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Taphonomic Processes of Some Intertidal Gastropod and Bivalve Shells from Northern Red Sea Coast, Egypt.

Byline: Abdelbaset S. El-Sorogy

Abstract

Disarticulation, encrustation, bioerosion, fragmentation and abrasion are the recorded taphonomic processes in the intertidal gastropod and bivalve shells from the northern Red Sea coast, Egypt. Balanoids, polychaetes, bivalves and bryozoans are the recorded encrusters. Bioerosion traces were predominately of clionid sponges and gastropods. They are represented by the ichnogenera Entobia and Oichnus. Fragmentation was mainly resulted from mechanical and biological processes. Many specimens are abraded, lacking-sheen, and showed evidence of corrosion. Most bivalve shells are dominated by disarticulated valves.

Key words: Taphonomy, Gastropoda, Bivalvia, Red Sea coast, Egypt.

INTRODUCTION

Taphonomy of marine skeletal organisms involves biological, chemical and mechanical processes that led to abrasion, bioerosion, disarticulation, dissolution, encrustation, fragmentation, precipitation, orientation, and sorting (Nielsen, 2004; El-Gendy et al., 2015). These taphonomic signatures may indicate that the processes happened in a certain time sequence controlled by life habits and environmental conditions (Barkati and Asif, 1984; Meldahl and Flessa, 1990; Viana and Richter, 1998). The taphonomic condition of shells varied with environment. Santos and Mayoral (2008) indicated that in intertidal and shallow sublittoral environments, the hard substrates, including the skeletons of living and dead organisms as well as rock clasts, may be colonised by a diverse array of endobionts (borers) and grazers, in addition to suspension feeding epibionts (encrusters).

Shell beds, lags, or concentrations can be a common feature of shelf and coastal areas, and considerable research has been devoted to understanding their formation through taphonomic analysis (Brett, 2003).

Many studies have been dealt with fauna, environmental assessment, and sedimentology of the Red Sea coast (Beltagy, 1984; Anan, 1984; Ziko and El-Sorogy, 1995; Mansour and Anwar, 2000, 2006; Nour et al., 2006; El-Sorogy, 1997a,b, 2008; Ostrovsky et al., 2011; Ziko et al., 2012; Abd El-Wahab and El-Sorogy, 2003; Nour and El-Naggar, 2007; El-Sorogy et al., 2012, 2013a, b, 2015). The works have been dealt with the postmortem processes of molluscan shells along the Red Sea coast are very rare, therefore, the aim of the present study is to document the taphonomic processes of the intertidal molluscan shells (bivalves and gastropods) collected from the northern Red Sea coast, Egypt and discuss the factors controlling them.

MATERIALS AND METHODS

More than 404 specimens of gastropods and bivalves have been collected at low tide from the intertidal zone of El Fanader lagoon, Hurghada, Makadi Bay, Al Qweh lagoon, and El-Humrawen harbor, northern Red Sea coast, Egypt (Fig. 1) Shell samples were washed, cleaned, sorted, identified and illustrated (Figs. 2-4). The identification of the present fauna is based on the classification of Moore (1960, 1969) and Bosch et al. (1995).

When necessary, the dead shells were carefully washed and brushed to remove sediments. All specimens are deposited in the collections of Department of Geology and Geophysics, College of Science, King Saud University.

Studied areas

Five localities were chosen for study (Fig. 1), namely: 1) El Fanader lagoon, 2) Hurghada, 3) Makadi Bay, 4) Al Qweh lagoon, and 5) El-Humrawen harbor. The following are description of the five studied areas:

El Fanader lagoon

This locality lies 10 km in the sea, facing to Hurghada city, between 270117-273816N and between 334049-334949E (Fig. 1). The site contains small reef patches and a barrier reef arises from the seabed to about 14 m height. This barrier reef includes some mixed species of stony corals and soft ones. Field survey illustrated high percentage of dead corals and faunal accumulation on the beach, indicating high water energy. Also, the study area is situated under human impacts in the form of diving and snorkeling activities.

