Estuarine and Coastal Marine Science (x98o) zo, 569-583

Zonation of Intertidal Molluscs on Rocky Shores of Southern Somalia a

G. Chelazzi and M. Vannini Istituto di Zoologla dell'Unlversltad Via Romans z7, 5oz25 Firenze, Italy

Received 3 May z979

Keywords: intertidal environment; rocky shores; Mollusca; Somalia; Indian Ocean

The zonation of two species of chitons, 7 ° species of gastropods and x9 species of bivalves inhabiting the rocky cliffs and erosion platforms of four southern SomaIian localities exposed in varying degrees to wave action, was plotted on the basis of quantitative collections along transects. Also included are data on the coastal morphology and algal covering. The compo- sition and zonation of the mollusc community, which is clearly related to wave exposure and cliff profile, corresponds to that of other known continen- tal and insular localities in the Western Indian Ocean.

Introduction

For many years, the Centro di Studio per la Faunistica ed Ecologia Tropicali of the Consiglio Nazionale delle Richerche has conducted research on the southern Somalian coast (Pardi, x976 ). This paper describes the zonation of molluscs inhabiting the rocky shores at Sar Uanle, Bender Mtoni, Gesira and ~iogadiseio (Figure x).

Taylor (i97ie), in his analysis of the molluscan assemblages in the Western Indian Ocean, comments on ' the large gap in the knowledge on the East African coast' in comparison to the data available on the islands. Hartnoll (x976), who studied some rocky shore~ in Tanzania, pointed out the lack of information on the coasts north of D a r e s Salaam. Our present knowledge on the ecology of the rocky shores of the southern East African coasts is due to the studies of Kalk (x958, I959a , i959b ) and Macnae & Kalk (x969) on Inhaca Island and some shores of northern Mozambique, and to those conducted on the eastern South African coast (cf. Stephenson & Stephenson, x97z).

Study area and methods

Data have recently been made available on the geomorphology, climate and tides of the Sar Uanle shore (Vannini et aL, x977), on the mieroclimate of its beach (~[essana et aL, I977) and on the algal vegetation of the area (Sartoni, i976 ).

The rocky shore between Bender IVltoni and Sar Uanle consists of limestone cliffs 3-4 m high rising out of the landward edge of an erosion platform lying at MLWN. At its seaward

* Publication of the 'Centro di Studio per la Faunistica ed Eeologia Tropieali' of the Consiglio Nazionale delle Ricerche. Researches on the coast of Somalia. The shore and the dune of Sar Uanle, No. 25.

569 o3o2-3524/8o/o5o569+ z 5 $o2.oo/o © x98o Academic Press Inc. (London) Ltd.

57o G. Chelazzi 6~ M. Vannini

edge a channel separates the platform from a barely living reef strewn with patches of sand and clumps of dead corals. South of Chisimaio is the Baguini Archipelago (or Somali Islands), a type of fossil coast lying 2- 3 km offshore and forming a shallow semi-lagoon. The rocky coast at Gesira and Mogadiscio, otherwise similar, lacks the coastal islands. As the general line of the Somalian coast on the Indian Ocean is north-east to south-west,

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Figure f. Map of Somalia showing the localities mentioned in the text.

exposure to the two seasonal monsoons is practically identical in all the areas studied. How- ever, the barrier of coastal islands in the southern Somalian area tends to attenuate wave action and has favoured the formation of large sand banks and mangroves at Bender Mtoni and other protected areas.

The four localities were inspected in July-August x975 and November-December x976. Quantitative sampling was made at diurnal low water along transects (from two to eight

Intertidal molluscs of Somalia 5 7 x

depending on the locality) on the cliff, working up from the base in unit areas (squares of xo x io, 5 ° × 5 ° or ioo × xoo cm) to where molluscs were no longer present. Non-quantitative sampling was made during intermediate tidal phases and nocturnal low water.

