Introduced versus native subspecies of Codium fragile: how distinctive is the invasive subspecies tomentosoides?

After its introduction, the green alga Codium fragile (Sur.) Hariot ssp. tomentosoides (van Goor) Silva has spread widely on several temperate-zone, rocky shores where non-weedy conspecific subspecies occur (N.E. Atlantic, N.E. Pacific, S. Pacific). To determine how phenologically and morphologically distinctive the invasive alga was relative to native subspecies, I compared marine intertidal populations of C. fragile ssp. tomentosoides and the native C. fragile ssp. novaezelandiae (J. Ag.) Silva (hereafter referred to as ssp. tomentosoides and ssp. novae-zelandiae respectively on New Zealand shores in 1992, 1993 and 1995. On the North Island, the invasive ssp. tomentosoides is sparsely distributed on low intertidal benches on wave-protected shores in the Hauraki Gulf (east coast) in spring and summer, and thalli die back to the perennial holdfast in autumn. In contrast, the native ssp. novaezelandiae forms dense beds within the low intertidal mussel zone on wave-swept shores of Maori Bay (west coast), and fronds are perennial. Whereas ssp. tomentosoides has only a few fronds arising from the spongy basal hold-fast, ssp. novae-zelandiae thalli are composed of many fronds. The ssp. tomentosoides from the Hauraki Gulf is significantly more branched than comparably sized native conspecifics from Maori Bay. These phenological and morphological differences were used to predict the subspecific identity of C. fragile from three other locations on the North Island, two locations on the South Island, and four locations on S.E. Australian shores; microscopic examination of utricles was used to check the predictions. Seasonality and number of fronds per thallus are the most reliable characters for field identification of native vs invasive subspecies: perennial intertidal thalli with large numbers of fronds are indicative of native subspecies for different geographic regions.     

Population dynamics of Mytilicola intestinalis in Mytilus edulis in south west England

The population dynamics of Mytilicola intestinalis Steuer in mussels (Mytilus edulis L.) from the River Lynher, Cornwall, England, have been studied over 3 years. By transplanting uninfested mussels from the River Erme, South Devon, into the Lynher mussel bed, the study was extended to the growth and development of new infestations under natural conditions. Female Mytilicola intestinalis are shown to breed twice, and two generations of parasites coexist for most of the year, with recruitment taking place in summer and autumn. One generation contributes its first brood to the autumn recruits before overwintering and contributing its second brood to the following summer's recruits. The other generation overwinters as juvenile and immature stages to contribute its two broods successively to the summer and autumn recruits. Environmental temperatures are believed to control the rates of development at all stages rather than acting as triggers in the onset or cessation of breeding at specific times. There is no evidence to support the contention that heavily infested mussels are killed, and parasite mortality is shown to be density-independent.     

Non-indigenous macroalga hosts different epiphytic assemblages to conspecific natives in southeast Australia

Codium fragile ssp. tomentosoides is one of the five most invasive macroalgae worldwide. We compared epiphytic assemblages between native (ssp. tasmanicum and ssp. novae-zelandiae combined) and non-indigenous (ssp. tomentosoides) subspecies of Codium fragile on three rocky intertidal shores in southeast Australia. Twelve species of epiphytes covered up to 20% of the surface of both native and non-indigenous Codium subspecies, but none were unique to either subspecies. For C. fragile subspecies, epiphytic cover declined from the lower (older) parts of the thallus to the branch tips. The abundance and species composition of epiphytes differed significantly between native and non-indigenous C. fragile, but differences varied among rocky shores. Results demonstrate that non-indigenous C. fragile does not play a functionally similar role to closely related native Codium subspecies. Spatial differences in epiphytic assemblages between Codium subspecies among rocky shores demonstrate that effects of non-indigenous species may be strongly location specific. Thus, our study emphasises the need to investigate variation in the effects of invaders across the regional landscape.     