Hurghada

This site lies at about 20 km to the north of Hurghada City. It is located within the main Hurghada port between 273812-275613N and between 330401-333950E (Fig. 1). Generally, faunal accumulation along the beach is rare in this site, may be due to many types of pollutants, such as reject brain water from desalination plants, light oil resulting from basin repair boats, the sewage output from the harbor Mosque and the port and fishing activities.

Makadi bay

Makadi Bay lies at 30 km to the south of Hurghada city, between 265946-270058N and between 334454-335417E (Fig. 1). It has semi-circular shape and receives all clastic sediments from main tributaries. Makadi Bay is sheltered from the direct currents and waves by barrier reefs off the bay, where it is bounded on the west by Pre-Cambrian metamorphic and igneous rocks and is separated from it by a sandy coastal plain covered with patches of post Miocene and limestone gravel banks (Shehata, 2011). The beach is relatively wide, while the intertidal zone is narrow with maximum depth of 1.5 m during high tide, and extends to a distance about 100 m from the shoreline. Its bottom floor is sandy inhabited with some spots of algae, followed by back reef zone which includes some coral patches.

Al Qweh lagoon

Al Qweh downstream lies at about 44 Km to the south of Safaga city. It lies along the main SafagaQuseir asphaltic road, between 26/36'/22"-26/39'/22"N and between 3458'07"-3408'00"E (Fig. 1). The beach is sandy in the northern part, while the middle and southern parts are of the rocky beaches. Tidal flat is shallow, wide, extends 2km and composed of biogenic sand inhabited with seagrass and algae. This zone is followed by high biodiversity of corals, molluscans and sponges.

El-Humrawen harbor

This site lies at about 20 km to the north of Quseir city. It is located between 2601'10"-2608'10"N and between 3402'12"-3405'12"E (Fig. 1). Beach zone is very narrow. The width of the intertidal zone is 30 m at the edge of coral reefs. Middle part of the marine area is rich with seagrass and algae. Marine fauna and flora are characterized by low diversity in this site. This may be due to spread of phosphate dust, especially during the shipping process.

RESULTS AND DISCUSSION

Disarticulation

Bivalve shells may serve as sensitive indicators of rapid, episodic sediment accumulation (Allen, 1992). The rate at which organisms skeletons disintegrate after the death is a function of their delicacy, environmental energy, temperature, oxygen level and residence time on the seafloor (Brett, 2003). Degree of articulation is a good indicator of relative exposure time, or energy of the depositional environment.

In the studied areas, More than 97% of the examined bivalve shells are disarticulated and the articulated ones are generally rare (Fig. 2A). In rocky shore along Al Qweh lagoon, articulation is present in comparison with sandy coastline of other areas. It is limited to specimens that are physically wedged within interstices of rock blacks (thus making the separation of valves difficult) and to species that have very thick ligaments or dentitions resistant to disarticulation. Most disarticulated valves occurred at El Fanader lagoon, due to the influence of high water energy.

The articulated shells are connected with ligament areas. No size sorting is recognized and shells are more commonly oriented concave-up than concave-down. Shells occur with others that are oriented obliquely and vertically. Boucot et al. (1958), Seeling and Bengtson (1999) indicated a transport of short duration in a moderately-energetic setting for such case. Valves are rarely found in their life position and appear as parautochthonous associations (transported slightly in the vicinity of their habitats (Seeling and Bengtson, 1999).

Encrustation

Encrusters on Cenozoic shells are very common, but the published studies of them are surprisingly few (e.g., Taylor and Wilson, 2003; Kidwell, 2013; El-Gendy et al., 2015).

Taphonomically, important encrusters whose have hard-parts include bryozoans, serpulid worms, foraminifera, corals and barnacles (Parsons and Brett, 1991; Perry, 2000; McKinney, 1996, El-Hedeny, 2005, 2007a, b). In the studied areas, four types of encrusters were recorded, in descending order of abundance they are: balanoid barnacles, polychaetes, bivalves and bryozoa.

