each population examined. conditions and hereafter referred to as type ‘B ’. The The largest individuals of Nerita undata w'ere ‘normal’ tj^pe of N. undaia replaces tjy'pe B and N. nearly always found at the upper edge of the zone, but plicata as the exposure to wave action increases, e.g. such behavioral differences among the different size towards the entrances of chamaels or on small islands groups have not yet been explained. in the lagoon where there is less shelter from the ■Kind. Biomass The most sheltered conditions occur in the tidal solution pools. Two such pools were sampled on West The size-frequency data in Table 2 were converted Island, namely Hawksbill Pool and Basin Cabri to biomass figures using the regression equations in (Fig. 1). The pools are bordered by champignon cliffs Table 3 A and B of log^, dry body weight (excluding about 2.5 m high (Fig. 2c). Littorina undulata extends operculum) on logi„ shell-aperture width. A control for 0.5 to 1.0 m above the iindercut (high spring-tide experiment showed that preservation for 4 months in level), which is often covered with filamentous green 4 % neutral saline formalin did not alter the dry body algae. Below this level, to the base of the cliffs, are weight of Littorina liltorea (L.) significantly (Table 4), dense populations of Nerita undata type B. The rock}'^ For aU species except Nerita textilis, regression floors of the pools are covered with a 1 to 10 cm layer coefficients were close to the value 3.0 expected if body of detritus consisting almost entirely of faecal pellets weight is proportional to the cube of any Hnear from N. undata, L. undulata, chitons {Acanthopleura dimension of the shell. N. textilis had unexpected! sp.) and the gastropods Cerithium morum Lajiai!.ck high regression coefficients, which were probably due and Planaxis sulcatus ( B o r n ) , which abound. to changes in shell shape as size increases. Table 5 gives the biomass/m length of shore represented bj^ the populations of Nerita at each sampling station. Shingle ridge on moderately sheltered shore At the entrance of Passe Femme on the north shore (Fig. 1) is a large bank of pebbles ranging from Behaviour mean high-tide level to low-water spring. Evidently, All species except Nerita textilis tend to hide in the bank has been formed by strong currents flowing depressions, crevices or under stones during the day. in. and out of the lagoon. Nerita undaia, N. polifa and C h i s l e t t (1969) and H u g h e s (in press) noted R. X. H pohks: Nerila populations on Aidabra

Table 1. Densities of Herita popvlations at different localities

Species Locality Numbers per Numbers/m 0.25 m» 1 m» strip of beach

N. pHcota Beach rock. W. Island level 1 2.9 ± 0.3 11.6 92.8 lev'e] 2 2.3 ± 0.2 9.2 73.6 268.8 level 3 , 3.2 ± 0.4 12.8 102.4 Passe Femme . -— 16.4 ± l.I 16.4 E. Channel . 6.2 ±1.0 24.8 24.8 Dune d’Messe . 5.0 ± 0.7 20.0 30.0 Dune Jean Louis 2.4 ± 0.4 9.6 14.4 Anse Malabar , 9.6 ± 1.0 38.4 38.4 Anse Cedres ■ 5.1 ± 0.6 20.4 20.4 ;

N. undafa Passe Femme ■■ 20.3 ± 1.0 20.3 ' Shingle ridge, Passe Femme 0.6 ± 0.2 2.4 — ' -' '' - ■ ' E. Channel 2.3 ± 0.5 9.2 9.2 Dune d’Mesae 5.0 ± 0.7 9.2 4.6 Anse Malabar 1.7 ± 0.2 6.8 6.8 Anse Cedres 3.0 ± 0.5 12.0 12.0

N . undata tj^pe B HawksbilJ Pool 3.2 ± 0.6 12.8 12.8 Basin Cabri 20.0 ± 3.0 40 40.0

N. textilis Dune d’Messe 3.1 ± 0.7 12.4 18.6 Dune Jean Louis 3.3 ± 0.6 13.2 19.8 Anse Malabar 1.5 ± 0.5 6.0 — - Anse Cedres 3.1 ± 0.4 12.4 18.6

N. albicilla Beach rock, W. Island _ 4.4 ± 0.9 110.0 Paase Femme 6.8 ± 0.6 27.2 27.2 Shingle ridge, Paase Femme 3.8 ± 0.9 15.2 — E. Channel 2.1 ± 0.3 8.4 8.4 Anse Malabar small numbers at base of cliffs Anse Cedrea 2.1 ± 0.6 8.4 8.4

