Reproductive cycle of the subarctic brooding asteroid Leptasterias polaris

The reproductive cycle of the large brooding seastar Leptasterias polaris Müller and Troschel was examined over an 18-month period in the St. Lawrence Estuary, Québec, Canada. There is a distinct annual cycle with spawning over several months in the autumn. The female has an unusual cycle in that the ovary only slightly decreases in size during spawning, and the size structure of the oocyte population is remarkably stable throughout the year. The major annual change observed in the oocyte population is the development of a small number of 600-to 800- oocytes prior to spawning and their loss during spawning. This stability, combined with the lack of evidence of phagocytosis, suggests that oocyte development takes place over many years. By contrast, the developmental cycle of the testis is similar to that of most echinoderms. The large reservoir of oocytes probably guarantees a steady annual recruitment, and brooding during the winter probably minimizes metabolic costs for the female and ensures the release of the juveniles when conditions are favourable in the spring and summer.     

Strong maternal effects and extreme heterogeneity of progeny development in the caridean shrimp Sclerocrangon boreas (Crangonidae)

Sclerocrangon boreas is uncommon among marine coastal carideans in having a non-dispersing, abbreviated (2-stage) larval phase. We investigated the implications of this strategy in terms of fecundity, offspring provisioning and brood care in S. boreas from the St. Lawrence Gulf and Estuary in 2009–2010. Fecundity scaled positively to female body size but was low due to the production of large, lipid-rich eggs. Offspring size at all stages of development was positively related to female size. Larval traits and lipid dynamics indicate obligatory lecithotrophic development from hatching to juvenile. The larva becomes a juvenile on the mother and remains associated with her for sometime after. The co-occurrence of early egg stages among many juveniles in some clutches raises the possibility that maternal care of juveniles includes the provisioning of trophic eggs or eggs reclaimed from other females.     

Mélange vertical et capacité photosynthétique du phytoplancton estuarien (estuaire du Saint-Laurent)

Time series of chlorophyll a, photosynthetic capacity and many physical parameters were sampled hourly for 167 h in August, 1975, at an anchor station located in the Middle Estuary of the St. Lawrence River, Canada. Sampling was carried out during the transition from neap tides to spring tides. The long-and short-term variations in chlorophyll a are coupled with the advection of water masses which depends on tidal currents. Vertical mixing also influences the chlorophyll a concentration of the cells, since it modifies the physiological state of the phytoplankton. Furthermore, circadian periodicities were observed in the photosynthetic capacity, suggesting that the phytoplankton of this area have a homogeneous light history due to strong vertical mixing. Under these conditions, the photosynthetic capacity is adapted to the mean light intensity in the mixed layer. The semimonthly (M _f) variations of the mean light intensity in the mixed layer depend on the M _f variations in the vertical mixing, whereas in the short-term, the variations in mean light intensity in the mixed layer are circadian.

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Mélange vertical et capacité photosynthétique du phytoplancton estuarien (estuaire du Saint-Laurent)

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Contribution au programme du Groupe interuniversitaire de recherches océanographiques du Québec (GIROQ)     

Toxin accumulation and feeding behaviour of the planktonic copepod Calanus finmarchicus exposed to the red-tide dinoflagellate Alexandrium excavatum

The planktonic copepod Calanus finmarchicus is a dominant member of the zooplankton community in the lower St. Lawrence Estuary in eastern Canada. Blooms of the toxic marine dinoflagellate Alexandrium excavatum which produces high cellular levels of paralytic shellfish poisoning (PSP) toxins, occur during the period of high C. finmarchicus production in summer in this region. To study the feeding behaviour of C. finmarchicus in the presence of Alexandrium spp., experiments were conducted in which female adult copepods collected from the St. Lawrence Estuary between May and September 1991 were exposed under controlled conditions to two toxic isolates of A. excavatum (Pr18b and Pr11f) from the estuary and to a non-toxic control (PLY 173) of a closely related species, A. tamarense isolated from the Tamar Estuary, Plymouth, U.K. Clearance rates on non-toxic A. tamarense cells averaged 5.5 ml ind^-1 h^-1 but were nearzero with either toxic isolate. When presented with a mixture of A. excavatum and the non-toxic diatom Thalassiosira weissflogii in varying proportions, C. finmarchicus fed upon the diatom but avoided the toxic dinoflagellate. Although feeding rates on A. excavatum were very low, toxin analysis by high-performance liquid chromatography with fluorescence detection (HPLC-FD) revealed that the PSP toxins were accumulated in copepods exposed to toxigenic dinoflagellates.The toxin composition in copepods was similar to that of the toxic dinoflagellate, but not necessarily identical, particularly after short-term (2-h) exposure, when relatively elevated levels of N-sulfocarbamoyl toxins were detected. The evidence suggests that C. finmarchicus ingests toxic dinoflagellate cells, either mistakenly or during exploratory bouts of feeding, and accumulates PSP toxins in its gut system and perhaps in other tissues.     

