Natural growth bands and growth variability in the sea urchin Echinus esculentus: results from tetracycline tagging

Growth of the European edible sea urchin Echinus esculentus L. was studied in a population held for 2 yr in cages on the sea bed, after labelling with the skeletal growth marker tetracycline. The final position of the tetracycline tag on the genital plates agreed with an annual periodicity in natural growth zones; two such growth zones appeared beyond the position of the tag on the ground surface of the plate as light-reflecting bands separated by narrow dark lines in the middle layer. Individual and group (pooled data) growth parameters were estimated from the growth increment shown in the genital plate, whose lateral growth displayed a linear relationship to the diameter of the urchin test within the size range of these measurements. Von Bertalanffy growth parameters (asymptotic size and growth-rate function K) fitted to the growth increment on each individual were highly significantly correlated to those fitted to the natural growth lines, assuming an annual periodicity. The inferred growth pattern agrees well with conclusions based on H. B. Moore's growth-band data. The caged urchins can be assumed to have experienced exactly similar conditions, yet the growth curves fitted to individuals showed considerable variability. The good agreement between estimated growth function parameters of individuals obtained by the two methods indicate that this reflects real variability in growth between individuals that probably is genetically rather than environmentally determined. The growth of E. esculentus, and the adaptational significance of high growth variability in the population is briefly discussed.     

Sedimentation of organic detritus in Lochs Etive and Creran,Argyll, Scotland

The rates of sedimentation of organic detritus were measured at 3 stations in two Scottish sea lochs, Loch Etive and Loch Creran, using sedimentation jars exposed at various depths. Details are given of the seasonal and depth distribution of sedimenting material, and its composition for a 1-year period. Differences in the pattern of sedimenting material collected at different depths and in the seasonal patterns of sedimentation at the different stations suggested that, in each case there were differences in the relative importance of detritus from various sources. At all stations, phytoplankton production made a relatively small contribution to the total detritus collected, either directly as dead cells, or indirectly as the faeces of zooplankton organisms. Near the head of Loch Etive there were contributions by filamentous algae and Enteromorpha sp., but a major source of detritus was terrestrial debris, mainly carried into the loch in the waters of the River Etive. In the lower basin of Loch Etive, terrestrial detritus also contributed to the total sedimenting near the surface, but at greater depths much of the material collected in the sedimentation jars probably resulted from short-term resuspension and re-deposition of bottom material, reflecting a net transport of fine sediment from the shallower to the deeper areas of the loch. Secondarily resuspended material was also a major source of material collected in the jars exposed in Loch Creran.     

Community structure of the benthos in Scottish sea-lochs. I. Introduction and species diversity

The diversity of the macrobenthos was measured, using the rarefaction method of Sanders (1968), from bottom samples from Loch Etive, Loch Creran and the Firth of Lorne (Scottish west coast). Each sample (representing 1.6 m² of bottom area) was accumulated as a series of separate hauls taken consecutively in a systematic pattern with a van Veen grab. Two habitats were considered: soft mud and muddy sand. Within-habitat comparison of species diversity clearly indicates that diversity is lower in Loch Etive than in Loch Creran or the Firth of Lorne. The reasons for this are probably connected with the relatively high freshwater runoff into Loch Etive, possibly limiting the survival of planktonic larval stages of the benthos. The values of species diversity measured for the soft-mud areas in Loch Creran and the Firth of Lorne are thought to be representative for this habitat along the west coast of the British Isles. They agree well with the diversity predicted by Sanders for such a boreal inshore area, where a maritime climate prevails, on the basis of his time-stability hypothesis and the results of his sampling elsewhere.     

