Comparison of multiple techniques to evaluate reproductive variability in a marine bivalve: application to the scallop Pecten novaezelandiae (original) (raw)
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Reproduction and Spawning in Calico Scallops, Argopecten Gibbus, from Bermuda
Journal of Shellfish Research, 2006
The gametogenic cycle and breeding season of the calico scallop, Argopecten gibbus was examined in Bermuda using quantitative and qualitative methods. Gonadal index varied seasonally, a rapid increase occurring in the winter months from January to March, and reaching maximum values in March and April. Greatest gonad growth and spawning were associated with low seawater temperature and low food levels. Variations in muscle indices implied a partial reliance on muscle reserves during the early stages of oogenesis. The later stages of ovarian growth (vitellogenesis and oocyte maturation) seemed to show a direct dependence on food supply. Histological analysis indicated the initiation of gamete development in early fall, when cell differentiation was first observed; oocyte ripening took place in late fall and early winter, reflected in increased oocyte diameters and gonadal indices. The lack of spawning activity during the hot summer months was seen histologically by the high proportion of spent cells in the gonads and a reduction in oocyte size, resulting in a minimal gonadal index. Individuals with gonad indices above 2 and mean oocyte diameters exceeding 50 m were induced to spawn with thermal shocks, exceeding 70% response rate in oocyte release in all trials. The present study extends our limited knowledge of the reproductive cycle in this subtropical scallop while providing a useful index to evaluate spawning readiness in cultured broodstock for routine hatchery purposes.
Journal of Shellfish Research, 2016
Monitoring gonad maturation for protandrous and functional hermaphrodite species such as the giant clam Tridacna maxima is difficult due to the juxtaposition and relative proportion of male and female tissues in the gonad [gonadal sex ratio (GSR)]. Here, the relevance of the widely used gonadosomatic index (GSI) as proxy of giant clam gonad maturation is tested with a large dataset (n ¼ 265). Gonadosomatic index is compared with other indices, namely the proportion of the male part harboring spermatozoids, the proportion of empty oocyte follicles, the mean oocyte diameter, and the oocyte elongation. At gonad scale, high index variability highlighted partial spawning. At individual scale, male and female maturation proxies were contrasted, showing either asynchronous emissions of male and female gametes or contrasted spermatogenesis and oogenesis duration. The GSI was mostly driven by the number and diameter of oocytes and therefore it is recommended here as primary proxy for female maturity. Except for the oocyte elongation, all indices were affected by the GSR, which ruled out drawing conclusions at population scale. These results highlight the need for maturation stage proxies that are optimized for functional hermaphrodite species.
Journal of Shellfish Research, 2008
The reproductive cycle of Pecten sulcicostatus is described as part of an investigation into the potential commercial culture of this species in South Africa. Scallops were collected monthly from False Bay, from August 2004 to October 2005, to determine seasonal variation in the gonadosomatic index (GSI) and to assess associated histological changes within the gonads. The reproductive cycle of P. sulcicostatus demonstrated clear seasonality. The mean GSI was highest from June to September (winter to early spring), and lowest from November to January (late spring to summer). The GSI findings were corroborated by histological analysis of the gonads with the mean oocyte diameter reaching a maximum in August and a minimum in November. Seasonal stratification and corresponding changes in phytoplankton biomass are considered to control the reproductive cycle of P. sulcicostatus in False Bay as winter spawning and the subsequent decline in the GSI coincide with the transition to spring upwelling conditions and a decline in bottom water temperature and food availability.
Marine Environmental Research, 2013
Understanding variability in reproductive schedules is essential to the management of recruitment limited fisheries such as that of Pecten maximus. Small scale (<5 km) variation in gonad condition and the onset of spawning of P. maximus were found among commercial scallop grounds in Isle of Man waters. Environmental and fishing drivers of these spatial patterns were investigated using a generalised additive model. Rate of change in temperature over the month prior to sampling was identified as the short term driver of gonad weight associated with the autumn spawning event. Long term drivers were average annual chlorophyll a concentration, scallop density, stratification index and shell size. The model explained 42.8% of deviance in gonad weight. Within site variation in gonad condition was high, indicating a "bet hedging" reproductive strategy which may decrease the chance of fertilisation especially at low densities. Therefore, areas protected from fishing, where scallop densities can increase may help buffer against reproductive failure. An increase in shell length from 100 mm to 110 mm equated to an increase of approximately 20% in gonad weight. Protecting scallops from fishing mortality until 110 mm (age four) compared to 100 mm (age three) may lead to an overall increase in lifetime reproductive output by a factor of 3.4.
Marine Ecology Progress Series, 1988
Quantitative and qualitative aspects of reproduction in the giant scallop Placopecten magellanicus (Gmelin) were compared between populations in the Damariscotta River (13 to 20 m depth) and the Gulf of Maine (170 to 180m depth). Gonad weight (p < 0.005) and gonad index (p < 0.01) were significantly greater in the shallow-water population dunng the period of gametogenesis. Oocytes of scallops in the shallow-water population developed gradually and spawning occurred in the fall. In spite of reduced gonadal production, scallops at the deep-water site completed gametogenesis. Oogenesis in the deep-water population was characterized by a greater initial rate of oocyte growth, resorption and a possible minor spring spawning, a period of redevelopment with continued resorption, and a later, more abrupt fall spawning. Gametogenesis was generally less synchronous among individuals in the deep-water population and oocytes were less dense withn gonadal tissue. The reduction in gonad production at the deep-water site was manifested as a reduced fecundity (number of eggs), rather than reduced egg size, as mean oocyte bameters were similar for both populations. In spite of apparent energehc hmitations, scallops at 180 m produce and spawn apparently viable ova, but in considerably lower numbers than those from shallower depths.
