Oysters Hitch a Ride

Common Saddle Oyster, Anomia ephippium

Unlike most other bivalves, the Common Saddle Oyster (Anomia ephippium) does not bury itself in the sand or mud of the sea bed. Instead they cement themselves to rocks or other hard substrates by chalky byssus threads that grow through a hole in the lower shell (1). They are experts at early colonisation of a substrate, and are often the among the first organisms to do so at a new location (2), something which is evident from the pictures included here, showing colonisation of an abandoned water drum by many A. ephippium individuals. So quick are they to colonise in fact, that smaller individuals can be seen growing on some of the larger ones. Unfortunately for them the drum had washed quite far up a beach and partially filled with sand, making a return to the water unlikely. Growing up to 6 cm across, younger, smaller individuals can appear almost translucent with older examples being white to pale brown in colour (1).

A water drum, washed up a a beach with Common Saddle Oyster, Anomia ephippium growing on the surface

 

References:

1. Challinor et al., 2003. A Beginner's Guide to Ireland's Seashore p. 123
2. Bramanti et al., 2003. Italian Journal of Zoology 70 pp. 175-178