Growth of the deep-sea irregular sea urchins Echinosigra phiale and Hemiaster expergitus in the Rockall Trough (N.E. Atlantic Ocean)

Growth in the deep-sea irregular sea urchins Echinosigra phiale (family Pourtalesiidae) and Hemiaster expergitus (family Hemiasteridae) was studied from deep-sea samples taken during the years 1973 to 1985 from two stations at 2900 and 2200 m depth in the Rockall Trough (N.E. Atlantic Ocean). Growth zones, similar to those described from sea urchins in shallow water, are present as a series of wide white bands separated by narrow, dark rings in the calcite stereom of the test plates after heating to 350°C. In shallow water, such growth zones seem to result from seasonally varying growth rates. In the supposedly constant conditions in the deep sea, a seasonal growth pattern is unexpected but may occur in response to recently discovered annual pulses in downward flux of detritus from the euphotic zone, providing a seasonally varying food supply for such deposit-feeding species living in the bottom sediment. On this assumption, growth curves were fitted to counts of growth zones (as representing age in years), in the larger lateral and ventral test plates of E. phiale and H. expergitus. The opportunity was also taken to fit growth curves derived from counts of growth zones in samples of the inshore spatangoids Spatangus purpureus and Echinocardium pennatifidum. Plots of counts against test length of Echinosigra phiale and H. expergitus, although scattered and not clearly asymptotic, indicate, growth to be slower than in the two inshore spatangoids, and than in the coastal species Echinocardium cordatum, for which there are good recent growth data, available.     

Acid-base status in the sea urchins Psammechinus miliaris and Echinus esculentus (Echinodermata: Echinoidea) during emersion

The acid-base status of two sea urchins, Psammechinus miliaris (Gmelin) and Echinus esculentus (L.) during experimental emersion has been investigated. Sea urchins were collected from the Firth of Clyde between August and September 1987. The carbon dioxide capacity of the coelomic fluid of P. miliaris was greater than that of E. esculentus, although both were low and only marginally greater than that of sea water. The pH of the coelonic fluid was also low (7.05 to 7.17) and was influenced mainly by the internal partial pressure of CO₂ (P_{CO 2}). Acid-base disturbance in the coelomic fluid of both species during emersion, although minimal, was more pronounced in E. esculentus than in P. miliaris and was due primarily to an increase in the internal P_{CO 2}, although there was an increase in the concentration of L-lactate in the coelomic fluid of E. esculentus. The coelomic fluid of both species was in a state of perfectly compensated respiratory acidosis. An increase in the concentration of divalent ions (Ca²⁺ and Mg²⁺) may be related to the dissolution of the test as a source of carbonate buffer.     

Growth and mortality of subtidal red sea urchins (Strongylocentrotus franciscanus) at San Nicolas Island,California, USA: problems with models

Red sea urchins, Strongylocentrotus franciscanus, were tagged with tetracycline in 1990 at subtidal sites off San Nicolas Island, California, USA. After one year in the field, the sea urchins were collected and growth increments were measured based on tetracycline marks, which indicated initial slow growth, a maximum rate, and finally a prolonged period of very slow growth. Small red sea urchins (4 cm diam) were estimated to be 3 to 4 yr old, which is much older than has previously been reported. It is estimated that about 12 yr would be required to attain 10 cm diam. Survival has previously been modeled assuming a constant rate. If the population of red sea urchins is assumed to be stable and stationary, annual survival rate was between 71 and 77% yr^-1. Census data for the two years of study have permitted annual survival to be estimated without assuming stable and stationary population structure. Under these conditions, annual survival rate was between 79 and 86% yr^-1. Analysis of transitions in the size distributions from 1990 to 1991 suggested that annual survival may have been sizespecific: 91% yr^-1 for individuals 1.1 to 4.0 cm diam, 82% yr^-1 for individuals 4.1 to 7.0 cm diam, and 63% yr^-1 for those of 7.1 to 10.0 cm diam. An alternative explanation to size-specific survival in our study is sizespecific immigration.     

Oxygen and acid-base status of the sea urchin Psammechinus miliaris during environmental hypoxia

The respiratory and acid-base responses to hypoxia of the sea urchin Psammechinus miliaris Gmelin have been studied both in the laboratory and in the field. Sea urchins were collected from the Clyde Sea area, Scotland, between March and July 1993. Individual urchins were unable to regulate their oxygen uptake during hypoxia. The partial pressure of oxygen (P_{O 2}) in the coelomic fluid was lower than ambient P_{O 2} during normoxia but still decreased with a decrease in environmental P_{O 2}. There was a significant increase in pH and in the concentrations of bicarbonate, magnesium and calcium in the coelomic fluid, but a significant decrease in the partial pressure of carbon dioxide (P_{CO 2}), in response to declining P_{O 2}s. The disturbance of respiratory parameters in urchins accompanying hypoxic exposure in rock pools was similar to that observed in the laboratory, although not as severe as might have been expected on the basis of the laboratory experiments.     

