Cochocelis distribution on the firth of clyde: estimates of the lower limits of the photic zone

A series of transects in the Firth of Clyde have shown that filaments of Conchocelis in shell substrata are always the deepest growing red algae. The abundance of calcareous material on the sea bed and its invariable association with the perennial Conchocellis filaments offers a reliable means of estimating photic limits. Some interactions of grazing molluscs with the shell-boring organism have been observed. Some of the pitfalls to be encountered in field determinationsof Conchocelis and its consequent use as an indicator organism are described.