Larval Adhesion, Releasing Stimuli and Metamorphosis

Abstract

Free living stages of sedentary organisms can be considered adaptations to enable immobile species to exploit a scattered or transient ecological niche. The task to prospect for and to identify a congenial habitat is consigned, as a rule, to larvae or larva-like buds, that is to those stages which actually transform into the sessile phase. This is the case even if a metagenetic life cycle has provided the species with a dominant swimming phase capable of brooding the eggs throughout development until the settling stage. Eventually, the larvae are set free and they have to find a suitable substratum themselves. But how can larvae comply with such a task? Their sensory equipment is very limited and does not qualify them to locate an appropriate habitat from a distance. This applies especially to coelenterate larvae. They depend, therefore, on a hierarchy of key or sign stimuli indicative for their adult environment. Viewed in terms of behaviour, the larva displays appetitive or searching activity which continues until the larva is presented with a specific releasing stimulus triggering fixation and metamorphosis. The effective stimulus must be derived from characteristic substrate properties which can be explored by mechanoreceptors and/or a chemical contact sense (Crisp, 1974). Recognition of an adequate substratum, therefore, depends on physical contact or, at least, on a close range approach. The word ‘close’ can be defined in this context in relation to the various forces of adhesion.