The role of disturbance in the evolution of life history strategies in the intertidal mussels Mytilus edulis and Mytilus californianus

Summary The intertidal mussels Mytilus edulis and M. californianus compete for space on the west coast of North America. An analysis of differences in size, growth rate, age at first reproduction, life span, mode of reproduction, niche and habitats between these two species demonstrates that their life history strategies are different in several important ways. M. californianus is a larger, sturdier, slower growing mussel which has effective predator-deterring mechanisms and is an overall superior competitor for space in the intertidal. Its reproductive strategy is based on continually spawning at a very low level throughout a yearly cycle. M. edulis is a classic fugitive species which rarely attains a large size but matures early and is characterized by a single massive reproductive output each year. The differences noted in their life history strategies are very likely the result of either a) a pre-adaptation for coexistence, or b) a competitively-induced regional evolutionary divergence.In Washington a band of small M. edulis (removed from most predation and competition) exists in a high intertidal spatial refuge, and contributes relatively little to the gene pool. Larger individuals exist in disturbed areas lower in the intertidal zone. These lower zones represent a relatively high risk, and usually only temporary, habitat for M. edulis because of mortality due to competition and predation. Individuals, however, colonizing these two intertidal patches contribute 43 times as much as their high intertidal counterparts in terms of reproductive fitness (calculated on a populationwide basis).     

A new gene family of single fibrinogen domain lectins in Mytilus

In molluscs haemolymph lectins bearing ?brinogen-like domain (FREP) act as immune pattern-recognition receptors. A full-length cDNAs of MytFREP1 and MytFREP2 cloned from haemocytes of blue mussel Mytilus edulis encoded putative polypeptides of 230 and 241 amino acids. Both polypeptides consist of signal peptide and C-terminal fibrinogen-like domain. Immune functions of these molecules may be extrapolated from the close-related and functionally characterized lectin AiFREP from bay scallop, Argopecten irradians. However, immune challenge experiments with zymosan particles, Escherichia coli bacterium and cercariae of Himasthla elongata (Trematoda) failed to modulate MytFREP1 and MytFREP2 mRNA expression in M. edulis haemocytes. Hypothetically, it argues into rather high specificity of mechanisms triggering a differential expression of MytFREP genes. The search in the EST database revealed orthologous copies for described genes and portion of relatively similar genes from two close-related mytilids, Mytilus galloprovincialis and Mytilus californianus. We document the new multigene family of FREPs from bivalves of genus Mytilus. MytFREP family currently represented by 2 genes from M. edulis, 4 genes from M. californianus and 7 genes from M. galloprovincialis.     

Long-term trends in algal-dominated rocky subtidal communities on the Swedish west coast — a transitional system?

Rocky subtidal communities on the Swedish west coast were monitored over approximately 16 years (1970–1985) by means of stereophotographic recordings, at intervals, of fixed sites.Dominating elements of community structure were studied at the 5- and 10 m depth levels of an outer archipelago locality. Excluding seasonal variations, the basic community structure, largely dominated by the perennial phaeophycean Halidrys siliquosa (L.), remained relatively stable over the first 7–8 years of study, but with an increasing trend in cover of filamentous epiphytic red algae. Later, the structure of the studied communities changed radically. The changes initially involved intense settling of the blue mussel Mytilus edulis L. and reduced cover of red algae. The mussels settled largely on Halidrys plants which eventually broke down almost completely. Later, there was intensive predation on Mytilus by the starfish Asterias rubens Linné. After the elimination of Mytilus, the studied test areas were dominated by filamentous ephemeral red algae for the rest of the study period, but with some regeneration of Halidrys at the 5 m level.The observed changes were thought to be indicative of a major disturbance in the coastal ecosystems off the Swedish west coast during the studied period. This notion is discussed in relation to studies of other components of the ecosystem during the same time period and in relation to possible causes. Among the latter may be mentioned a suspected eutrophication process, long-term temperature anomalies and resulting effects on biological interrelationships such as competition, predation and chemical defense.The observed trends included a shift in dominance from slow-growing perennial algae to fast-growing ephemeral algae. If this shift was a general phenomenon, for which there is some circumstantial evidence, it could have important repercussions on the cycling of nutrients and organic matter in the coastal ecosystem. Possible effects are discussed on hypothetical terms in relation to observed anomalies in phytoplankton blooms and benthic oxygenation.     