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Figure e. (a), section of the cliff at Sar Uanle and zonation of the commoner algal forms; (b) theoretical immersion times (in percentage) calculated on the basis of tide tables; (c) wave action estimated by the weight reduction (in percentage) of chalk balls; (d) temperatures registered during a diurnal low tide on the cliff surface in shaded points (circles), sunny points (triangles) and in the air eo cm from the surface (squares).

57z G. Chelazzi 6~ M. Vannlni

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Figure 3. Zonation of molluscs on the Sar Uanle cliffs; numbers above or below the kite diagrams indicate the specimens collected in a mean transect z m wide. A total of eight transect were actually inspected.

Zonation patterns

Rocky cliffs The scheme shown in Figure 2(a) (Sar Uanle) is valid for most .of the rocky shores south of Chisimaio. The nearly vertical cliffs rise to a height of 3-4 m, then gradually level off into the summit where the limestone is deeply pitte d and full of small spray pools up to 4-5 m inland. Intertidal notches (dashe d line) occur infrequently along the coast. The theoretical mean times of immersion (excluding the effect of wave oscillations) and wave action (influ- enced by monsoon intensity but affecting the upper cliff and the summit during spring tides even in the absence of strong winds) are shown in Figure 2(b),(c). Exposure to wave action ~ is greatest in the area 7o-80 cm from the foot of the cliff where, during low water, the least favourable thermal conditions also occur (Figure 2(d)). Scattered macroscopic algae grow down from about MWHS to the cliff base, while endolithic and epilithic Cyano- phyceae cover the upper levels.

"Chalk balls, 3 cm in diameter, were placed in brass cages 6 cm in diameter and 8"5 cm long, screwed into the cliff wall at selected levels. Exposure to wave action was estimated by the reduction in weight of these balls following two consecutive high waters.

Intertldal molluscs of Somalia 573

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Figure 4- (a) section of the Bender l~Itoni cliffs; (b) transect were actually inspected.

zonation of molluscs. Four

The mean distribution of the commoner species of molluscs at Sar Uanle is shown in Figure 3. According to the terminology introduced by Hartnoll (i976), there is an upper- shore group consisting of Truncatella, two Littorinidae and Nerita plicata, a mid-shore group with Nerita textilis, Siphonaria, Acmaea, Cellana and Acanthopleura of." borbonica, a lowest group with Acanthopleura spiniger and Nerita albicilla. The most numerous predator is Thais fusconlgra, especially common where Chthamalus barnacles are abundant, while less frequent predators are Thais aculeata, Purpura rudolphi, 3lorula granulata, M. margaritifera, M. marglnatra and Drupella cariosa. Bivalves are present in an upper Isognomon-Brachiodontes band and a lower, well defined Saccostrea belt, overlapping around M T L with the large barnacle Tetraclita rufotlncta Pilsbry. Occasionally found where the cliff base is more pitted, are Arca avellana, Barbitia lacerata and Isognomon perna.

Bender Mtonl, sheltered by the nearby rocks and islets of Zivazecundru, is only slightly exposed to wave action. Its cliffs, lower than those at Sat Uanle, are deeply undercut (Figure 4(a)). The only algae present on the upper cliff are Cyanophyceae while the intertidal notch is filled with Bostrychla tenella. The zonation pattern is clearly contracted towards the base of the cliff (Figure 4(b)) and molluscs are less numerous than at Sar Uanle due to the lack of important algal grazers such as Acanthopleura eL borbonlca and Siphonaria and to the reduction of Littorinidae, Nerita spp. and Thaisfusconlgra. Few bivalves occur on the cliff and Saccostrea is much smaller than that found on exposed shores. Typical inhabitants of this shore are Clypeomorus morus and the small predator Morula anaxeres.

574 G. Chelazzi & M. Vannini

( a }

MLWN

4 - (bl

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!

o

J ~ I00%

~ -~,

t~

Figure 5. (a) section of the Gesira cliffs; (b) zonation of molluscs. Two transects were actually inspected.

