A. Herry

Ultrastructure of the gill of the hydrothermal-vent mytilid Bathymodiolus sp.

Specimens of Bathymodiolus sp. were collected at 2 620 m depth during the “Biocyarise” Cruise (12°58′80″N; 13°56′60″W) in March 1984, and samples of the gill were fixed for ultrastructural observations. The gill of this hydrothermal-vent mytilid constitutes the main organ in the nutritional processes. The lamellae display abundant ciliation, normally comprised of frontal cilia, compound latero-frontal cirri and lateral cilia. At the ventral margin of each demi-branch, a longitudinal, ciliated, feeding groove is present. the lamellae are composed of numerous homorhabdic filaments connected by tufts of cilia. Each filament is made of a thin wall overlying a central lumen containing amoebocytes. Ultrastructural observations revealed the filament wall to be composed of four types of cells: (1) The ciliated cells of the frontal, latero-frontal and lateral ciliation, characterized by an abundance of mitochondria. (2) Mucous cells present to some degree among the ciliated cells, but more abundant on the distal edge and containing dense droplets of mucus. (3) Cells colonized at their apical pole by numerous bacteria enclosed in membrane-delimited clear spaces and composing the major part of the filament wall. (4) Thin ciliated cells separating the bacterial cells and characterised by a dense fringe of microvilli at their apical pole. The lumen of the filament contains amoebocytes of different morphological aspects which seem to accumulate electron-dense granules, possibly related to detoxification processes.

Feeding and digestive adaptations of bivalve molluscs to sulphide-rich habitats

Abstract From the standpoint of sulphide availability, marine bivalves are divided into three categories: Type 1, external environment and pallial cavities low in sulphides while submerged; Type 2, external environment containing sulphides but pallial cavity normoxic and low in sulphides due to irrigation via siphons; and Type 3, external habitat variably rich in sulphides and pallial cavity with some oxygen and also suphides, due to lack of siphons. This review presents the known anatomical and physiological adaptations associated with feeding and digestion in Type 3 bivalves, compared with Types 1 and 2. The principal anatomical adaptations of Type 3 bivalves to habitats of increasing sulphide availability are, in order of occurrence and degree: reduction of labial palps; reduction in length and volume of intestine; reduction in stomach volume; disappearance of crystalline style and style sac; reduction in digestive tubule volume; disappearance of digestive gland secretory cells; and total disappearance of the digestive system. In parallel with the above adaptations, an increasing physiological reliance on endosymbiotic bacteria is observed, as evidenced from δ 13C values. We conclude that, under similar conditions of environmental sulphide availability, similar adaptations are found in the feeding/digestive system, irrespective of taxon, depth or geographic location. The convergence between littoral and deep-sea vent and seep inhabitants is particularly striking.

Chemoautotrophic symbionts and translocation of fixed carbon from bacteria to host tissues in the littoral bivalve Loripes lucinalis (Lucinidae)

Specimens of Loripes lucinalis (Lucinidae) living in reducing sediments were collected near a sewage outfall at low tide on the Moulin Blanc beach, Brest, France, from January to March 1987. Electron microscope studies revealed numerous Gram-negative-type bacteria in the gill cells. Ribulosebiphosphate carboxylase, a diagnostic enzyme of the Calvin-Benson cycle of CO2-fixation was measured only in the gill extracts. Various tissues of L. lucinalis were examined for activity of APS reductase, (EC 1.8.99.2), ATP sulphurylase (EC 2.7.7.4) and rhodanese (EC 2.8.1.1), enzymes involved in sulphide oxidation. APS reductase was only found in symbiont-containing tissues, i.e., gills. These enzymatic studies characterise the symbionts as chemoautotrophic sulphide-oxidizing bacteria. Histoautoradiography demonstrated that part of the carbon dioxide fixed by symbiotic bacteria in the gills is translocated to symbiont-free tissues of the bivalve. The ultrastructure of the gill is detailed and a nomenclature based on established and new terminology is proposed to describe the various cellular types comprising the gill filament.

New observations of the gills of Placopecten magellanicus (Mollusca: Bivalvia), and implications for nutrition

The internal anatomy and microanatomy of the gill of Placopecten magellanicus Gmelin collected in May and November 1985 from Chamcook Bay, New Brunswick, Canada, was studied using thin-section light microscopy and transmission electron microscopy. Most of the spurs show no evidence of organic union, and hence do not participate in vascular exchange. However, the dorsal bend shows both ciliary and organic interfilamentar union. The internal structure and the hemocytes of the dorsal respiratory expansion are presented. The epithelium consists of three distinct cell types, bounded by apical microvilli. All regions of the gill contain an epithelial basal membrane, which is greatly convoluted in the interconnecting vessels of the dorsal respiratory expansion. The significance of these observations is discussed in relation to possible roles in respiration, transmembrane transport and nutrition. The apical surface of all ciliated cells is covered with an acellular matrix composed of clear spherical vesicles, which may serve a mechanical function for which mucus would be unsuited. The significance of the abundance of mucocytes on the abfrontal surface of the principal filaments is discussed in terms of the escape response of pectinid bivalves.

Nutritional strategies of the hydrothermal ecosystem bivalves

Studies of deep-sea hydrothermal bivalves have revealed that the species, which are strictly dependent upon the interstitial fluid emissions, derive their food indirectly via symbiotic relationships with chemosynthetic bacteria present in their gill tissues. As the gill plays the main trophic role, structural and ultrastructural modifications occur in the digestive tract. Scanning and transmission electron microscope studies reveal that the digestive system of species belonging to the genera Calyptogena, Bathymodiolus and Bathypecten have anatomical differences. In Calyptogena, the reduction of several parts of the digestive tract and the stomach content which is either empty or full, according to the various species examined indicate that the digestive system is hardly if at all functional. In Bathymodiolus, the labial palps are well developed, the stomach is always full with particles and the two cellular types, digestive and secretory, are present in the digestive gland. All these characteristics indicate that the digestive system is functional. In Bathypecten, the digestive tract is well developed and it seems that it plays the main trophic role. We conclude that the nutritional strategies of the hydrothermal vents bivalves are quite varied. They range from a normal trophic process, through a mixotrophic diet, to one based purely on chemoautotrophic bacteria. The strategy of each species is adapted to and influences its distribution.

Ultrastructural characteristics of typical and atypical spermatogenesis in the queen conch Strombus gigas (Mollusca, Gastropoda)

Summary As in many prosobranchs, two cell lines develop in the testis of Strombus gigas. The typical spermatogenesis is similar to that described in many species. The spermatic cell development is characterized by cell divisions. Typical spermatogenesis leads to a mature spermatozoa with a column-shaped nucleus. The acrosome consists of an acrosomal rod covered by an acrosomal cone and topped by a hemispherical vesicle. The middle piece is made up of nine mitochondria helically arranged around the axoneme. A ring centriole is present between the middle piece and the flagellum. The tail contains the axoneme and glycogen granules. During atypical spermatogenesis no division of the cells is observed. The cytoplasm of atypical spermatogonia increases in size while the nucleus degenerates. In the cytoplasm dark granules and vesicles with double membranes develop. The mature atypical spermatozoa are spindle-shaped. The cytoplasm contains large dark granules, but no remnants of the vesicles with double membranes...