ULTRASTRUCTURE DES STATOCYSTES CHEZ SCROBICULARIA PLANA ET TELLINA TENUIS (MOLLUSQUES LAMELLIBRANCHES TELLINACEA)

The statocysts of Scrobicularia plana and Tellina tenuis arecomposed of 8 sensory hair-cells separated by giant cells characterizedby a fibrous cytoskeleton. The sensory cells bear few ciliaespecially in S. plana. These cilia are responsible both forrotation of the statolith and for the transduction of stimuli.Their basal bodies bear a side rootlet facing a basal foot anda crown composed of 9 spokes. Such statocysts seem to receiveonly multidirectional stimuli and to allow a less diversifiedbehaviour than the statocysts of Pecten. (Received 16 September 1980;     

DEMOGRAPHY AND GROWTH OF ANOMALOCARDIA BRASILIANA (GMELIN) (BIVALVIA: VENERIDAE) IN A MANGROVE, IN GUADELOUPE (FRENCH WEST INDIES)

Anomalocardia brasiliana is a venerid clam which lives slightlyburrowed in sandy mud of shallow water environments throughoutthe Caribbean region. A 42-month study in a mangrove lagoonshows that the population is characterised by a variable recruitmentpattern on a year-round basis but with infrequent success. TheGompertz function gives a good estimation of growth. The weight-sizerelationship shows alternative isometric and subdeterminantallometric growth periods. A short life and variable demographicstructure seem to be characteristics of lagoon as well as oftropical populations. (Received 5 February 1990; accepted 26 July 1990)     

ETUDE ONTOGENETIQUE DE L'ORGANE SENSORIEL DU MUSCLE CRUCIFORME DES TELLINACEA

The closed cruciform muscle sense organ of Donax trunculus hasa similar origin to the open one of Scrobicularia plana. Bothdifferentiate as ectodermal pits before the cruciform muscleitself, which is issued from their wall. The sense organ ofDonax is closed immediately after the invagination of the pitand its closure is independent of the differentiation of theventral expansion which characterizes the closed sense organand of the canal which characterizes the open one. (Received 8 August 1983;     

Gill filament differentiation and experimental colonization by symbiotic bacteria in aposymbiotic juveniles of Codakia orbicularis (Bivalvia: Lucinidae)

Summary

Codakia orbicularis is a tropical lucinid harboring gill endosymbionts which are environmentally transmitted from a free living-symbiont form to the new host generation after metamorphosis. Structural changes occurring in the cellular organization from incomplete gill filaments in young aposymbiotic juveniles to full differentiated gill filaments containing bacterial endosymbionts in reared symbiotic juveniles, were analyzed for juveniles from 250 μm to 2 μm shell-length. Aposymbiotic juveniles possess differentiated gill filaments with ciliated, intermediary, and lateral zones similar to those described in wild juveniles, except for the bacteriocytes which are lacking. Granule cells, which progressively differentiate during the morphogenesis of the gill filament, do not appear as a consequence of symbiosis. Experimental colonization of aposymbiotic juveniles by the free-living symbiont form has been obtained through the addition of unsterilized sand collected from the natural habitat of C. orbicularis. Two days after exposure to crude sand, symbiosis-competent bacteria enter by endocytosis at the apical pole of undifferentiated cells which progressively differentiate into classical bacteriocytes similar to those found in the adult gill filaments. Undifferentiated cells of aposymbiotic gill filaments remain receptive to bacteria several months after metamorphosis, and become bacteriocytes when aposymbiotic juveniles get contact with the symbiont free-living form. Therefore, the environmental transmission of symbionts does not appear to be restrained to a defined period of time during post-larval development in C. orbicularis.