Molluscan Assemblages on Coral Reefs and Associated Hard Substrata in the Northern Red Sea

Coral Reefs *2001) 20: 107±116 DOI 10.1007/s003380100140

REPORT

M. Zuschin á J. Hohenegger á F.F. Steininger Molluscan assemblages on coral reefs and associated hard substrata in the northern Red Sea

Received: 15 January 2000 / Accepted: 16 December 2000 / Published online: 24 May 2001 Ó Springer-Verlag 2001

Abstract Information on spatial variability and distri- increasing eutrophication and physical damage in the bution patterns of organisms in coral reef environments study area *Riegl and Piller 2000) will result in a loss of is necessary to evaluate the increasing anthropogenic coral-associated molluscs in favor of bivalve crevice disturbance of marine environments *Richmond 1993; dwellers in dead coral heads and of encrusters on dead Wilkinson 1993; Dayton 1994). Therefore dierent types hard substrata. of subtidal, reef-associated hard substrata *reef ¯ats, reef slopes, coral carpets, coral patches, rock grounds), each Keywords Mollusca á Ecology á Substrate gradient á withdierent coral associations, were investigated to Coral reef á Recent á Red Sea determine the distribution pattern of molluscs and their life habits *feeding strategies and substrate relations). The molluscs were strongly dominated by taxa with Introduction distinct relations to corals, and ®ve assemblages were dierentiated. The Dendropoma maxima assemblage on The study of patterns in time and space is the mainstay reef ¯ats is a discrete entity, strongly dominated by this of ecology. The community structure of coral reef-as- encrusting and suspension-feeding gastropod. All other sociated organisms, for example, shows considerable assemblages are arranged along a substrate gradient of spatial variability and a better understanding of their changing coral associations and potential molluscan distribution patterns is necessary to evaluate the in- habitats. The Coralliophila neritoidea±Barbatia foliata creasing anthropogenic disturbances and devastations of assemblage depends on the presence of Porites and modern coral reefs *Richmond 1993; Wilkinson 1993; shows a dominance of gastropods feeding on corals and Dayton 1994; Edmunds and Bruno 1996). Molluscs are of bivalves associated with living corals. The Chamoi- a good example, because they frequently inhabit a dea±Cerithium spp. assemblage on rock grounds is variety of ecological niches in tropical±subtropical strongly dominated by encrusting bivalves. The Drupella reef-associated hard substrata environments. In the Indo- cornus±Pteriidae assemblage occurs on Millepora±Acro- Paci®c, however, most studies on their ecology focus on pora reef slopes and is strongly dominated by bivalves particular taxa *e.g., Frank 1969; Had®eld 1976; Kohn associated withliving corals. The Barbatia setigera± and Leviten 1976; Taylor 1976; Austin et al. 1980; Lee Ctenoides annulata assemblage includes a broad variety and Morton 1985; Arnoud and Thomassin 1990; Cum- of taxa, molluscan life habits and bottom types, but ming 1999) or cover the easily accessible intertidal areas occurs mainly on faviid carpets and is transitional *e.g., Taylor 1971; Ayal and Safriel 1981). Of the studies among the other three assemblages. A predicted de- that deal with regional distribution patterns of mollusc gradation of coral coverage to rock bottoms due to assemblages *Salvat 1970a, 1970b, 1971; Richard 1973; Kay and Switzer 1974; Mastaller 1978; Henon 1979; Sheppard 1984; Taylor and Reid 1984; McClanahan M. Zuschin *&) á J. Hohenegger 1990; LanctoÃ t et al. 1997; Zuschin and Piller 1997c), Institut fuÈ r PalaÈ ontologie, UniversitaÈ t Wien, most cover only the shallow reef parts and do not apply Althanstraûe 14, 1090 Vienna, Austria rigorous quantitative sampling methods. Zuschin and E-mail: [email protected] Tel.: +43-1-4277-53555 Piller *1997c) did not dierentiate between living and Fax: +43-1-4277-9535 dead molluscs in the quantitative analysis, and only one of the mentioned studies *LanctoÃ t et al. 1997) treated the F.F. Steininger Forschungsinstitut und Naturmuseum Senckenberg, molluscan associations withstatistical ordination Senckenberganlage 25, 60325 Frankfurt am Main, Germany methods, which enable the detection of environmental 108 gradients *Digby and Kempton 1987; Krebs 1989). The aluminium square frame. The sampling sites were chosen to cover present molluscan study is the ®rst to consider the full all major subtidal hard substrata *Fig. 1, Table 1). At each locality the location of the ®rst frame was selected haphazardly by a diver range of reefoidal environments down to 40 m depth, throwing the frame from a few meters above the substratum. The to apply statistical ordination methods and to apply subsequent frames were positioned contiguously along a line ex- life habit information for particular taxa in order to tending from that point. In mixed hard-substrata/loose-ground understand their distribution patterns. and/or soft-bottom habitats, frames were taken only from hard Most marine hard substrata are populated by en- substratum because the adequate sampling of soft substratum molluscs requires other sampling strategies. Inter®ngering with soft crusting and endolithic colonizers. Such hard substrata substrata, however, was generally of minor importance and oc- include hardgrounds *sensu Bromley 1975), rock curred mainly around coral patches and rock grounds. A detailed grounds *sensu Goldring 1995), cobbles *e.g. Wilson study on the distribution pattern of soft substrata molluscs was 1987) and a wide range of organismic skeletons *e.g. provided by Zuschin and Hohenegger *1998). A mean of 5.0 m2 *1.7) of sea¯oor was investigated per lo- Zuschin and Piller 1997a), most of which might be re- cality, witha range from 3 to 11 m 2 *Table 2). The number of ferred to as shellgrounds *after Dodd and Stanton 1990). frames investigated depended on mollusc density *low density re- Coral colonies, in contrast, are not only the primary quired a higher number of frames and vice versa) and on water frame builders of coral reefs *for a review see Scon depth *greater depths required shorter dive times). The mean water depthwas 14.3 m *9.9) witha range from 1 to 40 m *Table 2). 