Allocation of45calcium to body components of starved and fed purple sea urchins (Strongylocentrotus purpuratus)

Several lines of evidence in the literature indicate that environmental stress such as starvation may initiate reallocation of sea urchin endoskeletal tissue. For example, Aristotle's lantern enlarges under conditions of starvation, and sea urchins tagged with tetracycline and then fed develop a distinct growth line, while starved individuals develop a diffuse pattern. We designed anin vivo system to examine stress-related changes in calcification in the purple sea urchinStrongylocentrotus purpuratus. SmallS. purpuratus (ca. 2 cm test diam) were collected from the Mission Bay jetty or Imperial Beach (San Diego, California, USA) in 1987.⁴⁵Ca was incorporated from seawater into all body fractions including the organic tissue/coelomic fluid. In an initial experiment, sea urchins were fed or starved for 4 wk and then post-incubated in isotope. Overall, starved individuals deposited new calcite more slowly than did fed individuals; however, allocation was very different and calcification of teeth of starved sea urchins was nearly as great as in fed individuals. In a second experiment,S. purpuratus were first pre-labeled with isotope and then treated by feeding or starving. More of the labeled calcium was mobilized from the soft tissues and coelomic fluid into calcite in fed than in starved individuals. Growth of the teeth in starved sea urchins was significantly greater than in those fed. We conclude that starvation changes the metabolism of calcium in order to preferentially build teeth. However, we also found no evidence that calcium was resorbed from old skeletal calcite in order to build new skeleton.     

Food preferences and related behavior of the browsing sea urchin <Emphasis Type="Italic">Tripneustes gratilla</Emphasis> (Linnaeus) and its potential for use as a biological control agent

The short-spined toxopneustid sea urchin Tripneustes gratilla feeds on a wide variety of algal species and on sea grasses. However, the urchin does show preferences when offered a selection of macroalgal species, which it encounters in nature. Preferences among macroalgae were evident in field-collected urchins exposed to pair-wise tests where the variable was either the consumption rate of the algae or observation of which algal species the urchins chose to touch with their lantern teeth. Exposure of lab-housed urchins to one of five species of macroalgae for 5 months did not seem to alter preferences of urchins in three of the exposure groups, but those exposed to Padina sanctae-crucis seemed to show an enhanced preference for this species when offered a choice of the five species of macroalgae at the end of the exposure period, and those exposed to Gracilaria salicornia seemed to avoid the species when offered the choice of the five species. Perhaps more ecologically important than their preferences were two other observations on these urchins: first, when offered only a single species of algae, the urchins on four of five diets ate the same quantity per day. Second, when simultaneously offered the choice among the five macroalgal species, the urchins consumed more macroalgae per day than when offered only one species. These urchins move about a meter a day. They probably encounter food resources in a relatively coarse-grained fashion and have evolved to eat what is available. Because of their limited movements, their habitat overlap with grazing fishes, their acceptance of a wide variety of macroalgae and their preference for macroalgae, these native urchins are thought to have the potential to serve as biological control agents of alien and invasive macroalgae, which have come to dominate some reef zones normally occupied by corals in Hawaii.     