Brooding of the sub-Antarctic heart urchin, <Emphasis Type="Italic">Abatus cavernosus</Emphasis> (Spatangoida: Schizasteridae), in southern Patagonia

The reproductive cycle of the sub-Antarctic spatangoid sea urchin, Abatus cavernosus, was examined during a 2-year period in southern Patagonia, Argentina. The population studied is the northernmost known coastal population in the austral oceans, and is influenced by a greater annual range of seawater temperature than other Abatus species. The sex ratio of the overall A. cavernosus population did not differ significantly from 1:1, but was not homogeneous across size classes. A clearly defined annual reproductive cycle was found. Spawning occurred from May to July and was synchronous between sexes. Females were observed to be brooding within a 9-month period, from May to February. Juveniles were released during the austral summer, from January to February. The length at which 50% of the females were brooding occurred at a test length of 25.9 mm. A. cavernosus had a large egg size (mean diameter = 1.4 mm) and low fecundity (maximum = 57 eggs per female) compared to closely related species. The number of eggs within each brood pouch was highest in larger anterior brood pouch, which is close to two gonopores, as opposed to the other anterior and two smaller posterior pouches. Significant interannual variation was observed in gonad cycles, fecundity, and embryo development such as: brood size decreased during 2001; adjusted gonad dry weight and fecundity were higher during 2003. Hypotheses concerning the gonadal and brooding cycles and fecundity of sub-Antarctic and Antarctic Schizasteridae are discussed. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Movement and environmental preferences of Greenland sharks (<Emphasis Type="Italic">Somniosus microcephalus</Emphasis>) electronically tagged in the St. Lawrence Estuary,Canada

Three Greenland sharks (Somniosus microcephalus) were tagged with electronic tags, in Baie St. Pancrace, St. Lawrence Estuary, Quebec, Canada. One shark was tagged on 23 July 2004, with an acoustic telemetry tag. Two sharks were each tagged with a pop-up satellite archival tag (PSAT) on 27 August 2004. Two of the sharks remained in or close to the bay, one for 47 days and the other for at least 66 days. The third shark left the bay immediately after tagging on 27 August 2004. This shark entered the main channel of the St. Lawrence Estuary, and had moved 114.9 km upstream by 1 November 2004 when the tag reported to ARGOS satellites. The tags provided a total of 179 days of data on the movement and environmental preferences of Greenland sharks in the St. Lawrence Estuary. Sharks that reported depth and ambient water temperature data from the bay showed significant diel differences in depth preferences and corresponding ambient temperatures. The sharks remained near the bottom of the water column during the day and displayed increased vertical movements at night. The shark that resided in the main channel did not show this pattern, but generally remained at depths between 325 and 352 m. Sharks in the bay experienced water temperatures that ranged from −1.1 to 8.6°C at depths from 0 to 67 m. In the main channel the shark experienced temperatures that ranged from 1.0 to 5.4°C at depths from 132 to 352 m. This is the first report of numerous Greenland sharks inhabiting shallow near shore bays during summer and autumn.     

Costs of extended brood protection in the Antarctic sea star, Neosmilaster georgianus (Echinodermata: Asteroidea)

Changes in size, morphology, and biochemical composition in adults and embryos of a brooding sea star, Neosmilaster georgianus (Sladen), were studied in a population adjacent to Anvers Island, Antarctic Peninsula (64°46′S; 64°04′W) during the austral spring, 1991. Five morphological stages of development were designated in 24 broods, and for each the weight and biochemical composition of the brooding adults and their embryos were determined. Between Stage 1 and 2, the dry weight (dw) and organic weight (ow) of the embryo did not change. From Stage 2 to 3, the dw and ow increased significantly by 10%. Stage 2 and 3 embryos were in clusters of a few (2 to 10) to as many as 40 individuals. In the smaller clusters, individual embryos were attached by tissue cords to another, sometimes atrophied, brood member. In the larger clusters, they were attached to a central mass of tissue containing remnants of embryos. We interpret these interactions as a form of cannibalism which may account for the weight gains between Stage 2 and 3. During Stages 4 and 5, as juvenile form was approached, the dw and ash weight of the young increased significantly and the ow decreased significantly. The calculated energy content for the juvenile (Stage 5) was not significantly different from the energy content of the earliest undifferentiated stage (Stage 1), an indication that most of the organic matter in the egg is the primary contribution to the large juvenile. In brooding females, pyloric caeca indices declined by 52% from Stage 1 to Stage 5 and pyloric energy stores declined by 63% due to proportionately equivalent declines in protein and lipid. The ovary index was low and increased only slightly during brood protection, while the size of the largest oocytes remained approximately 23% that of ova. Energy stores in the pyloric caeca of brooding N. georgianus thus become depleted over a long period of incubation during which the adults apparently starve. This may delay oocyte development and ultimately limit the frequency of individual reproduction. Received: 15 September 1998 / Accepted: 9 March 1999     