Within- and between-patch variability of predation intensity on the mussel Mytilus trossulus Gould on a rocky intertidal shore in Oregon, USA

Variability of predation intensity is an important cause of spatial differences of community structure and organization in the intertidal rocky shore. Field experiments were conducted to evaluate the within- and between-patch variability of the effects of different types of predators (small invertebrates and birds) on Mytilus trossulus Gould, which occupies an intermediate position in the competitive hierarchy among sessile organisms in disturbance patches within a California mussel (Mytilus californianus Conrad) bed community on the central Oregon Coast. Predation by birds did not significantly affect the mortality of M. trossulus. On the contrary, predation by small invertebrate whelks (Nucella spp.) had a significant effect on M. trossulus mortality. Predation by whelks also caused between- and within-patch variability of mortality of M. trossulus. Within patches, M. trossulus mortality at patch margins was significantly higher than at patch centers only when invertebrate predators were present. Wave exposure did not cause between-patch variability of predation intensity. Between-and within-patch variability of predation intensity may be caused by the variability of supply of whelks from the surrounding mussel mat. The movement of predators between patches and surrounding matrices may play an important role in the patch dynamics of these communities.     

Larval settlement can explain the adult distribution of Mytilus californianus Conrad but not of M. galloprovincialis Lamarck or M. trossulus Gould in Moss Landing, central California: Evidence from genetic identification of spat

We investigated the spatial distribution of adult and newly settled mussels (Mytilus galloprovincialis Lamarck, Mytilus trossulus Gould and Mytilus californianus Conrad) on the shore at Moss Landing, California to test the hypothesis that adult distributions are a result of settlement patterns. Adult M. californianus were most abundant on a wave-exposed rocky jetty and adults of Blue mussels (M. trossulus and M. galloprovincialis) were more abundant inside the protected Moss Landing harbor. Using taxon-specific polymerase chain reactions, we monitored recruitment during continuous 1-2 week intervals on fibrous scrubbing pads for 12 months in 2002-2003. All mussel species settled in greatest numbers on the exposed jetty, and Blue mussels settled in greater numbers there than did M. californianus. Because Blue mussels settled abundantly where their adults were rare, post-settlement mortality appeared to be the strongest influence on adult distribution. In contrast, M. californianus settled mostly in their adult habitat.     

Lift as a mechanism of patch initiation in mussel beds

The mussel Mytilus californianus is the dominant competitor for space in the mid-intertidal zone of wave-swept rocky shores in the Pacific Northwest where it forms extensive tightly packed beds. The rate at which patches are formed in these beds, can play an important rôle in community ecology by controlling the establishment and persistence of fugitive species. Despite the biological importance of physical disturbance, the mechanism of patch initiation has not been adequately explained. Battering by logs can create patches, but is the predominant mechanism only on shores near active logging sites. In other areas, it has been speculated that the hydrodynamic forces associated with storm waves somehow cause patches to form. However, the forces acting along the direction of flow — drag and the acceleration reaction — are unlikely to initiate patch formation. Here, it is suggested that fluid-dynamic lift forces imposed on mussel beds by breaking waves are sufficient to dislodge individual mussels and trigger patch formation. Arguments are presented suggesting that the likelihood of dislodgment by lift is consistent with the observed rate of patch formation in the absence of log battering.     

