FACIES 38 229-254 PI. 56-57 14 Figs. 16 Tab. ERLANGEN 1998 i Subtropical Coral-reef Associated Sedimentary Facies Characterized by Molluscs (Northern Bay of Safaga, Red Sea, Egypt) Martin Zuschin and Johann Hohenegger, Vienna KEYWORDS: MICROMOLLUSCS- ASSEMBLAGES - DIVERSITY - FEEDING STRATEGIES - SUBSTRATE RELATIONS - TAPHONOMY - PALAEOECOLOGY - RECENT Summary The shallow marine subtropical Northern Bay of Safaga is composed of a complex pattern of sedimentary facies that are generally rich in molluscs. Thirteen diver- taken bulk-samples from various sites (reef slopes, sand between coral patches, muddy sand. mud, sandy seagrass, muddy seagrass, mangrove channel) at water depths ranging from shallow subtidal to 40m were investigated with regard to their mollusc fauna >lmm, which was separated into fragments and whole individuals. Fragments make up more than 88% of the total mollusc remains of the samples, and their proportions correspond to characteristics of the sedimentary, facies. The whole individuals were differentiated into 622 taxa. The most common taxon, Rissoina cerithiiformis, represented more than 5 % of the total mollusc content in the samples. The main part of the fauna consists of micromolluscs, including both small adults and juveni- les. Based on the results of cluster-, correspondence-, and factor analyses the fauna was grouped into several associations, each characterizing a sedimentary facies: (1) "Rhinoclavis sordidula - Corbula ervthraeensis - Pseudominolia nedyma association" characterizes mud. (2) "Mierocirce sp. - Leptomyaria sp. association" characterizes muddy sand. (3) "Smaragdia spp. - Perrilfia stellata - Anachis exilis - assemblage" characterizes sandy seagrass. (4) "Crenella striatissima - Rastafaria calypso - Cardites akabana - assemblage" characterizes muddy seagrass. (5) "Glycymeris spp. - Pamicardium sueziensis - Diala spp. - assemblage" characterizes sand between coral patches. (6) "Rissoina spp. - Triphoridae - Ostreoidea - assemblage" characterizes reef slopes. (7) "Potamides conicus - Siphonaria sp. 2 - assemblage" characterizes the mangrove. The seagrass fauna is related to those of sand between coral patches and reef slopes with respect to gastropod assemblages, numbers oftaxa and diversity indices, and to the muddy sand fauna on the basis of bivalve assemblages and feeding strategies of bivalves. The mangrove assemblage is related to those of sand between coral patches and the reef slope with respect to taxonomic composition and feeding strategies of bivalves, but has a strong relationship to those of the fine-grained sediments when considering diversity indices. Reef slope assemblages are closely related to that of sand between coral patches in all respects, except life habits of bivalves, which distinctly separates the reef slope facies from all others. Introduction [ndo-Pacific molluscan studies are generally rare be- cause investigations on ecology or facies distributions mainly focus on corals or coral communities. Moreover, most pub- lished molluscan studies concentrate on hard substrates and/ or easily accessible intertidal areas (e.g., Ale4OUD & THO~,tASSJN, 1990; AUSTINet al. 1980; AVaL & SAVmEL, 1981; FRANK, 1969; HADFIELD,1976; Kt_EEMANN,1990, 1992, 1995; LEE& MORTON, 1985; SAFRIELet al., 1980; SCHUHMACHER, 1993; TAYLOR, 1971, 1976; TAYLOR & REID, 1984; ZUSCHtN & PILLER, 1997a, b, c). Modern marine death assemblages in sediments comprise mostly molluscan remains (PoWELL et al., 1989) and, in most sediments, molluscs comprise the most abundant and diver- se of the geologically preservable macrofaunal remains e ~, EKDALE,1977). To date, however, only a few Indo- Pacific studies have treated the mollusc fauna of subtidal sediments (e.g., BaNDEL, 1991a; KAY & SWITZER, 1974; M.-XSTaLLER, 1978; SHEPPARD, 1984; TAYLOR, 1968); tO our knowledge none of the previously published studies dealt quantitatively with the total mollusc fauna of various sedimentary environments. Molluscs and their fragments are by far the most domi- nant particles > 250 btm in the Northern Bay of Safaga, generally comprising more than 50% (PmLER & MaNsoua, 1990). They range third among sediment constituents of a thin-section analysis including all grain size fractions (PIncER, 1994). Most of the mollvscs in our samples are "micro- Address: Dr. M. Zuschin, Prof. Dr. J. Hohenegger, Geozentrum, lnstitut for Palaontologie, Althanstr. 14, A- 1090 Wien; Fax: 0043-1 - 31336-784, E-mail: martin.zuschin@univie.ac.at