Introduction

Molluscs are the most diverse marine phylum and their diversity is particularly high in the tropical waters of the Indo-Pacific (Gosliner et al. 1996). They are also considered to be an indicator group for the rapid assessment of coral reef diversity (Wells 1998). In the Red Sea, they occupy almost every ecological niche provided by the complex structures of different reef zones and substrata (Mastaller 1987). General information is available about molluscan assemblages on various supratidal to shallow-water non-reefal hard- and soft substrata of the Red Sea (Fishelson 1971), and the molluscan composition of the rocky intertidal, shallow lagoons and reef flats are comparatively well studied (Hughes 1977; Mergner 1979; Ayal and Safriel 1981; Taylor and Reid 1984; Hulings 1986; Zuschin and Piller 1997a). Less information is available on deeper, subtidal molluscan composition (northern and central Red Sea: Mastaller 1978, 1979; Zuschin and Hohenegger 1998; Zuschin and Oliver 2003a), and only a few studies have systematically treated the molluscan distribution pattern on subtidal coral reef-associated hard substrata (Mergner and Schuhmacher 1974; Mastaller 1979; Zuschin et al. 2001). Although coral colonies provide important habitats for molluscs, most studies on coral reef-associated molluscs have focused on specific coral–mollusc interactions (e.g., Antonius and Riegl 1997; Schuhmacher 1992; Kleemann 1990, 1992; Al-Moghrabi 1996) or the autecology of specific molluscan taxa (e.g., Hughes and Lewis 1974; Kappner et al. 2000). Similarly, changes in coral composition with water depth are well known (e.g., Loya 1972; Riegl and Velimirov 1994), but only a few studies relate molluscan composition on reefoidal hard substrata to water depth categories (e.g., Zuschin and Piller 1997b).

The aim of this paper was to contribute to our knowledge of coral-associated molluscan communities in the Red Sea. Specifically, we wanted to study the dependence of molluscan assemblages on bottom types (i.e., different coral associations) and on water depth. We also investigate the pathways of empty shells—a prerequisite in estimating the fossilisation potential of molluscs (Kidwell and Flessa 1995; Martin 1999). Our approach underlines the importance of examining reefs in a more comprehensive faunistic manner and, from a management and conservation perspective, demonstrates that determining the status of a reef must go beyond the corals themselves to encompass other key invertebrate groups whose presence, abundance and distribution are intimately linked to reef health.

Materials and methods

Study area

The Gulf of Aqaba (Fig. 1) at the northern end of the Red Sea is a >1,000 m deep, oligotrophic marine basin (Braithwaite 1987; Medio et al. 2000) characterized by fringing reefs (Head 1987). Annual sea surface temperature in the gulf ranges from ∼21 to 27°C; salinity is about 40‰ (Edwards 1987). Due to its morphology, oceanography and hydrodynamics, the northern end of the Red Sea and particularly the Gulf of Aqaba are sometimes considered as a small sub-region of the Red Sea (Medio et al. 2000). For the Gulf of Aqaba, isolation, high salinities and strong temperature fluctuations have zoogeographic consequences: the coral communities differ somewhat from those further south (Sheppard and Sheppard 1991) and molluscan species numbers are somewhat lower than in the rest of the Red Sea (Mastaller 1987).

Fig. 1
figure 1

Study area with sampling stations. Station GPS coordinates from Google Earth: Saudi border N 29° 21′ 53′′, E 34° 57′ 42′′; Royal Diving Club 29° 24′ 02′′, 34° 57′ 58′′, Eel Canyon 29° 25′ 01′′, 34° 58′ 24′′, New Canyon 29° 25′ 24′′, 34° 58′ 15′′, Gorgonia II 29° 25′ 32′′, 34° 58′ 15′′, Gorgonia I 29° 25′ 36′′, 34° 58′ 17′′, Rainbow reef 29° 26′ 00′′, 34° 58′ 22′′, Black rock 29° 26′ 05′′, 34° 58′ 18′′