Growth rate as a measure of food value in thaidid gastropods: Assumptions and implications for prey morphology and distribution

Rates of body growth were measured for three species of rocky intertidal gastropods: Thais (or Nucella) lamellosa (Gmelin), T. (or N.) canaliculata (Duclos), and T. (or N.) emarginata (Deshayes). Three size classes of each predator species were grown experimentally in cages at two tidal heights on four size classes of each prey species: Semibalanus cariosus (Pallas), Balanus glandula Darwin, and Mytilus edulis L. Growth was also assessed for Thais canaliculata feeding on Mytilus californianus Conrad, and Thais emarginata feeding on Chthamalus dalli Pilsbry.Rates of body growth varied as a function of predator size, prey size and prey species. With one exception (large Thais canaliculata) intermediate sized Balanus glandula promoted the most rapid growth for all sizes of all three species of Thais; thus these potentially competing predator species have the same highest ranked prey. Among prey promoting slower growth, those encountered commonly in the normal habitats of the predator promoted more rapid growth than those encountered rarely, suggesting that past evolutionary experience has influenced present food value (= growth potential) of prey. For some predators, the ranking of prey species changed with predator size; Chthamalus dalli and similar sized Balanus glandula promoted comparable growth in small Thais emarginata but Chthamalus dalli promoted much slower growth than similar sized Balanus glandula in larger Thais emarginata. Rank differences based on prey size in Balanusglandula became more pronounced and in some instances changed with increasing predator size; larger B. glandula promoted relatively faster growth for large snails than for small snails in all Thais species. Finally, growth rates were correlated with two important attributes of fitness in T. canaliculata and T. emarginata; in general, prey promoting more rapid growth also resulted in an earlier age of first reproduction for initially immature snails and higher rate of egg capsule production for mature individuals. Thus, these growth rates provide a basis from which to examine quantitatively patterns of prey selection from an energy- or growth-maximization perspective.These patterns of predator growth also permit inferences about the possible influence of predation on the evolution of prey morphology, life-history, and microhabitat distribution. Size refuges from Thais predation were confirmed for both Semibalanus cariosus and Mytilus californianus. Both Balanus glandula and Mytilus edulis, on the other hand, were vulnerable to predation by all sizes of Thais examined (> 10 mm) regardless of prey size; thus, neither of these species achieves a size refuge. In addition, on rocky shores, vertical distributions of the prey species reflect general preference patterns of their predators: higher value prey species, i.e. those with less well developed defensive morphologies (Balanus glandula, Mytilus edulis), generally occur higher on the shore. This pattern would be expected if preferred prey are consumed first lower on the shore where they are more available.     

Hybridisations between Mytilus edulis and Mytilus galloprovincialis and performance of pure species and hybrid veliger larvae at different temperatures

The mussels Mytilus edulis L. and Mytilus galloprovincialis Lamark hybridise naturally in the wild along the Atlantic coast of Europe producing a patchwork of mixed pure species and hybrid populations. Individuals of both species were spawned in the laboratory and were hybridised in a series of reciprocal crosses. After 72 h, the proportion of eggs which developed into larvae (%yield) and the proportion of those larvae which had a normal veliger morphology (%normality) were estimated and compared between pure species and hybrid families. There were no significant differences in %yield or %normality between pure species and hybrids, but significant differences were evident between the offspring from different parents irrespective of whether they were hybrids or pure species. Therefore confirmation of hybrid heterosis in laboratory studies should not be based on a single, or a few reciprocal crosses. Hybrid and pure species veliger larvae were grown for approximately 4 weeks at 10, 14 or 20 °C. In all trials, pure M. galloprovincialis larvae grew significantly faster at 20 °C than either reciprocal hybrid or pure M. edulis larvae. Irrespective of temperature, in general, hybrid larvae grew slower than larvae of either pure species. Increased exposure to planktonic predation due to slow growth can be interpreted as selection against hybrids and this may play a role in the structure and distribution of mixed pure species and hybrid populations.     

