Evolutionary genetics of the Mytilus edulis complex in the North Atlantic region

Genetic relationships among Mytilus populations throughout the North Atlantic region, including the Mediterranean and the Baltic Sea, were studied using enzyme electrophoresis. Three distinct groups of populations, each of a remarkably wide distribution, can be recognised on the basis of their multilocus allelic composition: (1) M. galloprovincialis L. of the Mediterranean and western Europe; (2) a genetically distinct form of M. edulis Lmk. from both the Baltic Sea and some localities in the Canadian Maritime Provinces (here provisionally termed the trossulus type mussel); and (3) the traditional Atlantic M. edulis populations of northwestern European coasts and most of eastern North America. These groups are regarded as representing three relatively old evolutionary lineages, which all deserve separate and equal systematic status. The main part of the differentiation at most of the loci studied is accounted for by this major systematic pattern, but considerable geographical differentiation within each of the three principal groups was also detected. At single loci, different electromorphs were found to prevail in disjunct populations of M. galloprovincialis (Mediterranean/Britain) and of the trossulus-type mussel (Baltic/eastern Canada). Within the Atlantic M. edulis, a major part of the differentiation is transoceanic. At one locus (Ap), geographic differentiation appeared to be relatively independent of the systematic boundaries; the possible role of interlineage hybridisation in contact areas in regulating the pattern of geographical variation is discussed.     

Digestive rhythms in the mussel Mytilus edulis

Mytilus edulis L., collected from a mid-tide level on the shore, showed rhythmic changes in mantle fluid pH, crystalline style pH, style length and total protein, and in the amylase activity in the digestive gland. These changes were correlated with the changes in tidal height. Style size may be related to extracellular digestion in the stomach. Style size and amylase content of the style were not significantly correlated with each other. The changes in amylase activity in the digestive gland confirmed the existence of a tidal rhythm for intracellular digestion in M. edulis.     

Hydrocarbon uptake and loss by the mussel Mytilus edulis

The dynamics of accumulation and elimination of hydrocarbons by the blue mussel Mytilus edulis were studied in a continuous-flow system. Mussels were exposed for as long as 41 days to 200 – 400 g/l of diesel fuel adsorbed on kaolin particles. Hydrocarbons were accumulated in the tissues in excess of 1000 times the exposure levels. Upon termination of dosing, the mussels exhibited a rather rapid loss of hydrocarbons for the first 15 to 20 days (biological half-life=2.7 to 3.5 days). Subsequently, however, elimination was reduced to a minimum and a considerable fraction of the hydrocarbons could be recovered from the tissues after as long as 32 days of depuration. The mussels exhibited definite signs of physiological stress due to chronic exposure to diesel fuel, although recovery was rapid upon termination of dosing. It is concluded that mussels could be utilized as a test organism for monitoring long-term hydrocarbon pollution in marine waters. The implications for the mussel culture industry are discussed.     

Metal interaction during accumulation by the mussel Mytilus edulis planulatus

Mytilus edulis planulatus (Lamarck) were collected from Howden, South-east Tasmania in autumn 1981. Interaction effects of cadmium, copper and zinc during accumulation by mussels exposed for ten days to all three metals simultaneously were examined in a series of experiments in which each metal was tested at three concentrations. In general, interaction effects were most evident at the highest concentrations tested (20 g l^-1 Cd; 20 g l^-1 Cu; 200 g l^-1 Zn) and led to a reduction in the accumulation of cadmium and an increase in that of copper and zinc. More specifically, high levels of zinc caused a decrease in cadmium uptake and an increase in copper accumulation. The presence of copper resulted in depressed cadmium accumulation while zinc accumulation increased. Cadmium tended to enhance zinc accumulation, but copper accumulation was only affected to any great extent when zinc was also present.     