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Field work

Different subtidal hard substrata were sampled for shelled molluscs at eight diving sites in the Gulf of Aqaba (Fig. 1, Table 1) with a 0.25 m2 aluminium quadrat. Forty-four transects were laid to cover all major molluscan habitats on fringing reefs (reef flats, Millepora fringing reefs, fringing reefs with massive corals) and fore-reef hard substrata (coral patches, coral carpets and small patch reefs) in water depths ranging from intertidal to 26 m (Table 2). For each transect, a diver haphazardly dropped the frame from a few meters above the substratum. After this, the subsequent frames were positioned contiguously along a line extending from that point. Transects were laid parallel to the coastline. Five square meters of seafloor were investigated per transect (Table 1). Samples were taken during daylight (usually between 10 a.m. and 5 p.m.). The molluscs were identified either in situ (most bivalves) or were collected (many gastropods) for more reliable identification. The fauna was separated into living and empty individuals and hermit crab-occupied shells. The information on live and dead individuals was used for statistical analysis; shells occupied by hermit crabs were treated as an important descriptive feature affecting the distribution of dead gastropod shells. Fieldwork was performed in October and November 2001 and 2002.

Table 1 Basic data on sampling sites
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Table 2 Overview of the six molluscan habitats
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Habitat and water depth categories

Gulf of Aqaba coral habitats can be divided into fringing reefs and fore-reef hard substrata. The former consist of reef flats with depauperate coral assemblages and thick crusts of coralline red algae, reef edges and slopes dominated by massive corals (mostly Porites) and Millepora-dominated slopes (Table 2). Fore-reef hard substrata consist of coral carpets, coral patches and small patch reefs. Coral carpets (sensu Reiss and Hottinger 1984) build a framework of (mostly faviid) corals but lack distinct zonation; laterally they grade into coral patches, which are scattered aggregates of faviid coral colonies. Two patch reefs were studied, which elevate from sandy substrata, with diameters and heights of a few meters; they consisted of platy scleractinian associations (Table 2).

Water depth categories were divided into 5 m intervals with a notable exception being the shallowest category (<1 m), which was chosen to reflect the tidal influence in the study area. Tidal amplitudes in the Red Sea are generally small, with highest spring ranges occurring in the Gulfs of Suez (1.5 m) and Aqaba (1.2 m) (Medio et al. 2000). The deepest category (>20 m) included one diving site at 26 m.

Taxonomy

The molluscs were identified to species level wherever possible. Due to questionable taxonomies, Chamoidea, Spondylidae and Ostreoidea were identified to the family and superfamily levels only. Certain other taxa were also identified above the species level due to problems with identification in the field or poor preservation (e.g., Conus spp.).

It is difficult to recognize small molluscs when SCUBA diving. Therefore, to provide a consistent database, molluscs <1 cm were excluded from the quantitative treatment. In addition, most boring bivalves (e.g., gastrochaenids, lithophagines) have been excluded from the quantitative collection because they are difficult to identify using hole characteristics. Species identification followed Sharabati (1984), Oliver (1992), Bosch et al. (1995), and Zuschin and Oliver (2003a).

Analysis of faunal composition

The data set was explored to study differences between molluscan habitats (i.e., bottom types), water depth categories and living and dead fauna using analysis of similarity (ANOSIM, Clarke and Warwick 1994) based on the Bray–Curtis similarity coefficient (Bray and Curtis 1957). The important message of the pairwise tests of the ANOSIM analysis is less the significance level (which can often be low because of few replicates in each group), but the pairwise R-values; the latter give an absolute measure of how separated the groups were, on a scale of 0 (indistinguishable) to 1 (all similarities within groups are less than any similarity between groups). With R values >0.75, groups are well separated; with R values >0.5, groups overlap but are clearly different; with R values >0.25, groups strongly overlap; and with R values <0.25, groups are barely separable (Clarke and Gorley 2001). Non-metric multidimensional scaling (MDS, Kruskal 1964), used as an ordination method to provide a visual comparison of the pattern of Bray–Curtis values among the 44 transects, was run with 30 random starting configurations. ANOSIM and MDS were performed using the software package PRIMER (Clarke and Warwick 1994). Similarities between the living assemblages in Aqaba and Safaga (northern Red Sea), where a similar study has been conducted (Zuschin et al. 2001), were also explored by rank order correlation of the datasets using the software package SPSS 10.0 (SPSS 1999). Species contributing more than 1% to the living assemblages in Aqaba or Safaga were used to explain similarities and differences between these locations. These quantitatively important species are finally also compared with records from other Red Sea studies.

