Abstract
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.
Highlights
Clams have been part of shellfish fisheries in Europe since the XIX century
In the case of ITS-2 a single band of 565 bp corresponding to Manila clams, and another of 482 bp corresponding to grooved carpet-shell (GCS) clams, were found, as described previously by Hurtado et al (2011) and Hab temariam et al (2015)
The genetic identity inferred from siphon morphology coincided with that inferred from genetic markers in the great majority of the scored clams, and the only two dubious individuals were unambiguously identified as non-hybrids by genetic markers
Summary
Clams have been part of shellfish fisheries in Europe since the XIX century. Until the 1970’s, the main fished species was the native grooved carpet-shell clam, Ruditapes decussatus (GCS clam on). Its long growing period and high disease susceptibility favored the introduction of the Manila clam (R. philippinarum) in Europe. This species has established permanent, self-recruiting populations in many localities of the European Atlantic and Mediterranean coasts (Chiesa et al, 2016; Cordero et al, 2017). A stable hatchery-based aquaculture industry has developed (da Costa et al, 2020), and the two species coexist in the market and in the wild. In spite of these developments, the biological bases of differences and similarities in production traits and culture procedures have been only shallowly studied. The landings of the GCS clam in Spain, which is one of the main Euro pean producers, have decreased continuously from 1023 T in 2010 to
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