Copper-induced differential mortality in the mussel Mytilus edulis

Copper toxicity experiments were performed with 3 000 mussels (Mytilus edulis L.) from a population that is a mixture of individuals originating in the Kattegat and in the Baltic. Subsequently, the electrophoretic patterns of the enzymes phosphoglucose isomerase (PGI) and phosphogluco mutase (PGM) were analysed in the dead and surviving mussels in order to detect possible differential mortality. Significant differences in allele frequencies between dead and surviving mussels were found for PGI. The results are discussed in relation to genotypic differences in the PGI and in relation to the composite structure of the investigated population. It is concluded that the differential mortality is due to genotypic differences in the PGI locus and/or to different susceptibility to copper by mussels originating in the two geographic areas.     

Seasonal cycles in the carotenoid content in Mytilus edulis

The carotenoid content of Mytilus edulis was studied over a period of 3 years from 1965 to 1967; a seasonal cycle was observed which appeared to be controlled by the spring bloom of phytoplankton, and the sexual cycle of the mussel. Infestation by the copepod Mytilicola intestinalis and hours of exposure in the tidal cycle were found to contribute to variations in carotenoid content. Maturation of the gonads and spawning occurred in the late spring and early summer of the period studied. In addition, a second sexual phase was recorded in October 1967. The possible effect of exposure on the time of spawning is discussed. Lack of food affects both carotenoid content and the maturation of the gonads. M. edulis kept at 10 °C with little food showed no seasonal variation in the carotenoid content when compared with animals fresh from the shore. Lack of food resulted in regression in gonadal tissue after a period of time, and an absence of mature and spawning individuals.     

Bioseston production by Mytilus edulis and its effect in experimental systems

Mytilus edulis L. (Bivalvia: Mollusca) can produce appreciable quantities of 3.4 to 5.4 m diameter bioseston, when previously fed on Phaeodactylum tricornutum and Tetraselmis suecica. Bioseston production, observed in flowing filtered sea water, is shown to originate from resuspension of faeces and can cause filtration-rate underestimates of up to 35% under certain experimental conditions. Feeding with Thalassiosira pseudonana below a concentration of 4x10⁷ particles l^-1 caused no significant bioseston production and this alga would, therefore, be suitable for use in quantitative estimates of filtration rate.     

Accumulation of cadmium and bioenergetics in the mussel Mytilus edulis

Accumulation rates of cadmium, the amount of food ingested and assimilated, the amount of oxygen consumed and changes in dry flesh weight have been measured in Mytilus edulis L. exposed to 0, 10 and 100 ppb cadmium for 17 d in aquaria with seawater flowing continously and at constant algal concentration. The accumulation rates were linear at 10 and 100 ppb, amounting to 0.58 and 8.89 ppm d^-1, respectively. Body loads up to 150 ppm caused no effects on either clearance, ingestion, assimilation, respiration, or growth. High net growth efficiencies between 55–59% were obtained, indicating near optimal experimental conditions. It is suggested that the setup and experimental procedure provide an excellent tool in the study of accumulation and sublethal effects of environmental pollutants in suspension feeding bivalves.     

Ice nucleation in the blue mussel (Mytilus edulis)

The ice nucleation temperatures of different homogenised organs of the mussel Mytilus edulis L. were examined. The stomach and the hemolymph had the highest nucleation temperatures. In the homogenised stomach the nucleation temperatures were fairly constant throughout the year, whereas the nucleation temperatures of the hemolymph increased in the cold season. Bacterial growth experiments, microfiltration, and experiments using antibiotics indicated that the nucleators were not of bacterial origin. The nucleation temperatures of natural seawater were approximately −9 °C, whereas seawater made from distilled water and sea salt had nucleation temperatures of about −17 °C. The nucleation temperatures of the seawater were reduced when the seawater was filtered by the mussels. However, no clear indication that the nucleators in the stomach were obtained from the seawater was found. Stomach homogenates from mussels kept in nucleator-free water had the same supercooling points as stomach homogenates from mussels kept in natural seawater. This indicates that the nucleators in the stomach are not obtained from the seawater. The temperature and light conditions examined in the present study did not significantly influence the hemolymph ice nucleation temperatures of mussels kept in the laboratory. Received: 24 October 1996 / Accepted: 16 November 1996     