Results

A total of 1,665 molluscan individuals from 51 taxa was identified from 44 transects, which covered 220 m² at eight diving sites (Table 1). The quantitatively most important living molluscs were the coral-associated parasitic gastropod Coralliophila neritoidea (Lamarck, 1816), the encrusting gastropod Dendropoma maxima (Sowerby, 1825) and the coral-associated bivalve Pedum spondyloideum (Gmelin, 1791); together these species made up 67% of the living assemblage (Fig. 2). All three fringing reef substrata differed significantly from each other and from most fore-reef substrata in terms of the composition of living molluscs, an exception being the overlap between Millepora-fringing reefs, coral carpets and coral patches. Fore-reef hard substrata could not generally be distinguished from each other based on living mollusc composition (Table 3, Fig. 3). Similarly, the two shallower depth categories (<1 m, 1–5 m) differed significantly from each other and from the four deeper depth categories (6–10 m, 11–15 m, 16–20 m, >20 m), an exception being the lack of significant differences between the assemblages in 1–5 m versus 6–10 m. The four deeper depth categories were also indistinguishable based on their composition of living molluscs (Table 3). The global ANOSIM test shows that the compositions of molluscan living assemblages were better explained by hard substratum type than by water depth, as indicated by the higher R-value (Table 3).

Fig. 2
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Proportional abundance of species in the living and in the dead assemblage

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Table 3 ANOSIM (analysis of similarity) for molluscan habitats and water depth categories of living molluscs
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Fig. 3
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Ordination of abundance data of living and dead molluscs from 44 transects on six substrata- and water depth categories using non-metric multidimensional scaling (MDS). Points close to one another represent transects that are more similar in taxonomic composition than points farther away from one another

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Fringing reef habitats, occurring in shallow waters (<10 m), were characterized by high molluscan densities (typically >1 individual/0.25 m²) and were dominated by two gastropod species, Coralliophila neritoidea and Dendropoma maxima (Table 2, Fig. 4). Conversely, the three fore-reef hard substrata occurred in greater average water depths (>10 m) and were characterized by lower densities of living molluscs (typically <1 individual/0.25 m²) (Table 2). Coralliophila neritoidea and Pedum spondyloideum were the most abundant species here. Encrusting bivalves (Chamoidea, Ostreoidea and Spondylidae) and crevice-dwelling forms [Ctenoides annulata (Lamarck, 1819), Isognomon legumen (Gmelin, 1791), Barbatia setigera Reeve, 1844] were also important (Fig. 4). Based on their molluscan composition, fore reef hard substrata cannot be distinguished from each other, but fringing reef habitats are characterized by distinct assemblages (Fig. 3, Table 3).

Fig. 4
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Proportional abundance of species in the living assemblage on fringing reefs and fore reefs. Note absence of Dendropoma on deeper fore-reef substrata

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Reef flats are shallow (<1 m), had the second highest density of living molluscs and a high abundance of Dendropoma maxima, which accounted for more than 60% of the total molluscan fauna in this habitat (Table 2, Fig. 5). Only one additional species, the algal-grazing gastropod Turbo radiatus Gmelin, 1791 contributed more than 10% here (Fig. 5). Fringing reefs with massive corals were studied at an average water depth of ∼2 m and were characterized by the highest density of living molluscs (Table 2). By far the most abundant species is Coralliophila neritoidea, which makes up almost 78% of the total of molluscan fauna in this habitat. All other species here contributed <10% (Fig. 5). Millepora-dominated fringing reefs (average depth about 4 m) had a lower density of living molluscs than reef flats and fringing reefs with massive corals, but a higher density than that of any fore-reef bottom type (Table 2). Millepora-fringing reefs were about equally dominated by Pedum spondyloideum, Pteria spp. and Coralliophila neritoidea (Fig. 5).