1992) but also the major type of hard substrata char- Habitat features other than water depth *coral association, coral acterized by a living surface. Large parts of coral reefs coverage) were noted qualitatively. The present study is based on provide such a living surface *e.g. Liddell and Ohlhorst quantitative samples from the sea¯oor and from easily accessible 1988) and, especially among molluscs, a wide range of cavities. Due to methodological limitations, deeper cavities within taxa and dierent life habits *feeding strategies and coral reefs have not been considered *e.g. Scon 1992, and Fig. 6 therein). A taxa-sampling curve indicates adequate sampling of the substrate relations) are closely linked to suchsubstrata animals present in the study area *Fig. 2). *Had®eld 1976; Kohn 1983; Morton 1983b; Kleemann Samples were taken during daylight *usually between 10 a.m. 1990, 1992; Schuhmacher 1993). This close linkage al- and 5 p.m.). The molluscs either were identi®ed immediately in lows long-term changes of molluscan assemblages on their environment *most bivalves and sessile gastropods) or were collected *many vagile gastropods) for more detailed examination. fringing reefs to be related to changes in the nature of Due to a highly questionable taxonomy, Chamoidea *mainly the available substrata *Augustin et al. 1999). Chama brassica s.l. Reeve, 1847, and Chama imbricata s.l. Brode- The northern Bay of Safaga *Fig. 1) is composed of rip, 1835), Spondylidae *mainly Spondylus marisrubi s.l. RoÈ ding, various sedimentary facies *Piller and Mansour 1990; 1798) and Ostreoidea *mainly Hyotissa spp.), except for Lopha cristagalli, were not dierentiated to the species level; most of the Zuschin and Hohenegger 1998) that are closely associ- taxonomic studies in fact mainly describe ecophenotypes *Oliver ated withdierent types of hardsubstrata, including reef 1992). Some taxa were pooled due to problems withidenti®cation ¯ats, reef slopes, coral carpets, coral patches and rock in the ®eld or poor preservation *e.g. Conus spp.). bottoms *Piller and Pervesler 1989). These substratum It is dicult to recognize smaller molluscs in situ. Therefore, to categories are characterized by dierent coral associa- provide a consistent database, molluscs <2 cm were excluded from the quantitative treatment. Most boring bivalves *e.g., gastro- tions and by dierent degrees of surface coverage *Riegl chaenids, lithophagins) were also excluded from the quantitative and Velimirov 1994; Riegl and Piller 1997). The respec- collection because they are dicult to identify from the sur®cial tive molluscan assemblages exhibit strong dierences in appearance of the small openings of their boreholes. A qualitative the proportion of living and dead fauna *Zuschin et al. study of boring bivalves has been presented by Kleemann *1992). The original database contained 38 taxa, but statistical analysis 2000) and the distributions of bivalves on coral carpets was con®ned to the 15 taxa that each contributed more than 1% to are correlated withenvironmental gradients *Zuschin live mollusc abundance of all samples. Lima lima *0.8%), Drupella and Piller 1997b). The study area is under increasing cornus *0.9%) and Conus spp. *0.5%) were also included because environmental pressure from two sources: *1) diving they contribute considerably to the dead mollusc fauna encountered *compare Zuschin et al. 2000; Table 2). These 18 taxa con- tourism has led to an increasing number of visitors and tribute more than 97% of living individuals of the original tourist villages over the last few years, making eutroph- complete data set. ication a particular threat and *2) evidence of dynamite As most mollusc taxa within any one association are present in ®shing was observed locally along Porites reef slopes. other associations as well, ordination methods were used to clarify The objectives of the present study are *1) to docu- distribution patterns and the relations between taxa and sites. Because frequency data are represented in a contingency table, ment the distribution pattern of molluscs and their life correspondence analysis as an ordination method was used for data habits *substrate relations and feeding strategies) in a reduction *BenzeÂ cri 1992). The advantage of this method is the subtropical coral-dominated environment, *2) to discuss direct derivation of frequencies without transformations and the the correlation of substratum types on the distribution simultaneous representation of sites and taxa within the same system of axes in the form of biplots *Gabriel 1971). The ®rst three patterns and *3) to discuss the in¯uence of increasing dimensions of the correspondence analysis *e.g., Gauch 1982) were anthropogenic disturbance on the studied molluscan used to interpret the data. Sample groups were ®rst visually dis- assemblages. tinguished and then tested by numerical classi®cation using the K-means algorithm *Hartigan 1975). To retain data linearity, which is necessary for most numerical classi®cations, arcsine-root transformed proportions were used *Linder and Berchtold 1976). Material and methods An initial con®guration is necessary for the K-means algorithm, and the averages of taxa abundances of the visually de®ned groups Dierent subtidal hard substrata were sampled for shelled molluscs served this purpose. The numerical analysis led to 100% correctly at 68 localities in the northern Bay of Safaga with a 0.25 m2 classi®ed samples with regard to the visually distinguished groups. 109