N . polita Passe Femme 6.5 ± 0.6 2fi.O 26.0 Shingle ridge, Passe Femme — 20.8 — E. channel 1.7 ±0.3 6.8 6.8 Anse Malabar; a few at night on the rocks surrounded by sand

that Caribbean Nerita spp. confine their feeding At night, all species crawl over the rocks during activities to low tide at night. By morning, and low tide. Zones of Nerita plicata and N . undata which throughout the rest of the day, copious amounts of are about 0.5 m wide during daylight, expand to faeces are produced. about 1 m at night when animals are foraging. N. Nerita albicilla remains under stones, usually albiciUa come out to graze over the stones beneath covered by a few cm of water, while N. polita remains which they hide during dayhght. N . polita emerge buried in sand during daylight except on completely from the sand to graze over stones and champignon overcast days when numbers emerge to graze over cliffs to a height of 0.5 to 1.0 m. rocks. The heavily polished shell of N . polita (Fig. 3 a) Copulation of Nerita polita was observed at night is rounded off and, together with the large muscular throughout May/June and was similar to descriptions foot, enables the animal to burrow easily. of copulation in N . albicilla (Ibiki et al., 1963), N. Nerita textilis makes no attem pt to hide in crevices tessellata, N. versicolor, and N. peloronta (Ch is l e t t , or seek shelter from the sun at low tide, although there 1969). is a tendency for animals to collect in tide pools. Observations on all species except Nerita textilis, Tremaiode infection made while skin diving, revealed that the snails remain in their hiding places throughout high tide Incidence of trematode infection for the 6 Nerita during the day. No observations were made during species and the gastropods Littorina undidata, L. high tide at night. scabra, Planaxis sidcatiis, Cerithium Tnorum and Table 2. Size-jrequency structure of Nerita populalioTis from difjereni localities. The sizes refer to ahell-aperture width, (mm) except jor N. albicilla, where the sheU length was mmgared

Species Locality Means of Standard Frequency size deviation as % of total classes

iV. plicala Beach rock, W. Island 8.8 1.4 63 12.5 1.4 37 Passe Femme 14.8 0.9 34 16.0 0.8 62 18.6 0.3 4 E. Channel 8.4 0.7 11 11.2 0.9 74 15.3 0.6 9 17.6 0.4 6 Dune d’Messe 7.4 0.9 4 12..5 1.4 96 Dune Jean Ix)uis 10.1 0.7 5 14..5 1.3 93 17.3 0.5 2 Anse Malabar 10.5 2.6 35 15.1 0.6 65 Anse CMres 9.9 1.0 6 14.9 1.4 38 19.3 1.2 56 N. undata Passe Femme 8.4 2.1 15 22.4 1.6 16 14.6 1.4 69 Shingle ridge, Passe Femme 8.7 2.7 100 E. Channel 13.0 1.3 43 23.8 1.2 57 Dune d’Messe 20.2 2.9 50 23.9 0.9 50 Anse Malabar 20.1 1.2 100 Anse Cedres 25.0 0.9 100 N. undata Hawksbill Pool 8.0 1.0 10 type B 12.8 1.7 90 Basin Cabri 6.4 1.2 17 11.6 1.0 79 15.4 1.9 4 N. texlilis Dune d’Messe 21.7 1.0 8 24.6 0.8 14 27.8 1.1 78 Dune Jean Louis 30.9 2.5 8 34.0 1.1 60 38.1 1.7 32 Anse Malabar 31.0 1.6 40 37.3 1.9 60 Anse Cedres 27.9 4.0 8 33.0 1.3 92 N. albicilla Beach rock, W. Island 5.0 1.2 37 13.5 10.0 27 27.0 2.8 36 Passe Femme 20.7 3.3 100 Shingle ridge, Passe Femme 9.2 0.9 92 14.2 0.7 8 E. Channel 20.8 1.6 100 Anse Cadres 22.0 1.5 50 28.0 1.3 50 N. polita Passe Femme 10.5 3.0 6 11.2 1.3 . 15 14.4 0.8 21 21.4 1.6 58 Shingle ridge. Passe Femme 6.9 0.5 3.5 10.8 1.0 20.5 14.0 1.0 76 Table 3. Nerita species (A) Regressions of log dry body weight (y) on log shell aperture v,-idth (x) for each species of Nerita. (For N . cdbicilla x = log shell length). Standard errors are shown