Competition as a determinant of the upper limit of subtidal kelp Ecklonia stolonifera Okamura in the southern coast of Korea

Experiments in the subtidal zone of the southeastern coast of Korea examined the role of competition in determining the upper limit of perennial kelp, Ecklonia stolonifera, which is a dominant species in the subtidal community, but rarely found shallower than 5 m depth. Replicate clearings of dense algal turfs simulating natural disturbance at 1 and 3 m depth zones showed that E. stolonifera is able to settle and develop adult sporophytes in the upper zone. However, these adult sporophytes were eventually excluded from the clearings within one year after clearing. Two-year succession patterns at the clearings fell into three sequential categories: E. stolonifera and polychaete worms (Serpula vermicularis Linnaeus), mussels (Mytilus edulis Linnaeus), and turf-forming algae. A competitor removal treatment revealed that competition with M. edulis for primary space is the direct cause resulting in exclusion of E. stolonifera from the clearings. Recolonization of E. stolonifera after exclusion did not occur at clearings due to preemptive competition with other sessile organisms, which occupied 75-99% of bottom cover during the reproduction period of E. stolonifera (Autumn). Algal turfs, the final colonizer of the clearings, recovered more than 80% of bottom cover within 10 months after exclusion of M. edulis, and their species composition and abundance were not significantly different to those of adjacent controls. Experiments using artificial substrata showed that recruitment of E. stolonifera from spores is essentially impossible on the plates occupied by other sessile organisms (>86% cover). These results indicate that preemptive competition with dense algal turfs for the settlement and competition with M. edulis for survival in the course of succession are two most important factors determining the vertical distribution limits of E. stolonifera.     

Development of a real-time PCR assay for detection of Mytilus species specific alleles: Application to a sampling survey in Scotland

Shellfish aquaculture is a growing industry in Scotland, dominated by the production of the mussel Mytilus edulis, the native species. Recently the discovery of Mytilus galloprovincialis and Mytilus trossulus together with M. edulis and all 3 hybrids in cultivation in some Scottish sea lochs led to questions regarding the distribution of mussel species in Scotland. The establishment of an extensive sampling survey, involving the collection of mussels at 34 intertidal sites and 10 marinas around Scotland, motivated the development of a high-throughput method for identification of Mytilus alleles from samples. Three Taqman®-MGB probes and one set of primers were designed, based on the previously described Me 15/16 primers targeting the adhesive protein gene sequence, and samples were screened for the presence of M. edulis, M. galloprovincialis and M. trossulus alleles using real-time PCR. Mytilus edulis alleles were identified in samples from all 44 sites. Mytilus galloprovincialis alleles were found together with M. edulis alleles extensively in northern parts of the west and east coasts. Mytilus trossulus alleles were identified in samples from 6 sites in the west and south-west of Scotland. Because M. trossulus is generally undesirable in cultivation and therefore preventing the geographical spread of this species across Scotland is considered beneficial by the shellfish aquaculture industry, these 6 samples were further analysed for genotype frequencies using conventional PCR. Although distribution of the non-native species M. galloprovincialis and M. trossulus have proven to be more widespread than previously thought, there is no evidence from our study of either M. trossulus or M. galloprovincialis acting as an invasive species in Scotland. The real-time PCR method developed in this study has proven to be a rapid and effective tool for the identification of M. edulis, M. galloprovincialis and M. trossulus alleles from samples and should prove useful in future surveys, ecological or aquaculture management related studies in both unispecific and mixed species areas of these species.     

Mechanisms of successional dynamics: Consumers and the rise and fall of species dominance

Insight into potential mechanisms of succession following disturbance to an ecological community can be gained by considering processes that contribute to the rise (colonization, interactions with established species) and demise (differential mortality) of specific stages within the successional sequence. Most successional theories focus on the rise to dominance, assuming demise is the result of competition, but other factors can cause differential mortality among species, including physical disturbance, senescence, and consumers. In rocky intertidal communities on the coast of Washington state, USA, gaps in mussel beds exhibit a succession from predator-susceptible to predator-resistant species following disturbance, suggesting that differential consumption by mobile species may play an important role in the demise of early succession species and the eventual dominance of the mussel Mytilus californianus. Experimental manipulation of a dominant species earlier in succession, the blue mussel Mytilus trossulus, demonstrated that this species inhibits the invasion of M. californianus in the absence of predators. Experimental manipulation of predatory snails (Nucella emarginata and Nucella canaliculata), which feed heavily on M. trossulus but not M. californianus, greatly increased the rate at which M. californianus invaded gaps. These results and those of other studies indicate that consumers frequently have important effects on the dynamics of succession in benthic marine systems, and might also play a role in other settings.     

