Metabolic disposition of lactate in the horseshoe crab Limulus polyphemus and the stone crab Menippe mercenaria

The metabolism of ¹⁴C-labelled lactate was investigated in the horseshoe crab Limulus polyphemus and the stone crab Menippe mercenaria. When a bolus of (¹⁴C-U)-D-lactate was injected into L. polyphemus, there was substantial release pf ¹⁴CO₂ into the medium. In the case of M. mercenaria, ¹⁴CO₂ release was also observed after injection of (¹⁴C-U)-L-lactate into experimental individuals. Analysis of the distribution of radioactivity in whole body extracts of both species revealed easily detectable amounts of radioactivity in the glycogen fraction, although the bulk of the radioactivity was in the cation, anion/neutral and CO₂ fractions. To investigate the metabolism of lactate further, ¹⁴C-labelled lactate was injected into large individuals of L. polyphemus and M. mercenaria, and the distribution of radioactivity was determined in the hemolymph, muscle and hepatopancreas. Utilization of (¹⁴C-U)-D-lactate by L. polyphemus resulted in the accumulation of significant amounts of labelled glucose in all three body compartments as well as the production of labelled glycogen in the telson levator muscle and hepatopancreas. Utilization of (¹⁴C-U)-L-lactate resulted in a similar pattern of glucose and glycogen labelling in the hemolymph, cheliped muscle and hepatopancreas of M. mercenaria. These studies demonstrate that both L. polyphemus and M. mercenaria have the capacity for glyco- and gluconeogenesis using lactate as the substrate.     

Genetics and shell morphology in a hybrid zone between the hard clams Mercenaria mercenaria and M. campechiensis

Populations of Mercenaria mercenaria (L.) from South Carolina, USA, and M. campechiensis (Gmelin) from the Gulf of Mexico, Florida, USA, were sampled in 1987. The two species differed at all of seven enzyme loci tested, as well as in the thickness of shell ridges and nacre color. The difference in lunule shape was not great, although differences in relative shell width, shell weight, and lunule size make morphometric discrimination between the species possible. Shell ridges, nacre color, and multivariate morphometrics in a sample of clams collected from the Indian River Lagoon on the Atlantic coast of Florida in 1985 do not assort independently. Individuals with thick ridges, white nacre, and/or campechiensis-like morphometrics have significantly different allele frequencies at most enzyme loci from individuals with thin ridges, purple nacre, and/or mercenaria-like morphometrics. The deviations are in the direction predicted from the analysis of the allopatric populations of M. mercenaria and M. campechiensis. M. mercenaria outnumber M. campechiensis in the Indian River sample, but the majority of the clams seem to be hybrids.Contribution No. 235 from South Carolina Marine Resources Center     

Genetic variation and covariation during larval and juvenile growth in Mercenaria mercenaria

Quantitative genetic variances and covariances were estimated for shell length of the hard clam Mercenaria mercenaria (L.) at three larval stages (prodissoconch I, 2 d and 10 d post-fertilization) in 1987 and in 1988 after ca. 9 mo of growth. At each sample interval additive genetic variance was a highly significant component of the total size variation. Narrow sense heritability estimates for shell length ranged between 0.58 (±0.10) for prodissoconch I and 1.08 (±0.29) for 2-d-old larvae. There was significant and positive genetic covariance in prodissoconch I and 2-d larval shell length which resulted in a highly significant genetic correlation (r _g=0.74) between these two traits. This covariance is not surprising since the prodissoconch I comprises the majority of the larval shell of a 2-d-old larvae. The genetic covariances between 2-d-old and 10-d-old larvae and between 10-d-old larvae and 9-mo-old juveniles were low and not significantly different from zero. These results indicate that there is substantial genetic variation for shell growth in M. mercenaria but this variation is not stable during development; the genetic variation in shell growth at one stage of development is not strongly related to the genetic variation in growth during other ontogenetic periods. In this study there were no evident constraints to natural selection for increased shell growth rate during development, which coupled with the high levels of genetic variation may suggest that in nature high rates of larval growth may not be normally subject to significant selective pressure.     

