Peter G. Beninger

The use of physiological condition indices in marine bivalve aquaculture

Abstract The physiological condition indices most commonly used in bivalve aquaculture are reviewed and evaluated. Two general categories of indices may be distinguished: static and dynamic. Of the static indices reviewed, only one is recommended for use in the culture of early developmental stages — the dry ash weight: total dry weight ratio. In the case of juveniles and adults, the easily-measured dry tissue weight: dry shell weight is recommended for most routine work. The dynamic indices reviewed are based on production estimates, and hence reflect physiological changes over specified time intervals. Of these indices, only net growth efficiency is recommended for use in bivalve aquaculture, and it is applicable to all life stages. While this index gives the most information about the physiological state of the animals comprising a population, it is very difficult to evaluate and is thus most appropriate as a research tool.

Gill function and particle transport in Placopecten magellanicus (Mollusca: Bivalvia) as revealed using video endoscopy

The technique of endoscopic video observation was used to study feeding processes of Placopecten magellanicus (Gmelin), collected from Bull Arm, Newfoundland in August 1991 and 1992, under near-natural feeding conditions. The fate of captured particles depended on the extent of ingestive or handling capacity saturation. Under low (1 to 10 particles μl-1) to medium (10 to 20 particles μl-1) particle concentrations, most particles were incorporated in continuous anteriorly directed slurries in the dorsal ciliated tracts of the gill arch and dorsal bends. As particle concentration or exposure time to the lower particle concentrations increased, four endogenous mechanisms of ingestion volume control were increasingly observed: (1) rejection of dense mucus-particle masses from the principal filament troughs onto the ventrally beating cilia and associated currents of the ordinary filament plicae, counter to and below the incoming pallial current maintained by the principal filament cilia; (2) intermittent stopping of the anteriorward flow in the dorsal ciliated tracts; (3) reduction or cessation of input from the principal filaments to the dorsal ciliated tracts; (4) detachment of the dorsal bends from the mantle to establish a shunt from the infrabranchial to the suprabranchial cavity. Chemical and histochemical tests of purified fluid withdrawn from the dorsal ciliated tracts indicate that mucus is present at all particle concentrations. Mucus therefore participates both in normal feeding and in ingestion volume control on the bivalve gill, although different mechanisms, and types of mucus, effect transport of material in the dorsal (feeding) and ventral (cleaning) ciliated tracts.

Functional anatomy of the male reproductive system and the female spermatheca in the snow crab Chionoecetes opilio (O. Fabricius) (Decapoda: Majidae) and a hypothesis for fertilization

ABSTRACT To help elucidate the reproductive characteristics of the Atlantic snow crab Chionoecetes opilio, the functional anatomy of the male reproductive system and the female spermatheca was investigated using histological methods, transmission and scanning electron microscopic techniques, and microscopic observation of fresh material. Several fundamental corrections and additions to earlier descriptions of spermatozoan structure in C. opilio were made: radial arms are present, while a chromatin ring is not; the acrosome protrudes only slightly and spermatozoa are not mushroom-shaped. The spermatozoa and the matrix of the anterior vas deferens are packed into spermatophores and surrounded by a pellicle which appears to be secreted by the cells lining the anterior vas deferens proximal to the testis. The highly folded configuration of this pellicle may act as a safeguard against dehiscence induced by contact with sea water during copulation. The posterior vas deferens contains two distinct secretions which are probably ejaculated along with the spermatophores and the matrix of the more anterior vas deferens. Anatomical and in vitro observations suggest that fertilization is initiated by exposure of the spermatophores to a hypotonic medium. Such a medium may be generated within the spermathecae by dilution of the seminal fluids/spermatophore storage matrix with sea water prior to egg mass extrusion. Devagination of the liberated spermatozoa may be facilitated by the same mechanism. Most of the females processed for histology had distended spermathecae devoid of spermatophores or spermatozoa. It is thus impossible to deduce successful copulation and spermatophore storage without direct observation of spermathecal contents.