Hemolymph functions in Mytilus californianus: The cytochemistry of hemocytes and their responses to foreign implants and hemolymph factors in phagocytosis

The hemocytes of Mytilus californianus are of three types: small and large basophils and large granular acidophils. The basophils contain lysosomal enzymes and phagocytose colloidal carbon. Agglutinins for yeast and human A Rh+ve erythrocytes are present in plasma, but are not needed for effective phagocytosis; in vitro both acidophilic and basophilic hemocytes rapidly phagocytose these particles. Plasma proteins, analyzed electrophoretically, are under strong homeostatic control. When Mya arenaria mantle is placed orthotopically on M. californianus mantle, the implant is invaded by host hemocytes in a manner consistent with that described in other published reports on molluscan graft rejection. Steady state is achieved by 26 days postimplant. Second- and third-set implants are rejected more rapidly than are first-set implants, but this is not a specific response. Third-set implants elicit a host cellular response that is more localized than the response to first-set implants. These data do not permit conclusions with respect to memory in these molluscan immune responses, but do imply a qualitative “improvement” in this quasi-immune response of M. californianus.     

Estuarine habitats protect hybrid mussels from selection

The marine mussels, Mytilus edulis and Mytilus galloprovincialis, form an extensive hybrid zone in Europe where F₂ hybrids and mussels of mixed genetic ancestry are often locally abundant. Hybrid zones are maintained by the interplay of dispersal and selection on hybrid genotypes but there has been vigorous debate on the form of selection that may occur in these systems. Tension zone models argue that selection is against hybrids because of developmental misregulation and is independent of the external environment. Exogenous selection models argue that selection is habitat-dependent and the structure of the hybrid zone is due to the distribution of habitat patches that vary in selection intensity. We test this prediction by comparing the genetic structure of mussel populations in open-coast habitats, where selection on hybrids is strong, to those within two independent estuaries. We show that mussels within these estuaries are protected from selection and thus selection is strongly dependent on habitat, which supports the exogenous selection hypothesis. Hybrid mussel populations on the open-coast experience strong selection against M. edulis-like genotypes, which has been postulated to be the result of differential dislodgment by waves. This hypothesis is supported by our results since mussels within sheltered habitats are protected from selection. There was, despite previous suggestions, no evidence of selection in favor of M. edulis-like mussels within either estuary.     

Mytilus galloprovincialis Lmk. in Southern Africa

The Mediterranean mussel, Mytilus galloprovincialis Lmk. has been identified on the west coast of southern Africa using morphological and biochemical-genetic comparisons with samples of “pure” M. edulis L. from Denmark and M. galloprovincialis from the Mediterranean coast of Spain. Our results show that the southern African Mytilus more closely resembles M. galloprovincialis in anterior adductor, posterior adductor and hinge size. It also has a more ventrally located maximal shell width like the Mediterranean mussel. The electrophoretic examination of 23 proteins showed that many alleles diagnostic for M. galloprovincialis appeared in the southern African Mytilus. Nei's genetic distance between the southern African Mytilus and M. galloprovincialis was 0.010 ± 0.004 and the average genetic distance between these samples and M. edulis was 0.162 ± 0.077.The presence of M. galloprovincialis in southern Africa could be explained by biogeographic dispersal along the west coast of Africa during Pleistocene cooling, or by a recent inadvertant or intentional introduction by man. To test these two hypotheses, we examined ≈750 mussel shells collected from Koi-san middens and ≈350 shells from a raised-beach deposit all located on the west coast of Cape Province, South Africa. The ages of these deposits range from 1500 yr to 120000 yr and pre-date the arrival of Europeans to the Cape. This examination did not reveal any shells of Mytilus. We, therefore, conclude that the introduction of M. galloprovincialis to southern Africa was a recent event. We suggest that, since the first confirmed reference to Mytilus in southern Africa was made from a 1972 collection, the introduction occurred within the last two decades.     

