The podia, organs of adhesion and sensory perception in larvae and post‐metamorphic stages of the echinoid Paracentrotus lividus (Echinodermata)

Abstract

Competent larvae of Paracentrotus lividus have five primary podia and each consists of an extensible stem topped by a disc. The disc epidermis is made up of two areas, a peripheral area that encloses type 1 secretory cells, neurosecretory‐like cells and type 1 sensory cells, and a central area that contains type 2 secretory cells, neurosecretory‐like cells and type 2 sensory cells, the latter being mostly concentrated at the centre of the area (at this level a few type 1 secretory cells replace the type 2 secretory cells). During settlement, the competent larvae display a substratum‐testing behaviour in which the primary podia are involved in both cue perception and temporary adhesion. It is suggested that type 1 secretory cells produce an adhesive material whose physico‐chemical characteristics would allow contact chemoreception by ciliated sensory cells, while the secretion released by type 2 secretory cells would be a more effective adhesive, incompatible with chemosensory perception. In both cases, neurosecretory‐like cells would be de‐adhesive in function. This is confirmed by observations made on the two types of post‐metamorphic podia, viz. the locomotory coronal podia and the sensory peristomeal podia. Primary podia have a compromised structure between these two types which allows them to combine adhesion and sensory perception.