Genetic relations between the bivalves Cardium (Cerastoderma) edule,Cardium lamarcki and Cardium glaucum,studied by means of crossed immunoelectrophoresis

Rabbit antisera were raised against Cardium edule (L.) from the Waddensea locality, Skallingen, against Cardium lamarcki (Reeve) from the Danish locality, Dybsø Fjord, and against Cardium glaucum (Bruguière) from the French Mediterranean coast at Ile des Embiez. Allopatric and sympatric populations of C. lamarcki and C. edule from Swedish and Danish localities, and allopatric populations of C. glaucum from the Mediterranean Sea, Red Sea and Black Sea were studied by means of the described method and by starch gel electrophoresis of malate dehydrogenase (MDH). By means of crossed immuno-electrophoresis, using intermediary gels containing antibodies against one of the Cardium types and receiver gels containing antibodies against one of the others, type-specific and type-non-specific reactions were recorded. The results are compared with a similar investigation of Mytilus edulis, M. galloprovincialis and M. californianus. Based on immunological properties, it is shown that C. edule is a valid species different from the C. lamarcki/C. glaucum complex. It is further suggested that C. lamarcki and C. glaucum should still be referred to by these two names as significant genetic differences between them is revealed by this immunological study.     

Studies on the reproductive cycles of Cardium edule and C. glaucum

Two allopatric populations of Cardium edule and two of C. glaucum were sampled over a period of 16 months and their gonad maturity cycles compared by histological methods. The onset of both spermatogenesis and oogenesis in C. edule took place between September and October; in C. glaucum it was slightly later, between October and November. Development of the gonads continued slowly throughout the winter, C. glaucum developing marginally faster than C. edule. Maturation accelerated in April and May, with the gonads of both species becoming fully ripe in late May and early June. Spawning of the two species approximately coincided at all the localities investigated. Spawning in C. edule was better coordinated than in C. glaucum. Transplanting cockles of one species to the typical habitat of the other did not appreciably alter the spawning period. The separation of spawning periods in sympatric associations of the two cockles is discussed.     

Enzymatic and morphological criteria for distinguishing between Cardium edule and C. glaucum of the Portuguese coast

Cardium edule L. and C. glaucum Bruguière are two sibling species of cockles commonly found in littoral systems along the Portuguese coast, where they sometimes occur as mixed populations. The problem of morphological separation of these two cockle species has not been completely solved. Indeed, the distinctive morphological criterion proposed by Brock after identifying the specimens by means of electrophoretic patterns of hepatopancreatic malate dehydrogenase was not effective when applied to Portuguese Cardium spp. The object of the present study was to establish morphological discrimination criteria for distinguishing Portuguese C. edule and C. glaucum on the basis of initial identification of individuals through the enzymatic criterion established by Brock. Cockles were collected from 1988 to 1991 at 16 estuarine and lagoonal systems along the Portuguese coast. Hepatopancreatic extracts were submitted to horizontal starch gel electrophoresis following Brock's procedure. The malate dehydrogenase patterns obtained were identical to those found by Brock. Clear interspecific differences in the leucine aminopeptidase and esterase isozyme systems were also detected. The morphological study of the shells involved 7 morphometric and 9 qualitative and semiquantitative characters. Canonical variate analysis of the morphometric characters showed that these could not be used for identification purposes. However, a clear separation between the two species was obtained by a principal-component analysis of qualitative and semiquantitative characters. The characters mainly useful for distinguishing between the two species were valve profile, shape of ventral valve junction, type of calcareous scales and shell colour.     

Biological studies on Cardium glaucum,based on some Baltic and Mediterranean populations

The recent separation of Cardium glaucum Bruguière from C. edule L. and the scarcity of reports of populations containing both species has necessitated further ecological study to demonstrate differences in habitat preferences. Some mixed and unispecific populations of the cockles Cardium edule and C. glaucum, situated in fjords around the Kattegat and Baltic coasts of Denmark were studied; it was demonstrated that some degree of tidal influece was essential for the survival of C. edule, and that C. glaucum was being restricted by the exposure of the site. A study of the cockle populations of some étangs on the Mediterranean coast of France demonstrated that, despite earlier reports of its presence, C. edule was not present in that area. A brief survey of the Bassin d'Arcachon (France) showed that it contained a mixed cockle population. It was concluded that C. edule was unable to occupy localities having a tidal amplitude of less than 0.2 m, and that high temperatures also restricted occupation of semitidal habitats; similarly, the exposure of the site restricted colonisation by C. glaucum.     