The balanoid barnacles Balanus perforatus is the most common encrusters in the studied areas. Balanoid barnacles are recorded at nearly similar morphological sizes and same directions. They are usually found in all modern shallow marine environments, occupying shorelines of the continent (El Sorogy et al., 2003). They are well developed as aggregates on the external/somewhat the inner and outer surfaces of the bivalve Acrosterigma lacunosa and Marcia flammea (Fig. 2B-D). Their position of attachment may indicate the direction of flow of food-bearing water. It is supposed that balanoid barnacles are able to orientate to water movement in order to allow for efficient food collection over a wide range of conditions (El-Hedeny, 2007a). In addition, there are some gastropods species encrusted with balanoid barnacle as Cerithium caeruleum, SiphonarAa belcheri and Lunella coronata (Fig. 2E-G).

The second type of encrusters is the calcareous tube-forming worms (serpulid). They may be present judging from tube morphology, are moderately common on some bivalve shells. They sometimes appear as dense clusters (Fig. 2H) and in some cases develop as small aggregates on the inner and somewhat outer surfaces of some bivalve shells (Fig. 2I-L). The following are the recorded bivalves encrusted by serpulid worms: Pinctada margaritifera, Trapezium sublaevigatum, Mactra rochebrunei; Anadara antiquata, Glycymeris pectunculus, Barbatia setigera, Brachidontes variabilis, Madiolus cf. barbatus, Alectryonella plicatula (Fig. 2H-K).

The third types of encrusters are the bivalves. Some bivalves encrusted by other bivalves to form aggregates or banks. Examples from the study areas are Alectryonella plicatula, Chama asperella, Ch. reflexa and Spondylus hystrix (Fig. 2M-O). The fourth types of encrusters are the bryozoans. Bryozoans are small benthic aquatic invertebrates growing as colonies of connected zooids on submerged substrates (Massard and Geimer, 2008; El-Sorogy, 2015). In the studied shells, Parasmittina sp and biflustra sp encrusted parts of the inner smooth surfaces of dead disarticulated bivalves and gastropods drifted with the upper tide (Fig. 3A and B, Fig. 4F). Rare ones encrusted the outer surfaces. Surfaces of seashells act as hard substrate needed for larval settlement and then ostogenic development. The diameter of the examined bryozoan colonies ranges from a few millimeters to a few centimeters and somewhat covering the complete shell surface the Palaeozoic to Recent.

For whole specimens it is recorded the presence of predatory drill holes and identified them as either Oichnus paraboloides or O. simplex. The hole is circular, with paraboloid shape, 1.5-2.3 mm in diameter on the shells of Barbatia parva, B. setigera, Diodora funiculate and Pinctada margaritifera (Figs. 3G-J). It was most probably produced by a naticid gastropod. On the other hand, Oichnus simplex (Fig.4A, C) consist of smooth, vertical, circular to subcircular holes with axes oriented perpendicular to the host, piercing throughout the shell, diameters range from 2.5-3.1 mm.

Bioerosion

Bioerosion is an important process in both modern and ancient marine environments (e.g., Gibert et al., 2007; Fang et al., 2013). In the present study, borings are present in many specimens ascribed to either predators or endobionts (sponges and gastropods). Shells with galleries of the boring sponge Cliona were most common in the hardground samples. Clionid sponges are the most frequent borers in all environments, and environments with hard substrata have the highest diversities of borers (Best and Kidwell, 2000).

The majority of borings were small, rounded and closely spaced to connect chambers. Depth of penetration did not exceed three millimeters and apertures were always less than one millimeter in diameter. These bioerosion traces were identified as Entobia Bronn, 1837. It is a product of borings by the siliceous clionid sponges (Bromley and D'Alessandro, 1984). They are strongly developed on external and somewhat internal surfaces of Spondylus hystrix, Chama reflexa, Acrosterigma assimile and Marcia flammea (Fig. 3C-F).

The second type of bioerosion is represented by small borings, perpendicular to the shell surface, produced by carnivorous gastropods. Predatory gastropods have the ability to drill through the shells of other molluscans and feed on the animal inside. These predators (e.g., muricids and naticids) leave a distinctive beveled hole on their prey (Parsons, 2004).