Fig. 1 Location map, general bathymetry of the study area and *1994) and Riegl and Piller *1997, 1999); these are augmented by sample locations. Light grey ®elds on the right map are intertidal our own observations and modi®ed for the present molluscan areas. AM Aerial mast; H Safaga Hotel. *After Piller and Pervesler study. 1989) Reef ¯ats are rock bottoms colonized by a coral assemblage that is depauperate due to regular exposure during low tides. Thick Sample groups established by this analysis were described regard- extensive crusts of coralline red algae are a typical feature here. ing the dominating and characterizing molluscan fauna. All the Reef slopes border the fringing reefs and patch reefs of the statistical analyses were performed using the software package northern Bay of Safaga. They are associated with strong energy SPSS 8.0. gradients and/or steep relief and can be dierentiated into Acro- pora±, Porites± and Acropora±Millepora reef slopes based on their typical coral assemblages. Certain sites are characterized by atyp- ical coral associations and by dierent environmental conditions Hard-substrata types *Table 1). Two of these are located in very shallow settings: at sample location 14, Millepora dichotoma is superimposed on a base The hard substrata studied are distinguished by coral associations consisting of Porites. At sample location 64 Stylophora pistillata and coverage of living surface *Table 1). In general, the classi®ca- and Pocillopora damicornis are scattered on a rock ground. Sample tion of hard substrata utilized here follows the terminologies and locations 52 and 65 are deeper reef slope settings and consist of descriptions of Piller and Pervesler *1989), Riegl and Velimirov rock grounds witha patchyplatelike scleractinian cover. 110