Fig. 5
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Proportional abundance of molluscan species in the living assemblage differs on the three fringing reef habitats

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The most important molluscs in the dead assemblage were the encrusting bivalves Ostreoidea, Chamoidea, and Spondylidae, and the coral-predating gastropod Drupella cornus, which together accounted for 64% of the empty shells (Fig. 2). Most empty gastropod shells (76%) were inhabited by hermit crabs; the two most common species [Drupella cornus (Röding, 1798), Drupa ricnus hadari Emerson and Cernohorsky, 1793] and several others were almost exclusively occupied by crabs (Fig. 6). Fringing reef substrata differed significantly from fore-reef habitats, except for the strong overlap between Millepora-fringing reefs and fore-reef substrata. Among fringing reef substrata, significant differences occurred only between reef flats and Millepora-fringing reefs. No significant differences were recorded among fore-reef substrata (Table 4). Dead molluscan assemblages changed only little with water depth. Only those in <1 m differed significantly from all other depths; among the latter, however, no significant differences were evident (Table 4). The global ANOSIM test showed dead molluscan assemblages to be better explained by hard substrata than by water depth, as indicated by the higher R-value (Table 4).

Fig. 6
figure 6

Number of dead and hermit crab-occupied gastropod shells

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Table 4 ANOSIM (analysis of similarity) for molluscan habitats and water depth categories of dead molluscs
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In terms of abundance, most molluscs (83.1%) were alive; only 16.9% occurred as dead shells. The number of taxa, however, was higher for dead (40) than for living molluscs (33) (Fig. 2). For all substrata and water depth categories, the differences between living and dead assemblages were highly significant (Table 5, Fig. 3).

Table 5 ANOSIM (analysis of similarity) on living and dead molluscs for the total assemblage (Global test R-statistic), the six molluscan habitats and the six water depth categories
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Rank order correlations between the abundances of living assemblages of Aqaba and Safaga were highly significant (Spearman’s rho = 0.615, p < 0.01). The three most abundant species were the same in Aqaba and Safaga, although in different rank orders (Table 6). The proportional abundance of Coralliophila neritoidea was much higher in Aqaba than in Safaga, and the opposite was true for Pedum. Among the less important species, a striking difference was the much higher proportional abundance of Tridacna maxima (Röding, 1798) in Safaga. Lopha cristagalli (Linnaeus, 1758) was absent in our survey of Aqaba and certain other taxa [Barbatia setigera, Barbatia foliata (Forsskål, 1775), Cerithium spp.] were also distinctly more abundant in Safaga. Conversely, oysters, pteriid bivalves, and the gastropods Turbo radiatus and Tectus virgatus (Gmelin, 1791) were more abundant in Aqaba than in Safaga (Table 6).

Table 6 Species contributing more than 1% to the living assemblages in Aqaba (this study) or Safaga (Zuschin et al. 2000, 2001)
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Discussion

Hard substrata versus water depth

At our Red Sea site, the composition of both molluscan living and dead assemblages was better explained by hard-substratum type than by water depth. This is not surprising because coral reef molluscs depend on their coral habitat (e.g., Hadfield 1976; Morton 1983). While these do change with depth-related factors like light penetration and water flow, they are also influenced by other environment parameters (Perrin et al. 1995). Nevertheless, the two basic habitats investigated—fringing reef and fore-reef substrata—mark different depth categories and thus also differ strongly in terms of molluscan composition and density. Within fringing reefs themselves, the bottom types and their coral assemblages change with water depth. Accordingly, the three fringing reef categories feature distinct molluscan compositions. In contrast, depth is not a structuring variable for fore-reef hard substrata: water depths overlap in the three fore-reef categories, yielding similar coral compositions. Here, the rather uniform molluscan composition did not change within the depth range studied.

Life habits of the important taxa in Aqaba and the Red Sea

From the habitat perspective, coral assemblages and water flow are the key environment parameters influencing distribution patterns of the most important gastropod and bivalve species in this study. The comparison between the Aqaba and Safaga data sets (Table 6) reveals that molluscan communities on reefoidal hard substrata in the northern Red Sea are largely characterized by similar species-abundance patterns. Certain differences–—like those in rank order of the three most important species and higher proportional abundances in Aqaba for pteriid bivalves and oysters–—may reflect different sampling intensities at particular habitats.

