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; Lanctoà 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 *Lanctoà 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 encoun- tered *compare Zuschin et al.