Genotype-specific growth of hard clams (genus Mercenaria) in a hybrid zone: variation among habitats

Shell growth rate is an important component of fitness in bivalve molluscs. Using the parameter computed from the von Bertalanffy growth equation, we quantitatively compared rates of annual shell grwoth among the hard clams Mercenaria mercenaria, M. campechiensis, and their hybrids sampled from a variety of habitats in the Indian River lagoon, Florida, USA, a zone of species overlap and natural hybridization. Our results indicate that the classical paradigm describing hard clam growth, in which growth rate is fastest in M. campechiensis, intermediate in hybrids, and slowest in M. mercenaria is not supported in the Indian River lagoon. Instead, M. campechiensis has a growth advantage in deep-water habitats in the northern section of our study area. In the central and southern sections of our study area, hybrids have a growth advantage over M. mercenaria in shallow-water habitats, but M. mercenaria has a growth advantage over hybrids in deep-water habitats. In all other sampled habitats, either growth rate among genotype classes is equal, or M. mercenaria has a growth advantage. This complex relationship between genotype and habitat-specific growth provides a mechanism for selection to act on hard clams in the Indian River.     

Reproduction of the estuarine mysid Taphromysis bowmani (Crustacea: Malacostraca) in fresh water

We have monitored the incorporation of ³H-glycine into, and the excretion of, soluble tissue and extrapallial fluid proteins in the hardshell clam Mercenaria mercenaria in an attempt to follow some of the metabolic events that occur antecedent to shell deposition. After incubating at 20°C for 48 h, clams were killed and the distribution of incorporated and unincorporated tritium in seawater, mantle fluid, hemocoelic-tissue fluid, extrapallial fluid and tissue was determined. Most of the incorporated tritium was in the insoluble tissue proteins. Much more incorporated tritium was found in the hemocoelic-tissue fluid fraction than in the extrapallial fluid. We assume that most of the radioactivity we followed was due to free or incorporated radioactive glycine. The ratio of ³H-protein to ³H-glycine was greater in the extrapallial fluid than in the hemocoelic-tissue fluid, suggesting either protein secretion into or glycine removal from the extrapallium. We also observed that both ³H-protein and ³H-glycine concentrations were higher in the mantle fluid than in the external sea water, although the ratios of ³H-glycine to ³H-protein in these two fluids were not different.     

Clam predation by whelks (Busycon spp.): Experimental tests of the importance of prey size,prey density,and seagrass cover

In 57 l-m² samples within a meadow of Halodule wrightii in Bogue Sound, North Carolina, USA, densities of the clams Mercenaria mercenaria and Chione cancellata were positively associated with seagrass cover. Where seagrass was experimentally removed, marked individuals of both clam species exhibited high rates of mortality in fine sand sediments during two successive experiments spanning 13 months. In the unaltered (control) seagrass meadow, M. mercenaria density remained constant over 13 months and C. cancellata density declined at a slower rate than in the unvegetated plots. Seagrass provides these clams with a refuge from whelk (Busycon carica, B. contrarium, and B. canaliculatum) predation, the major cause of mortality and population decline in experimentally unvegetated plots. In 2 factorial field experiments in unvegetated substratum in which densities of M. mercenaria and C. cancellata were varied independently, first over 5 levels (0 X, 1/2X, 1 X, 2 X, 4 X) and subsequently over 4 levels (0 X, 1/4 X, 1 X, 4 X), there was no repeatable intra- or interspecific effect of density on percent survival, or on the rate of any mortality type. Whelk predation fell preferentially on larger size classes of both species, whereas factors which contribute to clam disappearance usually acted more intensely on smaller sizes. Experimental exclusion of large predators by caging demonstrated that even in unvegetated substratum survivorship of both clam species was high in the absence of whelks and other predators. Individuals of C. cancellata live closer to the sediment surface than those of M. mercenaria, which may explain why seagrass does not serve as effectively to protect them from whelk predation. The mechanism of whelk inhibition may depend upon sediment binding by the H. wrightii root mat, which produces a demonstrable decrease in the physical penetrability of surface sediments.     

Sclerotized protein in the shell matrix of a bivalve mollusc

The epsilon-amino groups of lysine and phenolic groups of tyrosine are most heavily concentrated in the newlydeposited organic matrix of the shell of the bivalve Mercenaria mercenaria. A phenoloxidase enzyme which oxidizes L-dihydroxyphenylalanine is present only in this new area of the shell matrix. Scanning electron micrographs of calcified secretions of the shell show that accretion lines, thought to be layers of organic matrix separating diurnal acceretions of calcium carbonate, are not developed until up to 4 d after deposition of shell material. These results suggest that the shell matrix is hardened by some kind of polymerization, and that lysine and tyrosine residues in the matrix are involved in the process. Accretion lines in polished and etched sections become visible only after complete hardening of the polymer occurs.     