The parasitic gastropod Coralliophila neritoidea, feeding on Porites (Robertson 1970; Schuhmacher 1992), was therefore mostly found on Porites-rich fringing reefs with massive corals in Aqaba. However, the high abundances we observed could be a relatively new phenomenon here: this gastropod was not listed by Mergner and Schuhmacher (1974), is mentioned as occurring only regularly by Mastaller (1979) and in only low abundances by Schuhmacher (1992) and Al-Moghrabi (1996).

The encrusting gastropod Dendropoma, inhabiting dead coral rock but also embedded in massive coral colonies, requires agitated water conditions to spread its mucous net for suspension feeding (Hughes and Lewis 1974; Kappner et al. 2000). Dendropoma was therefore only abundant on reef flats in Aqaba, with rare occurrences in the shallowest parts of fringing reefs. Its high densities and abundances are typical features in Aqaba (Mergner and Schuhmacher 1974; Mastaller 1979; Kappner et al. 2000) and are known elsewhere from the Red Sea (Hughes and Lewis 1974; Hughes 1977; Mastaller 1978; Taylor and Reid 1984; Zuschin et al. 2001), but have otherwise only been reported from the SE Pacific (e.g., Salvat 1971).

The host-specific bivalve Pedum bores into living corals, where it attaches with the byssus; it is associated with a variety of scleractinians in the Red Sea, but prefers Montipora (Kleemann 1990). Correspondingly, Pedum occupied all studied hard substrata, yet with distinctly lower abundances on reef flats. It was recorded as occurring in only scattered (Mergner and Schuhmacher 1974) and regular patterns (Mastaller 1979) for Aqaba.

Species of Chamoidea, Ostreoidea and Spondylidae encrust dead hard substrata, mostly dead coral colonies, but also bare rocky surfaces (Zuschin and Oliver 2003a). In Aqaba, they were found on all bottom types but preferred fore-reef hard substrata; oysters were also abundant on fringing reefs with massive corals.

Pteria spp. are bysally attached epizoic bivalves on Millepora fringing reefs, on fore-reef habitats and on octocorals at about 20 m depth (see also Zuschin and Oliver 2003a). The bivalve Ctenoides annulata is a crevice dweller in dead coral colonies (Zuschin and Oliver 2003a): in the study area it was restricted to fore reefs. The low abundance of Ctenoides and other bivalve crevice dwellers (B. setigera, I. legumen, Lima paucicostata Sowerby, 1843) in Aqaba is most likely a long-term feature because, except for B. setigera, all the above species were either listed as being rare by Mastaller (1979) or were apparently absent. The greater importance of crevice dwellers in Safaga (9%) than in Aqaba (3%) could reflect distinct habitat differences. Coral carpets are particularly well developed in Safaga, and bivalve crevice dwellers preferred such habitats, especially in areas tentatively associated with high suspension loads. No such areas were observed in Aqaba, perhaps explaining why Lima paucicostata was missing and Barbatia setigera occurred rarely in our Aqaba survey; Ctenoides and Isognomon were also present in distinctly lower proportions than in Safaga. This interpretation is supported by other observations. The same habitat was also preferred by Lopha cristagalli and Cerithium spp. in Safaga, and both taxa were either absent or quantitatively unimportant, respectively, in Aqaba. Finally, up to  30-cm-long specimens of Hyotissa hyotis (Linnaeus, 1758) were found in Safaga on this habitat type and tentatively related to the high suspension load: no such shells were found in Aqaba.

Turbo radiatus, Tectus dentatus (Forsskål, 1775) and Tectus virgatus are herbivores and algivores on rocky substrata (Mastaller 1979; Taylor and Reid 1984), each with distinct habitat preferences. Turbo radiatus was virtually restricted to reef flats, with a single occurrence on fringing reefs with massive corals, whereas Tectus virgatus preferred fringing reefs, being rare on the reef flat and occurring once on the fore reef. Tectus dentatus, finally, was about equally distributed among reef flat, fringing reef and fore-reef substrata.

The relatively high abundances of Turbo radiatus and Tectus virgatus in Aqaba are probably a long-term feature (see Mastaller 1979), and the distinctly lower abundances in Safaga may be due to shell collecting by diving tourists there.