Energetics of growth and reproduction in cockles (Cerastoderma edule): Seasonal and age-dependent variations

Seasonal patterns of growth were compared interannually between 1983 and 1985 for different ageclasses of an intertidal population of cockles,Cerastoderma edule L., in the Mundaka Estuary (Basque Country, North Spain). Growth data and simultaneous determinations of respiratory metabolism were integrated over defined periods of time in order to compute assimilation and net growth efficiency. Consistent differences between the exponents that scale both assimilation and metabolism to body size led to growth efficiency values that were a decreasing function of age. Total growth was partitioned into three components: somatic growth, reproductive output and reserve storage, and the corresponding effort indexes were calculated. Despite important differences in total growth, reproductive output was constant between years, indicating a higher reproductive effort under poorer growth conditions. Increased costs of reproduction that appeared to be associated with this higher reproductive effort did not affect somatic maintenance, since previously stored carbohydrate reserves were primarily used to fuel gametogenesis.     

Physiological responses of Mytilus edulis and Cardium edule to aerial exposure

Physiological responses of two bivalves (Mytilus edulis L. and Cardium edule L.) to intertidal conditions were studied. Specimens were collected from S. W. England in autumn/winter, 1980, and acclimatized to either intertidal or subtidal regimes before measurement of rates of heat dissipation and oxygen uptake during 5 h of air exposure, and rates of , particle clearance, ammonia excretion, and food-absorption efficiency during 7 h of reimmersion. Subtidal individuals were either intermittently or continuously fed in order to distinguish the effects of periodic food supply from the effects of air exposure. Specimens of M. edulis had low aerial rates of (14 to 20% of aquatic rate), and was greater than the energy equivalent of , indicating that they were largely anaerobic. In contrast, C. edule air-gaped and had higher aerial rates of and (50 to 75% of aquatic rate). There were behavioural and metabolic differences in the responses of intertidally and subtidally acclimatized mussels and cockles to air exposure. Intertidal individuals of both species were more quiescent, had lower aerial rates of and , and showed a conditioned response at the expected time of reimmersion. The reduction in aerial rate of was an energy-saving mechanism and the payment of the oxygen-debt within 2 h of reimmersion represented a significant cost. The heat increment associated with feeding and digestion was estimated as 15 to 17% of the oxygen uptake by M. edulis during all stages of recovery. M. edulis adapted to the intertidal regime by reducing its time-averaged aerial and aquatic rates of ammonia excretion. In contrast, C. edule maintained a high aerial and aquatic rate of ammonia excretion. The clearance rates of M. edulis recovered rapidly (0.5 to 1.5 h) following reimmersion, whereas those of C. edule recovered more slowly, particularly for the subtidal individuals following acute exposure (>4 h). There was no evidence of increased clearance rate or absorption efficiency by intertidal individuals to compensate for the loss of feeding time. Intertidally acclimatized individuals of M. edulis and C. edule had more energy available for growth (scope for growth) integrated over a 12 h period and higher growth efficiencies than subtidal individuals subjected to acute air exposure and intermittent feeding regimes. This was the result of reduced aerial and aquatic rates of energy expenditure, a relatively small cost in terms of the payment of oxygen-debt and a rapid recovery of clearance rate following reimmersion.     

Phylogeography of Cerastoderma glaucum (Bivalvia: Cardiidae) across Europe: a major break in the Eastern Mediterranean

Molecular variation of the lagoon cockle Cerastoderma glaucum (Poiret, 1789) was examined across the species range along European coasts, from the northern Baltic to the Black Sea and the Caspian Sea. A major phylogeographic break in mitochondrial COI gene sequences (divergence 6.2%) separated a group of Ponto-Caspian and Aegean Sea haplotypes from those to the west of the Peloponnese peninsula in the Mediterranean and in the Atlantic-Baltic sector. A similar subdivision, yet not entirely geographically coincident, was found at 16 allozyme loci (Nei's distance D=0.15). The results imply a long-term isolation of populations in parts of the Eastern Mediterranean or Black Sea basins through the Pleistocene. The subdivision does not concur with previous views on the systematics of C. glaucum complex, but the pattern is notably similar to that described earlier from some fish species. Marked phylogeographic structuring was also found at lower level within the major Mediterranean-Atlantic phylogroup of C. glaucum, which was divided into six regional or local haplotype subgroups. Divergence of these groups may date back one or several major Pleistocene climatic cycles. Local mtDNA diversity was particularly high in a sample from the Ionian Sea, whose mitochondrial identity was of the Mediterranean-Atlantic type, while its nuclear characters were more strongly associated with the Ponto-Caspian type. Patterns of shallower, star-phylogeny type diversity within the Ponto-Caspian phylogroup and in the Baltic Sea area may represent more recent, post-glacial generation of variation.     