Species Regression equation Number of animals

N . plirMta y = (3.71 ± 0,09) X - 5.23 ± 0.10 50 N. undala V = (3.40 ± 0.07) X - 4.86 ± 0.09 50 N . undala type B y = (3.19 ± 0.08) X - 4.67 ± 0.32 30 N . texiilis V = (6.04 ± 0.35) X - 9.00 ! 0.53 38 N . aihicilla V = (2.71 ± 0.07) X - 4.62 ± 0.09 49 N. polita y = (3.38 ± 0.09) x - 4.94 ± 0.12 50

(B) Regressions of log operculum weight (y) on log shell aperture width (x) for each species of Nerila. (For N. albicUla x = log shell length). Standard errors are shown

Species Regression equation Number of animals

A’, plicafu V = (2.89 ± 0.06) X - 4.87 ± 0.06 50 N. undala V = (3.13 ± 0.05) X - 4.55 ± 0.06 50 N. undala tj'pe B V = (2.85 ± 0.32) X - 4.38 ± 0.32 30 N. t.eztilis V = (5.01 ± 0.25) X - 7.54 i 0.38 38 N . aihicilla “y = (3.03 + 0.05) x - 5.23 ± 0,06 48 N . polita V = (3.17 ± 0.08) X ~ 4.79 ± 0.10 50

Table 4. L-iitoriw liUorea. Regressions of logj^ body weight (y) on log^^ shell len.glh (i-) before and after preservation in 4 % neutral, saline formalin for 4 months

Preservation Regression equation Sample Difference Difference size between slopes between positions

Treated y = 2.371 X - 3.8,35 50 t = 1.23 t : 1.46 P = 0.30 - 0.20 P = 0 .2 0 - 0.10 Untreated y = 2.797 X - 4„568 50

Fig. 3. (a) Nerita polita showing colour variation and the heavy, smoothed shell adapted for burrowing in sand; (b) shell apertures of Nerita plicata (left), N . polita (middle) and N . albiciUa (right). The strong, heavily calcified operculum is visible Ln N. polita and N. albiciHa, while the more fragile operculum of iV. plicata is withdrawn from sight. Here the shell aperture is guarded by robust ridges Table 5. Nerila sp. The biomass (dry fleah tveighl excludiyig operculum) represented hy each, species of Nerita at different localities. Fitjures in- parentheses are the weights of opercvlum,

Species Ix)c&lity Biomass g/m strip of shore

N. pUcata Beach rock, W. Island level 1 0.5 (0.1) 4.0 (0.8) level 2 0.4 (0.1) 3.2 (0.8) level 3 0.5 (0.2) 4.0 (1.6) Passe Femme 2.7 (0.6) 2.7 (0.6) E. Channel 1.5 (0.4) 1.5 (0.4) Dune d’Messe 1.4 (0.4) 2.1 (0.6) Dune .Jean Louis 1.1 (0.3) 1.7 (0.5) Anse Malabar 4.0 (1.1) 4.0 (1.1) Anse Cfedres 5.2 (1.0) 5.2 (1.0) N. unda ta Passe Femme 3.1 (3.3) 3.1 (3.3) Shingle ridge. Passe Femme 0.1 (0.1) — E. Channel 3.0 (3.4) 3.9 (3.4) Dune d’Mesae 4.9 (4.4) 2.5 (2.2) Anse Malabar 2.5 (2.2) 9.5 (8.2) N. undata type B Hawksbill Pool 0.9 (0.7) 0.9 (0.7) Basin Cabri 5.3 (3.4) 5.3 (3.4) N. textilis Dune d’Mesae 5.4 (5.2) 8.1 (7.8) Dune Jean Louis 28.6 (21.7) 42.9 (32.6) Anse Malabar 11.1 (9.7) __ Anse Cadres 17.8 (14.2) 26.7 (21.3) N. albicilla Beach rock, W. Island 1.3 (0.9) 19.5 (13.5) Paase Femme 0.6 (0.4) 0.6 (0.4) Shingle ridge, Passe Femme 0.2 (0.1) — E. Channel 0.8 (0.5) 0.8 (0.5) N. albicilla. Anse C^idres 1.3 (0.9) 1.3 (0.9) N. polita Passe Femme 6.2 (4.7) 6.2 (4.7) Shingle ridge. Passe Femme 1.6 (1.3) __ E. Channel 2.5 (1.8) 2.5 (1.8)