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.     

Turnover of photosynthetically fixed carbon in reef corals

Siderastrea siderea and Montastrea annularis were labeled in situ with NaH¹⁴CO₃. The corals were sampled over a period of 11 days and the radioactivity remaining in the ethanolsoluble and ammonia-soluble fractions measured. Total radioactivity in the corals fell to about 1/3 after one night and then to about 1/3 in the next 10 days. The ethanol-soluble radioactivity is probably converted to the less soluble, ammonia-extractable, material in the dark.Contribution No. 1514 from the University of Miami, Rosenstiel School of Marine and Atmospheric Science, 10 Rickenbacker Causeway, Miami, Florida 33149, USA.     

Assemblage shift following population collapse of a non-indigenous bivalve in an urban lagoon

The quahog Mercenaria mercenaria has been introduced repeatedly to the Pacific coast of North America, but only one population is known to have become established. In the 1970s, the population of M. mercenaria at Colorado Lagoon, in Los Angeles County, California (33^o46′16″N, 118^o08′05″W), was estimated at more than 300,000 individuals. To determine the current status of this non-indigenous species (NIS), in 2009, we sampled 57 intertidal and 20 shallow subtidal plots, identifying and quantifying collected bivalves. No quahogs were found among the 2,490 living bivalves in our plots, though two were found intertidally outside of our plots. The M. mercenaria population has thus collapsed since 1980, but the native community has not recovered. Six of the fourteen living bivalve species we encountered were NIS; three are new records for the location, including the clam Venerupis philippinarum, which made up 87.6% of collected individuals. Though M. mercenaria is likely on its way to extinction on the US Pacific coast, the bivalve assemblage at this location remains heavily dominated by NIS.     

Chromosomal mapping of major ribosomal rRNA genes in the hard clam (<Emphasis Type="Italic">Mercenaria mercenaria</Emphasis>) using fluorescence in situ hybridization

Karyotype and chromosomal location of the major ribosomal RNA genes were studied in the hard clam (Mercenaria mercenaria Linnaeus) using fluorescence in situ hybridization (FISH). Metaphase chromosomes were obtained from early embryos. Internal transcribed spacers (ITS) between major RNA genes were amplified and used as FISH probes. The probes were labeled with digoxigenin-11-dUTP by polymerase chain reaction and detected with fluorescein-labeled anti-digoxigenin antibodies. FISH with the ITS probes produced two to four signals per nucleus or metaphase. M. mercenaria had a haploid number of 19 chromosomes with a karyotype of seven metacentric, four metacentric or submetacentric, seven submetacentric, and one submetacentric or subtelocentric chromosomes (7M + 4M/SM + 7SM + 1SM/ST). Two ITS loci were observed: one located near the centromere on the long arm of Chromosome 10 and the other at the telomere of the short arm of Chromosome 12. FISH signals on Chromosome 10 are strong and consistent, while signals on Chromosome 12 are variable. This study provides the first karyotype and chromosomal assignment of the major RNA genes in M. mercenaria. Similar studies in a wide range of species are needed to understand the role of chromosomal changes in bivalve evolution.     

Mitochondrial gene variation in Mercenaria clam sibling species reveals a relict secondary contact zone in the western Gulf of Mexico

We investigated phylogeographic relationships among American Mercenaria taxa by assessing variation in a 444 nucleotide fragment of the mitochondrial 16S ribosomal gene in clams sampled from four representative sites in January to November 1994. Three of these sites were in the Gulf of Mexico, one was on the Atlantic coast in South Carolina. Direct sequencing of this amplified gene fragment in 85 individuals revealed 21 haplotypes. Phylogenetic analyses consistently resolved this variation into three well supported clades, and within-clade genetic divergence levels were markedly lower than among-clade values. One of the clades, A, was taxon-specific, in that it solely and exclusively contained specimens of M. mercenaria (Linnaeus, 1758) sampled in South Carolina. The other two clades, B and C, were the most divergent and both encompassed specimens of M. campechiensis (Gmelin, 1791) and of M. campechiensis texana (Dall, 1902), sampled from the three Gulf of Mexico sites. Clade B was found at high frequencies at all three Gulf sites, whereas Clade C occurred at low frequencies at two western Gulf sites. We interpret this pattern as resulting from the secondary contact and introgression of two allopatrically differentiated Mercenaria taxa in the western Gulf of Mexico. Clade C haplotypes may represent relict mitochondrial lineages from original Gulf Mercenaria spp. populations that predate massive mitochondrial introgression by M. campechiensis. We further propose that the M. campechiensis texana nuclear genome is a mosaic, heavily weighted toward M. campechiensis, but containing some relict alleles inherited from the precontact population, especially those governing shell characteristics, which may be adaptive in cohesive sediments of bays and estuaries in the northwestern Gulf of Mexico.     