Gesira has a high rocky shore undercut by a definite intertidal notch'which slopes into the erosion platform (Figure 5(a)). There are fewer Littorinidae than at Sar Uanle, no Thais fusconlgra and hardly any other predators. Found in greater numbers is a Siphonaria sp. while Nerita plicata outnumbers N. textilis. Despite the relative height of the cliff, zonation is again definitely contracted towards the base (Figure 5(b)).

North of Mogadiscio the cliffs lack the intertidial notch and are about 5 m high (Figure 6(a)), levelling off brusquely at the summit which is covered with a thick layer of light detritus. The mollusc community (Figure 6(b)) is similar to that found at Sat Uanle, with the exception of a very.reduced Saceostrea belt. Zonation also follows the Sar Uanle pattern up to the summit, where abrupt change from vertical to horizontal and very reduced spray on the roof halts the distribution of the species of the upper shore.

Erosion platforms Commonly found on raised limestone coasts of the Indian Ocean are low-eullttoral erosion platforms extending seaward from the cliff foot (Stoddart et al., x97z ; Hartnoll, x976 ). In the Sar Uanle area these platforms show three distinct belts (Figure 7(a)). The proximal one (a), hosting Cystosdra myrica, consists of shallow pools and narrow gullies filled with

Intertidal molluscs of ,Somalia 575

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~ ~ ~ ~ ~ ~. ~ .~ -~ .---~ o ~ - ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~.~ ~ - o o

Figure 6 (a) section of the Mogadiscio cliffs; (b) zonation of molluscs. T.wo transects were actually inspected.

water at low tide. The intermediate belt (b) wider than the proximal and relatively dry at low water, supports the algae Gelidiella aeerosa and Dictyosphaerla versluisii while the distal belt (c), frequently splashed by waves at low tide, is clothed mostly in Porolithon onkodes and Gelidium pusillum. Thougk molluscs are not strictly zoned, some species are more localized (Figure 7(b)). For instance, algal grazers of the lower cliff occur in the proximal belt to- gether with Haminoea gradlis and Cerithhon caeruleum, while predator species abound in the intermediate-distal part. The only bivalves present are small Brachiodontes variabills colonizing the Gelidiella turf.

These short platforms always terminate seaward in a ledge about 5o cm high, undercut and fissured, enerusted with P~,ssonnelia and other shade-dwelling algae. An increased species diversity characterizes the mollusc community of this ledge. Among the algal grazers Patella flexuosa and Siphonaria kurradleenffs are most common, together with Acantho- pleura splnlger and Nerita alblcilla. Common predators are Drupa morum, D. rlcinus and several Conus species, including abundant C. flavldus. Inhabiting the undercut are Cypraei- dae, especially C. anmdus, C. felina and C. mauritiana, rarely found on the platform. An

576 G. Chelazzi ~ M. Vannlni

( o ) PococMella Cladopharop$1$

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Haminoea gracili$ _ I I I 104

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Acanlhopleura spiniger l • I 14

Cerith/urn caeruleum 1 I I 1130

Conus mus/cus [ I 2 6

BrocModoates varlobili$ [ . . . . I I " - , 3 0 0 0

Engiaa mendicaria I 1 I 3 6

Canus ebraeu$ 1 1 I 2 7

Thais fusconigra 1 I 6 4

Morula granulafa I ] 2 8 4

Cellana radiata l I IO

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Figure 7. (a) schematic cross-section of the platform in front of the Sar Uanle cliffs with the belts (a-c) described in the text; (b) percentage frequencies of the commoner molluscs; numbers, specimens collected in a mean transect z m ,vide. A total of four transect were actually inspected.

increase in the species diversity is also evident in the bivalves which include Arca avellana, Lithophaga teres, Pinctada radiata, lsognomon perna, Hyotissa munisma and Fragura fragum, a typical assemblage of the ledge overhang.

