A functional interpretation of cilia and mucocyte distributions on the abfrontal surface of bivalve gills

The lack of fundamental data on the abfrontal surface of bivalve gills has prompted a comparative study of cilia and mucocytes on this surface, using scanning electron microscopy and histology on eight species of bivalves, representing seven families and the four major gill types: Mytilus edulis, Modiolus modiolus, Arca zebra, Placopecten magellanicus, Crassostrea virginica, Spisula solidissima, Mercenaria mercenaria, and Mya arenaria. Abfrontal cilia and mucocytes were found in all species studied, with types and densities differing within and between gill types. The three species of homorhabdic filibranchs presented different densities of abfrontal cilia and mucocytes, from very dense in M. edulis to sparse in A. zebra. The heterorhabdic gills had intermediate cilia and mucocyte densities, with highest concentrations of both abfrontal cilia and mucocytes on the principal filaments. The eulamellibranchs showed low ciliary densities together with high mucocyte densities, especially in S. solidissima, where the abfrontal mucocytes were glandular. These results indicate that: (1) the abfrontal surface is a vestigial mucociliary surface; (2) the abfrontal surface cannot participate in water pumping in most species, due to low ciliary densities; and (3) species with high densities of abfrontal mucocytes could utilize abfrontal mucus to reduce drag, especially in the highly fused gills, such as those of the eulamellibranchs. The differing distributions of abfrontal cilia and mucocytes may reflect different selective pressures acting on the gills within the various taxa. Received: 12 February 2000 / Accepted: 10 September 2000     

The role of mucus in particle processing by suspension-feeding marine bivalves: unifying principles

Contemporary research on bivalve suspension-feeding has revealed a diversity of particle processing mechanisms depending on the anatomy and functioning of the pallial organs involved. On the biochemical level, however, some evidence of homogeneity has emerged concerning the types of mucopolysaccharide associated with particle processing. The present study uses both previous data and original research combining video endoscopy and mucocyte mapping to further explore the relationships between pallial organ topography, functional correlates, direction of current flow, and mucocyte secretion type. Five species representing five different families and all four major gill types are represented: Mytilus edulis, Placopecten magellanicus, Crassostrea virginica, Mya arenaria, and Spisula solidissima. Viscous acid or acid-dominant mucopolysaccharides are used when particle transport occurs on an exposed surface, or on a structure leading directly to such a surface, counter to the prevailing current flow. Associated functions are indiscriminate transport in gill ventral particle grooves and rejection of pseudofeces. Lower-viscosity mixed mucopolysaccharides are used when particle transport is on an enclosed or semi-enclosed surface, leading to other such surfaces, and with the current flow. Associated functions are transport of particles destined for ingestion, and ingestion itself. Low-viscosity neutral mucopolysaccharides are found in regions where reduction of mucus viscosity is important, such as the areas of the labial palps responsible for fluidization of the high-viscosity mucus-particle cord of the gill ventral particle groove prior to particle extraction. There thus appears to be a specialization of mucus type corresponding to functional specialization of the various pallial organs in suspension-feeding marine bivalves. Received: 10 October 1996 / Accepted: 7 May 1997     

Peribuccal organs ofPlacopecten magellanicus andChlamys varia (Mollusca: Bivalvia): Structure,ultrastructure and implications for feeding