Energy relations of the bivalve Mercenaria mercenaria on an intertidal mudflat

Measurements of the filtration rate and oxygen consumption of the bivalve Mercenaria mercenaria (L.) have been made using flowing water systems, and then combined with growth and survivorship data to construct an energy budget for a tidal-flat population in Southampton Water. Estimated consumption (1292 kcal m^-2 year^-1) is mainly deposited as faeces and pseudofaeces (759 kcal) or excreted (160 kcal); 29% (374 kcal) is assimilated, of which 241 kcal are used for respiration, 72 kcal for flesh production and 61 kcal for gamete production. M. mercenaria contributes significantly to other trophic groups in Southampton Water; predators remove an estimated 55 kcal m^-2 year^-1, and 829 kcal pass to the scavenger/decomposer chain.     

Precipitation of aragonite by calcitic bivalves in Mg-enriched marine waters

To understand the relative importance of biological versus physicochemical control over biomineralization, we have tested if the chemical composition of the medium (i.e., the Mg/Ca ratio) can change the mineralogy of mollusk shells. The shells of mollusks are made of calcite and/or aragonite, which are by far the most common CaCO₃ polymorphs. Several species of bivalves with predominantly calcitic shells have been cultivated in artificial seawater with a Mg/Ca molar ratio within the range of 8.3–9.2, well above the present value for seawater (5.2). Four out of six species used (the scallop Chlamys varia, the oyster Ostrea edulis, the saddle oyster Anomia ephippium and the mussel Mytilus edulis) survived long enough to secrete significant amounts of calcium carbonate. The deposits (sometimes extensive) formed on the interior shell surfaces were predominantly aragonitic. Three individuals of C. varia also increased their length by adding new shell at the margin. Contrary to the internal shell deposits, these margins were high-Mg calcite. This implies that the marginal mantle is able to exert a more strict control on the secreted mineral phase than the mantle facing the internal shell surface. This is the first report on an in vivo experimentally forced switch in bivalve shell mineralogy, from calcite to aragonite due to a change in water chemistry.     

Microsensor studies of photosynthesis and respiration in larger symbiotic foraminifera. I The physico-chemical microenvironment of Marginopora vertebralis, Amphistegina lobifera and Amphisorus hemprichii

 The physico-chemical microenvironment of larger benthic foraminifera was studied with microsensors for O₂, CO₂, pH, Ca²⁺ and scalar irradiance. Under saturating light conditions, the photosynthetic activity of the endosymbiotic algae increased the O₂ up to 183% air saturation and a pH of up to 8.6 was measured at the foraminiferal shell surface. The photosynthetic CO₂ fixation decreased the CO₂ at the shell down to 4.7 μM. In the dark, the respiration of host and symbionts decreased the O₂ level to 91% air saturation and the CO₂ concentration reached up to 12 μM. pH was lowered relative to the ambient seawater pH of 8.2. The endosymbionts responded immediately to changing light conditions, resulting in dynamic changes of O₂, CO₂ and pH at the foraminiferal shell surface during experimentally imposed light–dark cycles. The dynamic concentration changes demonstrated for the first time a fast exchange of metabolic gases through the perforate, hyaline shell of Amphistegina lobifera. A diffusive boundary layer (DBL) limited the solute exchange between the foraminifera and the surrounding water. The DBL reached a thickness of 400–700 μm in stagnant water and was reduced to 100–300 μm under flow conditions. Gross photosynthesis rates were significantly higher under flow conditions (4.7 nmol O₂ cm⁻³ s⁻¹) than in stagnant water (1.6 nmol O₂ cm ⁻³ s⁻¹), whereas net photosynthesis rates were unaffected by flow conditions. The Ca²⁺ microprofiles demonstrated a spatial variation in sites of calcium uptake over the foraminiferal shells. Ca²⁺ gradients at the shell surface showed total Ca²⁺ uptake rates of 0.6 to 4.2 nmol cm⁻² h⁻¹ in A. lobifera and 1.7 to 3.6 nmol cm⁻² h⁻¹ in Marginopora vertebralis. The scattering and reflection of the foraminiferal calcite shell increased the scalar irradiance at the surface up to 205% of the incident irradiance. Transmittance measurements across the calcite shell suggest that the symbionts are shielded from higher light levels, receiving approximately 30% of the incident light for photosynthesis. Received: 6 July 1999 / Accepted: 28 April 2000     