Invertebrate Reproduction & Development, 2002
The reproductive cycle of Eurhomalea exalbida (Chemnitz, 1795) in Ushuaia Bay, Beagle Channel, was studied from October 1998 to December 1999. Clams were collected monthly by SCUBA diving at 3-5 m water depth. The degree of sexual maturity was determined histologically (n=338). The sex ratio was 1:1 (Chi-squared p>0.05). First sexual maturity occurred at 39 mm shell height in males and 40 mm in females (i.e., 4 years of age). Five distinct histological stages of sexual maturity could be established in males: (1) early active, (2) late active, (3) ripe, (4) partially spawned with recovery and (5) spent. Females had oocytes ranging from small and immature to large and fully developed in their ovaries throughout the year. Since we could not classify reproductive stages based on a qualitative histological scale of sexual maturity, quantitative measures using an image analyser were used. In males a high percentage of mature individuals were present between January and August. The percentage of early active and partially spawned individuals was highest in November 1998 (70%) and 1999 (60%). In females, the mean number of developed oocytes per unit of gonadal area in a cross section, the mean diameter (minor axis) of oocytes, and the percentage of gonadal area occupied by oocytes were lowest in November, indicating a spawning peak. These results indicate synchronous gonadal development and spawning in males and females. The more intensive spawning activity in November coincides with the higher biomass and production of phytoplankton in spring.
Invertebrate Reproduction & Development, 2002
The reproductive cycle of Eurhomalea exalbida (Chemnitz, 1795) in Ushuaia Bay, Beagle Channel, was studied from October 1998 to December 1999. Clams were collected monthly by SCUBA diving at 3-5 m water depth. The degree of sexual maturity was determined histologically (n=338). The sex ratio was 1:1 (Chi-squared p>0.05). First sexual maturity occurred at 39 mm shell height in males and 40 mm in females (i.e., 4 years of age). Five distinct histological stages of sexual maturity could be established in males: (1) early active, (2) late active, (3) ripe, (4) partially spawned with recovery and (5) spent. Females had oocytes ranging from small and immature to large and fully developed in their ovaries throughout the year. Since we could not classify reproductive stages based on a qualitative histological scale of sexual maturity, quantitative measures using an image analyser were used. In males a high percentage of mature individuals were present between January and August. The percentage of early active and partially spawned individuals was highest in November 1998 (70%) and 1999 (60%). In females, the mean number of developed oocytes per unit of gonadal area in a cross section, the mean diameter (minor axis) of oocytes, and the percentage of gonadal area occupied by oocytes were lowest in November, indicating a spawning peak. These results indicate synchronous gonadal development and spawning in males and females. The more intensive spawning activity in November coincides with the higher biomass and production of phytoplankton in spring.
Marine Ecology Progress Series, 2019
Broadcast spawners release their gametes into the water column for 'chance' fertilization events. However, detection of such events in near real-time is extremely difficult, but needed to improve fisheries and conservation management practices. It is practically impossible to distinguish the gametes of many species by microscopy; therefore, DNA-based techniques are preferable to detect and quantify gametes from field-collected plankton samples. We developed a quantitative PCR (qPCR) approach to detect and quantify broadcast spawning events in marine environmental DNA (eDNA) samples. We applied this approach to a commercially valuable broadcast spawning bivalve species, the sea scallop Placopecten magellanicus. Our approach includes (1) sequencing the internal transcribed spacer (ITS) region, (2) developing a novel species-specific probe and primer set, (3) testing the probe and primer set on a dilution series of sea scallop sperm to quantify the relationship between gamete abundance and DNA copy number, and (4) conducting dockside field tests of our method on plankton samples adjacent to naturally spawning sea scallops. Quantitative PCR revealed a clear relationship between DNA copy number and P. magellanicus sperm cell abundance, indicating that this method is reliable for detecting sperm release by male scallops during spawning events. Plankton samples collected during the scallop spawning season revealed spikes of scallop eDNA in both the < 20 µm (sperm) and > 20 µm (possible eggs) particle size-fractions. This method holds great potential to provide more efficient estimates of the timing, magnitude, and spatial scale of reproductive events than conventional methods for a wide range of broadcast spawners.
Reproductive phenology of coastal marine bivalves in a seasonal environment
Journal of Plankton Research, 2014
Environmental conditions during the larval phase (food concentration and temperature) impact recruitment success of marine bivalves by affecting growth and survival. We analysed the seasonal match between environmental conditions and larval presence of six coastal bivalve species over eight consecutive years (2006 -2013) in the western Wadden Sea, taking species-specific physiological responses to these conditions into account. The larval occurrence of four species overlapped with their optimum in environmental conditions as calculated following the Dynamic Energy Budget theory: the occurrence of larvae of Cerastoderma edule, Mya arenaria and Mytilus edulis matched with their optimum food conditions, while larvae of Crassostrea gigas were mainly found during the temperature optimum of this species. Larvae of Ensis directus and Macoma balthica, however, only occurred months after their most favourable food conditions. The first group probably uses the energy immediately after uptake for the production and release of gametes ('income breeders'), whereas the second group accumulates and stores energy reserves for later reproduction ('capital breeders'). Larvae of C. gigas and M. balthica were observed much earlier in the year than expected from previous occurrences and from formerly know threshold temperatures for spawning. These apparent changes in timing of spawning suggest that Pacific oysters and Baltic tellins have recently acclimated to new environmental conditions.