In order to better understand the structure of bivalve peribuccal organs and relate this to existing functional paradigms of their role in feeding, the labial palps of two scallop species,Placopecten magellanicus from the Bay of Fundy, Canada (1985 and 1986), andChlamys varia from the Bay of Brest, France (1986), were examined using histological techniques and electron microscopy. The ridged palp surface displays a uniformly dense ciliation with relatively few mucocytes; these are essentially concentrated in the region of the secondary ledge and may, through their secretory activity, determine the fate of particle masses in this area. The mucus secretions of the ridged palp surface are qualitatively different from those of the smooth palp surface. Mucocytes are much more abundant on the smooth palp surface, where it is suggested that their homogeneous secretions attenuate the potentially adverse effects of anteriorly-directed cleansing and swimming currents. Two other cell types are found in the palp epithelia: ciliated cells, which are very numerous on the ridged surface and relatively rare on the smooth surface, and non-ciliated epithelial cells, which are very numerous on the smooth surface and rare on the ridged surface, where they are confined to the palp margin. In addition to the mechanical role of the ciliated cells and mucocytes, the ultrastructural characteristics of the ciliated and non-ciliated epithelial cells indicate a dichotomy of function between the ridged and smooth surfaces. The ridged surface epithelial cells present an ultrastructural specialization in the absorption of dissolved and colloidal matter, suggesting an accessory nutritive role, whereas the smooth surface simple epithelial cells show signs of active molecular synthesis. No specialised sensory cells were observed on the ridged surface; it is therefore not yet possible to conclude whether the labial palps are capable of selection based on individual particle characteristics.     

Electrophoretic identification and genetic analysis of bivalve larvae

Taxonomic identification and genetic analysis of larval marine invertebrates have been vexing problems. We describe a polyacrylamide mini-gel electrophoresis technique for resolving proteins from individual larval bivalves (shell length 250 to 350 m) and apply this technique to three species of laboratory-cultured larval oysters [Ostrea edulis L., 1758, Crassostrea gigas (Thunberg, 1793) and c. virginica (Gmelin, 1791)] reared during summer 1989. Electrophoretic patterns of proteins clearly discriminate among the three species and allow genetic analysis of a polymorphic allozyme locus (Pgi) in field-collected larvae and juveniles of C. virginica. This technique provides an economical tool for largescale taxonomic, ecologic, and genetic studies of meroplanktonic stages of various species.     

Assessment of intertidal growth and capacity adaptations in suspension-feeding bivalves

Curves relating instantaneous growth rate to aerial exposure were determined for six species of bivalves in laboratory and shore experiments. A dimensionless index equatable with the relative intertidal growth performance of a species was calculated by integration of the growth curve after converting both growth rate and aerial exposure to decimal fractions. Intertidal growth performance of the bivalves tested corresponded well with their natural levels of occurrence on the shore, and improved in the following order: Modiolus modiolus < Argopecten irradians < Ostrea edulis < Mytilus edulis < Crassostrea virginica. Geukensia demissa, for which an index value could not be determined, grew faster intertidally than subtidally. The relative contributions made to intertidal growth performance by energy-conserving and energy-supplementing capacity adaptations were assessed by resolving the growth curves into energy-input and energy-loss components. The rate of energy loss due to intertidal exposure was lower in the high-shore species, and also less affected by harsher subaerial conditions, than in the low-shore species. Moreover, M. edulis and C. virginica showed abilities to supplement energy input such that growth per unit immersion time was better at certain intertidal levels than subtidally. Energy conservation and supplementation in these forms made roughly equal contributions to their improved intertidal growth relative to species occurring lower on the shore.     

The role of marine aggregates in the ingestion of picoplankton-size particles by suspension-feeding molluscs

