Early development of the gill and implications for feeding in Pecten maximus (Bivalvia: Pectinidae)

Despite the importance of understanding feeding in the early stages of bivalve development, little information is available concerning the organogenesis of the bivalve gill. The present study used histological and scanning electron microscopical techniques to present a detailed account of gill development in the early stages of the scallop Pecten maximus L. (Bivalvia: Pectinidae). Live specimens from larval cultures were observed daily using light microscopy, while five scallops were sampled for electron and light microscopy every 2 to 3 d from Day 18 to 35, then weekly to Day 56, with a final sampling on Day 58. Although development was continuous, four distinct stages were identified (1-primordia, 2-homorhabdic unreflected, 3-homorhabdic reflected, 4-heterorhabdic), partially recapitulating the presumed phylogenetic evolution of this character in the Pectinidae. The absence of a ventral grcove in all stages suggests that the particle transport mechanism of pectinids evolved independently of such a structure, which is found in other bivalve families. Similarly, the absence of latero-frontal cilia in all specimens up to the largest observed (4 mm) indicates that the single row found in adults is a later development, rather than a vestige of a more abundant ciliation in ancestral forms. The anatomical data, together with in vivo observations of feeding in postlarvae, suggest that the developmental stages of the P. maximus gill correspond to critical changes in gill function. The early life of P. maximus may thus be characterized by distinct functional changes in feeding.