Lying in front of the sandy stretches in the same area are rocky platforms extending in places more than zoo m seaward. These platforms (Figure 8) are analogous to those extend- ing from the cliffs, in the presence of a proximal Cystoseira belt (a), intermediate dry belt (b) and distal Porolithon belt (e), but are distinguished by some unvegetated (c)andvegetated (phanerograms) (d) sand accumulations. Inhabiting the Tlzalassla-Halodule beds and particu- larly the borders of large tide pools are small herbivores gastropods and some bivalves never found on the platforms in front of the cliffs (Table z). Beyond the distal belt of these platforms which never end in a seaward ledge, are Padina-dmansia-Ulva beds supporting many species of Conus.

Intertidal molluscs of Somalia 577

U/vo

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Pol"olithon

Oictyosphaer/o Volonia

H o l o d u l e Tholossio Gelid/ella

Cys/oseiro

4 - a ~.~ b ~.~ c ~ d ~ e Dune-beoch

I I I I I l I I I I I I I 0 I0 2 0 50 4 0 5 0 6 0

I1"1

Figure 8. Schematic cross-section of the platform in front of the Sat Uanle beach, showing the areas (a-e) described in the text. The relative extension of the areas is arbitrarily represented.

TABLE X. Distr ibution of the commoner species of molluscs on the different areas (a-e) on the erosion platform in front of the Sar Uanle beach (see Figure 8)

Species/Areas a b e d e

Acanthopleura spiniger Calliostoma plcturatus Turbo coronatus Nerita albicilla Smaragdia souverbiana Cerithium caeruleum Clypeomorus morus Cypraea annulus Cypraea tigris Cymatium muricbzum Purpura rudolphi Thais fusconigra Morula granulata Pyreneflava Galeodes paradlsiaca Nassarius albescens Nassarius echinatus Peristernia nassatula Ancilla sarda Chilotigma exlgua Conus cmtonicus Conus chaleleus Conus coronatus Conus encaustus Conus flavidus Conus imperialls Conus lividus Conus musicus Conus rattus Codakia punctata Codakla tigerina Gafrarh~m pectinatum

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+ + + +

+ + + +

578 G. Chelazzi & 3I. Vannini

The structure and ecology of the platforms at the northernmost localities fit this descrip- tion, with local differences due to diversity in exposure to wave action. I n front of the Gesira cliffs a layer of dead Thalassodendron and Sargassum leaves covers the rocky substrate which has almost no macroalgal vegetation. T h e mollusc community is consequently

H a l o d u l e "

- T h a l a s s l a

I I I ! ! I 5 I 0 15 2 0 2 5 3 0

m

Figure 9. Schematic cross-section of the platform in front of the Bender l~ { ton i cliffs, showing the belts (a-e) described in the text.

TAm~ z. Distribution of the commoner species of molluscs on the different belts (a-e) on the platform in front of the Bender Mtoni cliff (see Figure 9)

Species]Areas a b c d e

Acanthopleura spiniger + Calllostoma picturatus + PhasianeIla aethioplca + Nerlta alblcilla + + + Smaragdia souverblana + + Planaxi~ sulcatus + Cerithium caeruleum + + Clypeomorus morus + + + Strombus gibberulus + + Cypraea lynx + Cypraea moneta + + Cypraea vltellus + + Natica gaultierlana + + Pollnlces mamilla + + Thais fusconigra + + 3Iorula margariticola + + Nassarius albescens + Nassarius arcularius + + + gassarlus coronatus + Nassarius echlnatus + Nassarius margaritlferus + Ancilla sarda + Vasum turbhzellus + Conus ebraeus + Conus lividus + Dolabella scapula + + Area avellana + Area eythraeoensls + + Lithophaga nasuta + Pinna murieata + Isognomon perna + Lucina clausa + Cardita variegata + Quidnipagus palatam + Gafrarium pectinatum + +

Intertidal molluscs of Sonmlla 579

characterized by fewer herbivores (mainly Pyreneflava) and small predators (Engnla mendi- caria, 3forula granulata, M. margaritifera, Conus encaustus, C. lividus and C. sponsalis). The platform in front of the sandy stretches is covered by large accumulations of unvegetated sand supporting Galeodes paradisiaca, Mitra mitra and Terebra nassoldes. North of Mogadiscio the erosion platforms differ from those at Sar Uanle in the reduction or the absence of the Cystoseira belt and substitution of Patellaflexuosa by P. exusta on the distal belt.

