An unusual association of a Recent oyster and a slipper limpet

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.

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