The influence of light on the settling process.

The pigment spots of fully-developed larvae are eye-like structures as shown by Cole (1938) and Erdmann (1934). The black colour of the spots is due to heavy pigmentation in the epithelium. The spherical cup formed by the epithelium is filled with a gelatinous matrix, the aperture being closed by a lens-like body. The function of the eyespots has been the subject of considerable controversy. To date, observations on O. edulis and O. lurida indicate that the pigment spots are not light sensitive at all. On several occasions, fully-developed larvae of O. lutaria were placed in a glass tube and subjected to a bright light; half of the tube being shaded. The larvae did not exhibit a negative phototaxi. In contrast, the larvae of C. virginica have been demonstrated by some authors to be photosensitive. The larvae when stimulated continue to move away from the light until they reach a shaded site, and this is thought to account for the preference shown by the settling larvae for attachment in shade.

Experiments investigating the influence of light on settling and attachment have been carried out by many authors on various oysters and as yet there is no general agreement on the subject. Korringa (1941, p. 192) concludes that "although light appears to be no orientating factor in the settling behaviour of O. edulis under field conditions, it is not impossible that light influences the settling process in cases where the intensity of the light exceeds a certain degree."