No hybridization and marked interspecific differences in individual growth rate in mixed cultures of Manila clam (Ruditapes philippinarum) and grooved carpet-shell clam (R. decussatus)

The Manila clam (Ruditapes philippinarum) was introduced in Europe in the 1970's and in the following years it became naturalized. Interactions with the native species include hybridization with the grooved carpet-shell (GCS) clam (R. decussatus), which may have both useful and undesirable consequences. Here we report an attempt to produce hybrids in captivity by crossing 3 females and 4-5 males of each species in a two-step protocol that favored hybrid fertilizations. One-hundred animals were sampled at 15 months after fertilization, and scored for one morphological diagnostic trait (siphon fusion) and two diagnostic genetic DNA markers (ITS-2 and Fas-i1). No hybrids were detected, although the 0% hybridization rate has an associated 95% confidence interval of 3.3%. This result suggests that successful hybrid fertilization may be infrequent and/or the hybrid offspring may have very low survival rate. Abundant offspring of the two parental species were obtained and provided an unprecedented opportunity to study the innate differences in biological traits between the two species without the confounding influence of environmental variability. Individuals with ripe gonads were significantly less frequent in the Manila clam, suggesting an innate trend to earlier summer spawning in this species. Manila clam grew 20% faster than GCS clam and showed 80% heavier shells. However GCS clam showed almost twice as much variability in size as Manila clam, and some individuals of this species were as large as the largest Manila clams. The observed difference in growth variability may reflect a general loss of genetic variability in Manila clam during the introduction in Europe, although a random effect from using a small number of parents in the mixed cross cannot be discarded. Discrimination between these explanations, as well as determining more precisely the occurrence of hybridization in hatcheries by studying larger numbers of parents and offspring, may help improving clam aquaculture in Europe while preserving the genetic resources of the GCS clam.