Gastrochaenolites ichnofacies from intertidal seashells, Al-Khobar coastline, Saudi Arabia

https://doi.org/10.1016/j.jafrearsci.2020.103943Get rights and content

Highlights

  • The bioerosion structures and encrusters were studied from Al-Khobar coastline, along the Arabian Gulf, Saudi Arabia.

  • 9 ichnospecies were identified and encrusters of balaniid barnacles, serpulids, bryozoans and bivalves were recoded.

  • The studied bioeroders were defined as Gastrochaenolites-dominated ichnoassemblage.

Abstract

Bioerosional structures and encrusters on a total of 349 bivalves and 209 gastropod shells were studied from Al-Khobar coastline along the Arabian Gulf, Saudi Arabia. Nine ichnospecies belonging to five ichnogenera were identified, namely: Entobia geometrica, Entobia isp., Oichnus paraboloides, Oichnus simplex, Gastrochaenolites dijugus, Gastrochaenolites isp., Caulostrepsis taeniola and Maeandropolydora isp. Barnacles, serpulids, bryozoans and bivalves were recoded as encrusters on the internal and external surfaces of the investigated seashells. The studied bioerosional structures were defined as Gastrochaenolites-dominated ichnoassemblage, which is composed of Gastrochaenolites, Oichnus and borings of polychaete annelids and clionid sponges. It reflects marine conditions, in a shallow water depth and low rates of sedimentation. A postmortem colonization has been confirmed on the basis of the presence of different encrusters on the internal surfaces of many seashells, while those on the external surfaces indicated a colonization process mostly took place during the lifetime of the seashells.

Introduction

Bioerosion identifies every form of biological erosion of hard substrates and encrustation is the behavior by which organisms attach to hard substrates (Bromley, 1992; Demircan, 2012; El-Sorogy et al., 2019). Shells can provide the only hard substrate available for colonization by epi- and endoskeletozoans in areas where the bottom consists of unconsolidated sediments (Lopes, 2011; El-Sorogy, 2015). Formation of scrape traces, borings and other biogenic structures are the resultant byproduct of mechanical or chemical erosion of hard substrates by organisms. Modern biogenic hard substrates in intertidal and shallow subtidal environment may be colonized by a diverse endobionts, in addition to suspension feeding epibionts (Svane and Petersen, 2001; Santos and Mayoral, 2008).

Rocky shores represent major flooding surfaces in the geological record and offer excellent conditions for colonization by bioeroders and encrusters in shallow marine environments, as they associated with null or low sedimentation (Cachão et al., 2008; Demircan, 2012). The bioeroders including mechanical and/or chemical processes are the primary agent of shell destruction (Cutler and Flessa, 1995). They attack the living and dead shell materials, and their importance increases with productivity and decreases with higher rates of sedimentation (Lescinsky et al., 2002; Santos and Mayoral, 2008).

Al-Khobar coastline has subjected to three environmental studies (Alharbi and El-Sorogy, 2017; Alharbi et al., 2017; El-Sorogy et al., 2019). These studies assessed the anthropogenic sources of trace and heavy metals in coastal sediments, seawaters, and bivalve and gastropod shells. In general, the published articles on bioerosion and encrustation processes in the Aabian Gulf is very scarce (El-Gendy et al. 2015; El-Sorogy et al., 2018), therefore, the present contribution aims to: (1) identify and describe the bioerosion structures with their tracemakers on seashells in the coastal environment of the Al-Khobar area; (2) document the spatial distribution of borers and encrusters on mollusc shells in the study area, and (3) distinguish between the skeletobiont colonization of living and dead seashells.

Section snippets

Materials and methods

Seashells were collected from 15 stations in the intertidal zone of Al-Khobar coastline, Saudi Arabia, between 50°12′59″E − 26°15′49″N and 50°6′55″E − 25°57′15.497″N (Fig. 1). Al-Khobar coastline is subdivided into sandy dominated and rocky dominated shores (Fig. 2). The sandy dominated shores are composed of medium to coarse sands and gravels in parts. Seashells ranged from scarce to completely cover the coastline, and composed mostly of cerithiids, venerids, and other biogenic fragments. The

Bioerosion

The total percentage of bioerosion in the investigated samples is high, giving an average value of 78% for bivalves, and 47% for gastropods (Table 1, Fig. 3). A total of eight ichnospecies were identified on seashells from the study area, which were grouped into five ichnogenera. To characterize the documented biotic relationships, the present borings were divided by their producers, including structures produced by clionaid sponges (Entobia geometrica, and Entobia isp), duraphagous drillers (

Discussion

Shells of marine molluscs along the coastline of Al-Khobar, Saudi Arabian Gulf were subjected to bioerosion and encrustation by a wide array of organisms, while the individuals are still alive and also after death, provided that the shells exposed in the water-sediment interface long enough. Surfaces of the investigated seashells act as the hard substrate for larval settlement and ontogeny development. Skeletobiont larvae tend to settle in protected zones of disarticulated seashells, either in

Conclusions

  • 1

    The Al-Khobar coastline along the Saudi Arabian Gulf is differentiated into sandy dominated and rocky dominated shores, which are characterized by accumulation of seashells and other biogenic fragments. Shells of 14 bivalve species (A. lacunose, P. nigra, P. margaritifera, A. antiquata, D. ceylonica, G. pectunculus, C. rugifera, P. sinuosa, D. subrotundata, C. bicolor, C. divergens, F. fragile, M. flammea, and P. australis) and 6 gastropod ones (H. kuesterianus, F. arabicus, B. ampulla, T.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors extend their appreciation to the Deanship of Scientific Research at King Saud University for funding this work through research group No. (RG-1439-031).

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