Table 1 Descriptive classi®cation of the investigated hard substrata and corresponding assignment of sample sites. Descriptive classi®- cation of hard substrata modi®ed after Piller and Pervesler *1989), Riegl and Velimirov *1994) and Riegl and Piller *1997)

Bottom Coral associations Living coral Controlling environmental Sample locations types coverage *%) conditions

Reef ¯ats Stylophora association 30 Strong tidal in¯uence and 1, 2, 3 temperature variability Reef slopes Acropora association 60±80 Exposed to wave action 15, 16, 17 Porites association 80±100 Low energy, clear water 18, 19, 20 and plenty of light Acropora±Millepora 50 Exposed to currents 25, 26, 27, 38 association Porites±Millepora 80±100 Clear water and current-exposed 14 association Stylophora association <20 Strong tidal in¯uence and 64 exposed to wave action Coral carpets Porites association 80±100 Low energy, clear water 4, 21, 59, 62, 63, 66, and plenty of light 67, 68 Faviid association 25 Low energy, light availability 5, 7, 9, 11, 12, 13, 29, 30, decreasing 32, 33, 34, 35, 41, 43, 44, 47, 48, 51, 53, 58, 60, 61 Depauperate faviid <25 Strong sedimentation 6, 36, 37, 50 association Rock grounds Sarcophyton association <20±80 Highload of sediment and 10, 22, 31, 46, 55, 56, 57 organic material *POM) Platy scleractinian 30±50 Reduced amounts of light available 8, 23, 42, 45, 49, association 52, 54, 65 Coral patches Acropora patch 60±80 Tidal in¯uence and temperature 24, 28, 40 association variability Stylophora±Acropora 60±80 Tidal in¯uence and temperature 39 patchassociation variability

Table 2 Abundance, relative proportion, density *per 0.25 m2) and ecological properties of the 18 most abundant taxa. Corresponding classi®cation into six life habit groups using integrated taxonomic and life habit information *see text)

Taxon *life habit group) Molluscan no. and densities Substrate relations Feeding strategy

No. *%) Mean SD

Pedum spondyloideum *1) 548 25.4 0.40 1.42 Boring and bysally attached Suspension feeder in living coral Dendropoma maxima *4) 387 17.9 0.28 1.99 Encrusted to living corals/dead Suspension feeder hard substrata Coralliophila neritoidea *5) 338 15.7 0.25 1.57 Vagile on living Porites spp. Parasite Tridacna maxima *1) 257 11.9 0.19 0.53 Bysally attached among corals Various food sources or within corals Chamoidea *2) 142 6.6 0.10 0.39 Encrusted to dead hard substrata Suspension feeder Barbatia setigera *3) 87 4.0 0.06 0.33 Byssally attached in crevices Suspension feeder of dead hard substrata Ctenoides annulata *3) 74 3.4 0.05 0.30 Byssally attached in crevices Suspension feeder of dead hard substrata Spondylidae *2) 50 2.3 0.04 0.22 Encrusted to dead hard substrata Suspension feeder Streptopinna saccata *1) 40 1.9 0.03 0.17 Byssally attached in crevices Suspension feeder of living and dead hard substrata Lopha cristagalli *2) 36 1.7 0.03 0.26 Encrusted to dead hard substrata Suspension feeder Barbatia foliata *1) 35 1.6 0.03 0.20 Byssally attached in crevices Suspension feeder of living Porites spp. Ostreoidea *2) 34 1.6 0.02 0.20 Encrusted to dead hard substrata Suspension feeder Cerithium spp. *6) 30 1.4 0.02 0.19 Vagile on hard substrata Algal detritus feeder Pteriidae *1) 27 1.3 0.02 0.23 Byssally attached to living corals Suspension feeder Isognomon legumen *3) 25 1.2 0.02 0.15 Byssally attached in crevices Suspension feeder of dead hard substrata Drupella cornus *5) 20 0.9 0.01 0.31 Vagile on living corals *acroporans) Predator Lima lima *3) 18 0.8 0.01 0.13 Byssally attached in crevices Suspension feeder of dead hard substrata Conus spp. *6) 11 0.5 0.01 0.10 Vagile on hard substrata Predator 111

