Zonation of intertidal molluscs on rocky shores of Southern Somalia

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  • 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 Dares 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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  • 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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    I i I I i ' I I V i i i 29 31 33 35 37 39

    Temperoture (C)

    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

    4 ............. (a)

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    3 (b)

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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 MTL 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 }

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    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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    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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    Marcia margarilicola 1 I ] 176

    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 26

    BrocModoates varlobili$ [ . . . . I I " - ,3000

    Engiaa mendicaria I 1 I 36

    Canus ebraeu$ 1 1 I 27

    Thais fusconigra 1 I 64

    Morula granulafa I ] 284

    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

    Aman$ io - -

    Pod /no

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    Holodu le Tholossio Gelid/ella

    Cys/oseiro

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    I I I I I l I I I I I I I 0 I0 20 50 40 50 60

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    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. Distribution 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

    + +

    + + +

    + + + + +

    + + + +

    + +

    + + + + -b + + + + + + + +

    +

    +

    +

    +

    + + +

    q. +

    + q- +

    + + + +

    + + + +

  • 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. In front of the Gesira cliffs a layer of dead Thalassodendron and Sargassum leaves covers the rocky substrate which has almost no macroalgal vegetation. The mollusc community is consequently

    Halodu le "

    - Tha lass la

    I I I ! ! I 5 I0 15 20 25 30

    m

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

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