Yves-Marie Paulet

Differential δ13C and δ15N signatures among scallop tissues: implications for ecology and physiology

There have been several studies where the isotopic composition of organisms has been determined seasonally, but fewer have examined separate organs. In this context, separate organs (e.g. gonad, digestive gland and muscle) of a suspension-feeder, the scallop Pecten maximus, were used to assess seasonal changes of both stable isotopes and biochemical components. Our study used multiple indicators [stable carbon and nitrogen isotope ratios, biochemical components and seston chlorophyll-a (chl a)] to track nutritive activity and energy allocation in P. maximus from the Bay of Brest (France). In addition to seasonal variation in the isotopic composition of P. maximus tissues, we found strong differences in the mean isotopic signatures of different organs. This has serious implications for interpretation of animal diets and potential use in animal physiology. Furthermore, we present evidence that seasonal variations of metabolism will cause changes in the isotopic composition not related to changes in the diet. Interpretation of isotopic data may require consideration of values from several separate organs. Finally, y 15 N appears powerful to track metabolite fates in the scallop P. maximus. D 2002 Elsevier Science B.V. All rights reserved.

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.

Growth anomalies in Pecten maximus from coastal waters (Bay of Brest, France): relationship with diatom blooms

The aim of this study was to characterize the daily shell growth of Pecten maximus from early 1998 to late spring 1999 in the Bay of Brest with a careful qualitative and quantitative description of the pelagic primary production. Our results, in accordance with previous studies, demonstrate that there are episodic declines in the growth rate. Concurrent events of low growth rate and large bottom-concentrations of algae following diatom blooms (Cerataulina pelagica or Rhizosolenia delicatula), suggests that this high concentration of phytoplankton on the bottom layer may affect food intake or respiratory activity of the scallops by gill clogging or oxygen depletion. In this study, silicic acid or phosphorus are inferred to be limiting factors responsible for the collapse of the spring bloom. Further, we suggest that, in an N-enriched ecosystem, Si or P-limitation and the changing sinking velocities of phytoplankton, could affect the biology of benthic suspension-feeders.

Monoamines and reproduction in Pecten maximus, a preliminary approach

Summary The distribution of serotonin-like immunoreactivity was studied in the central nervous system and the gonad of Pecten maximus. Cerebral and pedal ganglia contain a well developed serotonin-immunoreactive neuronal subpopulation, whereas positive neurons are scarce in the visceral ganglion. The distribution pattern of immunoreactive elements in the gonad indicate that serotonin is involved in peripheral neurotransmission of this organ. Seasonal variations of monoamines (serotonin, dopamine and noradrenaline) have been investigated in the nervous system using HPLC. Lower concentrations of serotonin are observed during winter in the central nervous system; dopamine levels of the visceral ganglion are correlated to gonadal growth.

Survival of Nucella lapillus in a tributyltin-polluted area in west Brittany : a further example of a male genital defect (Dumpton syndrome) favouring survival

A male genital defect was reported in a Nucella lapillus (L.) population at Dumpton Gap (England). This defect was termed “Dumpton syndrome” (DS) and appears to be a genetic feature. Its main characteristic is the absence of penis (aphally) in males and in females. In 1992, such a phenomenon was discovered in populations in the vicinity of Brest (Brittany, France). DS-affected females exhibit fewer tributyltin-induced imposex characteristics than expected in normal individuals. The percentage of female sterilization is thus lower, favouring population survival. In consequence, the DS is considered to be a “pollution-resistance” feature. Comparison with the Dumpton population revealed similarities and differences in the DS characteristics. It is thus hypothesized that the DS observed at the two locations is due to two different biological mechanisms. Indeed, aphallic males with a split prostate were observed but no underdevelopment of their vas deferens and testis was noted in the present study. In this gonochoristic gastropod species, the most DS-affected males in Brest possess an ovotestis and it is thus hypothesized that feminity is remnant in N. lapillus. Incidence of abnormality is ten times higher in females than in males. This suggested that a sex-difference operates in the Brest phenomenon.

