Gérard Thouzeau

Effects of environmental factors on the daily growth rate of Pecten maximus juveniles in the Bay of Brest (France)

The study of the effects of environmental factors on the daily growth of Pecten maximus juveniles (one- and two-year olds) in the Bay of Brest was prompted by the decline of the scallop fishery in the Bay. Scallops over 30 mm in shell length were collected monthly from October 1994 to November 1995. Daily shell growth rings were counted using image analysis. Multiple stepwise regression analyses were performed to identify environmental parameters significantly affecting mean daily shell growth rates of one- and two-year-old juveniles in 1994 and 1995. Normal growth of Pecten shells is mainly regulated by bottom-water temperature, salinity and, to a lesser extent, river flow, rather than food. In 1995, three major toxic blooms of Gymnodinium cf. nagasakiense were recorded, leading to major reductions in shell growth rates. The effect of the toxic blooms on the natural Pecten populations was all the more important as no wild scallop spat settled on artificial collectors during the 1995 summer toxic events, and no pre-recruits originating from the 1995 spawnings were sampled on the bottom, despite good-quality spawnings. Two growth retardations were observed for one-year-old juveniles during the summer of 1995: the first one appears to be related to the sedimentation of a Rhizosolenia delicatula–Chaetoceros sociale bloom. It is suggested that the large aggregates of these diatoms led to clogging of the scallop gills. The second growth decrease was explained by the first bloom of G. cf. nagasakiense. The second and third dinoflagellate blooms were not associated with daily growth rate decreases in one-year-old scallops. The toxic effect of the dinoflagellate blooms was greater for two-year-old than for one-year-old juveniles; higher filtration rates and/or a differently oriented metabolism, compared with younger individuals, would explain these variations.

Gregariousness and protandry promote reproductive insurance in the invasive gastropod Crepidula fornicata: evidence from assignment of larval paternity

According to the size‐advantage hypothesis, protandric sequential hermaphroditism is expected when the increase in reproductive success with age or size is small for males but large for females. Interestingly, some protandrous molluscs have developed gregarious strategies that might enhance male reproductive success but at the cost of intraspecific competition. The gastropod Crepidula fornicata, a European invading species, is ideal for investigating mating patterns in a sequential hermaphrodite in relation to grouping behaviour because individuals of different size (age) live in perennial stacks, fertilization is internal and embryos are brooded. Paternity analyses were undertaken in stacks sampled in three close and recently invaded sites in Brittany, France. Paternity assignment of 239 larvae, sampled from a set of 18 brooding females and carried out using five microsatellite loci, revealed that 92% of the crosses occurred between individuals located in the same stack. These stacks thus function as independent mating groups in which individuals may reproduce consecutively as male and female over a short time period, a pattern explained by sperm storage capacity. Gregariousness and sex reversal are promoting reproductive insurance in this species. In addition, females are usually fertilized by several males (78% of the broods were multiply sired) occupying any position within the stack, a result reinforcing the hypothesis of sperm competition. Our study pointed out that mating behaviours and patterns of gender allocation varied in concert across sites suggesting that multiple paternities might enhance sex reversal depending on sperm competition intensity.

Experimental collection of postlarvae of Pecten maximus (L.) and other benthic macrofaunal species in the Bay of Saint-Brieuc, France. I. Settlement patterns and biotic interactions among the species collected

Settlement patterns of Pecten maximus (L.) postlarvae and 105 other species of benthic macrofauna in the Bay of Saint-Brieuc were studied from spat collectors in order to clarify biotic interactions at the postlarval stage. Between-sites differences in taxonomic group abundances reflected the bottom sediment type beneath the collectors and were partly explained by hydrographic mechanisms; larval retention occurred in the collection areas. Other factors regulating settlement process were the water turbulence and trophic resources. Temporal variability in collection occurred between and within the taxonomic groups. Optimal collection for all the species was within 2 m above the seabed for an immersion period of <1 month. Anomia ephippium L., Crepidula fornicata L. and Chlamys varia L. were the main spatio-trophic competitors of P. maximus collected in the bay while brachyuran crustaceans accounted for 96% of the potential scallop predators. Possible ways of improving scallop spat collection and cultivation in the bay are discussed.

Rôle des interactions biotiques sur le devenir du pré-recrutement et la croissance de Pecten maximus (L.) en rade de Brest

Biotic interactions within benthic megafauna were studied from quantitative dredge samples to investigate their role in the persistence of low scallop, Pecten maximus, stocks in the Bay of Brest. The distributions of 148 species of the megafauna and of four age classes of Pecten maximus were sampled. Suspension feeders dominate all megabenthic assemblages within the bay. This domination can be related to Crepidula fornicata spreading in the bay; however, no direct competition for food between scallops and the slipper limpet (introduced species) has been observed. On the other hand, Crepidula spreading leads to major changes in sediment type (silting) in the bay owing to biodeposit production, and silting would be a major factor inducing the decrease in Pecten maximus distribution. Predation would not be limiting. Within 10 years, one may predict a strong decrease in the area colonized by scallops in the bay.Biotic interactions within benthic megafauna were studied from quantitative dredge samples to investigate their role in the persistence of low scallop, Pecten maximus, stocks in the Bay of Brest. The distributions of 148 species of the megafauna and of four age classes of Pecten maximus were sampled. Suspension feeders dominate all megabenthic assemblages within the bay. This domination can be related to Crepidula fornicata spreading in the bay; however, no direct competition for food between scallops and the slipper limpet (introduced species) has been observed. On the other hand, Crepidula spreading leads to major changes in sediment type (silting) in the bay owing to biodeposit production, and silting would be a major factor inducing the decrease in Pecten maximus distribution. Predation would not be limiting. Within 10 years, one may predict a strong decrease in the area colonized by scallops in the bay.