High intertidal community organization on a rocky headland in Maine,USA

A mosaic patchwork of the barnacle Balanus balanoides L., the mussel Mytilus edulis L., and the alga Fucus vesiculosus L. was found in the transitional region between the mid and high intertidal zones on a rocky headland on Mount Desert Island, Maine, USA. The development of the mosaic was observed by following recruitment and survival of B. balanoides in denuded patches located at the same tidal level along a 60 m stretch of shore. Barnacle recruitment was least under canopies of F. vesiculosus and greatest in open areas kept moist at low tide by surf. Barnacle survival after settlement was least under the F. vesiculosus canopy due to the whiplash effect of the algal fronds in the surf and greatest in open areas free from competition from mussels. In open areas, early mortality was correlated with settlement density. In areas of dense settlement (60 spat cm^-2) up to 90% mortality resulted within 5 months from crowding associated with growth. In older individuals crowding produced hummocks of elongated, weakly attached barnacles which were more prone to removal by surf than uncrowded barnacles. Mussels exerted competitive dominance over barnacles for space and the presence of mussel beds prevented further barnacle recruitment. Mussels suffered extensive mortality during winter storms when surf removed dense mats of weakly attached mussels. The patchy distribution of mussels and barnacles results from irregular rock substrata producing numerous environmental patches with respect to wave exposure and drainage at low tide, and from densitydependent mortality of both mussels and barnacles which creates patches of new colonizable space within each environmental patch.     

The effect of selection treatments on <Emphasis Type="Italic">Mytilus edulis</Emphasis>, modifications of genetic and physiological characteristics

This study examined the effects of two selection treatments (elevated water temperature and air exposure) on the genetic and physiological characteristics of the juvenile marine mussel, Mytilus edulis (<10 mm). Genetic effects were measured on five allozymes and fitness assessed using physiological tests to estimate energy balance (scope for growth) as well as size, growth and survival. The in vitro treatments resulted in 48% mortality from an air exposure of 11 h at 27°C and 76% mortality from a 6-h exposure to 33°C water. Survivors (n = 1,152) of each treatment along with controls (n = 2,304) were measured and randomly placed in compartmentalized cages. Mussels were deployed to three bays in Prince Edward Island, Canada and monitored over a 10-month period. Initially, both of the treatments had an effect on mussel size and increased the heterozygosity of the surviving mussels. Physiological analyses after 3 months in the field showed that the two treated mussels showed lower metabolic rate that the control group. After 10 months in the field, the treated mussels were larger and had lower mortality than the untreated control mussels. Unexplained environmental interaction in each of the bays had an effect on allelic frequencies and heterozygosity. Overall, the results demonstrate that simple husbandry techniques can be used to increase the productivity of mussel seed and heterozygosity measures can be used to assess fitness. However, more field data is needed to determine the consistency of the increased productivity and if the increased productivity justifies the costs of a selective treatment. Furthermore, because the level of heterozygosity in juvenile mussel populations can vary considerably, both spatially and temporally, it may be effective as a warning of future natural mass mortality when overall heterozygosity levels are found to be low.     

Nitrogen limitation of Codium fragile ssp. tomentosoides as determined by tissue analysis

The critical nitrogen concentration of Codium fragile ssp. tomentosoides (van Goor) Silva was 1.90% on a dry weight basis. Internal nitrogen levels of thalli collected from three Rhode Island populations ranged from 0.75±0.08 to 3.72±0.08%. Internal nitrogen content was minimal in summer and maximal in winter, indicating nitrogen limitation during the summer and nitrogen storage during the winter. Part of this nitrogen storage appeared to support new growth of C. fragile in the spring. Carbon:nitrogen (C:N) ratios were inversely related to the internal nitrogen concentration. Low internal nitrogen content indicated nitrogen limitation better than C:N ratios because the C:N ratio could be influenced by both carbon and nitrogen metabolism.     