Physiological energetics of cultivated mussel (Mytilus edulis) populations in two Scottish west coast sea lochs

Physiological responses of suspended cultured mussels, Mytilus edulis L., in two Scottish sea lochs (Lochs Etive and Leven) were investigated in their native and transplanted environments, after 15 d, 4.5 mo and 1 yr acclimatization, during the main growing season of May to September 1992. These measurements were integrated by means of the balanced energy equation, and scope for growth was calculated to assess the performance of each stock. Transplanted mussels showed clear signs of stress during the first 15 d after transfer, with low clearance rates and energy retention and high rates of respiration and nitrogen excretion. There were significant differences in some of the physiological responses (clearance, respiration and excretion rates), scope for growth and growth efficiency between the native populations, with the responses of mussels in Loch Etive being more favourable than those in Loch Leven. With newly transplanted mussels after 15 d acclimatization, almost all these measurements also differed significantly from native mussels in their host site and the original stocks but, with the exception of ammonia excretion rates, all the variables of cross-transplanted mussels after 4.5 mo acclimatization were the same as those of the native stock at the host site. Good agreement between observed long-term growth rates and estimated scope for growth suggests that, like growth rate, differences in physiological responses are mainly controlled by environmental factors; i.e., stock or origin had no significant influence on variations in physiological response (except ammonia excretion). The results further indicate that scope for growth estimated during the main growing season can be used to assess the actual growth rate and to compare sites for on-growing.     

Fungi in corals: black bands and density-banding of Porites lutea and P. lobata skeleton

 Dark coloration of coral skeleton forming black bands is commonly observed in fractured, massive-coral colonies (Porites lutea and P. lobata) collected from May- otte Island in the Mozambique Channel and Moorea Island in French Polynesia. Black-banding was similar in corals from the two areas and was associated with an assemblage of microbial endoliths: Ostreobium queketti, a common siphonal chlorophyte, and a type of Aspergillus-like fungus. Fungi of coral skeletons are capable of euendolithic growth entirely within the skeleton, and of cryptoendolithic growth whereby they spread from the skeleton into the skeletal pores. The morphology and size of fungal hyphae differs significantly between euendolithic, cryptoendolithic and reproductive phases. Reproductive phases involve formation of conidiophores. Insoluble residues in black bands involve a dark pigment and a dark membranous veil. When attacked by fungi, the algae are usually destroyed. They darken and are threaded by dense, dark-brown, fibrous excrescences. The fungi excrete a dark pigment that stains the surrounding skeletal carbonate black. The pigment is organic, and its presence correlates with higher concentrations of polysaccharides. Black bands match high-density bands of the coral skeleton. Both black bands and high-density bands form at the end of the rainy season in Mayotte. Thus, black-banding in the corals studied is caused by a series of events, beginning with an increase in the abundance of endolithic algae followed by an increase in skeletal density. The algae are then attacked by fungi, which produce more cryptoendolithic hyphae and conidia that are associated with production of the dark pigment. Received: 29 January 1999 / Accepted: 29 September 1999     

Waterborne cues from crabs induce thicker skeletons, smaller gonads and size-specific changes in growth rate in sea urchins

Indirect predator-induced effects on growth, morphology and reproduction have been extensively studied in marine invertebrates but usually without consideration of size-specific effects and not at all in post-metamorphic echinoids. Urchins are an unusually good system, in which, to study size effects because individuals of various ages within one species span four orders of magnitude in weight while retaining a nearly isometric morphology. We tracked growth of urchins, Strongylocentrotus droebachiensis (0.013–161.385 g), in the presence or absence of waterborne cues from predatory Jonah crabs, Cancer borealis. We ran experiments at ambient temperatures, once for 4 weeks during summer and again, with a second set of urchins, for 22 weeks over winter. We used a scaled, cube-root transformation of weight for measuring size more precisely and for equalizing variance across sizes. Growth rate of the smallest urchins (summer: <17 mm diameter; winter: <7 mm diameter) decreased by 40–42% in response to crab cues. In contrast, growth rate of larger urchins was unaffected in the summer and increased in response to crab scent by 7% in the winter. At the end of the 22-week experiment, additional gonadal and skeletal variables were measured. Cue-exposed urchins developed heavier, thicker skeletons and smaller gonads, but no differences in spine length or jaw size. The differences depended on urchin size, suggesting that there are size-specific shifts in gonadal and somatic investment in urchins.     