Suspension-feeding molluscs are important members of coastal communities and a large body of literature focuses on their feeding processes, including the efficiency of particle capture. Some molluscs, such as bivalves, capture individual picoplankton cells (0.2–2.0 μm) with a retention efficiency of less than 50%, leading to the assumption that such particles are not an important food resource. Picoplankton, however, are often concentrated in particle aggregates of much larger size. This study investigates the ability of suspension feeders to ingest picoplankton-size particles (0.2–2.0 μm) bound in marine aggregates. We fed clams (Mercenaria mercenaria), mussels (Mytilus edulis), oysters (Crassostrea virginica), scallops (Argopecten irradians) and slipper snails (Crepidula fornicata) 1.0- and 0.5-μm fluorescent particles (either polystyrene beads or bacteria) that were (1) dispersed in seawater, or (2) embedded within laboratory-made aggregates. Dispersed 10-μm beads were also delivered so that feeding activity could be determined. Ingested fluorescent particles were recovered in feces or isolated digestive glands and quantified. Results indicate that aggregates significantly enhance the ingestion of 1.0- and 0.5-μm beads by all species of bivalves, and enhance the ingestion of bacteria (greatest cell dimension ca. 0.6 μm) by all suspension feeders examined. Differences among species in their ability to ingest aggregates and picoplankton-size particles, however, were evident. Compared to mussels and clams, scallops and oysters ingested fewer aggregates with 1.0-μm beads or bacteria, and slipper snails ingested the most dispersed beads and bacteria. These differences may be a consequence of variations in gill structure and mechanisms of particle processing. Our data demonstrate that suspension feeders can ingest picoplankton-size particles that are embedded within aggregates, and suggest that such constituent particles may be an important food resource.     

Potential effects of a non-indigenous predator in its expanded range: assessing green crab, <Emphasis Type="Italic">Carcinus maenas</Emphasis>, prey preference in a productive coastal area of Atlantic Canada

The non-indigenous green crab (Carcinus maenas) is an important predator on bivalve wild beds in coastal areas worldwide. This study explored size-dependent green crab prey preference on American oysters (Crassostrea virginica), blue mussels (Mytilus edulis), and soft-shell clams (Mya arenaria) in a productive coastal system of Atlantic Canada. Using two sizes of prey and three different experimental manipulations, small, medium, and large green crabs were given a choice among these three bivalves, and their daily feeding rates were monitored over the course of 3 days. For both prey sizes, green crabs showed an early feeding preference for soft-shell clams and, only as they declined in numbers, a switch toward mussels and subsequently toward oysters. We found that such changes in the timing (order) of prey preference are related to prey differences in shell thickness, a fairly reliable indicator of prey shell strength.     

Comparison of feeding and biodeposition of three bivalves at different food levels

Experiments on the edible mussel Mytilus edulis, the American oyster Crassostrea virginica, and the hard clam Mercenaria mercenaria, using flowing systems, showed that the feeding and biodeposition rates were affected by food concentration. At all levels of food concentration, the order of increasing feeding rate (both the percent of available particulate carbon and the actual amount of carbon removed) was: clam < oyster < mussel. All bivalves exhibited lower feeding rates (both precent and actual) at low food concentrations. However, the precent of available food removed quickly increased to a maximum at food concentrations typical for the natural environment. This maximum remained constant for the mussel and oyster, but declined with increasing food concentration for the clam. However, because this percentage was for increasing levels, the actual carbon removed continued to increase up to the highest food level for all three bivalves. In increasing order of biodeposition rate, the bivalves were: clam < oyster < mussel. The biodeposition rates of the three bivalves increased logarithmically with increased food concentration as a result of the production of pseudofeces. The feeding and biodeposition data were used to calculate assimilation rates, and this pointed out the higher efficiency of the oyster compared to the mussel and clam.Contribution No. 2993 from the Woods Hole Oceanographic Institution.     

Assimilation efficiencies and turnover rates of trace elements in marine bivalves: a comparison of oysters, clams and mussels

