Introduction
The striped Venus clam is an infaunal, subtidal species living in the Mediterranean Sea and its neighbour Atlantic coasts, and in the Black Sea. The species is exploited as a fishery but is facing a reduction in catches. In the Gulf of Valencia (Spain) and neighbouring areas, the fishery has collapsed and has been closed, but in spite of the ban, it has not recovered (Baeta, 2021). Supplementation of wild populations with clam seed obtained in captivity by using bivalve aquaculture methodology might help the recovery of populations and favour an aquaculture industry. A problem with this strategy is that the high fecundity and large variance of family size in bivalves may lead to large numbers of spat that have less genetic variability and higher relatedness than the supplemented population. Therefore, a strict control of the parental contributions and the genetic variability in the offspring is recommended. Here, the successful production of striped Venus clam spat in hatchery under controlled conditions, and the characterization of its genetic variability, are reported.
Materials and Methods
Clams (N = 350) were collected near Gandía (Valencia, Spain) in May 2023, before the peak of the reproductive season (summer), in order to have as much control as possible of the gonad maturation process. They were placed in 150L PVC tanks with running seawater, and were subjected to two conditioning treatments, with two replicates each: food only (FO) and food plus temperature (FT, average 2°C above ambient). Food ration was 3% of average clam dry meat weight, with 50% Isochrysis glabana, 30% Chaetoceros calcitrans, and 20% Tetraselmis suecica. Gonad maturation was studied by histology and condition indices (CI). The gametogenic stages were classified as in Delgado et al. (2013). A CI = dry meat weigh / dry shell weight was used. After 4-5 weeks of conditioning, the broodstock was subjected to thermal shocks (17-27°C ) to induce spawning. Larvae were cultured in 200 L conical tanks. Water was changed three times a week. When most larvae had reached the pediveliger stage, they were moved to 200 µm mesh-bottom containers for settlement. Larvae and postlarvae were fed Isochrysis galbana (105 cells/ml).
The 11 parents and 72 spat were genotyped for 11 microsatellites with the primers designed by Papetti et al. (2016). Allele frequencies and gene diversity were calculated with the program Arlequin (Excoffier et al., 2025).
Results
Histology and condition indices of the broodstock showed that most clams were sexually mature after conditioning with the FO treatment, while before conditioning most animals were immature. The 17-27°C thermal shocks induced spawning only in clams from the FO treatment. Six females and five males spawned, and three million eggs were recovered. Ca. two million larvae were alive at 48 hours after fertilization. The size of the larvae increased up to an average of 220 µm and reached a plateau 21 days after fertilization. Survival at this point was 25%. Larvae were then moved to the settlement tanks. A few days later water temperature raised suddenly to 29°C, coinciding with a summer heat wave, and mass mortality of the larvae occurred. Surviving spat were collected for further study.
All scored microsatellites were polymorphic. Out of the 57 alleles observed in the parents, 49 were present in the offspring, and nine new alleles appeared by mutation. Gene diversity in the parents was 0.624±0.040 and 0.554±0.007 in the progeny.
Discussion
Rearing larvae obtained from spontaneous spawning in captivity of clams collected at the peak of the reproductive season was achieved by Joaquim et al. (2016). However, this approach provided no control over the parental contribution to the offspring. These authors also tried inducing spawning in controlled conditions by thermal shock without success. Here, it has been shown that conditioning of clams collected before the natural reproduction peak during four weeks at ambient temperature results in large numbers of larvae, and potentially large numbers of spat. However, too high a seawater temperature can be a problem, inducing spontaneous spawning before the gonad is completely mature, and mortality during the larval to juvenile metamorphosis stage.
The genetic characterization of the parents and the progeny showed that 14% of the parental alleles and 11% of gene diversity were lost due to due to the variable contribution to offspring among parents. Clearly much larger numbers of parents are needed to produce clam seed that meets the recommended standards for supplementation of wild populations. Prolonging the conditioning period, increasing the food ration and changing the diet composition are three factors that should be explored to achieve this goal.
References
Baeta, M., Solís, M.A., Ballesteros, M., Defeo, O., 2021. Long-term trends in striped venus clam (Chamelea gallina) fisheries in the western Mediterranean Sea: The case of Ebro Delta (NE Spain). Marine Policy 134, 104798.
Delgado, M., Silva, L., Juárez, A., 2013. Aspects of reproduction of striped venus Chamelea gallina in the Gulf of Cádiz (SW Spain): Implications for fishery management. Fisheries Research 146, 86–95.
Excoffier, L., Laval, G., and Schneider, S. (2005). Arlequin ver. 3.0: An integrated software package for population genetics data analysis. Evolutionary Bioinformatics Online, 1:47-50
Joaquim, S., Matias, D., Matias, A.M., Gonçalves, R., Chícharo, L., Gaspar, M.B., 2016. New species in aquaculture: are the striped venus clam Chamelea gallina (Linnaeus, 1758) and the surf clam Spisula solida (Linnaeus 1758) potential candidates for diversification in shellfish aquaculture? Aquac Res 47, 1327–1340.
Papetti, C., Schiavon, L., Milan, M., Lucassen, M., Caccavo, J.A., Paterno, M., Boscari, E., Marino, I.A.M., Congiu, L., Zane, L., 2018. Genetic variability of the striped venus Chamelea gallina in the northern Adriatic Sea. Fisheries Research 201, 68–78.
Acknowledgements
This study was financed by MCIU (Spain), with funds from European Union Next Generation EU (PRTR-C17.I1), by Generalitat Valenciana (THINKINAZUL/2021/005; P.I.: M.R.), by the AQUAEXCEL3 project VENUSREAR (PID23137; P.I.: C.P.) and by a Senior Research Award from the Malacological Society of London to C.P.