Clearly related to the sheltering of the coast is the structure and ecology at Bender Mtoni (Figure 9) where the platform consists of large accumulations of muddy sand covering a layer of wrack and dead shells. The underlying rocky substrate crops up sporadically through the sand, but is never covered by macroalgae such as at Sat Uanle. Proximally the cliff slopes gently seaward (a), gradually disappearing Under ever-thickening layers of sand (b) where blue-green algae appear. Cropping up 15-2o m from the cliff foot are patches of limestone (c). Distally are beds of phanerogams, never as thick as those at Sar Uanle, where Thalassia hempriddi and Halodule wrightii are dominant (d), while further seaward (e) only occasional tufts of Halodule appear among heaps of sand which indicate the presence of many burrowing species, particularly Thalassinidae decapods, The distribution of the commonest forms, shown in Table 2, reveals the presence of shelter-dwelling gastropods (Natica, Polinices, many Nassarius spp.) and bivalves (Lucina, Quidnlpagus) typical of sandy shores rich in organic detritus. The species diversity clearly increases passing from the bare sand to rock outcrops and phanerogam beds.

Discussion

The extreme uniformity of the littoral fauna inhabiting the Western Indian Ocean was first stressed by Taylor (I97IC) on the basis of reports from the islands of Aldabra (Taylor, I971a), Diego Garcia (Taylor, x97xb), Madagascar (Plante, x965), Mah6 (Taylor, 1968 ) andMauritius (Balssac et al., 1962; Hodgkin & Michel, 1963). Hartnoll (1976) substantiated this picture of general uniformity with his study on the species present at Dares Salaam, the majority of which are common along the western coast of the Indian Ocean and some of the Red Sea coasts.

The intertidal mollusc communities in Southern Somalia correspond well to those of other known continental and insular localities, though it should be remembered that such conclusions are based on the still relatively incomplete taxonomic knowledge of some forms. For instance, Thaisfusconigra--which is undoubtedly a determining species in the intertidal community in Somalia--has been reported only on the Cocos Keeling Islands (Maes, x967), Aldabra Atoll (Taylor, 1976 ) and Kenyan coast (Taylor, personal communication) as it was previously confused in the Thais hippocastanum group. Similar uncertainties mar the identifi- cation of other extremely important forms such as Patella and Siphonarla. As well there is a tendency in almost all reports to omit quantitatively irrelevant species which could nonethe- less be of possible zoogeographical interest.

On the basis of his study of the fauna, and algal data given by Lawson (1969) for the coasts of Kenya and Tanzania, and according to thegeneral classification set up by Stephenson (1949) and Lewis (1964), Hartnoll (x976) divided the flora and fauna at Dares Salaam into an 'upper-shore group' of the littoral fringe, a 'mid-shore group' of the upper eulittoral and a ' lowest group' of the lower eulittoral. Usihg the same criteria for Sar Uanle, the upper limit of the littoral fringe is 4-6 m above MHWS, that of the upper eulittoral o.5-t. 5 m above bIHWS and the upper limit of the lower eulittoral is between M T L and MHWN.

580 G. Chelazzi & M. Vannini

S.U. GES. B.M. MOG. More Less

Exp. Seml-exp, Shelt. exp. exp. 19 n

Acan/hopleura cf. borbon/¢o [ ] I I 16 ~ 15

Aconthopleura spiniger [ ] i I 14 ~ 4

Acmoeoprofvndo ~ I I 84. ~ 181

Cellona rodiolo • ] I 41 ~ 29

IVerita alblcl/la i i I I 20

Herlto debili$ • I I II ~ 7

Heritaplicata [] I ] 91 ~ 13

Nerita text//is i I I 87 ~ 13

Net/to undoto I • J 17 ~ 3

LHlorinokraus$1 I I ' , ,3000 ~ 30

Nodili/lorinanotalensls I I ' r i O 0 0 0 ~ 710

Clypeomorus moru$ [ I 57

Thai$ fusconigrO I 1 I 25 ~ 7

Morula anaxeres I t I0

Siphonar/o spp. 1 l l = , I I 53 ~ 12

Isognomon dentifer 1 I I I 57 ~ 4.

