Observations on Byssus Systems in the Spat of Cerastoderma Glaucum and C. Edule
Observations made of the byssus apparatus of Cerastoderma glaucum and C. edule are reported and include an account of the mobility of the spat in the laboratory cultures. In their typical natural habitats both species are capable of an extended pelagic existence following initial settlement from the plankton. During this period C. edule alternates pedal crawling with byssal drifting to give horizontal dispersal; C. glaucum combines pedal crawling with byssal climbing of submerged vegetation resulting in vertical dispersal. It is suggested that the behaviour of young C. glaucum spat enables them to escape unfavourable conditions at the muddy bottom of their typical lagoon habitat, and to compete more favourably for food. The structural adaptations for these activities are in the byssus gland system which, in C. edule is relatively simple and secretes a thin unbranched thread; but in C. glaucum is more elaborate and secretes a stronger thread with anchoring side branches. These differences have incidental taxonomic value in separating these closely related species.