Growth and survival differences among native,introduced and hybrid blue mussels (<Emphasis Type="Italic">Mytilus</Emphasis> spp.): genotype,environment and interaction effects

The Mytilus species complex consists of three closely related mussel species: Mytilus trossulus, Mytilus edulis, and Mytilus galloprovincialis, which are found globally in temperate intertidal waters. Introduction of one or more of these species have occurred world-wide via shipping and aquaculture. Stable hybrid zones have developed in areas where these species have come into contact, making the invasion process complex. On the east coast of Vancouver Island (VI), British Columbia (BC), Canada, the native (M. trossulus) and introduced species (M. edulis and M. galloprovincialis), as well as their hybrid offspring, occur sympatrically. This study used a common environment experiment to quantify growth and survival differences among native, introduced, and introgressed mussels on VI. Mussels were collected from an area of known hybridization and reared in cages from May to August 2006. The cages were deployed at a local site as well as a remote site (approximately 150 km apart), and the mussels were genotyped at two species-specific loci. Growth and survival, as fitness measures, were monitored: native, introduced, and introgressed individuals were compared between and within sites to determine whether growth and survival were independent of site and genotype. Overall, mussels reared at Quadra Island performed better than locally-reared mussels at Ladysmith. Specifically, introgressed mussels reared at Quadra Island performed better than all genotypes reared at Ladysmith, as well as better than native mussels reared at Quadra Island. Differences in survival and growth among the native, introduced and introgressed mussels may serve to explain the complex hybridization patterns and dynamics characteristic of the VI introgression zone.     

Ontogenetic changes in small-scale movement by recruits of an exploited mussel: implications for the fate of larvae settling on algae

Many South African populations of the brown mussel Perna perna have been depleted through over-exploitation by subsistence harvesters. This is problematic because recovery after disturbance is very slow, partly because recruits are largely associated with adult mussels. However, unlike large recruits of 3.5–10 mm that exhibit spatial structure related to that of adults, a very high proportion of settlers and small recruits (0–3.5 mm) occur on foliose algae. We tested the hypothesis that recruits on algae move to adult mussel beds after a period of growth, with the null hypothesis that they die at a certain size. We conducted an indirect field study comparing the ratios of large to small recruits in 100% cover mussel patches at locations with high and low algal cover. A second laboratory experiment analysed whether the size of recruits on algae affects their active movement behaviour in response to nearby mussel patches. Large/small recruit ratios were slightly, but not significantly greater in high than low algal cover locations. Both small (2–2.5 mm) and medium (4.5–5.5 mm) recruits remained on algae and moved very short distances throughout the laboratory experiment, while larger recruits (9–10 mm) moved significantly further distances and more often into mussel patches. The results suggest that very large recruits are able to migrate actively to nearby mussel patches, indicating ontogenetic shifts in this behaviour. However, the absence of a significant difference in ratios between field locations with high and low algal cover suggests many large recruits are accidentally dislodged from the algae and presumably die. Thus settlement of P. perna onto algae is likely to be wasted, with consequences for sustainable management of the mussel resource.     

Mechanisms of habitat segregation between an invasive and an indigenous mussel: settlement,post-settlement mortality and recruitment

The mussel Mytilus galloprovincialis is highly invasive worldwide, but displays varying degrees of local and regional coexistence with indigenous mussels through spatial habitat segregation. We investigated the roles of settlement, post-settlement mortality, juvenile growth and recruitment in partial habitat segregation between the invasive M. galloprovincialis and the indigenous mussel Perna perna on the south coast of South Africa. We used two study locations, Plettenberg Bay and Tsitsikamma, 70 km apart, with two sites (separated by 300–400 m) per location, each divided into three vertical zones. There were no significant effects in Tsitsikamma, where daily settlement and monthly recruitment were significantly lower than in Plettenberg Bay. In Plettenberg Bay, settlement (primary and secondary) and recruitment of both species decreased upshore. Post-settlement mortality was measured over two consecutive 6-day periods during a spring tide and a neap tide. For both species mortality was low on the low-shore. High-shore mortality was consistently low for M. galloprovincialis, but increased dramatically for P. perna during spring tide. No data were obtained for growth of P. perna, but juvenile M. galloprovincialis grew more slowly farther upshore. P. perna recruited mainly in spring and summer, with a peak in summer far greater than for M. galloprovincialis. Recruitment of M. galloprovincialis was more protracted, continuing through autumn and winter. Thus local coexistence is due to a combination of pre- and post-recruitment factors differing in importance for each species. P. perna is excluded from the high-shore by recruitment failure (low settlement, high mortality). High survival and slow growth in juveniles may allow large densities of M. galloprovincialis to accumulate there, despite low settlement rates. With no differences between species in settlement or mortality on the low-shore, exclusion of M. galloprovincialis from that zone is likely to be by post-recruitment processes, possibly strengthened by periodic heavy recruitments of P. perna. At larger scales, larval retention and protracted recruitment contribute to the success of M. galloprovincialis at Plettenberg Bay, while recruitment limitation may explain why M. galloprovincialis is less successful at other sites.     