The giant clam Tridacna maxima is usually bysally attached within or between living coral colonies (Zuschin and Oliver 2003a). In the study area it preferred fore-reef hard substrata, with few occurrences on reef flats and fringing reefs. Its strikingly low abundance in Aqaba versus Safaga seems to be a long-term feature at the former site (Mergner and Schuhmacher 1974; Mastaller 1979). More recent studies, however, consider Tridacna species as endangered due to souvenir collecting (Kilada et al. 1998; Al-Horani et al. 2006). The low abundance of Barbatia foliata in Aqaba is also a long-term feature (see Mastaller 1979) but more difficult to explain because Porites colonies in very shallow water are abundant. In Safaga (Zuschin et al. 2001) and the central Red Sea (Taylor and Reid 1984), the abundant B. foliata is bysally attached to these corals. The low abundance of both bivalves could reflect the northern, somewhat isolated, position of the Gulf of Aqaba, with its characteristic high salinities and fluctuating shallow-water temperature (Mastaller 1987).

The influence of over-fishing by local fishermen and tourists is probably responsible for the low abundance of three other attractive species in the northern Red Sea: the pearl oyster Pinctada margaritifera (Linnaeus, 1758) and the gastropods Strombus tricornis Humphrey, 1786 and Lambis truncata (Kiener, 1843) were absent in our quantitative survey of Aqaba. Scattered and rare occurrences are recorded for Aqaba (Mergner and Schuhmacher 1974; Mastaller 1979) and rare finds in Safaga (Zuschin et al. 2001).

In Aqaba, the coral-predator Drupella was widely distributed on reef flats, fringing reefs and fore reefs, but larger individuals only on acroporans. Drupella cornus was not listed by Mergner and Schuhmacher (1974) or by Mastaller (1979). Abundances were low in the 1970s and 1980s—partly estimated based on distinct feeding marks—and increased distinctly, mostly in the shallowest reef parts, in 1992 and 1993 (Schuhmacher 1992; Schuhmacher et al. 1995). Al-Moghrabi (1996), using a more destructive sampling method, which enabled him to collect many cryptic specimens in 1994 and 1995, recorded a Drupella outbreak in the northern Gulf of Aqaba. Based on semi-quantitative sampling strategies, a Drupella outbreak was recorded for 1996 further south on the Sinai side of the Gulf, around Ras Mohammed (Antonius and Riegl 1997). Our study, and a similar survey in the northern Red Sea at Safaga (Zuschin et al. 2001), which are methodologically more comparable to that of Schuhmacher (1992), now suggest that Drupella regularly occurs on acroporans in this region.

Conus spp., invertebrate predators that favour reefs with <20% living coral cover (Kohn 1983), were found mostly on reef flats, with two occurrences on fore-reef substrata. This reflects their reported occurrence on reef flats with rocky substrata, sand pockets and sparse coral coverage here and elsewhere in the Red Sea (Mergner and Schuhmacher 1974; Hughes 1977; Taylor and Reid 1984). In Safaga they were also abundant on subtidal rock bottoms of the fore reef (Zuschin et al. 2001).

Streptopinnasaccata (Linnaeus, 1758) was typically embedded in living massive corals, but sometimes also occurred in dead coral head crevices (see also Zuschin and Oliver 2003a); it was about equally distributed among the studied hard substrata. Around Port Sudan, the muricid Drupa ricinus hadari feeds on a broad range of invertebrates (mostly crustaceans) (Taylor and Reid 1984), and in Aqaba it preferred reef flats, with two occurrences on Millepora-fringing reefs and on the fore reef, respectively.

Differences between living and dead assemblages

Comparable studies—using the same field methods—on the degree of coincidence between molluscan living and dead assemblages on coral reef-associated hard substrata are available from Safaga, northern Red Sea (Zuschin et al. 2000; Zuschin and Oliver 2003a) and the Seychelles (Zuschin and Oliver 2003b). Water depths and molluscan habitats were similar to the Safaga study and somewhat different from the Seychelles one (see Zuschin and Oliver 2003b). All three studies, including the present one, however, revealed strong differences in the abundances of living and dead molluscs, largely due to distinct biases in the dead assemblage.