Assimilation efficiencies (AEs) and physiological turnover-rate constants (k) of six trace elements (Ag, Am, Cd, Co, Se, Zn) in four marine bivalves (Crassostrea virginica Gmelin, Macoma balthica Linnaeus, Mercenaria mercenaria Linnaeus, and Mytilus edulis Linnaeus) were measured in radiotracer-depuration experiments. Egestion rates of unassimilated elements were highest during the first 24 h of depuration and declined thereafter. Significant egestion of unassimilated Co, however, continued for up to 5 d in Macoma balthica, Mercenaria mercenaria and Mytilus edulis. With the exception of the extremely low values for ^110 mAg, ¹⁰⁹Cd, and ⁶⁵Zn in C. virginica, physiological turnover-rate constants (k) showed no general pattern of variation among elements, bivalve species or food types, and were relatively invariant. Values from  ≤0.001 to 0.1 d⁻¹ were observed, but excluding those for Co, most values were  ≤0.04 d⁻¹. In all four species, the AEs of Ag, Am, and Co were generally lower than those of Cd, Se, and Zn. The AEs of Ag, Cd, Se, and Zn in these bivalves are directly related to the proportion of each element in the cytoplasmic fraction of ingested phytoplankton, indicating that >80% of elements in a prey alga's cytoplasm was assimilated. C. virginica, Macoma balthica, and Mercenaria mercenaria assimilated ∼36% of the Ag and Cd associated with the non-cytoplasmic (membrane/organelle) fraction of ingested cells in addition to the cytoplasmic fraction. The ratio of AE:k, which is proportional to the consumer–prey trace-element bioaccumulation factor (concentration in consumer:concentration in prey) was generally greater for Cd, Se, and Zn than for Ag, Am, and Co. This ratio was lowest in Mytilus edulis, suggesting that this bivalve, the most widely employed organism in global biomonitoring, is relatively inefficient at accumulating important elements such as Ag, Cd, and Zn from ingested phytoplankton. Received: 7 February 1997 / Accepted: 24 February 1997     

Feeding current characteristics of three morphologically different bivalve suspension feeders, Crassostrea gigas, Mytilus edulis and Cerastoderma edule, in relation to food competition

Introduced Pacific oysters (Crassostrea gigas) have shown rapid expansion in the Oosterschelde estuary, while stocks of native bivalves declined slightly or remained stable. This indicates that they might have an advantage over native bivalve filter feeders. Hence, at the scale of individual bivalves, we studied whether this advantage occurs in optimizing food intake over native bivalves. We investigated feeding current characteristics, in which potential differences may ultimately lead to a differential food intake. We compared feeding currents of the invasive epibenthic non-siphonate Pacific oyster to those of two native bivalve suspension feeders: the epibenthic siphonate blue mussel Mytilus edulis and the endobenthic siphonate common cockle Cerastoderma edule. Inhalant flow fields were studied empirically using digital particle image velocimetry and particle tracking velocimetry. Exhalant jet speeds were modelled for a range of exhalant-aperture cross-sectional areas as determined in the laboratory and a range of filtration rates derived from literature. Significant differences were found in inhalant and exhalant current velocities and properties of the inhalant flow field (acceleration and distance of influence). At comparable body weight, inhalant current velocities were lower in C. gigas than in the other species. Modelled exhalant jets were higher in C. gigas, but oriented horizontally instead of vertically as in the other species. Despite these significant differences and apparent morphological differences between the three species, absolute differences in feeding current characteristics were small and are not expected to lead to significant differences in feeding efficiency.     

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.     

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.     

Labial palps of the blue mussel Mytilus edulis (Bivalvia: Mytilidae)

As a basis for understanding the functions of labial palps in the blue mussel Mytilus edulis, the structure and histology of palps were studied using light and scanning electron microscopy. Mussels used in the present study were collected in August 1993 and April 1994. The palp ridged surface is characterized by the presence of a smooth but densely ciliated dorsal fold, upon which rests the corresponding demibranch ventral region. The underside of the dorsal fold and the palp ridges fuse to form vestigial ciliated tracts. The dorsal fold is capable of contraction, allowing it to cover variable amounts of the ridged surface. Two different types of mucocyte are present on the palp ridged surface: subepithelial, glandular, acid-dominant secretion mucocytes and epithelial mucocytes characterized by neutral secretions. In histological section, these mucocytes appear to be concentrated on anatomical features known to intervene in particle handling. The anatomical and histological features of the smooth surface are typical of bivalve labial palps, except that the dense ciliation of the dorsal fold begins in the dorsal region of the smooth surface, indicating the possible origin of this feature. Previous studies on M. edulis point to the palps as the probable site for both ingestion volume control and particle selection; the anatomical basis of the present study should facilitate further research on these aspects.     