Short-term shell growth in bivalves: Individual,regional, and age-related variations in the rhythym of deposition ofCerastoderma (=Cardium) edule

The influence of potential sources of variations (age, site, region) on the production of shell microgrowth increments was studied in the cockleCerastoderma edule (L.) from the shores of Denmark, Jersey Island and South Wales between 1978 and 1986. Microgrowth increments in the shell of the cockle were counted between annual growth marks corresponding to the second, third, and fourth yearly growth period of the cockle. The number of microgrowth increments per year showed a decrease with increasing age. Increments formed after deposition of the last annual mark in cockles sampled at different dates indicate that the rate of increment formation differed between age classes and populations. Furthermore, we observed pronounced intrapopulation differences in the number of increments for specimens of the same age, and differences in both number and clarity of increments in subtidal and intertidal populations. Our results do not support the hypothesis of a general endogenous rhythm of microgrowth that closely corresponds to tidal rhythms inCerastoderma edule. Instead, they emphasize the plasticity of deposition according to the origin and age of the experimental specimens.     

Potential of bivalves' secondary settlement differs with species: a comparison between cockle (Cerastoderma edule) and clam (Ruditapes philippinarum) juvenile resuspension

Juvenile bivalves may be dispersed by entering a bysso-pelagic phase where they drift through the water mass aided by a long thread. The ability to resuspend and control the specific weight in two bivalve species, the cockle Cerastoderma edule (L.) and the Japanese clam Ruditapes philippinarum (Adams and Reeves), was documented with juveniles through flume and still-water experiments. Cockle juveniles initially placed on an unsuitable substratum were exposed to two shear velocities (u _*). At the end of the experiment, 42 (±15)% (for u _*=0.51 cm s⁻¹) and 79 (±9)% (for u _*=0.99 cm s⁻¹) of individuals were retrieved from the sand area which represents only 8% of the total flume surface. Most juveniles (70.5%) with shell lengths <2.5 mm migrated from the unsuitable Plexiglas substratum to the sand array by resuspension in the water column. The percentage was lower (21.5%) for larger individuals. The same experimental design was applied to clams, which immediately adhered to the Plexiglas substratum and remained attach to it. Sinking rates of live and dead specimens of both species were measured in a 1 m long transparent PVC tube. Cockle fall velocities showed severe deceleration, probably due to byssus secretion (up to 15-fold slower than dead cockles), sometimes interrupted by brutal acceleration probably due to byssal rupture. Cockles were able to reduce their sinking rate for shell lengths up to 4.25 mm. By contrast, clam sinking rates were constant, and similar to dead clam sinking rates. Specific weights of all experimental juveniles were calculated in relation to their lengths, and their passive motion into the boundary layer was theoretically assessed with Shields curve. In short, C. edule and R. philippinarum can both exhibit dense populations in the field with a good capacity to colonize, although juveniles display different abilities to resuspend in the water column. Received: 27 January 1997 / Accepted: 13 February 1997     

Genetic structuring and gene flow in Cerastoderma glaucum (Bivalvia: Cardiidae): evidence from allozyme variation at different geographic scales

Genetic variation at 16 enzyme loci was analysed in 20 Mediterranean and 1 Baltic population of Cerastoderma glaucum. Spatial genetic variation at different geographic scales was investigated. In general, this species was fairly genetically structured (over all loci Š=0.088), according to the fragmentary nature of its habitat. Spatial structuring of genetic diversity was shown to follow different models, depending on the geographic scale considered: a stepping-stone model provided a good fit at a wide scale, with gene flow inversely related to geographic distance, whereas at small scale, genetic relationships among samples could not be interpreted as simply the effect of physical distance among populations. Results are discussed taking into account the intrinsic and extrinsic factors which are most likely to affect the patterns of genetic structuring in coastal marine animals. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00227-001-0753-x     

Interannual variation in the reproductive cycle and biochemical composition of the cockle Cerastoderma edule from Mundaca Estuary (Biscay,North Spain)

Seasonal determinants of body weight, biochemical composition and reproductive condition in the cockle Cerastoderma edule L. from the Mundaca Estuary, Spain, were performed from December 1983 to July 1985. Interannual differences in timing of gametogenesis and spawning between 1984 and 1985 were correlated with temporal variations in the cycles of storage and utilisation of carbohydrate reserves. Young individuals exhibited larger annual fluctuations in soft-body weight, displaying higher growth rates in spring and greater diminishment of energy reserves during autumn and winter than older individuals. Carbohydrate content displayed a similar tendency, levels being lower in young individuals throughout the year.     