These bioerosion traces were identified as Oichnus, Bromley, 1981 and they were known from Figure 3K, L indicates biological erosion on the outer and inner surfaces of Barbatia parva and Spondylus marisrubri. These bioerosions may be of muricid gastropods, which typically hunt and drill prey epifaunally (Ziegelmeier, 1954; Carriker, 1981).

Fragmentation

Shell fragments are important components of many modern and fossil marine benthic ecosystems (Zuschin et al., 2003). They are very common in modern death assemblages (Tauber, 1942; Hollmann, 1968; Pilkey et al., 1969; Zuschin and Hohenegger, 1998; El-Gendy et al. 2015). In general, fragmentation is associated with high-energy environments such as beaches and tidal channels (Parsons and Brett, 1991), although Best and Kidwell (2000) observed high levels of fragmentation in all environments, even those with low energy. Fragmentation is potentially derived from a range of both physical and biological processes (Zuschin et al., 2003).

Physical agents consider the primary agent determining fragmentation in the study area, where Red Sea waters have high currents and tidal range. Also, fragmentation process may come next to other preceding operations such as abrasion and bioerosion that may reduce the density and thickness of the shells and consequently facilitate breaking. Predators and other organisms are other likely agents of fragmentation in such environment.

Shell holes, caused by bird pickings (Fig. 4A-F) were observed in many specimens (n=35) of the studied shells. Previously, MacArthur and Wilson (1967) have noted the importance of bird attack in fragmentation processes. Predators (biological factor) are enhanced fragmentation and should take into account in taphonomic studies and depend mainly on shell strength (Zuschin et al., 2003).

The recorded gastropods showing fragmentation by bird pickings are Hexaplex kuesterianus, Cypraea annulus and Cerithium caeruleum (Fig. 4A-C) and the bivalve ones are Asaphis violascens, Anadara antiquata and Acrosterigma lacunose (Fig. 4D-F).

Abrasion

Abrasion is another important taphonomic process, being probably associated with shell damages (Kotzian and SimAes, 2006). Shell abrasion is often associated with physical transport, but it also can occur when the predator uses its radula and/or shell to abrade the shelled prey (Glaub et al., 2007). In the present study, abrasion is seen in the loss of surfaces ornamentation such as growth lines and radial ribs of the following bivalves and gastropods: Glycymeris pectunculus, G. livida, Marcia flammea and Ancilla (Sparella) castanea (Fig. 4G-M); by a mottling of the shell surface and a lack of luster on the inner surface (Fig. 4G and O). The processes of abrasion often leave shells with a glassy appearance (Driscoll and Weltin, 1973).

The hard rocky shore area of Al Qweh lagoon exhibits considerably higher levels of abrasion than the other studied sandy shores. Mechanical abrasion results from the movement of the shells on the substrate and due to imbricate over each other's by wave actions. Shells on hard grounds are exposed to current and wave activity for longer periods of time, and therefore, hard ground samples tend to display greater mechanical abrasion.

The loss of color occurs when the outer layers of the shell are either abraded off, or the colors simply fade with time. Color loss is higher on average on the hardground substrates than at sand-bottom sites.

CONCLUSIONS

The coastline of the northern Red Sea coast is rich with intertidal molluscan shells that are affected by several taphonomic processes, such as disarticulation, encrustation, bioerosion, abrasion and fragmentation. The bivalve accumulation is totally dominated by disarticulated valves, except very few which are articulated. Fragmentation is mainly caused mechanically and the biological one is somewhat effectible due to increase energy gradient in the study area. Encrustation was common in molluscan shells by balanoids, polychaete tubes, some ontogenically bivalve shells and bryozoa. Balanoid barnacles represent the most common encrusters, and are probably the most abundant ones in the terms of area occupied and skeletal biomass. Bioerosion traces are predominately of clionid sponges and gastropods. Abrasion is observed in the loss of surfaces ornamentation and loss of shells color. Abrasion increased in rocky shore of Al Qweh lagoon.

ACKNOWLEDGEMENTS

This work was funded by the Deanship of Scientific Research, College of Science Research Center, King Saud University, to whom the authors are greatly thankful.

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