Results

At 68 sample locations a total area of 340.5 m2 was investigated and 2,211 individuals were counted. The overall density of living molluscs was 1.62 individuals/ 0.25 m2. Densities tended to have high associated standard deviations *Table 2), indicating patchy distribution within and between sites. Molluscs were dominated strongly by bivalves associated withliving corals, especially Pedum and Tridacna, the encrusting gastropod Dendropoma, and gastropods feeding on corals, mainly Fig. 2 Plot of cumulative molluscan diversity versus number of represented by the parasitic Coralliophila. Overall, in the sampling sites indicates adequate sampling of Mollusca present on reduced data set gastropods make up 36% of the mol- hard substrata in the study area. The reduced data set contains luscan fauna *the corresponding value for the original more than 97% of individuals and was used for statistical analysis data set is 37%). The ®rst three dimensions of the correspondence Coral carpets *sensu Reiss and Hottinger 1984) are extensive in analysis can be well interpreted and explain 53.6% of the the northern Bay of Safaga down to 40 m; they are up to 3 m thick variation in the data set *Table 3). The ordination re- in shallow settings. Coral carpets build a framework, but lack a vealed a distribution pattern of living molluscs involving distinct zonation since they only grow in areas without distinct ®ve distinct assemblages *Fig. 3), eachof whichcanbe gradients *Riegl and Piller 1999). They can be dierentiated into Porites and faviid carpets, which grade into one another. Depau- related to speci®c hard-substratum types. perate faviid carpets are characterized by less than 20% of living The Dendropoma maxima assemblage is located on coral coverage *Table 1). On coral carpets, the highly variable reef ¯ats. Dendropoma, which is nearly restricted to this heights of individual coral colonies create diverse molluscan habi- habitat *Fig. 3, Table 4), clearly has no special substrate tats. Rock grounds *sensu Goldring 1995) consist of non-frame- preference, but lives cemented to dead hard substrata or work-building Sarcophyton carpets *Riegl and Piller 1999) on embedded into the tissue of living corals. bare rocky surfaces and platelike scleractinian carpets, where The Coralliophila neritoidea±Barbatia foliata assem- distinct molluscan habitats are lacking except for the high blage is located on coral carpets and reef slopes with amount of exposed coral limestone between platelike faviid corals *Table 1). Porites associations and characterized by a strong Coral patches *sensu Piller and Pervesler 1989) are a special reef dominance of gastropods feeding on corals and of type, which is widespread in the northern Bay of Safaga, commonly bivalves associated withliving corals: Coralliophila occurring in shallow water on sandy substrata. Coral patches are neritoidea and Barbatia foliata are nearly restricted to only a few meters in diameter and do not show a clear ecological this habitat *Fig. 3, Table 4). B. foliata inhabits crevices zonation *Riegl and Piller 1999). They can be dierentiated into two major groups based on their coral composition: Acropora± and of living Porites spp., usually withdirect contact Stylophora±Acropora coral patches; coral patches typically grade between the bivalve and the coral tissue. C. neritoidea is into faviid carpets. a parasitic gastropod that feeds on Porites and Porites *Synaraea) *Robertson 1970; Schuhmacher 1993).