Terebralia palustris (L.) is given in Table 6. Nerita Discussion were not infected, perhaps because they do not form part of a suitable food chain for the transmission of Ecology trematode parasites. However, samples were too Nerita textilis is almost entirely confined to low, small to detect very low incidences of infection such Jagged, champignon subjected to heavy wave action as were found by Ch isl e t t (1969) in 3 Nerita spp. on on very exposed shores, or to pinnacled champignon Barbados (0.6 % mature animals infected). L. undulaia, at the tops of taller cHlfs on slightly less exposed 0. morum, and P. sulcatus, taken from mangroves and shores. Occasional specimens were found on medium an inland tidal solution pool, showed a low level of energy shores on West Island. N. plicata occupies a trematode infection. band just above N. textilis on low exposed cham­ Dr. J. F arley (Dalhousie University) identified pignon, and is found on the tops of pinnacles between the parasites as spineless echinostome cercariae. All tide pools. There is little overlap between N. textilis parasitised animals were infected with redia stages and N. plicata, but large specimens of N. undata are containing mature cercariae. Litiorina scabra and occasionally found together with N. textilis. On L. uniidctta contained rediae of the family Philoph- exposed shores with tail champignon cliffs, N. plicata thalmidae. Adults of this family are typically found is abundant above the undercut at high spring-tide in the gut and cloaca of birds, and usually pass through level, while N. undata occupies a zone immediately a second intermediate host. The rediae in Cerithium below this. There is a certain amount of overlap be­ morum were echinostomes of a different, unidentified tween the zones of the two species. Zones of N. plicaia group. and N. undata extend to more sheltered shores but. R. X. Hughes: Nerita populations on Aldabra

Table 6. Iricidence of tremalodr infeciion

Species Locality Sample Xumber size infected

.V. plicata Beach rock. \V. Island 50 0 Passe Femme 50 0 y . undata Beach rock, W. Island 50 0 Passe Femme 40 0 E. Channel 50 0 y . und.aia tvpe B Havrksbill Pool 50 0 Basin Cabri 50 0 y . albicilla Beach rock, W. Island 40 0 Passe Femme 50 0 E, Channel 30 0 y . polita Passe Ferame 50 0 Shingle, Passe Femme 20 0 E. Channel 50 0 y . iextilis Anse Cedres 30 0 Dune Jean Louis 40 0 Littorina. uruhilala Beach rock. W. I.sland 50 0 Passe Femme 30 0 Mangroves. Passe Femme 28 2 E. Channel 30 0 Hawksbill Pool 40 0 L. scabra Mangroves, E. Channel 50 0 Ceritkiuw morurn Hawksbill Pool 40 2 Terehralia palustris ^langroves, E. Channel 40 0 Planaoi'is sulcaluf! Hawksbill Pool 50 0 Mangroves, Passe Femme 40 2 Mangroves, E. Channel 30 0