Direct observations of feeding structures and mechanisms in bivalve molluscs using endoscopic examination and video image analysis

A new technique is described for observing the structures and mechanisms of suspension feeding in bivalves using endoscopic examination and video image analysis. This method permits direct in vivo observations of whole, intact structures of relatively undisturbed specimens. No surgical alterations of shell or tissue are required for most species. Pallial organ activity can be recorded for future observations and analysis. Using this technique we examined three bivalve species, each with different degrees of mantle fusion:Mya arenaria L.Mytilus edulis L., andPlacopecten magellanicus (Gmelin). The specimens were collected between April and September 1990 at various locations in Trinity Bay, Newfoundland, Canada. Particle retention by the gill and transport of material to the palps was observed, and velocity of particles moving on the gill was determined. We demonstrate that the endoscope-video-analysis system is an efficient and affordable technique suitable for studies of pallial organ function and mechanisms of feeding.

Multiple reproductive strategies in snow crab, Chionoecetes opilio: Physiological pathways and behavioral plasticity

Abstract We review the three major reproductive pathways displayed by snow crab, Chionoecetes opilio . Females can copulate in either a soft- or hard-shelled condition and also fertilize eggs with sperm stored in their spermathecae. Spermatophore partitioning and preservation mechanisms in the spermatheca plus postulated spermatophore removal abilities in males play central roles in shaping mating behavior. However, fitness consequences of the various strategies are enigmatic. Fecundity is influenced by both spermatophore source and female characteristics; also, egg production estimates are confounded by high loss rates and uncertainties surrounding brooding period. Males challenge the concept of “maturity” as they copulate both as morphometrically immature (MI) with small chelae and morphometrically mature (MM) with large chelae individuals. Reproductive opportunities for each male morph vary according to long-term population cycles influencing inter-male competition levels. Thus, during phases when MM males are abundant, reproductive opportunities for MI males are probably low; however, the window opens when MM males become scarce. We explore possible mechanisms that control whether males with small chelae either continue to molt and remain MI or attain full sexual maturity and, hence, probable terminal molt-status.

Reproductive Processes Revealed by Spermatophore Dehiscence Experiments and by Histology, Ultrastructure, and Histochemistry of the Female Reproductive System in the Snow Crab Chionoecetes Opilio (O. Fabricius)

ABSTRACT Histological, histochemical, ultrastructural, and microbiological techniques were used to elucidate the structure and function of the female reproductive system in the snow crab Chionoecetes opilio (0. Fabricius). The anatomy of the ovary and oviduct conform to that of other brachyuran species. The spermatheca comprises a dorsal (glandular) and a ventral (chitin-lined) region, with no intervening anatomical separation. Ultrastructural and histochemical bases are presented for the dual role of the spermatheca in secretion and sperm storage. Proteinaceous polysaccharides are the dominant holocrine secretion of the glandular epithelium. A dense, morphologically homogeneous bacterial population was observed in the spermathecae of all of 25 crabs examined. A discrete layer of nonspermatophore-containing acid mucopolysaccharides appears to be transferred to the vagina and ventral spermatheca region by the male after insemination. A functional partitioning of spermatophores seems to occur at insemination; some dehisce immediately (probably due to mechanical forces), while those resisting initial dehiscence are stored in the spermatheca. In vitro experiments show that storage in the spermatheca greatly increases the tendency of spermatophores to dehisce when exposed to sea water. Functional correlates to these data are explored, with emphasis on roles and interactions of secretions, bacteria, spermatophore storage, and dehiscence, and mechanisms of ensuring lastmale precedence under conditions of sperm competition.