Life habits of molluscs Table 3 Proportions explained by the 17 dimensions of the correspondence analysis The 18 taxa considered in the statistical analysis consist of 13 bivalves and 5 gastropods, which are classi®ed using integrated Dimension Proportion explained Cumulative proportion taxonomic and life habit information *e.g., Robertson 1970; Go- reau et al. 1973; Leviten and Kohn 1980; Reichelt 1982; Morton 1 0.245 0.245 1983a, 1983b; Fankboner and Reid 1990; Zuschin et al. 2000; Ta- 2 0.171 0.417 ble 2). 3 0.119 0.536 Group 1 consists of bivalves that have a close substrate rela- 4 0.101 0.637 tion to living corals: these taxa bore into living corals *Pedum), 5 0.071 0.709 settle in crevices of living corals *Barbatia foliata and Streptopinna 6 0.062 0.771 saccata), are epizoic on living corals and the hydrozoan Millepora 7 0.045 0.816 *Pteriidae) or live in close association withliving corals * Tridacna 8 0.039 0.855 maxima). Group 2 consists of bivalves that are encrusters on dead 9 0.034 0.889 hard substrata *Chamoidea, Ostreoidea and Spondylidae). Group 10 0.033 0.922 3 consists of bivalves that are crevice dwellers of dead hard sub- 11 0.020 0.942 strata *Barbatia setigera, Ctenoides annulata, Isognomon legumen, 12 0.013 0.955 Lima lima). Group 4 consists of the encrusting gastropod Den- 13 0.012 0.967 dropoma maxima, which shows a rather indierent relation to 14 0.010 0.977 living or dead hard substrata because it occurs on both types. 15 0.009 0.986 Group 5 consists of vagile gastropods that feed on living corals 16 0.008 0.994 *Coralliophila neritoidea and Drupella cornus). Group 6 consists of 17 0.006 1.000 vagile gastropods that typically avoid living corals *Conus spp. and Total 1.000 1.000 Cerithium spp). 112 113 b Fig. 3 Ordination of samples and taxa and relationships between

taxa and sites using the ®rst three dimensions of the correspon- Total dence analysis. A and C show sample groups established by distribution pattern of molluscs in B and D. Reef ¯ats, Porites associations, Millepora±Acropora reef slopes and rock grounds are characterized by distinct faunal associations. The molluscan Conus spp. assemblage on faviid carpets shows a transitional character. Circles

indicate sample locations and taxa spp. Cerithi- um Avoid living corals Streptopinna is typically embedded into living massive reef slopes, rock grounds and coral colonies. The host-speci®c Pedum is associated Drupel- la cornus witha variety of scleractinians *preferentially Montipora spp.) in the northern Red Sea *Kleemann 1990). Coral- liophila neritoidea Tridacna maxima is a coral-reef-associated bivalve *e.g., Feeding on corals Yonge 1974) usually found bysally attached within or between living coral colonies. Millepora-Acropora Den- dropoma maxima The Chamoidea±Cerithium spp. assemblage inhabits En- crusting rock grounds. It is characterized by a strong dominance

of encrusting bivalves and vagile gastropods that nor- Lima lima mally avoid living corals, mainly Chamoidea and

Cerithium spp. *Fig. 3, Table 4). The suspension-feeding associations,

Chamoidea were mostly encountered on bare rocky Isogno- mon legumen surfaces, where even dead coral heads are rare. Cerithium spp. *C. echinatum, C. nodulosum, C. ruÈppelli), Porites which are well-known algal-detritus feeders *Houbrick Cteno- ides annulata 1992), and Conus spp., which prefers <20% cover of living coral *Kohn 1983), are mostly found on this hard Barba- tia setigera substratum type. Other important constituents of this Byssally attached to deadsubstrata hard assemblage are the byssate bivalves Barbatia setigera, Ctenoides annulata, Isognomon legumen and Lima lima, Spond- ylidae which inhabit crevices in dead hard substrata, mostly dead coral colonies *compare Had®eld 1976; Morton