Phytoaccumulation of iron by callus tissue of Clerodendrum indicum (L)

Clerodendrum indicum (L.) is one of the important medicinal plants, originating in India. The plant is used as medicine for bronchitis, asthma and different immunological disease. Micro-propagation is a quick method, producing a huge number of plants. Apical leaf of Clerodendrum indicum (L.) was used as an explant for callogenesis. The combinations of different plant growth regulators (2,4-D, NAA, KN) in MS (Murashige and Skoog) medium produced two types of calli: one was friable and loose and the other was green, embryogenic and compact. Heavy metals cause pollution and health hazards. The plant absorbs heavy metals, which is very useful for controlling environmental pollution. Iron is a major nutrient for both plants and animals, but hyper accumulation of iron is injurious to health. Calli were used to study the optimum conditions of iron uptake in presence of different chemical environments. The different environments were created by using different chemical reagents such as (HCL, HNO₃, H₃PO₄ and KNO₃). Iron (III) was spiked with radioactive Fe-59 and dynamics of uptake was followed by measuring radioactivity by gamma-ray spectrometer. The best condition of iron uptake corresponded to 1 mol L^?1 KNO₃ after 4 hour of equilibration.     

Growth of juvenile Mercenaria mercenaria and the effect of resuspended bottom sediments

The influence of silt on growth of juvenile hard clams Mercenaria mercenaria (L.) (9 mm in mean shell length) was investigated in the laboratory using mixed suspensions of algae (50x10⁶ Pseudoisochrysis paradoxa cells l^-1) and fine-grained bottom sediments (0 to 44 mg l^-1). Growth rates, expressed as percent increase in ash-free dry tissue weight, were not significantly affected by sediment concentrations up to 25 mg l^-1. Significant reduction in growth (by 16% relative to controls fed only algae), and condition of clams, occurred at 44 mg silt l^-1. The results of the 3-week growth experiment agree well with predictions made in an earlier study by integrating results of shortterm physiological measurements. Growth rates obtained with experimental algal-silt diets at 21°C (2.6 to 3.3% increase in dry tissue weight d^-1) were comparable to those determined at ambient concentrations of Great South Bay particulates at 20°C (0.9 to 4.0% d^-1). Levels of particulate inorganic matter in seawater from Great South Bay, New York, exhibited pronounced daily changes, and ranged from 6 to 126 mg dry weight l^-1. Growth enhancement by the addition of silt to an algal diet, reported in mussels, surf clams and oysters, was not found in M. mercenaria. It is suggested that these three species are better suited than hard clams for culturing efforts in inshore turbid waters above uncompacted, muddy bottoms.Contribution No. 452 from the Marine Sciences Research Center, State University of New York at Stony Brook, USA     

Ultrastructural analysis of dissolution of shell of the bivalve Mytilus edulis by the accessory boring organ of the gastropod Urosalpinx cinerea