Detrimental effects of the isopod, <Emphasis Type="Italic">Edotia doellojuradoi,</Emphasis> on gill morphology and host condition of the mussel, <Emphasis Type="Italic">Mytilus edulis platensis</Emphasis>

There are many reported associations between mussels and other invertebrates, such as pea crabs, polychaetes, turbellarians and copepods, which live in their mantle cavities. The boundary between commensalism and parasitism is often indistinguishable because of insufficient knowledge or because the interaction is variable. Preliminary evidence led to a closer examination of the relationship between the mussel, Mytilus edulis platensis, and an isopod, Edotia doellojuradoi, previously described as commensalism. Monthly intertidal samples of mussels were taken from September 2004 to August 2005 at Caleta Cordova Norte (45°43′S, 67°22′W) in southern Argentina and assessed for the prevalence and abundance levels of isopods. Mussels with and without isopods were measured, examined for evidence of gill damage and their condition (soft tissue dry weight) was determined. The overall isopod prevalence in mussels was 57.9% and infestation varied with mussel length, with maximum occurence at 30.2 mm (medium-sized mussels). Experimental evidence indicated that the position of the isopod inside the mussel depended on the feeding activity of the mussel. Female isopods were observed grasping the ventral food groove of the gill demibranchs and feeding on the mucous food strand produced by the mussel. Juveniles and males were observed clustered together on the dorsal side of the single female in each occupied mussel, suggesting extended maternal care. Gill damage was observed in 58.2% of mussels at the Argentine site and was significantly associated with isopod occurrence. Categorical regression analysis showed that the most important factor associated with the degree of gill lesions was the number of male and juvenile isopods per mussel, followed by the length of female isopods and the developmental stage of juveniles. Conversely, the degree of gill damage decreased with increasing mussel length. Overall, E. doellojuradoi had a significant effect on mussel condition throughout the year, with low flesh weight in mussels with isopods, except during the austral summer and early autumn. In contrast to previous studies, which concluded that the isopod was a commensal, the present study clearly demonstrates that E. doellojuradoi is a parasite of M. edulis platensis. Other symbiotic interactions formerly classified as commensal might not be innocuous on further investigation, especially if samples are taken at multiple sites and at different times of the year.     

Growth patterns of Codium fragile ssp. tomentosoides in response to temperature,irradiance, salinity,and nitrogen source

Seasonal patterns of growth, reproduction, and productivity of Codium fragile spp. tomentosoides (van Goor) Silva were monitored at 3 locations in Rhode Island. Maximal growth occurred during the summer and was more significantly correlated with temperature than any other factor measured in this study. Multiple correlation models suggested an interaction between temperature, irradiance, and available nitrogen. Maximal reproduction occurred in late summer and early fall. The maximal productivity, based on harvested quadrats, was 2. 10 g dry weight m^-2 day^-1. A large amount (up to 87.3%) of the annual production entered the detrital food chain during the winter by fragmentation of the thallus. Culture studies examined the effects of temperature (6° to 30°C), irradiance (7 to 140 E m^-2 sec^-1), daylength (8 h light: 16 h dark to 24 h light: O h dark) and salinity (6 to 48) on growth. Differentiated thalli grew over a broad range of experimental conditions, with maximal growth at 24°C, 24 to 30 S, a minimal irradiance of 28 E m^-2 sec^-1, and 16 h daylength. The effect of increasing daylength was due to increased total daily irradiance rather than to a true photoperiodic effect. Undifferentiated sporelings survived and grew in a narrower range of environmental conditions than thalli. c. fragile spp. tomentosoides grew equally well with nitrate, nitrite, ammonium, and urea as a nitrogen source. The addition of NaHCO₃ stimulated growth at levels of 2.4 to 4.8 mM, suggesting an inorganic carbon limitation in static cultures. This study supports the hypothesis that the in situ seasonal growth pattern of c. fragile spp. tomentosoides is primarily due to the interaction of temperature and irradiance.     