1983b; Taylor 1971; Zuschin and Piller 1997b, 1997c). Ostreo- idea Spondylidae and Ostreoidea typically encrust dead coral

heads *Zuschin and Piller 1997b). Lopha cristagalli The Barbatia setigera±Ctenoides annulata assemblage ) of molluscan assemblages on reef ¯ats,

shows a very heterogeneous composition of bottom 2 Chama sp. types, molluscan taxa and molluscan life habits. More Encrusters on dead hard substrata than half of the sample locations, however, belong to coral carpets withfaviid or depauperate faviid associa-

tions, which also show a transitional position among Trid- acna maxima coral associations in Safaga *Riegl and Piller 1997). The very high coral diversity enables a broad variety of molluscan life habits on these bottom types. One group Strepto- pinna saccata of characteristic taxa distinctly prefers living corals

*Streptopinna, Pedum and Tridacna) and therefore Pterii- dae marks a transition to molluscan assemblages on Porites associations. A second group of characteristic taxa is distinctly related to dead hard substrata *Barbatia Pedum spondyloideum setigera, Ctenoides annulata, Isognomon legumen, Lima lima, Spondylidae, Ostreoidea, Lopha cristagalli, Conus Barba- tia foliata spp.) and therefore marks a transition to molluscan as- Closely associated with living corals semblages on rock grounds *Fig. 3, Table 4). The Drupella cornus±Pteriidae assemblage is located %Density 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.7 0.2 3.2 0.1 1.2 0.0 0.0 0.0 0.0 0.0 0.2 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 5.1 91.6 0.0 0.0 0.0 0.0 0.0 0.0 0.1 1.2 100.0 5.6 Total no. 28%Density 160 0.2% 4.8Density 0 1.1 0.0 27.2 0.0 10 0.0 0.0 0.0 0.1 1.2 1.7 36 0.0 0.0 0.3 0.0 6.1 1 0.0 0.0 0.2 0.0 0 2.4 0.0 0.0 0.5 72.9 0.0 1 0.2 0.0 0.0 0.0 0.0 0 0.0 0.0 0.0 0.0 0.0 1.2 0 0.0 1.0 0.0 1.2 6 0.0 0.0 0.0 0.0 0.0 0 0.0 0.0 1.2 0.0 0.3 0.0 0 2.4 2.3 57.0 0.0 0.0 2 0.0 0.2 0.0 2.4 336 0.0 0.0 0.0 0.0 1 0.0 0.0 0.1 10.6 0 100.0 0.0 1.2 4.1 0 100.0 0.7 589 Total no.% 4Density 0.0 374 0.4 13 0.4 35.0 26 0.0 1.2 0.0 2.4 202 74 0.2 18.9 0.1 6.9 36 0.0 3.4 29 0.0 2.7 47 0.0 4.4 84 0.1 7.9 68 0.1 6.4 24 0.0 2.2 16 0.0 1.5 0.0 0.8 9 0.0 0.0 0 0.0 0.3 3 0.0 2.0 21 0.0 0.5 5 100.0 1.1 1069 on Acropora±Millepora-dominated reef slopes and Total no.% 3Density 0.0 13 4.9 0.2 14 21.3 0.2 23.0 1 0.0 1.6 4 0.1 6.6 0 0.0 0.0 0.0 0.0 0 0.1 6.6 4 0.0 1.6 1 0.0 3.3 0.0 0.0 2 0.0 0.0 0 0.0 0.0 0 0.0 4.9 0 0.0 0.0 26.2 0.2 3 0.0 0.0 0 0.0 0.0 16 100.0 0.8 0 0 61 characterized by a strong dominance of bivalves associated withliving corals, mainly Pteriidae * Electroma Composition *total number, percentage, density per 0.25 m alacorvi and Pteria spp.) and Pedum, and the gastropod associations carpets Drupella. Drupella cornus and Pteriidae are nearly Acropora reef slopes Reef ¯ats Total no.Porites 0 0 0 3 13 5 0 0 1 0 0 0 0 373 0 0 0 5 407 Rock grounds Total no. 0Mainly faviid 1 0 0 2 62 0 0 1 1 0 1 2 0 2 0 9 1 85 Millepora± faviid carpets Table 4 114 restricted to this habitat *Fig. 3, Table 4). Pteriidae are be very important for a depth-related dierentiation of epizoic bivalves which we found bysally attached to bivalve assemblages on coral carpets in the study area living branching corals *mostly Acropora) and the *Zuschin and Piller 1997c). hydrozoan Millepora. Drupella is a coral predator The northern Red Sea is one of the world's most *Robertson 1970; Schuhmacher 1993), which we only visited diving