Assimilation of 210Po by the mussel Mytilus edulis from the alga Isochrysis galbana

Marine invertebrates are thought to accumulate ²¹⁰Po primarily from their food. In this study, a pulse-chase methodology was used to examine the assimilation and depuration of ²¹⁰Po by Mytilus edulis from the common marine alga Isochrysis galbana. The digestion of ²¹⁰Po from I. galbana occurred via a biphasic process, characteristic of a rapid (extracellular) and slow (intracellular) digestion typical of marine bivalves. The mantle/gill and foot have no known digestive role, yet their ²¹⁰Po specific activities increased after 24 h. It is proposed that this increase in ²¹⁰Po specific activity was related to ²¹⁰Po being incorporated into these tissues from ²¹⁰Po assimilated from I. galbana during extracellular digestion. It is proposed that the linear loss of ²¹⁰Po previously accumulated by control mussels was related to the continual state of renewal and replacement of cellular proteins, with ²¹⁰Po turnover and metabolism governed by protein turnover and metabolism. M. edulis' assimilation efficiency of ²¹⁰Po from the ²¹⁰Po-labelled alga was calculated to be 17.2 ± 2.1%, and thus similar to that of Ag, Cd, Co, Se and Zn by bivalves from other marine algae species. It is proposed that the assimilation efficiency of ²¹⁰Po is a function of protein assimilation. Received: 27 August 1998 / Accepted: 3 September 1999     

Clinal variation and heterozygote deficit at the lap-locus in Mytilus edulis

Electrophoretic investigations of Mytilus edulis populations on the New York and Connecticut coasts of Long Island Sound (USA) revealed a cline in the frequency of the Lap ⁹⁴-allele at a 20 km zone at the entrance to the estuary. Significant deficits of heterozygotes were found in the region of the cline and are discussed in terms of a model of mixing between oceanic mussel populations of high gene frequency and estuarine populations of low frequency. The pattern of estuarine circulation is assumed to provide a mechanical basis for isolation between oceanic and estuarine populations and to determine the position of the gene frequency cline.     

Oogenesis in the marine mussel Mytilus edulis: an ultrastructural study

Ultrastructural changes occurring during the course of development in oocytes of Mytilus edulis are described for mussels collected at monthly intervals over a period of one year (September 1981 to October 1982) from a site in Cornwall, England. During early stages of oogenesis the oocyte is surrounded by a small number of follicle cells but, as development proceeds, the follicle cells are restricted to the stalk region which attaches the oocyte to the acinar wall. Contact between the follicle cells and the developing oocyte is maintained by means of desmosomelike gap junctions. Organelles and inclusion bodies present in the ooplasm during oogenesis include rough endoplasmic reticulum (RER), Golgi bodies, mitochondria, free ribosomes, Balbiani's vitelline body, annulate lamellae and yolk and cortical granules. The RER, in particular, varies considerably throughout the course of development. Evidence for uptake of exogenous macromolecules into oocytes by pinocytosis is presented; it occurs in the basal region of previtellogenic oocytes prior to the formation of the vittelline coat. Lipid-yolk granules invariably have mitochondria in close association and, during the winter months, develop in close proximity to small, apparently glycogen-rich vesicles possibly suggesting that conversion of glycogen to lipid takes place in developing oocytes. Oocyte degeneration was commonly observed and involves initial breakdown of the plasma membrane followed by rupture of the vitelline coat. The oocyte contents once released into the acinar lumen are resorbed by the epithelial cells of the gonoducts, which are prevalent throughout the mantle of ripe individuals.     