(1) Dead bivalves that lived in close association with living corals (mainly byssate pteriomorph bivalves) are easily overgrown after death. They were therefore overlooked by our sampling regime. (2) Some gastropods were under-represented in the studied dead assemblages, presumably because of rapid post-mortem transport into surrounding sediments or into crevices within corals (mainly parasitic coralliophilids).

(3) Most gastropods, on the other hand, were strongly overrepresented in the dead assemblage (mostly cerithiids and thaidids). Most were inhabited by hermit crabs, and thus important—as secondary inhabitants—in forming the dead assemblage.

The comparison of living and dead molluscs yielded important insights into the temporal and spatial dynamics of the taphonomic fate of reef biota. Taxa preferentially overgrown by a living substratum should provide considerable temporal and ecological information: they will be preserved within a rapidly growing reef framework (e.g., Crame 1980, 1981). Fauna preferentially transported into surrounding soft substrata will be affected by the processes of time-averaging and taphonomic disintegration that typically occur in such sediments: much temporal information will be lost (e.g., Perry 1996; Kidwell et al. 2005). Gastropod shells inhabited by hermit crabs may strongly alter the fossil gastropod community structure (for review see Walker 1989). Almost all dead gastropod shells in Aqaba were occupied by hermit crabs, except for those that are structurally unsuitable (e.g., Cypraea) to house the crabs’ abdomen.

Such complex relationships between ecology and taphonomy have to be considered when evaluating the quality of the fossil record of molluscs in coral reef environments.

Conservation implications

Molluscs are increasingly being recognized as a major component in the overall biodiversity of coral reefs (Bouchet et al. 2002; Zuschin and Oliver 2005). Accordingly, molluscs should be better used in evaluating the status of reefs (Wells 1998) and in making prognoses about the effects of reef degradation (Zuschin et al. 2001). Our results have clear management and conservation implications. First, the comparison of this and other recent studies (Schuhmacher 1992; Al-Moghrabi 1996; Zuschin et al. 2001) with data from the literature (Mergner and Schuhmacher 1974; Mastaller 1979) suggest that coral predators (Coralliophila and Drupella) have increased considerably during the last two decades in the northern Red Sea. This points to higher stress levels for corals and the prevalence of coral diseases (Antonius and Riegl 1998). Second, the major substrate-related changes in molluscan composition in reef flats, Millepora-fringing reefs, fringing reefs with massive corals and fore-reef substrata mean that any loss of these habitat types will impact distinct molluscan assemblages. Third, the strong depth-related changes in the shallow fringing reef substrata means that damage in shallow water will have the greatest impact on the overall mollusc community. These depths are precisely the most vulnerable to an array of anthropogenic impacts. Tourism-related impacts in the shallowest zones range from trampling (wading beachgoers or scuba divers entering the water), fin damage by inexperienced snorkelers, to boat- and footbridge-related damage. Currently, the Gulf of Aqaba is unevenly affected by anthropogenic factors, with the Israeli coast from Eilat southward having deteriorated extensively during the last decade and being heavily impacted (Fishelson 1995; Wielgus 2003; Loya 2004; Loya et al. 2004). At present the Jordanian coast is in good condition (Al-Horani et al. 2006). Those authors, however, identify rapid development of tourism, industry and construction sectors along the coast as key problems. This coast may be one of the fastest growing resort areas in the world (Shackley 1999). Specifically, the major current and planned tourism development along the Jordanian coast, including the creation of major hotel complexes directly on the shoreline, artificial harbors and new lagoons, threaten the reefs here. Our study demonstrates that the potential degradation of such shallower coral habitats will pose the greatest threat to molluscan biodiversity. Degradation of coral cover would result in a loss of coral-associated molluscs in favour of bivalve crevice dwellers in dead coral heads and of encrusters on dead hard substrata (Zuschin et al. 2001). The results once again underline a general conclusion that coral reef decline is directly correlated to a concurrent decline in the full range of associated fauna. This study, conducted in a pre-exploitation phase, also provides a baseline for future evaluations of the status of coral reef-associations–—a particular concern in an area at the northernmost limits of coral reef distribution in the Red Sea. This calls for a more inclusive approach to evaluating reef status, one that considers molluscs and other invertebrate groups in monitoring studies, reef surveys and general research efforts.