Mussel glue fromMytilus californianus Conrad: a comparative study

Summary Marine mussels secrete a byssus in order to attach to solid surfaces in the sea. The polyphenolic protein is the glue in the adhesive plaques of the byssus. InMytilus californianus, the polyphenolic protein has an apparent molecular weight of 85,000±5,000 and is rich in the amino acids lysine, 3,4-dihydroxyphenylalanine, serine, threonine, and hydroxyproline. In composition it resembles the polyphenolic protein ofM. edulis (M _r=125,000), although theM. edulis protein contains significantly less isoleucine and more alanine. Tryptic digestion ofM. californianus polyphenolic protein revealed two types of repeating decapeptides (1) (Ser/Thr)-Thr-(Tyr/Dopa)-Hyp-Hyp-Thr-Dopa-Lys-Hyp-Lys and (2) Ile-(Thr/Ser)-(Tyr/Dopa)-Hyp-Hyp-Thr-Dopa-Lys-Hyp-Lys. Residues 2 to 8 are identical with residues 4–10 inM. edulis decapeptides.Abbreviation Dopa 3,4hydroxyphenylalanine     

Vulnerability of newly settled plaice (Pleuronectes platessa L.) to predation: effects of habitat structure and predator functional response

Plaice (Pleuronectes platessa) nursery grounds on the Swedish west coast have been subject to increasing cover of annual green macroalgae during recent years, with growth of algae starting at the time of plaice settlement in April to May. A laboratory experiment was performed to investigate how the vulnerability to predation of metamorphosing plaice was affected by the presence of filamentous algae. Predation by shrimps (Crangon crangon) on settling plaice larvae was higher on sand than among algae, whereas predation by crabs (Carcinus maenas) was unaffected by habitat type, suggesting a lower overall mortality of plaice in the vegetated habitat. When predators and prey were presented with a combination of the two habitats, predation by shrimps was as high as that in the sand treatment alone, whereas predation by crabs was lower than that in the two treatments with one habitat. Based on these results, an additional experiment was performed, investigating the functional response of shrimps to six densities of juvenile plaice in a sand habitat with alternative prey present. The proportional mortality of juvenile plaice (12-16 mm total length (TL)) was density-dependent and was best described by a type III (sigmoid) functional response of the predatory shrimps. The results suggested that the combined predation pressure from shrimps and crabs was lower among algae than on sand, but settling plaice and predatory shrimps chose the sand habitat. Plaice densities in the sigmoid part of the obtained functional response curve represented normal to high field densities of plaice on the Swedish west coast, suggesting that shrimp predation could have a stabilising effect on plaice recruitment. The formation of macroalgae mats could therefore lead to a concentration of plaice juveniles in the remaining sand habitat and increased mortality through density-dependent predation by shrimps.     

Genetic Divergence Detected by ISSR Markers and Characterization of Microsatellite Regions in Mytilus Mussels

The wide distribution of microsatellites in mussels of the Mytilus edulis complex (Mytilidae) enables the analysis of inter-simple-sequence repeat (ISSR) markers. The aim of this investigation was to assess genetic differentiation in six sampling localities distributed along the European Atlantic coast to expose the potential of these markers in genetic studies requiring the detection of low polymorphism and as a source of sequences for developing microsatellite markers. We detected low genetic structuring within each member of the Mytilus edulis complex. Nei and Li distances and AMOVA clustered the individuals studied into two groups. On the basis of these results two sampling localities coming from the M. edulis × M. galloprovincialis mosaic hybrid zone in Western Europe were assigned to one species. On the other hand, mussels of a sampling locality in the Baltic Sea were not significantly different from a pure M. edulis locality supporting an extensive introgression of M. edulis in these individuals. Finally, 148 microsatellites were found in the sequences of 51 ISSR markers, and two polymorphic microsatellite markers were developed.     