Brooding and development of <Emphasis Type="Italic">Anasterias minuta</Emphasis> (Asteroidea: Forcipulata) in Patagonia,Argentina

Brooding, embryonic and larval development, and the influence of environmental and biological factors in tidepool habitats were studied in the sea star, Anasterias minuta, at various sites along ~220 km of the Patagonian coast. This species has a benthic, lecithotrophic development that includes eight distinct developmental stages. A larval organ, the connection cord, is developed from a small preoral lobe at early stages of development and becomes larger and thinner at advanced stages. Fecundity and average egg size increased with female body size. The regression of log egg number to log sea-star size and weight at different sites had a slope significantly less than 3.0, resulting in negative allometry and indicating that brood capacity was limited in large females. Development was generally synchronous among sites, but varied within each brood at advanced stages, with more developed brooded larvae located at the periphery of the brood mass. Brooding was synchronous among various populations at different years and spatial scales, and extended over a period of 8 months. The highest proportion of brooding females occurred during May and June (austral winter). Juveniles were released mainly during September. The likelihood of finding brooding sea stars decreased with increasing sea water temperature, tidal height, and wave exposure, and increased with increasing body size. Both body size of brooding females and brooding rate were higher in the infralittoral fringe than at midlittoral levels. A revision of the current model of brooding behavior and development among forcipulate sea stars is given.     

Seasonal,geographical and size-induced variability in mercury content of Mytilus edulis in an estuarine environment: a re-assessment of mercury pollution level in the Estuary and Gulf of St. Lawrence

Seasonal variability of mercury content of Mytilus edulis (L.) was studied at monthly intervals for 18 mo (June 1978–December 1979) in a natural population from a typical estuarine environment (St. Lawrence Estuary). Body-size-induced variability was observed in the same population in spring 1980. The regression coefficients of log-log relationships between mercury content and body size of the mussels varied with time. However, the regression coefficients were never higher than 1; this indicates, therefore, no net accumulation of mercury with age of the bivalve. A normalization method, taking into account shell length and soft-tissue dry weight, is proposed to minimize the metal variations due to size differences of the mussels. The normalized mercury contents vary seasonally by a factor of two. Minimum values were observed during summer. This factor (2) is taken as the minimal difference which can be accepted as indicating a significant geographical trend in a mussel watch-type study in estuarine environments. In the St. Lawrence Estuary and Gulf, size-normalized concentrations range from 69 to 378 ng Hg g^-1 (dry weight); highest levels were observed in areas where freshwater influence is maximum, while lower contents were encountered in regions of highest Atlantic influence. In the Gulf, mercury in mussel soft tissue is 3x10⁴ times that of the seawater. There is some indication of a decrease in the previous measured levels near the mouth of the Saguenay fjord in the Lower St. Lawrence estuary.     

Circadian variations in photosynthetic assimilation and estimation of daily phytoplankton production

In August 1984, hourly measurements of photosynthetic characteristics were carried out during 96 h, at 5 and 10 m, on a natural population of phytoplankton in the St Lawrence Estuary. Synchronous circadian variations of similar amplitude (max./min.: 2 to 3) were observed at the two depths in both the photosynthetic capacity (P _m ^B ) and the photosynthetic efficiency (^B). Maximum values occurred at around noontime and minima during the night. Estimates of daily specific productivity were computed with and without the observed circadian variability. Large differences (15 to 70%) were evidenced between estimates.Contribution to the programs of GIROQ (Groupe interuniversitaire de recherches océanographiques du Québec) and of the Maurice Lamontagne Institute (Department of Fisheries and Oceans)     