Accumulation of polychlorinated biphenyls in the cockle Cerastoderma edule and the tellin Macoma balthica

Two species of marine bivalve, Cerastoderma edule (Linnaeus) and Macoma balthica (Linnaeus), were exposed to particles contaminated with three commercial polychlorinated biphenyl (PCB) mixtures, Aroclors^® 1242, 1254 and 1260. The accumulation of PCB in bivalve tissue was monitored for periods of up to 40 days. In addition to measuring total PCB concentrations, which reached 60 ppm of Aroclor 1242 in M. balthica, the fate of individual PCB homologues of different chlorine content, ranging from 2 to 8 chlorine atoms per biphenyl molecule, was also determined. Selective accumulation of PCBs with 5 chlorine atoms per biphenyl molecule occurred, whilst isomers of very low and very high molecular weights were accumulated at much slower rates.     

Dynamics of heavy-metal transfer between biotic (Cytheria chione and Cerastoderma edule) and abiotic (water and sediment) components in marine environment (Bay of Martil, Moroccan Mediterranean coast)

This study deals with the transfer of heavy metals (Hg, Cd, Pb, Cr, Ni, Ti, Zn, and Cu) between abiotic and biotic components (Cytheria chione and Cerastoderma edule) in the inshore intertidal zones of Tetouan/Martil over 2 yr of investigation (1992-1994). Analysis of the heavy-metal accumulation kinetics in Cytheria chione and Cerastoderma edule shows that their highest contents occur during the spring-summer period when an important proliferation of the plankton biomass occurs. However, the accumulation of Cd is higher during the period between winter and spring. In general, heavy-metal concentrations are higher at Cytheria chione than at Cerastoderma edule. These results, according to previous studies, suggest the presence of significant correlations and dynamic reciprocal transfer of heavy metals among seawater, sediment, and molluscs. They also suggest that the level of contamination of decreasing heavy metals follows the sequence: sediment, organisms, and water. Among the molluscs investigated, the variability of the thallium (Tl) contents is difficult to detect due to the very weak presence of this metal in the analysed tissues.     

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.     

Occurrence of lysozyme in the common cockle Cerastoderma edule and the effect of the tidal cycle on lysozyme activity

In 1983 a sample of cockles [Cerastoderma edule (L.)] was removed from the mid-tide level at Booterstown Strand, County Dublin, Ireland at 2-h intervals, for a period of 24 h. The gills, mantle and visceral mass tissues contained significant levels of lysozyme, with the gills containing the highest, weight-specific levels. A tidal rhythm of lysozyme activity in the gills, mantle and visceral mass was inversely related to the tidal cycle. The data were best described by a tidal curve shifted-5 hours out of phase, with maximum and minimum levels of activity occurring one hour after low and high tides, respectively. In marine bivalves, lysozyme may be involved in the extracellular digestion and absorption of bacteria during low tide. In this context the present use of bivalves as biological indicators of bacterial pollution is discussed.Constribution No. 6282 from the Woods Hole Oceanographic Institution     

Geographical range and taxonomic divergence in North Atlantic Calanus (C. helgolandicus,C. finmarchicus and C. glacialis)

The known distribution of Calanus helgolandicus in the North Atlantic Drift is difficult to explain in the absence of a reproductively active population inhabiting continental waters off eastern North America. New evidece indicates that this population, overlooked in the past, does in fact exist. The species has been found in a study of zooplankton samples from a number of MARMAP (Marine Resource Monitoring, Assessment, and Prediction Program) cruises surveying ichthyoplankton between Cape Hatteras and the New York Bight. Sexual activity in these stocks of C. helgolandicus was indicated by the ripeness of ovaries, the frequency of males, the presence of sperm in the females' seminal receptacles and the appearance of a female bearing a Calanus spermatophore. The new records provide a likely origin for the presence of the species in the vicinity of the Labrador Grand Banks as well as further east in the North Atlantic Drift. C. finmarchicus, similarly sexually active, was found to be sympatric with C. helgolandicus in the MARMAP collections which were taken from a region contiguous with the southernmost, known distribution, of C. glacialis. The distribution of integumental organs (i.e., pore signature patterns) was examined in the three species to determine whether they would be taxonomically useful. Strikingly different patterns were found on the female urosome. Pore signature differences between the polar species C. glacialis and the temperate C. helgolandicus proved to be as pronounced as those between the boreal C. finmarchicus and its two neighboring species. The successively overlapping ranges and the distinctive differences in pore signature patterns suggest that divergence from the generic pattern of integumental organ distribution has been a product of selection against hybridizing among the three species. If this is in fact the case, the reproductive range of C. helgolandicus has overlapped with those of C. finmarchicus and C. glacialis for appreciable periods in the history of the three species.     