The shell penetrating mechanism of the muricid gastropod Urosalpinx cinerea follyensis Baker was examined with reference to solubilizing effects of the secretion of the accessory boring organ (ABO) on the ultrastructural organic and mineral components of the shell of the bivalve Mytilus edulis Linné. The fine structure of shell etched by the secretion was contrasted with normal shell and shell solubilized artificially by acids, a chelating agent, and enzymes as an aid in interpreting the pattern etched by the secretion. A synoptic series of scanning electron micrographs of representative regions of the normal shell of M. edulis was prepared to serve as a standard for ultrastructural interpretation of the patterns of dissolution. Intercrystalline conchiolin of the mosaicostracal, prismatic, myostracal, and nacreous strata was dissolved as readily as the periostracum by the ABO secretion. In the prismatic region, maximum depth of dissolution of intercrystalline organic matrix occurred when long axes of prisms were approximately perpendicular to the surface being dissolved. Microscopic solubilization of organic matrix noticeably preceded dissolution of mineral crystals, revealing subsurface prisms and lamellae similar in size and form to the distinctively shaped normal shell units. After solubilization of intercrystalline conchiolin, further dissolution by the ABO secretion revealed a variety of what appeared like internal structures in prisms and lamellae. The form of these subunits varied from that of platelets to nodules in prisms and from laths to nodules in lamellae. These intracrystalline patterns of dissolution probably resulted from preferential etching of (a) soluble intracrystalline conchiolin membranes or other internal aggregates of nacrin, (b) heterogeneously distributed trace and minor elements, or (c) from both. Carriker and Williams (1978) hypothesized that a combination of HCl, chelating agents, and enzymes in a hypertonic mucoid secretion released by the ABO dissolve shell during hole boring. The similarity of patterns of dissolution etched by the ABO secretion and those produced artificially by HCl and EDTA in the present study support the hypothesis that these chemicals, or chemicals similar to them, are constituents of the ABO secretion. Lactic and succinic acids and a chitinase-like enzyme were also suggested by Carriker and Williams as possible agents in shell dissolution. Alteration of the shell surface by experimental application of these, and other, chemical agents was not sufficiently comparable to that etched by the ABO secretion to support the suggestion. Preferential dissolution of shell matrix by the ABO secretion is functionally advantageous to boring gastropods because it increases the surface area of mineral crystals exposed to solubilization and facilitates removal of shell units from the surface of the borehole by the radula.     

Effect of tidal currents,seston, and bottom sediments on growth of Mercenaria mercenaria: results of a field experiment

Tidal currents, seston, and sediments separately influence growth of the hard clam, Mercenaria mercenaria, but it is uncertain how these factors may interact. A 3×3 factorial field experiment, carried out in Great Sound, a coastal lagoon in Southern New Jersey, USA, between May and September 1986, determined the relative effects of three sediment types and three site-specific seston/tidal current regimes on the individual growth of M. mercenaria. Analysis of variance of the change in shell length after 15 wk (differences in initial and final lengths) demonstrated a significant difference (P=0.0064) in growth among sites, but no significant differences (P=0.1331) for growth in different sediments, although trends were evident. Effects of sites were independent of sediment type (P=0.2621). Shell growth rates differed by 10.7% between the slowest and fastest sites, but only differed by 5.7% between sediment types, with fastest growth in sand and slowest in mud. Tidal current speeds and four measures of seston (chlorophyll a, particulate inorganic and organic matter, PIM and POM, and energy content) were measured >20 times in near-bottom waters at each site. Horizontal fluxes of POM exhibited higher correlation coefficients with growth rates, than did seston concentrations or current speeds alone. We attribute significant site differences to differences in horizontal seston fluxes fluxes among sites. We suggest that horizontal seston fluxes may be a major factor affecting individual growth of suspension-feeding bivalves.No. 216 of the Jackson Estuarine Laboratory Contribution Series     

Morphological variation of Mytilus edulis and Mytilus trossulus in eastern Newfoundland

Allopatric populations of Mytilus species show distinct shell morphology which may be due to genetic and/or environmental effects. Sympatric populations of Mytilus species show similar shell morphology which may be due to hybridization eroding morphological differences and/or the influence of common environmental conditions. The present study examined shell morphology and shell shape from 16 sites in eastern Newfoundland where M. edulis L. and M. trossulus Gould coexist in common environments with limited hybridization. Shell morphology was based on measurements of eight characters, and shell shape was quantified by elliptic Fourier analysis of shell outlines. Significant differences were observed between species for both shell morphology and shell shape across 16 sites sampled. The relatively small differences in morphology and shape between the species were probably due to exposure to common environments rather than hybridization. Shell shape for M. edulis was more eccentric compared to M. trossulus which was more elongated. Shell shape analysis of a range of size classes at one site showed a change from an eccentric to an elongated shape going from the smaller to the larger size classes. Both species showed a similar trend, with the larger M. edulis more eccentric and the larger M. trossulus more elongated. Received: 17 July 1998 / Accepted: 6 January 1999