Variabilité à court terme du phytoplancton de l'estuaire du Saint-Laurent

Within the framework of a general study on time variability in the St. Lawrence Estuary, phytoplankton was sampled at 10-min intervals for 13 h. A symmetric, cosine, tapered filter was applied to the time series of cell counts, the cyclical trend being interpreted in relation to physical variables. Autocorrelation coefficients were computed for residual phytoplankton data, in order to investigate fine-scale effects in the series. The main pattern (trend) of variation in the phytoplankton data had a frequency of about 1 cycle/190 min and was related to physical variations, following therefore the tidal movement of water masses in the estuary. On the other hand, the correlogram suggests that phytoplankton is aggregated in patches of an average diameter of about 0.75 km.

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Contribution au programme du Groupe interuniversitaire de recherches océanographiques du Québec (GIROQ).     

Tidal variations in the photosynthesis of estuarine phytoplankton isolated in a tank

In 1981 two large (1 200 1) seawater samples from the St. Lawrence Estuary were kept under constant temperature and light conditions for periods of 50 and 68 h, respectively. In both tank experiments, semidiurnal variations in NH₄ were observed that could be related to cyclical NH₄ uptake by the phytoplankton. Semidiurnal cycles in photosynthetic efficiency (^B) and intracellular chlorophyll a in the tank, phased on tides at sea, were also evidenced in both experiments. These results support the hypothesis that variations in phytoplankton photosynthetic activity, which are possibly endogenous, can be phased on semidiurnal variations in vertical tidal mixing (variations of the mean light conditions in the mixed layer). In addition, observed variations in intracellular chlorophyll a suggest the possibility of endogenous cycles of phytoplankton light and shade adaptation.Contribution to the program of GIROQ (Groupe interuniversitaire de recherches océanographiques du Québec)     

Reproductive habits and life cycle of the small clam Kingiella chilenica (Bivalvia: Cyamiidae) in an estuarine sand flat from the south of Chile

A population of the small clam Kingiella chilenica Soot Ryen 1959 was studied from March 1986 to December 1988 in an intertidal flat at Queule River, in the south of Chile. The life cycle and life history pattern of the bivalve were established contrasting population structure and dynamics to its reproductive habits. Individuals are gonochoristic and semelparous, presenting a typical annual life cycle. The species is a sequential brooder whose embryos undergo direct development. After the brooding season (summer through autumn), the adults disappear gradually (autumn through winter). Recruited juveniles overwinter during a relatively long period, undergoing rapid growth during the spring to attain the adult stage during the summer. The number of brooded embryos increases in proportion to adult length cubed. Life history traits of this bivalve are compared to those reported for other small brooder clams. Some basic tendencies become apparent when traits for semelparous versus iteroparous species are contrasted. As in other semelparous sequential brooders (Transennella tantilla, Gaimardia bahamondei), the relationship between brood size and shell length obseved in K. chilenica does not fit the allometry hypothesis for marine brooding invertebrtes that allometric constraints on the brooding space limit the fecundity of larger individuals. Also contrary to theoretical predictions, small body size does not limit the diversity of a clam's developmental patterns. Possible explanations for this finding are discussed.     

Long-distance dispersal of a subantarctic brooding bivalve (Gaimardia trapesina) by kelp-rafting

The probability of successful dispersal by sessile benthic invertebrates is thought to strongly influence their geographic distribution and population genetics. Generally, species with long-lived planktonic larvae are expected to exhibit wider distribution patterns than those species which brood their young, due to their presumably greater potential for dispersal. In some cases, however, brooding species exhibit broad distributions and show evidence of genetic exchange with geographically distant populations. One potential factor that has been invoked as an expianation is dispersal by floating and rafting of adults and egg masses. Several studies have shown that it is possible for sessile adults to disperse on the order of several to many thousand kilometers by rafting on debris in ocean currents. With very few exceptions, however, direct evidence of rafting in the open ocean has been lacking. We present evidence of long-distance (1300 to 2000 km) dispersal of a brooding pelecypod,Gaimardia trapesina (Lamarck, 1819), in the Southern Ocean in the vicinity of Cape Horn, the Falkland Islands, and the antarctic island South Georgia (54°S; 37°W). Data on survival and fecundity rates ofG. trapesina and the prevalence of kelp rafts collected during the austral winter of 1993 indicate that dispersal by rafting can occur over ecologically relevant time scales and could potentially serve as a significant means of genetic exchange between populations.     