Feeding and cleaning mechanisms in the suspension feeding bivalve Mytilus edulis

Feeding and pseudofeces formation were studied in intact Mytilus edulis clearing suspensions of graphite particles, and the processes were compared with activities observed in mussels with a severed adductor muscle. Most particulate material in the stomach was present in the suspended state. Ingestion of particles in suspension could take place concurrently with the production of pseudofeces. Severing of the adductor muscle resulted in profuse secretion of mucus that gradually subsided to a constant low level. Addition of graphite particles at concentrations that did not cause mucus secretion in the intact mussels strongly stimulated secretion in the operated mussels, resulting in the accumulation of highly viscid, mucusparticle aggregates. These aggregates were mechanically stable, in contrast to the fragile pseudofeces. It is indicated that normal feeding depends upon hydromechanical mechanisms that produce highly concentrated suspensions of particles for ingestion, and that mucociliary mechanisms serve to clean the gills and other organs of the mantle cavity for excess particulate material. It is further indicated that intact mussels secrete mucus only in the amounts needed to consolidate excess particulate material, and that lesions affect the normal balance between particles in suspension and mucus secretion by enhancing the sensitivity of the mechanisms that control mucus secretion.     

Genetic and nutritional control of the gametogenic cycle in Mytilus edulis

Individuals of Mytilus edulis were collected from two populations in easterm Long Island Sound, USA, at 6 wk intervals from January 1982 to February 1983, and the hypothesis that nutrient availability controls the timing of gametogenic events in this mussel was tested using genetic variants at the Lap locus. In eastern Long Island Sound, Lap genotypes differ in the net rate of nitrogen accumulation. A highly significant difference in reproductive cycle among genotypes occurred in a population of M. edulis where Lap genotypes differ in nitrogen budget. Genotypes with lower rates of nutrient accumulation delayed the reproductive cycle by approximately six weeks relative to genotypes with high rates of nutrient accumulation. No significant difference in reproductive cycle occurred at a locality where Lap genotypes do not differ in nitrogen budget. These data indicate that nutritional status can exert significant control over the timing of gametogenesis in M. edulis. Differences in reproductive cycle among Lap genotypes provide a mechanism of non-random mating among genotypes which may result in heterozygote deficiencies at this locus.     

Antigenic characterization of hemocyte subpopulations in the mussel Mytilus edulis by means of monoclonal antibodies

In order to characterize in more detail hemocytes from Mytilus edulis and establish links between histological and cytological featues, monoclonal antibodies (MABs) were prepared using hybridoma technology. Five MABs were identified and their reactivity patterns were analyzed with both immunoperoxidase and ultrastructural immunogold labelling. Four MAB classes were identified by their immunostaining patterns. Optical and ultrastructural examination revealed that Class I MAB reacted specifically with basophilic granulocytes (granulocytes containing small granules). Class II and III MABs differed in the intensity of their indirect immunofluorescence and degree of immunoperoxidase labelling, but all recognized both basophilic and eosinophilic granulocytes. Epitopes recognized by Class IV MABs were not always present in basophilic granulocytes, whereas they were always expressed in the eosinophilic granulocytes (granulocytes containing large granules) and could constitute differentiation antigens which progressively form during the maturation process. Molecular weights of the proteins recognized by these MABs were determined by Western blotting. It is concluded that immunostaining can identify at least three hemocyte types in M. edulis.     

Resource allocation in Mytilus edulis on the shore and in suspended culture

The annual cycle of carbon and nitrogen content of the flesh of wild and cultivated mussels (Mytilus edulis L.) in Killary Harbour, Ireland, was measured over two years, starting in February, 1980 and ending in November, 1981. The carbon and nitrogen contents of mussel gametes were determined and the allometry of growth of wild and cultured mussel shells was examined with respect to length, weight and organic content. The carbon and nitrogen contents of the organic fraction of the shell were determined. These data were combined with those we had previously published on growth rate, gametogenesis and the annual cycle of ash-free dry weight (AFDW) of mussels in the same locality. Estimates were made of fecundity, reproductive effort and the partitioning of carbon and nitrogen between soma, gametes and shell. In suspended culture, cumulative production after eighteen months is equal to cumulative production after six to seven years on the shore. For comparison, partitioning of carbon and nitrogen resources between soma, shell and gonad is estimated when total cumulative production by wild and cultivated mussels is approximately equal. Differences in resource allocation are considerable. Wild mussels allocate some 57% of their carbon budget and 52% of their nitrogen budget to gamete output. In culture, mussels allocate only 22% of their carbon budget and 19% of their nitrogen budget to gamete output. It is concluded that in response to a higher production rate, cultivated mussels increase allocation of resources to somatic growth.     