areas and, in conjunction withdestructive encountered on acroporans *compare Turner 1994; ®shing practices, serious environmental changes are en- Cumming 1999). visioned over the next few years, with degradation of coral carpets and reef slopes to rock bottoms being the most serious threat *Jameson et al. 1999; Riegl and Piller Discussion 2000). Coral colonies provide some of the most important molluscan habitats in the study area, and their de- Most Indo-Paci®c molluscan studies have focused on the crease will result in a loss of coral-associated molluscs habitats and diets of particular taxa in speci®c envi- and in a shift towards bivalve crevice dwellers in dead ronments. Only a few have attempted to describe the coral heads and towards encrusters to bare rocky sur- regional distribution patterns of assemblages, mostly by faces. providing species lists from qualitative or semiquanti- In summary, the investigated molluscan taxa have tative observations: Mastaller *1978), for example, re- conspicuous preferences for various types of living or ported reduced mollusc species diversity and lower dead hard substrata. The ordination of our samples in- numbers of individuals in areas withlow water ex- dicates that, apart from reef ¯ats, molluscan assem- change, high sedimentation rates and turbid waters in blages in Safaga are mainly developed along a distinct the region of Port Sudan. Salvat *1967) related the dif- substrate gradient along which the coverage by living ferent molluscan assemblages of atoll lagoons to the corals decreases and molluscan habitats change. A pre- presence or absence of connections to the open ocean. dicted environmental change and associated loss of Kay and Switzer *1974) associated mollusc distribution molluscan habitats due to anthropogenic disturbances patterns in Fanning Island lagoon withclear and turbid will reduce coral-associated molluscs in favor of bivalve water areas. Sheppard *1984) stated for the Chagos crevice dwellers in dead coral heads and encrusters on Archipelago that mollusc distributions on hard sub- dead hard substrata. strata parallel those of corals with respect to diversity and separate along depthor depth-related parameters. Acknowledgements Thanks are due to A.M. Mansour, W.E. Piller, Taylor *1968) recorded a zonation parallel to the M. Rasser and B. Riegl for help with ®eld work. R. Janssen, shoreline in response to a variety of environmental fac- E. Neubert and P.G. Oliver helped with taxonomic problems. tors around MaheÂ , Seychelles. Taylor and Reid *1984) K. Kleemann, W.E. Piller, B. Riegl and M. Stachowitsch provided attributed dierences in faunas between fringing and stimulating discussions on coral±mollusc interactions and the classi®cation of hard substrata. The comments and suggestions of patchreefs in theSudanese Red Sea to major dierences B. Salvat, P. Sale and an anonymous reviewer improved the in the distribution of habitats due to terrestrial in¯u- manuscript greatly. Financial support was provided by Project ences. McClanahan *1990) related dierences in density P10715-Geo of the Austrian Science Foundation. The manuscript and species richness between reefs along the Kenyan was completed during a Max Kade fellowship of M.Z. at Texas coast to stronger river discharges in the north, but dif- A&M University. ferences within reefs to dierent predation rates between environments. 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