Genetic structure of hybrid mussel populations in the west of Ireland: two hypotheses revisited

In Ireland, mussels on exposed rocky shores constitute an interbreeding mixture of two forms of mussels, the blue mussel, Mytilus edulis, and the Mediterranean mussel, M. galloprovincialis. Results from an Irish study in the 1980s, using partially diagnostic allozyme markers, indicated that mussels higher up the shore were more galloprovincialis-like than those lower down. In this study we set out to test two hypotheses: (a) recruits arriving on the shore are composed of genetically distinct cohorts that settle preferentially at different levels on the shore, and maintain genetic distinctiveness into adulthood; (b) recruits are genetically homogeneous, but once settled they diverge genetically over time, due to within-habitat site specific-selection. The diagnostic Me 15/16 DNA marker was used to analyse the genetic composition of newly-settled spat recruiting to artificial substrates, which were placed at two-week intervals from May–October 2002, on the mid- and low shore areas of two exposed sites in Galway Bay. Adult mussels were also collected on each sampling date. Results did not support the preferential settlement hypothesis, i.e., the genetic composition of primary settlers (≤ 500 μm) was similar between tidal heights and shores. Neither was there evidence of post settlement selective mortality, as adults were genetically similar to settling spat. In spat and adults the frequency of the M. galloprovincialis allele was high (0.56–0.80), due to high frequencies of M. galloprovincialis (> 37%) and hybrid (> 33%) genotypes, and correspondingly low frequencies of the M. edulis genotype (< 11%). Adult mussels from a nearby sheltered estuarine site, while significantly different to exposed shore mussels, still had low frequencies of the M. edulis genotype (< 17%), indicating no apparent advantage for the genotype in this environment. There are indications that the genetic composition of mussels may be changing on the Atlantic coasts of Ireland.     

Ecological studies of Codium fragile in New England,USA

The seasonal growth and reproduction of Codium fragile were studied at Boothbay Harbor, Maine, and at Woods Hole and Wings Neck, Massachusetts (USA), in relation to several environmental parameters. Maximum growth and reproduction occur during periods of highest water temperatures and insolation. The effects of various light intensities and temperatures on the photosynthesis rates of C. fragile were determined and correlated with the field results. Optimal conditions for net photosynthesis are 21° to 24°C and 900 to 1100 foot-candles. It is suggested that C. fragile is a warm-temperate plant that is growing near its northern limits in New England, but one that has not extended to its potential southern limits.Published with the approval of the Director of the New Hampshire Agricultural Experiment Station as Scientific Contribution Number 625.     

Adaptive host choice and avoidance of superparasitism in the spawning decisions of bitterling (Rhodeus sericeus)

Choice of a site for oviposition can have fitness consequences. We investigated the consequences of female oviposition decisions for offspring survival using the bitterling, Rhodeus sericeus, a freshwater fish that spawns inside living unionid mussels. A field survey of nine bitterling populations in the Czech Republic revealed a significantly lower rate of release of juvenile bitterling from Anodonta cygnea compared to three other mussel species. A field experiment demonstrated that female bitterling show highly significant preferences for spawning in A. anatina, Unio pictorum, and U. tumidus. Within a species, female bitterling avoided mussels containing high numbers of bitterling embryos. Mortality rates of bitterling embryos in mussels were strongly density dependent and the strength of density dependence varied significantly among mussel species. Female preferences for mussels matched survival rates of embryos within mussels and females distributed their eggs among mussels such that embryo mortalities conformed to the predictions of an ideal free distribution model. Thus, female oviposition choice is adaptive and minimizes individual embryo mortality. Received: 6 October 1999 / Received in revised form: 7 January 2000 / Accepted: 13 March 2000     

