Adult oysters cannot move of their own volition but they have a free-swimming larval stage. The larvae are curious little creatures, a fraction of a millimetre across, that drift in the sea with plankton for a couple of weeks after they have been expelled from the body cavity of the adult female oyster. The water temperature has to be at least 15 degrees C for spawning to take place – and more than that over a sustained period for successful incubation and settlement. In addition to a tiny hinged shell and the basic body parts that will develop in the maturing oyster, there are several organs that will disappear once they have served their purpose.
The larva swims upside down with the straight hinge of the two shell valves lowermost. Protruding between the two shells on the upper side is a fleshy lump with a circlet of hair-like cilia. This organ is called the velum and the whipping action of the cilia moves the larva through the water – although the general direction taken by this microscopic organism is dictated by the currents.
The other organ unique to the oyster larval stage, and absent from the settled oyster, is a muscular foot that sticks out between the valves at the top near to the velum. Inside the foot is the byssus gland that produces a type of adhesive. The velum and the foot can be manoeuvred, or extended from the shell, or withdrawn into the shell, by the contraction of strong muscle bands. The larva will sink if it retracts the velum and rise if it extends it.
The larva swims with the plankton from one to two and a half weeks depending on the temperature. By this stage of its development it has sense organs capable of detecting and responding to changes in light and gravity. The protruding foot will grip onto any solid object it touches and the larva will then withdraw the velum and crawl about to investigate whether it is a suitable place to settle. Oyster larvae are fussy about where they will settle. If the place is not right, they will withdraw the foot, extend the velum and swim off.
The movements of a European Flat Oyster larva, when it has eventually decided to settle after these exploratory wanderings, has been described as “curiously reminiscent of that of a dog preparing its bed”. The larvae rocks backwards and forwards and from side to side with the foot gripping the hard surface. It is squeezing out adhesive from the byssus gland, via a duct, onto the substrate. The larva then topples over sideways so that the left shell lands on the drop of glue – which quickly hardens on contact with water and cements the larva permanently onto the substrate. The velum and the foot are resorbed by the body after settlement as they are then redundant.
Oyster larvae become known as spat once they attach to a substrate; and the action of the larvae settling like this is called spatfall. Oyster larvae have marked preferences for particular settlement locations. They prefer the under surfaces of objects near the bottom of the water column; they use light and gravity sensing organs to help them find these places. They like to settle on surfaces to which other oysters are already attached; they do this by detecting chemicals given off by other oysters. They also like surfaces which are clean but not too clean; a slight growth of bacteria, diatoms, and hydroids is ideal.
The photographs in this post show as examples Flat Oyster shells recovered from archaeological excavations of the Saxon settlement in Southampton, UK, which existed around 1,200 years ago. Each oyster is still attached to the hard object on which it first settled. They include shells of a Sting Winkle (Ocenebra erinacea), Saddle Oyster (Anomia ephippium), Common Cockle (Cerastoderma edule), Edible Mussel (Mytilus edulis), and acorn barnacles, plus a small flint pebble.
In oyster cultivation, hard materials are sometimes placed in the water at suitable locations to encourage spatfall. These might includes bundles of sticks, for example, or pottery tiles coated with lime. Such materials are termed cultch. This word may also be used used to describe the hard objects on which oysters settle naturally. They include both natural substrates like shells and rocks as well as man-made objects like shipwrecks, bottles and old boots on the seabed.
This blog posting is one of a series in the category Oyster Variations.
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