Stephen K Donovan (The Netherlands)
My young son and I both have a taste for oysters, and have a favourite restaurant in which we like to eat them. It is in the Spui district of Amsterdam, which is also an area with a high density of bookshops. Therefore, there is a double incentive to visit the area. When eating oysters, we are always keen to examine the shells for interesting encrustations or borings, but have never before found anything quite as interesting as the specimen described below (Fig. 1), which was eaten and enjoyed by my son. Although not a fossil specimen, this shell is considered instructional and shows a number of features that would excite interest, if found in a fossil shell.
The oyster is preserved attached to its substrate, a gastropod shell. Crepidula fornicata (Linné), the slipper limpet, “… is a serious pest in oyster beds, and was introduced from America with imported oysters” (Campbell, 1976, p. 154). “Crepidula can actually settle on top of the oysters, almost smothering them …” (Beedham, 1972, p. 48), but in this example the tables are turned: an oyster has used a dead shell as a hard substrate.
The adductor muscle scar of the oyster is a deep brown colour, with a purple patch towards the umbo and the plicate valve is moderately long, indicating that it is a Crassostrea, most probably the Portuguese oyster, C. angulata (Lamarck), also introduced (Beedham, 1972, p. 160). Therefore, the association is of two species that were separated by the Atlantic Ocean before man’s interference, although Crassostrea spp. are widespread in the Americas (Morris, 1975, pp. 36-37) and can be locally common fossils in the Neogene (Littlewood & Donovan, 1988).
The oyster has both encrusted the gastropod and, in turn, been encrusted itself. The oyster spat attached to the platform within the aperture of the gastropod (Fig. 1A, B) and now completely conceals it (compare with, for example, Beedham, 1972, fig. top right on p. 49; Morris, 1975, pl. 45, fig. 10). Its further growth was impeded by the lip of the aperture and the oyster had to grow out of this depression, and over it, resulting in a distinct, semi-circular ridge inside the shell of the bivalve (Fig. 1B). Once it was more fully grown, the external surface of the oyster shell then, in turn, served as a hard substrate for other oyster spatfalls; evidence of least six attached examples are apparent, the most prominent three shells being indicated in Fig. 1A by asterisks.
The Crepidula shell acted as a hard substrate, not just for the oyster, but also for boring annelid worms. Although the annelids themselves are not preserved, their morphologically distinct borings are referred to the ichnospecies Caulostrepsis taeniola Clarke, 1908 (Fig. 1C, D). These are sinuous, U-shaped borings made in the plane of the shell; the boring is roofed by a thin layer of shell in life that has ‘popped off’ to show the internal structure in some examples (best seen in Fig. 1D).
In conclusion, what was a tasty meal for my son is seen to preserve an association of organisms that dwelled on hard substrates. The dead Crepidula shell formed the basic substrate for this ecological association. It was infested by both a Crassostrea and annelid worms, although there is no indication as to the relative timing of these events; that is, the annelids did not infest the oyster shell and the bivalve did not overgrow the borings.
While the oyster could only have attached to the dead shell of the gastropod, the annelids could have infested the snail when either alive or dead. The oyster later formed a hard substrate for other oysters while it was still alive. We know this to have been the case (my son ate the oyster), but, in an analogous fossil example, we would note that all encrustations of the oyster are on the external surface only, at least suggesting infestation in life. The specimen will be deposited in the collections of the Naturalis Biodiversity Center in Leiden.
Beedham, G.E. 1972. Identification of the British Mollusca. Hulton Educational Publications, Amersham, 239 pp.
Campbell, A.C. 1976. The Country Life Guide to the Seashore and Shallow Seas of Britain and Europe. Country Life Books, London, 320 pp.
Clarke, J.M. 1908. The beginnings of dependent life. New York State Museum Bulletin 121, 146-169. [Not seen.]
Littlewood, D.T.J. & Donovan, S.K. 1988. Variation of Recent and fossil Crassostrea in Jamaica. Palaeontology, 31: 1013‑1028.
Morris, P.A. (edited by Clench, W.J.). 1975. Shells of the Atlantic. Houghton Mifflin, Boston, xxviii+330 pp.