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.

Effect of photoperiod on conditioning of the great scallop

A 45 days experiment was conducted by exposing adult scallops with empty gonads to three different photoperiod regimes under constant food and temperature conditions: Simulated natural/decreasing photoperiod (N), constant photoperiod (C) and increasing photoperiod (I). At the end of the experiment the I and C groups had significantly higher gonad indexes and larger oocyte diameters compared to the N group. No significant changes were found in the N group during the experiment. The adductor muscles and digestive glands did not change significantly during the experiment and no differences were found between the groups. These results suggest that photoperiod affects early rebuilding of gonads in scallops from Western Norway.

Histology, histochemistry and enzyme biochemistry in the digestive system of the endosymbiont-bearing bivalve Loripes lucinalis (Lamarck)

The structural and functional aspects of the digestive gland in the endosymbiont-bearing bivalve Loripes lucinalis were examined. Specimens were collected over a one year period (September 1991 to September 1992) from the Moulin Blanc beach, Bay of Brest, (Brittany, France). The two apertures leading from the stomach into the digestive diverticula are very large as are the digestive ducts and tubules. The main and secondary ducts possess no ciliated tracts and present a definite brush border. The structural aspects of the cells lining the ducts, in addition to the presence of abundant acid phosphatase activity, suggest that these cells are actively involved in absorption and intracellulgr digestion of food material. Lipid and glycogen are abundant throughout the digestive diverticula suggesting that this organ is the site of significant energy stores. The tubules contain the two classical cell types, namely the secretory and digestive cells. Also present within the tubules are extremely large acid polysaccharide-filled vacuoles which originate from the lysis of digestive and/or secretory cells. These vacuoles suggest an extensive process of tubule autophagy and regeneration. The intestine is lined with cilia and microvilli. The cytological features of the intestinal cells, together with an abundant acid phosphatase activity, suggest that these cells are capable of absorption and intracellular digestion. The wet weight of the gonad-digestive gland complex varied throughout the year between 12 and 61.2 mg with an average wet weight of 31.8 ± 8 mg. Following spawning, total wet weights dropped by 50%. The α-amylase activity was measured and an average β-amylase to protein ratio of 5.34 ± 1.09 U/g protein was obtained. It was observed that amylase activity expressed per g protein may vary with the animals physiological status. The temporal evolution of absolute α-amylase activity throughout the sampling period revealed that the variations in activity were dependent on a combination of environmental factors, including temperature and food levels. Digestive activity, however, did not seem to correspond to reproductive efforts.

Changes in tissue concentrations of the vitamins B1 and B2 during reproductive cycle of bivalves. Part 1 : The scallop Pecten maximus

Abstract Vitamins B1(thiamin) and B2 (riboflavin) in eggs of the scallop Pecten maximus are highly utilized during larval development. Possible storage of these vitamins, for later transfer to gonads and eggs, was studied over the natural reproductive cycle of the scallop. Vitamin concentrations were measured during hatchery conditioning at three periods of the year. Vitamin B1 was found mainly in muscle, gonad and eggs, while B2 was detected in the digestive gland, gonad and eggs. They varied simultaneously in the three organs during the annual cycle. Two main peaks were observed for digestive gland and gonads in spring and summer. The first peak corresponded to vitellogenesis and decreased in gonads in June as the scallops spawned and the vitamins were lost with the eggs. The second peak was interpreted as vitamin storage in gonad tissue in fall. The large vitamin decrease occurring from November to February coincided with gonial mitoses observed by histology. Possible high utilisation of thiamin and riboflavin during winter was supported by hatchery conditioning at three periods during the year: during the same winter period, when broodstock were fed a high-vitamin diet, vitamin levels stayed low in all organs, as vitellogenesis was in progress. However, in spring and summer vitamin incorporation increased. We concluded that attention should be paid to winter-conditioning diets, as riboflavin and thiamin, even when provided at high levels in the food, did not accumulate in organs, suggesting intense utilisation when mitoses in the gonads take place.