Les caryotypes de quelques espèces de bivalves et de gastéropodes marins

The karyotypes of the bivalves Ostrea edulis (2 n=20), Crassostrea gigas (2 n=20), Mytilus galloprovincialis (2 n=28) and M. edulis (2 n=28) were obtained from mitotic metaphases of branchial tissue by means of a cellular suspension technique. The occurrence of metacentric, submetacentric and telocentric chromosomes in O. edulis as well as differences in size between Pair 1 and Pair 10 emphasize the cytological interest of this species compared to C. gigas which possesses only metacentric and submetacentric chromosomes. The variation in Pair 2, which is telocentric in M. galloprovincialis and metacencentric in M. edulis, has been observed for the first time and may support taxological separation of these two species. The karyotypes of the mesogastropods Rissoa ventricosa (2 n=32) and Littorina neritoides (2 n=34) were obtained from mitotic metaphases of gonadal tissue by the same technique. Sexual chromosomes were identified in the karyotypes of R. ventricosa. At the beginning of spermatogenesis, the gonad of L. neritoides shows both haploïd and polyploïd meïotic metaphases which correspond to typical (germinal cells) and atypical (nurse cells) forms, respectively.     

Attraction to prey and stimulus to attack in the predatory gastropod Urosalpinx cinerea

Urosalpinx cinerea (Say), accustomed to feeding on Balanus balanoides, were strongly attracted in a choice chamber by the effluents of B. balanoides and B. eburneus, were indifferent to the effluents of Crassostrea virginica, Crepidula fornicata and Mytilus edulis, and responded negatively to the effluent of their own species. Oyster drills from a population feeding on Crassostrea virginica were attracted to oyster effluent; when these snails were offered a choice between Crassostrea virginica and B. balanoides effluents, they responded in equal numbers to the two effluents. Concentrations of NH₄Cl-ammonia in the range 18.2 to 73.3 mol did not attract the snails, and their responses to animal effluents were not correlated with the ammonia and amino-acid concentrations of the effluents, which ranged from 11.8 to 21.0 mol. It is argued that these results deny ammonia the role of a nonspecific distance attractant. Confined separately with various potential prey species, Urosalpinx drilled Balanus balanoides, B. eburneus, Crassostrea virginica, Crepidula fornicata, C. plana, Mercenaria mercenaria, Modiolus demissus, Mya arenaria, Mytilus edulis, Spisula solidissima, and Yoldia limatula, but not Anomia simplex. The prey species that were the least attractive in running water were generally rendered attractive and subject to attack in standing water. Freshly shucked Crepidula fornicata shells were scarcely drilled unless continuously perfused by C. fornicata effluent, and then only in small numbers. Altering the texture of living C. fornicata shells by wrapping them in nylon netting, by polishing, or by roughening, did not make them immune to drilling. Oyster drills in contact with C. fornicata attacked only if they received the effluent of the living prey, proving that tactile stimuli alone are not adequate to induce drilling.     

Particle sorting and formation and elimination of pseudofaeces in the bivalves <Emphasis Type="Italic">Mulinia edulis</Emphasis> (siphonate) and <Emphasis Type="Italic">Mytilus chilensis</Emphasis> (asiphonate)

