Oyster shells as habitat: Encrusters and borings

Stephen K Donovan and Werner de Gier (The Netherlands)

“The talk [between Nero Wolfe and Lon Cohen] had covered the state of the Union, the state of the feminine mind, whether any cooked oyster can be fit to eat, structural linguistics, and the prices of books” (Stout, 1975, p. 13).

Oysters have a close association with humanity, worthy of discussion as the above quotation demonstrates. They form the focus of several semi-popular books (for example, Stott, 2004; Kurlansky, 2007). The prevalence of oysters in shell middens on both sides of the Atlantic Ocean (Purdy, 1996; Milner et al., 2007; Hirst, 2019), and elsewhere (for example, Renfrew and Bahn, 2012, pp. 294-295), shows that they have been an important food source since prehistory. If the importance of oysters as food is today diminished in these areas, several factors must contribute, such as overfishing, pollution and reduced dependence of urban populations on fresh seafoods.

This does not make an oyster any less interesting to the zoologist and palaeontologist (Yonge, 1960). Their robust valves make their preservation potential particularly high, so oyster-rich sedimentary deposits are locally common (Littlewood and Donovan, 1988; Donovan et al., 2014a; and Figs. 1 and 2 in this article). Even on stretches of modern coastline where oysters are rare, their valves are still tough enough to be found washed up on beaches after storms (Fig. 3). Even if bivalves that burrow into sandy substrates, such as razor shells, cockles and their kin, are numerically dominant as dead shells along many strandlines, they are nonetheless smaller, thinner-valved and commonly more delicate than the valve of an epifaunal oyster.

Because they are commonly large in size, oysters are obvious to collectors. Size matters – a large and robust valve will probably have a longer residence time on the seafloor than a small and more delicate shell. Further, oysters may be locally common in oyster banks (Fig. 1). The large valves of oysters will also present a greater surface area for invasive organisms, such as encrusters and borers. Thus, when assessing invertebrate diversity on a beach, anyone who just notes ‘oyster’ is likely to be missing evidence for other taxa.

A valve washed up on a beach is likely to be the free valve. Examine your specimen. Is it fresh? The inner surface is more likely to be ‘clean’, suggesting only a brief, post-mortem residence on the seafloor. If there are borings or acorn barnacles (balanids) attached to the inner surface, then you know that the valve most probably had a period of residence on the seafloor for some little time after the death of the oyster.

Oysters and their disarticulated valves are used as a substrate by two functionally distinct groups of invertebrates: the borers and encrusters (cementers). We shall consider these two distinct groups separately, as an introduction to the oyster as a ‘trap’ for palaeoecological data. The following notes introduce several common ichnogenera and groups of invertebrates, but it is not exhaustive.

Some common borings in oyster shells

Caulostrepsis Clarke (Figs. 3B, C and E, and 6B and C)

Caulostrepsis is commonly produced by polychaete worms, such as the extant spionid Polydora spp. The boring is U-shaped, elongate and straight or sinuous, with the two parallel tubes separated by a central raised vane (obvious in Fig. 3C). The aperture of the boring has a figure-of-eight outline; the figured specimens are exposed in plan view, due to exfoliation of shell layers. It is commonly found parallel to the layered structure of the oyster valve.

Entobia Bronn (Figs 2B-D, 3B-D and 4)

Commonly complicated (Fig. 4) sponge borings, with a series of globular chambers connected by canals and with external apertures, which appear as pores in shell surfaces (Figs. 2D, and 3B-D and G). Apertures are invariably multiple, either linear (Fig. 3C) or pepper pot-like in their distribution. It is commonly found parallel to the layered structure of the oyster valve.

Gastrochaenolites Leymerie (Figs. 2B and 5)

Club-shaped borings, commonly produced by bivalves boring into a substrate, such as a limestone or a robust shell (such as an oyster). A well-preserved Gastrochaenolites may preserve the valves of the borer, but beware – after the death of the boring bivalve, the hole may be invaded by other invertebrates, including nestling bivalves, but also encrusters such as bryozoans and worm tubes (Donovan, 2017a). It is commonly found perpendicular to the layered structure of the oyster valve.

Maeandropolydora Voigt (Fig. 7)

Long and meandering borings produced by polychaete worms (Häntzschel, 1975, fig. 79.5). It is commonly found parallel to the layered structure of the oyster valve.

Oichnus Bromley (Figs 3B, D and F, and 6C-E)

Small, solitary, round holes in shells produced by a variety of organisms. Recent borings are most likely to be the spoor of predatory bivalves. Oichnus may be either penetrative (successful predation) or incomplete (failed predation), and commonly found perpendicular to the oyster valve.

Rogerella de Saint-Seine (Fig. 2A)

These are small, shallow, solitary borings made by acrothoracian barnacles. Their longitudinal section is sock-like and their aperture is teardrop-shaped (Häntzschel, 1975, figs 80.1, 80.2). It is commonly found perpendicular to the surface of the oyster valve.

Trypanites Mägdefrau (Fig. 3A and C)

Trypanites are slender, cylindrical or subcylindrical, unbranched borings with a single entrance, and are commonly straight or curved. Trypanites ichnospecies may be perpendicular or parallel to the host shell.

Invertebrates that are common encrusters of oysters

Acorn barnacles (balanids) (Fig. 6)

Acorn barnacles are gregarious and, if present, are commonly represented by many individuals. They are common on live oysters – next time you have them in a restaurant, examine the shell for balanids (Donovan, 2017b, fig. 1A, C). Post-mortem, the inner surface of the oyster will be available for infestation as soon as the soft tissues rot away (compare with Donovan et al., 2014b).

Bryozoans

The calcified colonies of encrusting bryozoans, mainly cheilostomes, are easily overlooked. They hug the surface of the shell and are most likely to be preserved in protected areas, in overhangs, depressions and between ribs on the external surface. If present with balanids, it is likely that a complex palaeoecological succession may be decipherable, with barnacles overgrowing bryozoan or bryozoan overgrowing barnacles or both.

Cementing bivalves (Figs 1B, 6A, D)

The most likely cementing bivalves on any oyster shell will be a younger oyster, most likely of the same species. Oysters cement to a range of substrates, not least the shells of non-oyster bivalves (Donovan et al., 2020, figs 1, 3).

Tube-secreting ‘worms’

Calcareous worm tubes on oysters are likely to be spirorbids (more or less planar coiled) or serpulids, which are commonly larger, single or gregarious (various figures in Donovan, 2017a). They may adorn either surface of a dead oyster shell and may infest, in association with cheilostomes, large empty borings in a valve such as Gastrochaenolites.

References

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