Avith increased shelter in the lagoon, both species axe Most Nerila species have a thick calcified oper­ replaced by N. undaia tj'p e B. X . albicilla a lw a y s culum which is approximately equal in weight to the occupies a zone among stones and pools at the base of whole dry tissue of the animal (Table 5). Since L e w is the champignon cliffs and does not usually overlap (1963) has shown that evaporation is an important other species of Nertia. polita occupies the same factor in keeping the temperature of Nerila temellaiM level on the shore as N. albicilla, but is confined to dowTi to a suitable level, it is unlikely that heavy sandy areas among stones and boulders and seems to calcification of the operculum has evolved to reduce be absent from extremel,y exposed shores. water loss. The heavj^ operculum probably defends It is interesting to note that Nerila 'polita a n d N . the animal against attacks by crabs which abound albicilla both have very variable and often highly throughout the Nerit-a zones. N . plicala has a relatively coloured shells, yet both remain completely covered fragile horny operculum, which is not heavily calcified by sand or stones dm'ing d a y li g h t . In contrast, other (Table 5). The shell aperture is protected by very crypticalljr coloured species are commonly visible prominent ridges or “teeth”. Moreover, the disturbed during daylight. N. albicilla is usually dull grey, but animal withdraws deeply into its sheU until the shells with w'ide longitudinal bands of black, white, operculum is out of sight, unlike other species where red and dull yellow are frequent. N. 'polita, w h ic h the operculum is always visible (Fig. 3 b). Probably alwaj'S has a whitish background colour, is even more the shell teeth take over the role of the heavy oper­ v a r ia b le . Common varieties have longitudinal bands culum in protecting the animal from predators. of black, red, yellow, brown or p a t c h e s and fle c k s o f these colours together with olive greens and browns. Co7n.parison of Nerita. spj). on Aldabra and Barbados M ajhdihassan (1960) noticed that populations of iV . wyzarum ( R e c l u z ) at Bombay were very variable Barbados is fringed with cUffs derived from old in colour but, within a particular area, all anim als were coral reefs and showong much the same structui’e as of the same colour type and matched the colour of the Aldabra champignon. The mixed, semidiurnal tides of r o c k s . Barbados are smaller (about 1.5 m at springs) than

39 Marine Biology, Vol. 9 the Kemidiornal tides of Aldabra (about 3 m at Below Nerita versicolor on moderately exposed to sfjrings), so that the vertical extent of the intertidal sheltered shores is a zone of N . tessellala extending zone is correspondingly less. down to low tide level. N. tessellata has a black shell, On Barbados, crypticaUy coloured Neriia feloront^ which is very conspicuous and more robust than those occupy a 1 to 2 m wide band in the splash zone, just of the other Barbadian species. Many shells of both above high water spring on very exposed shores. The N. tessellata and N. undata type B (Fig. 4) are eroded corresponding habitat on Aldabra is occupied by N. severely. No explanation could be found for this textilifi, also cryjjtically coloured, and by N. plicata. condition, which was never seen in any of the other species. The equivalent zone of N. tessellata is occupied b3^ N. albirAUa on Aldabra. Unlike JV. tessellata, N. albicilla remains under ledges and boulders during daylight and is confined to situations offering such hiding places. Nerita polita replaces N. albicilla where sand occurs at the base of the cliffs or among boulders, and has no equivalent on Barbados. According to Kolipinski (1964), the maximum life span of Nerita peloronta and N. versicolor is slightlj' over 3 3^ears, while that of N. tessellata is about 3.5 j^ears. This is supported by the data of L e w i s et al. (1969), who found that all 3 species approach their maximum size in 2 to 3 years. F ean k (1969) found that N . albicilla on the Great Barrier Reef at Heron Island was slow growing, wdth an average life span of 6 to 7 years, and a maximum life of 12 years, while N. polita grew faster wdth a maximum life span of 4 years. Both N. albicilla and N. polita ceased growth in spring and summer on Heron Island, causing them to lay down growth rings on the shell. Apart from a few N. albicilla, growth rings were absent from all Nerila spp. on Aldabra and Barbados.