Soccostreo cucullo/o I 1 • I 22

0 50 I00 i I I

%

Figure zo. Relative frequencies (in percentages) of the commoner molluscs at Sat Uanle, Gesira and Bender Mtoni. Numbers refer to specimens present in an overall transect resulting from the sum of mean transects of the three sites. On the right are the relative frequencies on two transects at l~1ogadiscio.

Both its species pattern and zonation place this area between the semi-exposed and exposed stations in the gradient proposed by Taylor (z97xb) for Aldabra Atoll. Belonging in the same category is Mogadiscio, somewhat modified by the brusque passage between cliff face and summit which greatly reduces the range of the Iittoral fringe. Gesira is midway between a semi-exposed and a sheltered shore, while Bender 1Vltoni is definitely a sheltered shore.

Nonetheless the definition of zonal boundaries and the comparison between different stations must take into account that many intertidal forms (especially gastropods) have a dynamic rather than a simple static zonation pattern. The complex migratory behaviour of Nerita textilis (Chelazzi & Vannini, z976; Vannini & Chelazzi, i978 ) is probably present in many other intertidal species whose position on the shore thus varies in periods of short (diurnal and/or tidal) or long (synodic or seasonal) duration.

Once the exposure gradient of the various stations has been established, the preference of each mollusc species for exposure to wave action can be derived (Figure io). Mogadiscio was considered separately due to its particular cliff morphology and because molluscs were sampled in two differently exposed transects. Species clearly preferring exposure are Acantho- pleura cf. borbonlca, Littorina kraussi, Nodillttorlna natalensis and Thais fusconlgra, while Nerita undata, Clypeomorus rnorus and l~lorula anaxeres clearly avoid exposure to wave action and the remaining tolerate different degrees of wave exposure.

Intertidal molluscs of Somalia 58x

In comparison to the cliff wall, species diversity greatly increases on the erosion platforms due to an increase in the mean immersion times and greater complexity of the substrate. The importance of the latter element in imposing a higher degree of species diversity-- underlined by Kohn (I967)--is demonstrated by the flowering of forms in correspondence to irregularities in the substrate such as the borders of tide pools and the seaward ledge of the cliff-foot platforms. Of great importance in describing the distribution of lower eulittoral fauna on the platforms is the identification of facies corresponding to changes in substrate, microclimate, algal or phanerogam distribution. Our schematic arrangement of data, which obviously needs further verifications on the basis of distribution of other organisms, consti- tutes a first attempt to describe this part of the eulittoral, typical of the raised limestone coasts in this area.

List of species:

Molluscs Acanthopleura cf. borbonica (Deshayes) Acanthopleura splnlger (Sowerby) Acmaea profunda (Deshayes) Patella exusta Reeve Patellaflexuosa (Quoy & Gaymard) Cellana radiata (Born) Calllostoma plcturatus (A. Adams) Turbo coronatus Gmelin Phasianella aethiopica Philippi Nerita albicilla L. Nerita debilis Dufo Nerita plicata L. Nerlta textilis Gmelin Nerita undata L. Smaragdia souverbiana (Montrouzier) Littorina kraussi Rosewater Nodilittorlna natalemis (Philippi) Truncatella guerinii A. & J. Villa Planaxis sulcatus (Born) Cerlthium caeruleum Sowerby Clypeomorus morus (Lamarck) Strombus gibberulus (L.) Cypraea annulus L. Cypraea felina Gmelin Cypraea lynx L. Cypraea mauritiana L. Cypraea moneta L. Cypraea tigris L. Cypraea vitellus L. Natica gaultieriana Recluz Polinices mamilla (L.) Cymatium muricinum (Roeding) Purpura rudolphl (Lamarck)