Tidal height,rather than habitat selection for conspecifics,controls settlement in mussels

Settlement is a major determinant of intertidal populations. However, the energy costs of lost larvae are very high. Accordingly, arrival and attachment on suitable substrata are essential requirements for species’ survival. On the intertidal, the presence of cues left by adult or juvenile conspecifics could be vital for the successful establishment of larvae arriving on the shore. Two mussel species, the indigenous Perna perna and the invasive Mytilus galloprovincialis, co-occur on the lower eulittoral zone on the south coast of South Africa. P. perna dominates the low and M. galloprovincialis the high mussel zones, with co-existence in the mid mussel zone. This study tested the hypothesis of settlement selectivity for conspecifics in these two mussel species, to understand whether the final adult distribution of mussels on the shores is determined by active behavioural and chemical mechanisms. Preferential selection by larvae for conspecifics was tested in the field during the peak settlement period in 2004 in natural mussel beds across zones and through manipulative experiments in the mid-zone where the species co-exist. On natural beds, settlement was determined by counts of settlers attached over 48 h onto artificial collectors. Collectors were placed on beds of P. perna and M. galloprovincialis present at both high- and low-adult densities, as well as in mixed beds. On such natural beds, settlers of both species consistently favored low-zone P. perna beds. Settlement patterns over 24 h onto experimentally created mussel patches consisting of P. perna, M. galloprovincialis or the two species combined beds, set in the mixed zone, did not conform with the results of the natural beds study: settlers of both species settled with no discrimination among different patches. The results indicate that mussels, which are sedentary, lack attraction to conspecifics at settlement. This highlights the importance of tidal height in setting settlement rates, and of post-settlement events in shaping populations of these broadcast spawners.     

Differences in somatic and gonadic growth of sea urchins ( Stronglyocentrotus droebachiensis ) fed kelp ( Laminaria longicruris ) or the invasive alga Codium fragile ssp. tomentosoides are related to energy acquisition

The rocky subtidal community off the Atlantic coast of Nova Scotia has historically undergone a cyclical transition between Laminaria-dominated kelp beds and sea urchin-dominated barrens. Since the introduction of the invasive alga Codium fragile ssp. tomentosoides, a third community state has emerged: Codium-dominated algal beds. We conducted a 42-week feeding experiment in the laboratory, which mimicked the quantity and quality of food available to urchins (Strongylocentrotus droebachiensis) in each of these community states. Feeding rate, growth, reproduction, and survival of urchins fed either Laminaria longicruris or C. fragile ad libidum, or L. longicruris 2 days per month, were measured. Although the ad libidum feeding rate on C. fragile was higher than that on kelp, energy intake was lower. Urchins in the ad libidum kelp treatment were larger and had larger gonads than those in the C. fragile treatment. Urchins fed kelp infrequently exhibited little somatic and gonadic growth over the course of the experiment. Regression analysis revealed that urchin performance on these diets was strongly related to energy intake. Diet treatment had no effect on survival or gonad maturation. Although urchins can consume substantial amounts of C. fragile, it appears that they cannot, or do not, feed quickly enough to compensate for its lower nutritional value. Our results suggest that, although urchins feeding on C. fragile are capable of surviving, growing, and reproducing, the replacement of kelp by C. fragile in some areas might negatively affect urchin populations as they continue to repopulate the shallow subtidal zone.     