Mating systems in the sea anemone genus Epiactis

Four morphologically similar species in the sea anemone genus Epiactis exhibit overlapping distributions on the Pacific coast of North America; E. prolifera, E. lisbethae, E. ritteri and E. fernaldi. All brood their offspring up to the juvenile stage, but each has a different combination of internal versus external brooding and hermaphroditism versus gonochory (separate sexes). Specimens were collected from sites ranging from British Columbia to southern California between December 1988 and July 1992. Mating systems were inferred from genetic comparisons of mothers and offspring histological analyses of sex expression and observations on brooding and spawning behavior. Allozyme and multilocus DNA fingerprint analyses of the gynodioecious hermaphrodite E. prolifera showed that offspring were all identical to their mothers, a result consistent with either asexual reproduction, self-fertilization or extreme biparental inbreeding. In the gonochore E. lisbethae, mothers and offspring were also electrophoretically identical, but variation in DNA fingerprints indicated cross-fertilization. Similar DNA fingerprint differences between mother and offspring in the gonochore E. ritteri implied that cross-fertilization also occurs in this species. No mother-offspring comparisons were performed on E. fernaldi, as this species was not observed brooding offspring during this study. Although incomplete, the results of this study increase our knowledge of the very unusual combination of reproductive modes in the genus Epiactis, and argue for further investigations of the evolution and genetic consequences of mating systems in these species.     

Reproduction,development and population biology of the Caribbean ophiuroidOphionereis olivacea,a protandric hermaphrodite that broods its young

Reproduction and development of the ophiuroidOphionereis olivacea H. L. Clark, a species that broods its young, were investigated in Florida from January through December 1985 and in Belize in June 1985 and April 1986.O. olivacea has a maximum disc diameter of 5.2 mm and is a protandric hermaphrodite. Sex reversal occurs at disc diameters ranging between 2.2 and 4.0 mm; it has 400 µm diameter oocytes and is ovoviviparous. As expected from an obligate out-crosser, this ophiuroid has a similar allocation to male and female reproduction, in terms of gonad volume. Reproduction of the Florida population was monitored for one year.O. olivacea is a sequential brooder and incubates its young during the coldest months of the year. The females produce a single clutch of juveniles each year. Spawning starts in November and peaks in January and February. The ova are spawned synchronously from all the ovaries of each female into the bursae and are retained there. Fertilization is accomplished by intake of sperm into the female bursae.O. olivacea develops through a ciliated embryonic stage that appears to be a modified vitellaria larva lacking ciliary bands. These modified larvae swim in seawater in the bursae. Direct development in this species has resulted from heterochrony in the loss of ophiopluteus features. The number of brooding females in the population increases in November and reaches a maximum in March. JuvenileO. olivacea emerge from the bursae at a disc diameter of 480 µm and each arm has three segments. Recruitment was first detected in April, and juveniles were particularly abundant in June. Small size inO. olivacea is associated with paedomorphic heterochrony with precocious maturation of the males at a disc diameter of 1.5 mm, ~2 mo after taking up their free existence. Brooding as a life history mode inO. olivacea is examined and the adaptations of this species to its warm-water habitats are discussed.     

Influence of substratum heterogeneity and settled barnacle density on the settlement of cypris larvae

On the Atlantic coast of Canada, Semibalanus balanoides (L.) is widely distributed in the mid-intertidal zone, whereas in the Gulf of St. Lawrence, this species is mostly limited to crevices. We tested the hypothesis of regional differences in microhabitat selection by barnacle larvae at settlement in 1984 and 1985 at St. Andrews, New Brunswick, Canada. Since larvae settle in microhabitats already colonized by adults, the relative influence of settled barnacle density and of different scales of substratum heterogeneity on settlement were evaluated experimentally at Capucins, Québec, (Gulf of St. Lawrence) and at St. Andrews, New Brunswick (Atlantic coast). On a large scale (>10 cm deep crevices) of heterogeneity, results show that, in the Gulf, cypris larvae settled nearly exclusively (93%) in natural crevices rather than on adjacent horizontal surfaces. On the Atlantic coast, settlement was more important outside than inside of crevices, when the substrata were either natural or artificial. This result is unique and contrasts sharply with all known reports on barnacle settlement in relation to surface contour. The influence of barnacle density on settlement was greater than that of large scale heterogeneity. On a small scale (<1.5 cm deep cracks), the presence of conspecifics had a stronger effect on settlement than heterogeneity in both regions. Field observations showed a relationship between larval settlement density and percentage of adult cover. Settlement increased up to 22 or 30% (Gulf and Atlantic coast) of adult cover and decreased afterwards. The results confirm the hypothesis of larval selection for cryptic habitats in the Gulf and the opposite behaviour (preferences for horizontal surfaces) on the Atlantic coast. This microhabitat selection is apparent at large scales of heterogeneity, whereas at small scales, the presence of conspecifics is the predominant factor.Contribution to the programme of GIROQ (Groupe interuniversitaire de recherches océanographiques du Québec)     