Diet,behaviour and reproduction of the whelk Buccinum undatum in the northern Gulf of St. Lawrence,eastern Canada

A laboratory experiment was used to examine whether a decrease in the width of sutures is the basis for shrinking of the test in echinoids under conditions of low food availability and whether the body condition of sea urchins is likely to be retained when shrinking occurs. This experiment was performed with H. erythrogramma specimens collected in January 1987 from Pt. Lillias in Corio Bay, Australia. Suture widths in the test and gonad volume were significantly less (relative to body volume) in Heliocidaris erythrogramma (Val.) in the low food treatment than the high food treatment after 4 mo. These results imply that sea urchins which shrink may have a poorer body condition and be less able to reproduce than sea urchins that do not shrink. A comparison of the expected to the observed changes in suture width showed that a decrease in suture width is involved in the shrinking of the test in H. erythrogramma and may be of sufficient magnitude to fully explain the observed reduction in test size. This suggests that the amount a sea urchin may shrink under low food conditions is constrained by the degree to which the sutures can be reduced in width.     

Skeletal growth markers in the deep-sea brittle starsOphiura ljungmani andOphiomusium lymani

Growth bands have been found in the calcitic vertebral arm ossicles of the commonly occurring deep-sea brittle starsOphiura ljungmani Wyville Thomson andOphiomusium lymani (Lyman) (Echinodermata: Ophiuroidea) trawled from the Rockall Trough (N.E. Atlantic) at 2 200 and 2 900 m depth from 1973 to 1982. InOphiura ljungmani, the study of ossicle microstructure by SEM shows that growth bands reflect differences in stereom porosity and surface relief, similar to that previously found amongst shallow-water brittle stars. The pattern inOphiomusium lymani was much less clear from the microstructure, but could be revealed by heating the ossicle to 450 °C and clearing in xylene. The bands showed up as fine, translucent rings separated by more opaque, wider zones, perhaps reflecting differences in organic material incorporated within the calcite. Both growth-banding patterns probably reflect an annual cycle in skeletal growth rate. On this assumption, the number and spacing of the banding in the two species indicates contrasting growth strategies. WithOphiura ljungmani, a rather regular annual growth increment and perhaps shorter lifespan (up to ca. 10 yr) thanOphiomusium lymani is indicated. The latter shows a relatively wide spacing of early bands, followed by tight clustering of the outermost bands corresponding to adult sizes. This growth pattern is characteristic of species escaping from predation by rapid growth to relatively large adult size. AdultO. lymani probably grow slowly, some perhaps reaching 20 yr of age. Growth curves were fitted to size-at-age corresponding to measurements of the size and ordering of growth bands. These corroborate age structure previously estimated from analysis of size frequencies in time series from the stations sampled in the present study. Such skeletal growth markers should be of value in analysis of the demography of deep-sea populations.     

The dual functions of sea urchin gonads are reflected in the temporal variations of their biochemistry

Fatty acid analyses are emerging as a powerful technique to probe trophic interactions between organisms. In this paper, the application of both this procedure and gonad index (GI) determination on two populations (intertidal and subtidal) of the echinoid Psammechinus miliaris is reported. The investigation spanned the 3-month spawning period of Scottish west coast populations. In both populations a progressive decrease in the GI was found, coupled with an increasing maturity stage (from mature to spent). Sexual maturation and decrease in GI was synchronous between the two populations. In conjunction, there were distinct changes in gonad biochemistry. Differences in the fatty acid composition of the gonad reflected the changes in sexual maturation. Mature males and females had significant differences in the fatty acid composition of their gonads, whereas post-spawned individuals showed no gender differences. Male urchins had higher levels of polyunsaturated fatty acids (PUFAs) compared to females, and there was a dramatic reduction in the fatty acids 22:6(n−3) and 20:5(n−3) with increasing maturity stage. Using multivariate statistical techniques, these changes in the fatty acid composition of the sea urchin gonad were linked to habitat related diet differences combined with gender differences. These changes in the fatty acid signatures clearly reflect the dual function of the gonad as both a nutrient store and a reproductive organ.     