Variations in food intake of Pecten maximus (L.) from the Bay of Brest (France): influence of environmental factors and phytoplankton species composition.

Previous studies carried out in the bay of Brest on daily shell growth of Pecten maximus have demonstrated that temperature is a major control on daily shell growth in contrast to food supply. However, repeated events of slow growth have been observed during diatom and dinoflagellate blooms. The aim of this study was to determine how fluctuations in environmental parameters influence P. maximus food intake and daily shell growth rate. In 1995, P. maximus food intake and growth were highest when Cerataulina pelagica (diatom) blooms occurred and lowest during Gymnodinium cf. nagasakiense (dinoflagellate) blooms. During blooms of other diatom species, P. maximus food intake and growth were high when the algal concentration did not exceed a critical threshold, dependent upon the dominant species and sedimentation rate of diatoms. These results demonstrate that the morphological and physiological features of phytoplankton bloom species strongly affect benthic microphytophagy, a component of benthic-pelagic coupling.

Experimental collection of great scallop postlarvae and other benthic species in the Bay of Brest: settlement patterns in relation to spatio-temporal variability of environmental factors

Settlement patterns of great scallop (Pecten maximus) postlarvae and other benthic species in the Bay of Brest were studied from 2-D and 3-D collectors to clarify biotic interactions at the postlarval stage. Plane tiles became colonized mainly by epifaunal suspension feeders, while the 3-D substrata collected a more diverse fauna. Both species diversity and total abundance were higher in Netlon bags, compared with sandstone tiles. However, hydroids, bryozoans, tubicolous polychaetes, a number of epifaunal molluscs and macroalgae were more abundant on tiles. Between-sites differences in taxonomic group abundances mostly reflected the benthic fauna and flora beneath the collectors and would be explained by differential settlement and survival, according to local environmental conditions. Sediment type, depth, salinity, water turbulence, food and biotic interactions would be the factors regulating settlement. Bryozoans, hydrozoans, Anomia ephippium, Crepidula fornicata, mussels, tubicolous polychaetes, Pisidia longicornis, Hiatella arctica, Aequipecten opercularis and Chlamys varia were the main potential competitors of Pecten maximus collected. Crabs and Nassaridae would account for most of the potential scallop predators. Possible ways of improving scallop spat collection and cultivation are discussed with reference to the efficiency and placement of collectors, to environmental characteristics and to the intensity of biotic interactions at the postlarval stage.

Experimental collection of postlarvae of Pecten maximus (L.) and other benthic macrofaunal species in the Bay of Saint-Brieuc, France. II. Reproduction patterns and postlarval growth of five mollusc species

Spawning seasonality and postlarval (spat) growth of five mollusc species taken in spat collectors in the Bay of St-Brieuc in 1985 were analysed from size-frequency distributions. There is evidence for a short spawning period and a major spawning for all species. The respective abundance peaks of the three pectinid species in the collectors were shifted with time; the settlement of Pecten maximus occurred 15–30 days before those of Chlamys sp. A spatial intracohort growth variability was found in P. maximus, depending on the collector height from the seabed. This variability appeared as early as the first benthic stages (mean height < 500 μm), with growth being lower by 25–30% in the collectors located nearest to the bottom. Postlarval growth of P. maximus, Anomia ephippium and Crepidula fornicata also varied seasonally between cohorts, with growth rates decreasing during the reproductive season. Temperature and trophic resources were the main factors determining these variations. In P. maximus, another source of growth variation, the gamete quality, is suggested. These results, added to those obtained in a study of biotic interactions within the collectors (Thouzeau, 1991), help define ways to improve collection of pectinid spat in the Bay of St-Brieuc.

Variabilité spatio-temporelle de la biomasse microphytobenthique en rade de Brest et en Manche occidentale

Abstract Microphytobenthic biomass was estimated in the Bay of Brest (1994) and the western English Channel (Trezen Vraz, 1993) using spectrophotometry. Best results (42 % difference) were obtained with an instantaneous extraction procedure at room temperature, compared with the cold extraction procedure. Chlorophyll biomass in sediment was higher in the western English Channel (65–215 mg Chl a m−2) than in the Bay of Brest (10–113 mg Chl a m−2), in contrast to total pigment biomass (Chl a + pheo a : 88–254 mg m−2 at Trezen Vraz vs. 131–934 mg m−2 in the Bay of Brest). This study emphasized decreasing pigment biomass from the estuary to the open sea. Low benthic pigment biomass in the Bay, compared to eutrophic ecosystems, agrees with previous studies on the pelagic system, suggesting that there is no eutrophication in the Bay of Brest. Microphytobenthic blooms were strongly correlated with environmental factors, such as river floods, light intensity, water temperature and the spring/neap tidal cycle. Turbidity and light could be limiting factors at Roscanvel, i.e. the deepest site facing gyre circulation. The latter would favour sedimentation of silt particles at the sediment-water interface. Microphytobenthos population dynamics would be also dependent on the spring/neap tidal cycle, since maximum chlorophyll biomass occurred at neap tides at all three sites in the Bay of Brest, in contrast to the western English Channel. Lower temporal variations of pigment biomass in sediment were observed in the Channel, compared with the Bay of Brest; maximum chlorophyll biomass in sediment was found to occur three to four weeks after the phytoplankton blooms.