Mode of particle ingestion in five species of suspension-feeding bivalve molluscs

In order to elucidate the mode of particle ingestion and the functional anatomy of the oesophagus in bivalves, a histological study was performed onMytilus edulis (Mytilidae),Crassostrea virginica (Ostreidae),Placopecten magellanicus, Chlamys varia, and juvenilePecten maximus (Pectinidae). Specimens were sampled from various sites in New Brunswick, Canada, and Brittany, France, from 1987 to 1989. The buccal, peribuccal, and oesophageal epithelia of all species contained a dense distribution of actively secreting mucocytes, although these were somewhat less abundant inCrassostrea virginica, which also has the shortest oesophagus. Mucocyte morphology, while constant within a family (Pectinidae), showed clear differences among families. Both acid and neutral mucopolysaccharides were secreted by the epithelial mucocytes of all species. Mucus and mucus-particle masses were observed in the peribuccal and buccal regions, as well as in the oesophageal lumina of all species, even in those specimens which had been maintained without feeding (Placopecten magellanicus) or held out of water for 48 h (C. virginica) prior to dissection and fixation. These results indicate that a basal level of mucus production and transport is continuous on the peribuccal, buccal, and oesophageal ciliated epithelia, regardless of the particle concentration in the external medium. Buccooesophageal glands, generally thought to be absent in the Bivalvia, were observed in one of the species examined (M. edulis). It is concluded that the mode of particle ingestion in these suspension-feeding bivalves is via ciliatransported mucus masses; the presence of buccooesophageal glands inM. edulis suggests a digestive role for the oesophagus in this species.

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.

Gonopods of the Majid Crab Chionoecetes Opilio (O. Fabricius)

ABSTRACT To further elucidate the reproductive processes of the snow crab Chionoecetes opilio (O. Fabricius) the first and second gonopods of adult specimens were examined using scanning electron microscopy and histology. Surface features and setal types are described for both gonopods. The gonopods are highly modified compared to other brachyuran crabs and lower crustaceans. Suggestions for setal function are presented, based on size, orientation, location, fine structure, and cuticular insertion. Tubulation of the endopod is observed in both the first and second gonopods. Rosette glands are abundant in the basal half of the first gonopod, but are absent in the second gonopod. A duct network from the rosette glands terminates in cuticular pores that are restricted to the ejaculatory canal, indicating that the rosette glands function in copulation, probably contributing to the seminal fluids. A mechanism for transfer of seminal fluids is proposed, based on the relative sizes of the gonopods and the morphology of the second gonopod.

Ontogeny of Female Primary Sexual Characters in the Majid Crabs Chionoecetes Opilio and Hyas Coarctatus

ABSTRACT To investigate the development of primary sexual characters in the family Majidae, the spermathecae, ovaries, and oviducts of juvenile female Chionoecetes opilio and Hyas coarctatus were examined, using histology, histochemistry, and electron microscopy, and compared to those of mature females. Females were divided into 3 developmental categories, with corresponding size ranges, based on anatomical criteria: immature, premature, and mature. Immature crabs were still temporally remote from the terminal molt, had white ovaries, and the spermatheca presented a thin tissule separation between the dorsal and ventral regions. No stratified glandular epithelium was present; only a columnar epithelium lined the lumen. This epithelium tested slightly positive for amine-containing substances. Premature females were temporally close to terminal molt, had orange ovaries, and retained the tissule separation between the dorsal and ventral regions of the spermatheca. In addition, they presented a stratified glandular epithelium which developed beneath the columnar epithelium lining the lumen. The latter degenerated and sloughed into the lumen as the former developed. Mature females had undergone the terminal molt, and were examined both prior to and following egg extrusion. No trace remained of the columnar epithelium which was present in the preceding two categories. In mature females which had extruded eggs, the tissule separation between dorsal and ventral regions of the spermatheca was also absent. Neutral mucopolysaccharides dominated in the secretions of the dorsal glandular epithelium of both types of mature female. These results establish the primary sexual characters in female C. opilio and H. coarctatus, and also document the sequence of aquisition of these characters.