Isolated and combined impacts of blue mussels (Mytilus edulis) and barnacles (Balanus improvisus) on structure and diversity of a fouling community

Effects of two presumably dominant competitors, the blue mussel Mytilus edulis and the barnacle Balanus improvisus on recruitment, population dynamics and community structure on hard substrata were experimentally investigated in the subtidal Kiel Fjord, Western Baltic. The hypothesis that blue mussels and/or barnacles are local dominants and strongly influence succession and community structure was tested by monitoring succession in the presence and absence of simulated predation on either or both species. Manipulations included blue mussel removal, barnacle removal, combined blue mussel and barnacle removal, as well as a control treatment for natural (non-manipulated) succession. In the second part of the experiment, recovery from the treatments was monitored over 1 year.During the manipulative phase of the experiment, blue mussels had a negative effect on recruitment of species, whereas barnacles had no significant effect. Even so, a negative synergistic effect of blue mussels and barnacles was detected. Calculation of species richness and diversity H′ (Shannon Index) showed a negative synergistic effect of blue mussels and barnacles on community structure. Additionally, diversity H′ was negatively affected by the dominant competitor M. edulis. These effects were also detectable in the ANOSIM-Analysis. The non-manipulative phase of the experiment brought about a drastic loss of diversity and species richness. Blue mussels dominated all four communities. Barnacles were the only other species still being able to coexist with mussels. Effects of simulated predation disappeared fast.Thus, in the absence of predation on blue mussels, M. edulis within a few months dominates available space, and diversity of the benthic community is low. In contrast, when mussel dominance is controlled by specific predators, more species may persist and diversity remains high.     

Microgeographic allozyme differentiation in the hybrid zone ofMytilus galloprovincialis Lmk. andM. edulis L. on the continental European coast

The EuropeanMytilus galloprovincialis Lmk. andM. edulis L. coexist and hybridize in different proportions in extended areas of the British and Atlantic French coasts.M. galloprovincialis typical allozymes seem to predominate in wave exposed areas, at high levels of attachment and in larger mussels in the British hybrid zone. Mussel samples from exposed and sheltered areas, 200 m apart, and from high and low levels of attachment were collected from a location of the French hybrid zone in 1988–92. PureM. galloprovincialis andM. edulis populations were also taken as controls. Diagnostic enzyme loci for bothMytilus (EST-D ^*, LAP-1^*, MPI^*, ODH^*) andAP-1 ^*, LAP-2^* andPGM ^* loci were studied. The frequencies of theM. galloprovincialis typical alleles were significantly greater in exposed populations than in sheltered samples (e.g. 0.729 to 0.803 vs 0.192 to 0.581 forEST-D ^*90), and at high level of attachment than at low level for the sheltered area (e.g. 0.581 vs 0.192 forEST-D ^*90). PutativeM. galloprovincialis was more abundant on the exposed coast (0.591 and 0.702) than on the sheltered shore, where it predominated at the high shore but not at the low shore location (0.371 vs 0.045). Significantly positive correlations between shell length and typicalM. galloprovincialis compound allele frequencies were found only for populations from exposed areas. Relationships between theMytilus genetic differentiation and ecological factors are discussed.     

Algal-induced spawning in the marine mussel Mytilus californianus 4

The characteristics of algal-induced spawning of the marine mussel Mytilus californianus were studied. Exposure of mature individuals to culture suspensions of the unicellular alga Pseudoisochrysis paradoxa elicited copious and synchronous release of gametes. Alkaline conditions were necessary to make the animals responsive to the spawning stimulus provided by the algae. The filtered, cell-free fraction of the algal suspensions also stimulated spawning, suggesting that an active principle is secreted into the culture media by the algae.

A key requirement for the development of an efficient and economically viable molluscan mariculture program is the capability to induce copious, synchronous spawning of gravid individuals. This procedure should be easily managed and have no deleterious effects on the gametes, fertilization, or the ontological development of the animals. Miyazaki [1] induced spawning in male oysters by exposing them to an extract of green algae. Recently it was reported that the marine mussels, M. californianus, could be induced to spawn by exposing them to cultures of the marine algae, P. paradoxa [2], This paper summarizes our efforts to characterize this spawning response of mussels to high concentrations of cultured marine algae.