Abundance of Fucus spiralis and ephemeral seaweeds in a high eulittoral zone: effects of grazers,canopy and substratum type

According to a widely accepted view of intertidal community organization in the north-west Atlantic Ocean, the abundance of perennial seaweeds is regulated by the activities of herbivores (which control ephemeral algal competitors) and carnivores (which control populations of competing filterfeeders). This concept was examined in 1987–1988 in a eulittoral belt on the Atlantic shores of Nova Scotia, Canada, dominated by a closed canopy of Fucus spiralis. A factorial design was used to test the effects of (a) grazers (there were no carnivores present, (b) adult F. spiralis canopy, and (c) substratum type on the abundance of perennial and ephemeral seaweeds. Grazers had no significant (p>0.05) effect on the density or cover of juvenile F. spiralis, but significantly reduced canopy cover of adults through the winter. The presence of grazers significantly enhanced the cover of ephemeral algae in early spring. Barnacle presence enhanced the cover of juvenile F. spiralis, but reduced the canopy of adult plants through abrasion. The most important regulator of recruitment density in F. spiralis was the presence of a canopy of conspecific adults. Canopy greatly reduced juvenile development. There was no significant relationship between the covers of perennial F. spiralis and ephemeral seaweeds. This study demonstrates that the recruitment of the perennial rockweed population examined is not dependent on the activities consumer animals. The results contrast with work in the midshore intertidal of New England, where the activities of herbivores are thought to regulate the abundances of perennial seaweeds.     

Population dynamics of the endangered fan mussel <Emphasis Type="Italic">Pinna nobilis</Emphasis> in a marine lake: a metapopulation matrix modeling approach

A metapopulation, time-invariant, stage-classified matrix model was developed to assess the dynamics of an important Pinna nobilis population in the marine Lake Vouliagmeni (Korinthiakos Gulf, Greece). The main aim of the study was to provide insight on the life cycle of the fan mussel and reveal influential factors for its population dynamics, with a special focus on the effect of poaching. The size of the fan mussel shell was selected as a state variable, and the model consisted of five size classes. The lake was divided in two regions, a shallow region of high (illegal) fishing mortality and high recruitment (region 1) and a deeper region of low mortality and low recruitment (region 2). The estimation of the transition matrix (stage-specific growth and mortality probabilities) was based on a tagging survey between 2005 and 2006, while independent annual surveys for abundance estimation using distance sampling techniques were utilized for the estimation of recruitment and stage-specific fertilities. The population was found to be increasing with an intrinsic rate of increase r = 0.038; however, r was not statistically different from zero. The life expectancy and expected lifetime offspring production of individuals in region 1 was markedly lower than that of individuals in region 2. Due to poaching, the life expectancy of a yearling fan mussel was less than 2.5 years in region 1, while it was almost 12 years in region 2. The highest expected annual natural mortality of fan mussels occurred on their first year of life after settlement (~43%) and greatly declined at greater sizes. Perturbation analysis revealed that the population growth rate was most sensitive to the vital rates of the larger size classes in region 2 and to fertilities corresponding to offspring that settled in the same region. The spatial distribution and abundance of the species was greatly dependent on the extent of poaching, which caused a size segregation of individuals, with small and young individuals being abundant in region 1, and larger and older individuals being restricted in region 2. If poaching ceased, the fan mussel population would be increasing with a significantly higher intrinsic rate of increase (r = 0.186), while if region 2 was also illegally exploited at the same intensity as region 1, the fan mussel population would be decreasing with r = −0.364 and would eventually collapse. The existence of refuge areas, where fan mussels may grow and reproduce, providing adjacent areas with offspring, seems crucial for the viability of local populations. Transplantation of fan mussels from high mortality areas to low mortality refuges might prove to be an effective measure to protect local populations of the species.     

Age,growth and population structure of <Emphasis Type="Italic">Modiolus barbatus</Emphasis> from the Adriatic

