Pol Gosselin

From competent larva to exotrophic juvenile: a morphofunctional study of the perimetamorphic period of Paracentrotus lividus (Echinodermata, Echinoida)

Abstract The perimetamorphic period in Paracentrotus lividus lasts for 8–12 days. It starts from the acquisition of larval competence, includes the change in form (metamorphosis) and the endotrophic postlarval life, and stops with the appearance of the exotrophic juvenile. All major postlarval appendages already occur in competent larvae being either grouped into the echinoid rudiment (terminal plates, early spines and primary podia) or scattered within the larval integument (genital plates and sessile pedicellariae). Competent larvae show particular behaviour which brings them close to the substratum. The latter is tested by primary podia protruding through the vestibular aperture of the larva. Primary podia are sensory–secretory appendages that are deprived ampullae. They are able to adhere to the substratum in order to allow evagination of the echinoid rudiment (i.e. metamorphosis) and substatum adhesion of the postlarva. Particular spines are borne by the postlarva; these are multifid non-mobile appendages forming a kind of protective armour. Like those of the larva, all characteristic structures of the postlarva (primary podia, multified spines and sessile pedicellariae) are transitory and regress either at the end of postlarval life (primary podia) or during early juvenile life (multifid spines and sessile pedicellariae). Other appendages that develop during postlarval life (i.e. podia with ampulla, point-tipped spines and sphaeridiae) are similar to those borne by the adults and become functional when the individual enters its juvenile life. Thus, the perimetamorphic period appears to be a fully fledged period in the life-cycle of P. lividus, and presumably in the life-cycle of any other sea-urchin species.

Effects of delayed metamorphosis and food rations on the perimetamorphic events in the echinoid Paracentrotus lividus (Lamarck, 1816) (Echinodermata)

Effect of delayed metamorphosis and food ration on late (competent) larvae and postlarvae of Paracentrotus lividus were investigated. Metamorphosis of competent larvae was either not delayed or delayed from 1 up to 4 days. Larvae were starved or submitted to two different food rations of the algal species Phaeodactylum tricornutum. Larvae during the prolonged competence period and the resulting postlarvae were characterised by: (1) the size of the larval body, (2) the size of the rudiment, (3) the rate of metamorphosis, (4) the size of postlarvae 24 h after metamorphosis, (5) the rate of opening of mouth and anus, (6) the rate of survival, and (7) the growth rate of early postmetamorphic individuals. Both the width of the larval body and the diameter of the echinus rudiment grew in competent larvae that were fed. Unfed larvae did not grow. There was no significant difference in growth between the two food rations. The rate of metamorphosis was higher with larvae that metamorphosed soon after they became competent. Lower capacity of larvae to metamorphose during the delay period was associated with treatments yielding a greater larval width and rudiment diameter during the same period. Postlarval development was affected by a delayed metamorphosis treatment inflicted on competent larvae before metamorphosis. Acquisition of exotrophy happened earlier in postlarvae issued from prolonged competent larvae whatever the larval food rations. The delay treatment negatively affected the development of the digestive tract though it positively affected the growth of early postmetamorphic individuals during the first 6 days following metamorphosis. However, selective mortality occurred afterwards as bigger individuals died preferentially.

THE PODIA, ORGANS OF ADHESION AND SENSORY PERCEPTION IN LARVAE AND POST-METAMORPHIC STAGES OF THE ECHINOID PARACENTROTUS LIVIDUS (ECHINODERMATA)

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, ne...