Mulinia edulis and Mytilus chilensis are suspension-feeding bivalves with homorhabdic gills that live in different sedimentary habitats in the lower and upper intertidal, respectively, in Yaldad Bay, Chile. They are faced with different suspended particle size distributions when feeding, and both eliminate most of the inorganic particles by pseudofaeces production. This study used histology, scanning and transmission electron microscopy, and video endoscopy to compare particle processing on the labial palps and the mechanisms of particle sorting, acceptance, and rejection in the two species. In both species, disaggregation of mucus-bound particles occurs on the plicate surface of the palps. Particles destined for ingestion pass anteriorly from crest to crest and reach the mouth via the palp acceptance tract. Those destined for rejection enter the troughs between the plicae and move ventrally to the palp rejection tract. The palps manipulate the pseudofaeces into a mucous ball, which is transferred to the mantle rejection tract. In Mulinia edulis (an infaunal, siphonate mactrid), the pseudofaeces are stored in a chamber at the base of the inhalant siphon until expelled by intermittent contraction of the siphon wall. In contrast, Mytilus chilensis (an epifaunal, non-siphonate mytilid) releases pseudofaeces continuously when submerged.     

Photoacoustic determination of the thermal properties of bivalve mollusk shells

We used the photoacoustic technique for characterization of the thermal properties of adult shells of American oyster Crassostrea virginica (Gmelin, 1791) and the mussel Ischadium recurvum (Rafinesque, 1820). Heat transport is more efficient in C. virginica than in I. recurvum shells, reflecting the different architecture, microstructure, and composition of the two kinds of shells. Published online: 4 July 2002     

Acquisition of particle processing capability in juvenile oyster <Emphasis Type="Italic">Crassostrea gigas</Emphasis>: ontogeny of gill mucocytes

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.     

Foraging behavior of juvenile Carcinus maenas (L.) and Cancer pagurus L.

Information concerning the way juvenile crabs choose their diet from a variety of prey types can be useful for a better understanding of community dynamics, as well as for the adequate management of natural resources. Prey size and species selection by juvenile Carcinus maenas (15-35 mm carapace width, CW) and Cancer pagurus (20-40 mm CW) feeding on four bivalves of contrasting shell morphology were investigated. When offered a wide size range of Mytilus edulis, Ostrea edulis, Crassostrea gigas, and Cerastoderma edule presented individually, crabs generally showed evidence of size-selective predation. Cancer pagurus selected larger mussels relative to the size of their chelae (relative prey size, RPS) than did Carcinus maenas of similar and even larger carapace width. However, the RPS of selected O. edulis and Cerastoderma edule were similar for all crabs, suggesting that certain prey features constitute effective barriers even to the powerful chelae of Cancer pagurus. When offered a wide size range of mussels and oysters simultaneously, all crabs consistently selected mussels. When offered O. edulis and Crassostrea gigas, crabs consumed both these oyster species in similar numbers. Carcinus maenas consumed similar numbers of mussels and cockles; Cancer pagurus, however, showed no preference for either prey in the smaller size classes but selected more mussels than cockles as prey increased in size. Although previous studies report that adult Carcinus maenas select prey species according to their profitability (amount of food ingested per unit of handling time, milligrams per second), consumption rates of the size classes of prey selected by juvenile shore crabs did not always parallel prey value. Although variations in crab strength can account for many of the differences between the foraging strategy of juvenile and adult C. maenas, our results suggest that juvenile crabs are less species selective than adults as a result of the restrictions imposed on small individuals that have limited access to larger prey.     

Rate of excretion of ammonia by the hard clam Mercenaria mercenaria and the American oyster Crassostrea virginica

Two species of bivalve molluscs, Mercenaria mercenaria Linné and Crassostrea virginica Gmelin, were maintained in an identical laboratory environment and fed the same diet for 1 month prior to measuring the quantity of ammonia which they excreted per gram dry weight. Data derived from juveniles and adults of these species were fitted to a log-log equation. M. mercenaria excreted more ammonia per gram body weight than C. virginica, and considerably more scatter is evident in the fit of the data from M. mercenaria to the log-log relationship. Experiments showed that neither temperature fluctuation during the experiments nor decomposition of organic nitrogen in the test water account for the scatter of points. Behavioral differences between the two species of organisms may explain these differences.This work is the result of research sponsored by NOAA Office of Sea Grant, Department of Commerce, under Grant No. 04-3-158-30.