Age, growth and population structure of Modiolus barbatus from Mali Ston Bay, Croatia were determined using modal size (age) classes in length frequency distributions, annual pallial line scars on the inner shell surface, internal annual growth lines in shell sections of the middle nacreous layer and Calcein marked and transplanted mussels. The length frequency distributions indicated that M. barbatus attain a length of ∼40 mm in 5–6 years indicating that a large proportion of the population in Mali Ston Bay is <5 years old. Some mussels of ∼60 mm were predicted to be 14 years old using the Von Bertalanffy growth (VBG) equation. Up to the first 6 pallial line scars were visible in young (<6 years) mussels but in older shells the first scars became obscured by nacre deposition as the mussel increased in length and age. The age of the older shells (>6 years) was determined from the middle nacreous lines in shell section, which formed annually in winter between February and March; the wider dark increments forming during summer (June to September). The oldest mussel, determined from the middle nacreous lines, was >12 years, with the majority of mussels aged between 3 and 6 years of age. The ages of mussels ascertained using the growth lines were not dissimilar to the ages predicted from the length frequency distributions. Age at length curves produced using modal size class data were not different from the data obtained using the pallial scar rings and internal growth lines. Taken together these data suggest that M. barbatus attains a length of 40 and 50 mm within 5 and 8 years, respectively. Eighty one percent of individual M. barbatus injected with a Calcein seawater solution (300 mg Calcein l⁻¹), into their mantle cavity successfully deposited a fluorescent line, which was visible in suitably prepared shell sections under ultra violet light. Incorporation of Calcein into the mussel shells was seasonally variable with the lowest frequency of incorporation in mussels marked in February and recovered in May. Seasonal shell growth was observed with significantly higher growth rates in mussels marked in May and removed in August (ANCOVA, F _3,149 = 23.11, P < 0.001). Mussels (∼18 to 22 mm) marked in May and recovered in August displayed maximal growth rates of >2.5 mm month⁻¹ compared with a mean mussel growth rate of 1.2 ± 0.6 mm month⁻¹. At other times of the year mussel shell growth ranged from immeasurable to 1.48 mm month⁻¹.     

Quantitative studies on gonad cycle,fecundity, reproductive output and recruitment in a baltic Mytilus edulis population

Since Mytilus edulis L. is a biomass dominant in the Baltic much interest is focused on the ecology of the species. In this paper an attempt is made to quantitatively cover the reproductive cycle of a Baltic M. edulis population in order to provide data for energy flow models and to discuss aspects of recruitment in this species. Histological preparations of gonads showed that gametogenesis started with declining temperatures in autumn and proceeded very slowly through winter. At the beginning of March when food was supplied during the spring phytoplankton bloom, rapid maturation took place. This was also revealed by an increase in meat weight of the mussels. Only one spawning period was recorded, from the middle of May until the beginning of June, due to food being strongly limited to the population during the rest of the year. The length of the larval period was estimated as being 5 to 6 wk and settlement was registered from the end of June through July. In general the large annual variations found in the Baltic with regard to temperature and food abundance give rise to a more marked annual pattern in the reproductive cycle than is encountered in other seas. Fecundity was assessed for two populations from 4-and 15-m depths from studies covering two annual cycles of the changes in the relation of shell length — meat weight. The size-related fecundity was found to be equal in both populations and related to food abundance and not to growth or age. Fecundity, expressed as weight loss at spawning, ranged from 0% in 2-mm mussels and increased from 38 to 52% in 10-to 30-mm mussels. The fecundity as percentage of biomass in full-grown Baltic M. edulis is of similar magnitude as in full-grown mussels from other areas despite the smaller size of Baltic mussels. The reproductive output for the total 160 km² research area was calculated as being 1 200 tons dry weight or 80% of the standing stock, which, due to the particular features of the Baltic M. edulis population probably represents the larger part of the total mussel production. This reproductive output, calculated as 8·10⁷ eggs·m^-2 and corresponding to 50% of the total annual zooplankton production, may thus consitute an important food source for herring larvae and carnivorous zooplankton. Recruitment was divided into two phases: (1) Recruitment of juveniles (=settlement of larvae), and (2) recruitment to breeding stock. Monitoring studies of settlement on ropes and the year round presence of high abundances of mussels <2 mm indicate that settlement is in excess of the demands for maintaining population size and that most settled mussels form a pool of competitively suppressed non-growing individuals. Not until death of an already established mussel will these become recruited to the breeding population. Thus recruitment is possible throughout the year which stabilizes the population and maintains it near the carrying capacity of the area with regard to food and space availability.