Acquisition of particle processing capability in juvenile oyster Crassostrea gigas: ontogeny of gill mucocytes

Abstract

Acquisition of particle processing capability in postlarval oysters depends upon the structural development of the pallial organs, as well as the development of cilia and mucocytes used (either directly or indirectly) in particle capture and transport. Mucocyte mapping was therefore used to identify mucocyte types and distributions throughout gill development in juvenile oyster Crassostrea gigas (Thunberg 1793) specimens from 2.9 mm to 2.4 cm in shell length. Three categories of gill filaments were identified: apical, lateral and principal filaments, corresponding to filament location or future location in gill plicae. Mucocyte densities were recorded per linear μm (l μm) of frontal surface, and converted to potential total volumes, using the mean volumes of each of the two major mucocyte types: acid mucopolysaccharide (AMPS)-mucocytes and mixed mucopolysaccharide (MMPS)-mucocytes. While AMPS secretions were dominant up to 1.0 cm (flat homorhabdic gill, to semi-heterorhabdic differentiation and plication), MMPS secretions increased progressively, dominating in 2.4 cm and adult specimens (fully heterorhabdic and plicated). Mucus composition, and hence mucus viscosity, thus appears to evolve in relation to the degree of enclosure of the gill frontal surfaces. Total (AMPS + MMPS) potential mucus secretion increased allometrically with juvenile growth, characterized by a sharp increase between 10 and 24 mm shell length, suggesting a marked improvement in particle processing capability. Mucocyte distributions on the gill were heterogeneous from the onset of heterorhabdic differentiation (7.5 mm): the apical filaments of the plicae contained much greater mucocyte total volumes, compared to the lateral and principal filaments. In addition to mucus composition, total potential mucus volume thus also evolved in relation to the degree of enclosure of the gill frontal surfaces. These results show that functional specialization in mucocyte distribution precedes the complete anatomical heterorhabdic differentiation. The completely functional adult gill system is thus attained in 2.4 cm juveniles. This information should be of use in understanding the dynamics of juvenile feeding, growth, and mortality, both in natural systems and in rearing operations.