Further studies of short-term variation in the pigment composition of a spring phytoplankton bloom

Phytoplankton sampled at the chlorophyll maximum during the spring bloom at Loch Ewe, Scotland during 1986, were analysed using high-performance liquid chromatography for chloropigments and carotenoids. The day-to-day proportions of the pigments remained relatively stable throughout the bloom, with good preservation of chlorophyll a even at the sediment surface. Chlorophyllide a and phaeophorbide a appeared towards the end of the bloom. The major carotenoids remained stable throughout the bloom, but became rapidly degraded on sedimentation under the oxic conditions. The pigment data is related to the nutrient status of the bloom, when slow steady growth occurred without nutrient limitation. These data contrast sharply with data from the 1983 spring bloom at Loch Ewe when acute nutrient limitation took place.     

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.     

Relative size of Aristotle's lantern in Echinometra mathaei occuring at different densities

Like species of sea urchins in Zanzibar and Oregon (USA), Echinometra mathaei (de Blainville) at Rottnest Island, Western Australia, displays variation in the size of Aristotle's lantern relative to the maximum diameter of the test. This variation was associated with local variations in density of urchins at each of two sites in each of two years (1980 and 1981); this association with density was consistent with the proposal that relatively larger lanterns are a response to decreased food availability. Furthermore, variation of relative lantern size associated with local density was similar in magnitude to the variation displayed between sites and between years. This temporal variation demonstrated the plasticity of the relative lantern size over periods as short as 12 mo. Further experimental studies are required before relative length of lanterns can be used as estimates of food availability.     

Otolith age validation and growth-rate variation in flyingfish (Hirundichthys affinis) from the eastern Caribbean

A study of otolith aging and growth-rate variation in the flyingfish Hirundichthys affinis (Günther) was conducted in the eastern Caribbean (10–16°N; 58–62°W) in 1987–1989. Daily otolith-increment formation was validated in laboratory-reared larvae, confirming the usefulness of otolith-increment counts for age determination of H. affinis juveniles (<150 mm fork length, FL). A mark-recapture programme to validate increment formation in wild adults was unsuccessful due to tetracycline-linked mortality and insufficient tetracycline uptake in slow-growing adult otoliths. A von Bertalanffy growth curve fitted to juvenile size-at-age data gave preliminary growth-curve parameters of t ₀=2.85 d and k=0.00854 on a daily basis, with an asymptotic length, L, of 245 mm FL, for eastern Caribbean flyingfish. Juvenile growth rate in H. affinis is sensitive to spatial and temporal variation in temperature. Growth rates were higher where sea-surface temperatures were higher, and were higher for juveniles hatched in warmer months (April–July) than in colder months (November–March). Growth rates were also higher near islands than at more oceanic locations. Variation in juvenile growth rates may influence the spatial and temporal variation in spawning frequency observed in H. affinis.     

Growth,mortality and shell morphology of cultivated mussel (Mytilus edulis) stocks cross-planted between two Scottish sea lochs

One-year-old rope-grown blue mussels (Mytilus edulis L.) were held in experimental lantern nets and cross-transplanted between two sea lochs (Lochs Etive and Leven) on the west coast of Scotland. Growth and mortality were monitored from May 1991 to May 1992, as well as shell morphology in native and transplanted mussels. There were highly significant differences (P0.001) in growth rates between lochs; both native and transplanted mussels performed better in all shell and tissue growth-parameters in Loch Etive than in Loch Leven. Stock also had a significant (P0.05) influence on shell length and live weight, but its contribution to total variance was considerable only in the latter case. Wet and ash-free dry meat weights were governed mainly by site and to some extent by site x stock interaction. Mortality rates were quite low (6 to 7% yr^-1) and did not show any significant variation either between sites or stocks. There were significant morphological differences (ratios of shell length, height and width) between the Loch Etive and Loch Leven populations (P0.001) which persisted for one year after transfer. Stock appeared to be the main factor affecting shell morphology, as opposed to site. Linear regressions of shell height and width on length were significantly different between the native stocks, but became indistinguishable from those of the recipient populations one year after reciprocal transfer. This did not, however, conceal the effect of stock origin on dimensional ratios. It is concluded that site differences related to environmental factors, notably chlorophyll a levels and currents, and also possible stress caused by high levels of Zn and Cu, are major determinants of growth, which are of main interest to aquaculture, while morphological differences most probably result from genotypic variation.