Timing of eclosion in marine insects on Mediterranean shores – studies on Clunio mediterraneus, C. ponticus and Thalassomyia frauenfeldi (Diptera: Chironomidae)

In three Clunio stocks, two from the Mediterranean Sea (Banyuls, Rovinj) and one from the Black Sea (Sosopol), an identical circasemilunar eclosion pattern in combination with a circadian eclosion time of day was evoked in breeding experiments under an artificial moonlight cycle (four nights of 0.3 lux every 30 d, 12 h light:12 h dark). The eclosion peaks occurred on days with artificial moonlight and approximately 2 weeks later, and with regard to the time of day between mid-night and lights-on. In spite of the weak tidal amplitudes on Mediterranean shores, extreme low water levels occur when a nocturnal low-water time coincides on days around full and new moon (spring-tide situation) with winds from land to sea. Field observations at Banyuls and Rovinj agree with the experimental results. In contrast to Clunio spp. adults (life span only a few hours), Thalassomyia frauenfeldi adults (life span up to several days in the laboratory) demonstrated an unsynchronized eclosion pattern in the experiments (no lunar concentration, wide diurnal eclosion gate). Although inhabiting the same intertidal area, T. frauenfeldi walking on exposed intertidal substrates can delay egg-deposition for days and can probably wait for favorable low water levels. The taxonomical status of European Clunio populations is reviewed; minute morphological distinctions are presented for C. mediterraneus in the otherwise morphologically uniform, but ecophysiologically differentiated genus. Received: 15 May 1997 / Accepted: 16 June 1997     

Genetic divergence of Maldivian and Micronesian demes of the damselfishes Stegastes nigricans,Chrysiptera biocellata,C. glauca and C. leucopoma (Pomacentridae)

Samples representing the damselfishes (Perciformes: Pomacentridae) Stegastes nigricans, Chrysiptera biocellata, C. glauca, and C. leucopoma from North Male Atoll, Republic of Maldives, and Guam, Southern Marianas, were surveyed electrophoretically. A sample representing C. leucopoma from Ngemelis Island, Republic of Palau, was included in the analysis to provide a within-Micronesia comparison. On average, products of 23 presumptive protein-coding loci were examined in each sample. In comparisons of Maldivian and Guamanian samples, absolute or virtually fixed allelic differences were detected at: (1) ADA ^* and PEPS ^* for Stegastes nigricans; (2) ADA ^*, AAT-1 ^*, AAT-2 ^*, and mMDH ^* for C. biocellata, and; (3) ADA ^*, AAT-1 ^*, and AAT-2 ^* for C. glauca. At three polymorphic loci, ADA ^*, sMDH-1 ^*, and sMDH-2 ^*, similar allele frequencies were found in Palauan and Guamanian samples representing C. leucopoma, suggesting that these samples share a gene pool. Significantly different allele frequencies at ADA ^* and sMDH-1 ^* and a virtually fixed allelic difference at sMDH-2 ^* were detected between the Maldivian samples and each of the Micronesian samples representing C. leucopoma. The observed patterns of allele frequency differentiation suggest that Maldivian and Micronesian samples of each of the four study species represent separate demes.     

Vertical distributions of Calanus finmarchicus and C. helgolandicus (Crustacea: Copepoda)

The vertical distributions of the spring populations of Calanus finmarchicus (Gunnerus) and C. helgolandicus Claus are described and compared. The differences we observed between the two species have probably confused the understanding of the vertical distribution and development of the populations of Calanus spp. in the shelf seas around the United Kingdom where the species occur together. The results imply that these two congeneric species have different behaviour patterns which minimise interspecific competition where the species have sympatric distributions. C. finmarchicus has its younger development stages overlying the older stages in the water column. In C. helgolandicus the converse is true; i. e., the majority of the populations of Stage I and II copepodites of the first spring generations are found below the thermocline. It is also suggested that the different behaviour patterns lead to different feeding regimes and strategies.JONSDAP Contribution No. 52