Population genetic structure in brooding sea anemones (Epiactis spp.) with contrasting reproductive modes

Effects of dispersal and mating systems on the genetic structure of populations were evaluated by comparing five sea anemones: four Epiactis species that brood their offspring to the juvenile stage and one Anthopleura species that broadcasts gametes and has pelagic, planktotrophic larvae. The anemones were sampled at sites ranging from British Columbia to southern California between 1988 and 1992 and were analyzed by enzyme electrophoresis and by multilocus DNA-fingerprinting. Results were only partially consistent with expectations. While all four brooding species had lower observed heterozygosities than the broadcasting species, not all brooding species had greater population subdivision than the broadcasting species. The self-fertile E. prolifera had the expected evidence of intense local inbreeding ( f = 0.955); unexpectedly, the cross-fertile E. lisbethae and E. ritteri also had similar departures from random mating ( f = 0.957 and 0.831, respectively) probably due to biparental inbreeding among near neighbors in small, highly subdivided populations. Received: 24 May 1996 / Accepted: 12 July 1996     

Dynamics of external brooding in the sea anemone Epictis Epiactis prolifera

The sexually produced young of the externally brooding actinian Epiactis prolifera Verrill, 1869 are attached to the parent's column just above the base. A transitory brood groove may be formed around the limbus when the parent contracts. In the population studied on the coast of Sonoma County, California, USA, from 27 to 49% of the adult anemones were brooding at any time, the proportion being inversely related to seawater temperature and directly related to size of the anemones. A brood usually consisted of young of various sizes. Overall, number of young being brooded was directly related to parent size and inversely related to size of the brooded juveniles. The growth rate of juvenile anemones was inferred to average about a millimeter in pedal disc diameter per month. Juveniles dislodged from the parent when smaller than 4 mm in basal diameter apparently cannot survive in the intertidal zone. Juveniles which successfully make the transition to adulthood have, therefore, been brooded for at least 3 months. Fewer than 50% of the smallest juveniles survived to a size at which they could live independently. The transition to adulthood was accompanied by a mortality of 80%. The survival rate, from egg to adult, was calculated to be about 1% per season.This work was done in partial fulfillment of the requirements for the Ph.D. degree in the Department of Zoology, University of California, Berkeley, and was supported by a U.S. National Science Foundation Traineeship.     

Comparison of spatial overlap between the polychaetes Nereis virens and Nephtys caeca in two intertidal estuarine environments

An intensive study of spatial overlap between the polychaetes Nereis virens (Sars) and Nephtys caeca (Fabricius) was conducted in 1992 on two tidal sand flats, which differ by their exposure to dominant winds and residual currents in the Lower St. Lawrence Estuary (Québec, Canada). Results showed that spatial overlap (Lloyd's index and spatial distribution) was higher among adults of both species at the lower tidal elevation of l'Anse-à-l'Orignal (north-east oriented). Results also suggest weaker interspecific interactions among juveniles of both species because of limited spatial overlap. In Baie-du-Ha!Ha! (south-west oriented), spatial overlap was greater than that observed in l'Anse-à-l'Orignal and appeared important in adults as well as in juveniles. Levins' directional measure of competition indicated an asymmetric spatial overlap between N. virens and N. caeca in Baie-du-Ha!Ha! and a symmetric overlap in l'Anse-à-l'Orignal. The intraspecific encounter values, estimated from Lloyd's mathematical expression, were significantly higher than values of interspecific interactions only in l'Anse-à-l'Orignal. Moreover, no vertical stratification was found inside the sediment, with no effect of the densities and individual body weights of the polychaetes. Juveniles of both species mainly inhabited the organic-rich upper portion of the sediments (0 to 12 cm), while adults colonised greater depths (>25 cm) where organic matter content was lower. A complementary field experiment was conducted in 1993 to investigate interspecific interactions (predation and competition) existing between N. virens and N. caeca. Results from this experiment depend on which species was first-introduced and showed an important variation in mortality rates between allopatric and sympatric conditions. The influence of competition and predation on the structure of these populations is also discussed in relation to their spatial overlap.