Thais aculeata Deshayes Thais fusconigra (Dunker) Drupa morum Roedlng Drupa ricinus (L.) Drupella carlosa (Wood) )$lorula anaxeres (Kiener) Moruta granulata (Duelos) Morula margariticola Broderip 3Iorula marginatra (BIainville) Pyrene flava (Brugui~re) Engina mendicaria (L.) Galeodes paradisiaca (Roeding) Nassarius albescens (Dunker) Nassarius areularius (L.) Nassarius eoronatus (Brugui~re) Nassarius echinatus (A. Adams) Nassarius margaritiferus (Dunker) Peristernia nassatula (Lamarck) Vasum turbinellus (L.) Ancilla sarda (Reeve) Chilotigma exigua (Sowerby) Mitra mitra L. Conus canonlcus Hwass Conus chaldeus (Roeding) Conus coronatus Gmelin Conus ebraeus L. Conus encaustus Kiener Conus flavldus Lamarck Conus intperialis L. Conus lividus Hwass Conus musicus Hwass Conus rattus Hwass Conus sponsalis Hwass Terebra nassoides Hinds

58z G. Chelazzi & 2~1. Vannini

Molluscs Haminoea gracilis Sowerby Dolabella scapula Martyn Siphonaria ? asghar Biggs Siphonaria kurracheemis Reeve Siphonaria sp. tlrca avellana Lamarck Arca erythraeoensis Jonas Barbatia lacerata (L.) Brachiodontes variabilis Krauss Lithophaga nasuta (Philippi) Lithophaga teres (Philippi) Pinna muricata L. Pinctada radiata (Leach) lsognomon dentifer (Krauss) Isognomon perna (L.) Hyotissa numlsma (Lamarck) Saccostrea cucullata (Born) Luclna clausa (Philippi) Codakia punctata (L.) Codakia tlgerina (L.) Cardita variegata (Brugui~re) Fragum fragum (L.) Quidnlpagus palatam Iredale Gafrarium pectinatum (L.)

Algae Scytonema eL hofmannii C. Agardh Ulva rigida C. Agardh Cladophoropsis sundanemis Rinbold Dictyosphaerla vershtysii Weber van Bosse Valonia aegagrophila C. Agardh Padlna gymnospora (Kuetzing) Vickers Pocockiella variegata (Lamouroux) Papenfuss Cystoseira myrica (S. G. Gmelin) C. Agardh Sargassum sp. Gelidiella acerosa (Forsskfil) J. Feldmann &

Hamel Gelidiella adnata Dawson Gelidium pusillum (Stakhouse) Le Jolis Gelldlum sp. Peyssonnelia sp. Porolithon onkodes (Heydrich) Foslie Amansia glomerata C. Agardh Bostrychia tenella (Vahl) J. Agardh

Angiosperms Thalassodendron dliatum (Forsskfil) den

Hartog Halodule wrightii Ascherson Thalassia hemprichii (Ehrenberg) Ascherson

Acknowledgements

We are extremely grateful to Dr John D. Taylor of the British Museum (Natural History) for his invaluable aid in identifying the molluscs and for his comments on the manuscript. Dr A. J. Southward of the Marine Biological Association of the United Kingdom for having identified the barnacles and Prof. L. Pardi and Prof. A. Ercolini of the Isti{uto di Zoologia dell'Universit.~ di Firenze for their suggestions during the preparation of the manuscript. Our thanks also to Dr G. Sartoni of the Istituto di Botanica dell'UniversitA di Firenze for identifying the algae and phanerogams, and to our colleagues at the Centro di Studio per la Faunistica ed Ecologia Tropicali del C.N.R.--in particular Dr S. Turillazzi and Mr R. Innocenti--for their help in collecting the material.

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