Ecological gradients and relative abundance of native (Mytilus trossulus) and invasive (Mytilus galloprovincialis) blue mussels in the California hybrid zone

Marine communities are experiencing unprecedented rates of species homogenization due to the increasing success of invasive species, but little is known about the mechanisms that allow a species to invade and persist in a new habitat. In central California, native (Mytilus trossulus Gould 1850) and invasive (Mytilus galloprovincialis Lamarck 1819) blue mussels and their hybrids co-exist, providing an opportunity to analyze the mechanisms that determine the distributions of these taxa. Spatial and temporal variation in temperature and salinity and the relative frequencies of these mussel taxa were examined between 2000 and 2004 at four sites in San Francisco Bay and four in Monterey Bay, which were chosen for their different positions along inferred estuarine/oceanic gradients in the hybrid zone. Mussels were genetically identified as the parent species or hybrids by amplifying regions of two species-specific loci: the adhesive byssal thread protein (Glu-5′) and the internal transcribed spacer region of ribosomal DNA (ITS 1). The proportion of M. trossulus at the eight hybrid zone sites correlated negatively with average salinity (R ²=0.60) and positively with maximal temperature (R ²≥0.72), a somewhat unexpected result given what is known about the phylogeography of this species. The proportion of M. galloprovincialis showed the opposite pattern. The proportion of hybrids was correlated neither with habitat temperature nor salinity. Genotypes of mussel populations at an additional 13 sites from Coos Bay, Oregon (latitude 43.35°N) to Long Beach, California (latitude 33.72°N), sampled at various intervals between 2000 and 2004, were also determined. This survey confirmed previous reports that the hybrid zone lies between Monterey and the Cape Mendocino region (latitudes 36.63°N–40.5°N). Within Monterey and San Francisco Bays, however, the temporal comparisons (1990s vs. 2000s) revealed abrupt changes in the proportions of the two parent species and their hybrids on annual and decadal time scales. These changes indicate that the blue mussel populations are in a highly dynamic state. The survey also showed that, regardless of habitat, M. trossulus is consistently of smaller average size than either M. galloprovincialis or hybrids.

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Induced defensive responses by the bivalve Mytilus edulis to predators with different attack modes

Predators such as crabs, whelks, and sea stars attack their bivalve prey in different ways, and predator-induced defenses are an important means of protection. The degree to which induced defenses are specific to different predators, however, remains largely unknown. In laboratory experiments (June to August 1998), we raised mussels (Mytilus edulis L.) in the presence of a drilling predator [the whelk Nucella lapillus (L.)] or a crushing predator [the crab Carcinus maenas (L.)] to determine whether induced changes in prey shell thickness, size, or shape occurred and whether changes were predator-specific. Over a 2 month period, juvenile mussels were exposed to waterborne cues from actively feeding crabs or whelks. Mussels produced thicker shell lips in response to both predators relative to control mussels raised in their absence, and the difference was significantly greater in response to whelks than to crabs. Mussels exposed to whelks showed significantly smaller increases in shell length and width and total wet weight than did mussels exposed to crabs. Thus, there may be a trade-off between shell thickness and linear shell growth and a potential delay in attaining a size refuge from predation. Received: 4 August 1999 / Accepted: 31 January 2000     

Effects of salinity on the net uptake of zinc by the common mussel Mytilus edulis

The net uptake of zinc by the common mussel Mytilus edulis (L.) has been investigated under different natural and artificial salinity stresses. The effects of stable and fluctuating salinities on the uptake of zinc by the mussel are discussed in terms of three possible modes of action. Under certain highly-stressful conditions, salinity may affect the uptake of zinc by the mussel. This factor should be considered when the mussel is used as an indicator of environmental pollution by zinc in estuarine areas, or spurious conclusions may result.