Comparison of the two ecophenotypes of Nerita undata Type B Nerita undata are easUy distinguished from the narrnal type by their black colour, often eroded shell, and irregular teeth on the inner lip of the shell (Fig. 4). The maximum size of type B is much less than Fig. 4. A comparison of the two ecophenotj'pes of Nerila that of the normal type; most of the type B specimens undata. Top row: Eroded shell of type B (left); uneroded, have an aperture width of II to 12 mm, while the typically dark shell of type B (middle); normaJ type (right); majoritj^ of the normal tjrpe have an aperture width middle row: Apertures of 3 specLmens of type B showing variation in the t«eth of the lower lip of the shell aperture; greater than 20 mm (Table 2). bottom row: Normal type from above and below Populations of type B are only foimd in extremelj^ sheltered locations, and were never found to overlap with the normal type. The latter species which also extends to moderately exposed situations, is conspicuous in situ but is pro­ tected by an unusually robust shell. Summary Nerita versicolor is a cryptically coloured species 1. Population, size-frequencj' structure and density occupying a 1 to 2 m wide zone immedia.tely below of Nerita undata, N. plicata, N. textilis, N. polita and N. peloroTtia on very exposed shores, but it also N. albicilla were ascertained at 9 stations on Aldabra. extends to the most sheltered situations. This be­ 2. N . plicata is found at high-water spring level on haviour is paralleled by N. undata on Aldabra. There very exposed to moderately sheltered shores. N. is no equivalent of N. wndata type B on Barbados, textilis occurs below N. plicata on very exposed shores which lacks situations offering the extreme shelter of and is replaced by N. undata in moderately exposed to the tidal solution pools on Aldabra. sheltered localities. N. albicilla occurs among stones at the base of champignon cliffs, except in the Jagoon Ffi.iSx, r. W.: Growth rates and longevity of some gastropod and in extremely exposed places, polUu replaces moUusks on the eoral reef at Heron island. Oecologia 2, 232— 250 (19(59). N. albicilla on sand patches at the base of the cliffs. Hughes. R. Ecological energetics of Iffrita (An'haeoga- 3. A distinct ecophenotype of undata occius in stropodn; Neritacea) populations on Barbados. Westlndies. the extreme shelter of tidal solution pools and parts Mar. Biol. (In press). of the lagoon. lK5Tii, S.. S. NisHiWAKi and T. TocinMOTO: On the peculiar mode of the spermataphore transfer in Nerita alhiciUa L. 4. N. albicilla and N. polita, wliich hide under (Prosobranchia, Keritidae). V'enus. Kvoto 22, 290—292 stones and sand, respectively, during daylight, are (1963). ■ more variably and bi'ightly coloured than most other KoLEprxsKi, M. C.: The life liistory, growth, and ecology of species which remain visible. four intertidal gastropods. Unpubl, Ph. D. thesis, Uni­ versity of Miami 1964. 5. All samples of A^eriia spp. were free of trematode L e w i s . J. B .: Environmental and tiss\ie temperatures of some infection. tropical intertidal marine snails. Biol. Bull. mar. biol. Lab., 6. Habitats of Aldabran Neritfi. spp. are compared Woods Hole 124, 277-284 (1963). \vdth those of 3 species on Barbados. — Comparative respiration of some tropical intertidal gastropods. J. exp. mar. Biol. Ecol. (In press). AcknowleJ-gemenis. I am indebted to the Royal Society of - F. Axklsok, I. Goodbody, C. P age, G. Chislett and London for supporting this study, and to J. D. Tayi.or for iT. Cho'ddhoury : Latitudinal differences in groM th rates identification of gastropods. of some intertidal marine molluscs in the Caribbean. Afar. Sci. Manuscr. Rep. 12, McGill Univ^ersity 1969. MahdihaSSaK, s .; The crvptic colouration of an Indian snail. Literature cited Zool. Anz. 164, 377—.382 (1960). CaSsie, R. M. : Some uses of probability paper in the analysis STonD.wi, D. R. (JEd.): Ecology of Aldabra Atoll, Indian of size frequenuv distributions. Aust. J. mar. Freshwat. Ocean. Atoll Kes. Bull. 118, 1—141 (1967). Res. 5, 513—522 (1954). S\ ZTJKI, S.: Notes on the behaviour of Nerila japonira Dun- CtnsLETT, G. R.: Comparative aspects of the ecology of three KER. Sci. Rep. Toholiu Univ. (Ser. 4) 9, 297- 314 (1935). Nerila (Mollusca; Gasteropoda) species from different T.iYLOR, J, D.; Cora! reef and associated invertebrate com­ locations in Barbados, W. I. Unpubl. jM. Sc. thesis. McGill munities (mainly molluscan) around Mahe, Se^'chelles. University 1969. Phil. Trans. R. Soc. (Ser. B) 254, 129—2(>6 (1968)’. F ischer, P. H.: Repartition des Xeritidae sur les cotes de la region Indo-Pacifique. Int. Congr. Zool. (Lond.) 15, 778— Author’s address: Dr. R. N. H t'O H E S 780 (1958). Department of Zoology — and A. Brunel: Relation entre Thabitat des gasteropodes University College of is'orth Wales et leur processus d’excretion. C. r. sonim. Seanc. Soc, Bangor, Caernarvonshire Biogeogr. 30, 36—37 (1953). N. A^'ales, UK

Date of final manuscript acceptance: January 11, 1 9 /1. Communicated by J. B u n t, Miami