Other Species Associated with the Sedentary Forms on the Test Blocks

Species of two other groups of mollusc—viz., the tectibranchiate Pteurobranchus sp. and the nudibranch Acolidia gracilis (T. W. Kirk) were found frequently associated with the fixed species of the test blocks. There is little doubt that A. gracilis was feeding on the hydroid polyps as sea slugs of this type are known to feed extensively on simple ascidians, and it was probably the presence in some numbers of these animals that caused the appearance of this species on the blocks. The small blennie Tripterygion varium (Forster) was twice found browsing among the algae and polyzoans.