Natural hybridization between Calanus finmarchicus and C. glacialis (Copepoda) in the Arctic and Northwest Atlantic
Limnol. Oceanogr., 57(4), 2012, 1057-1066 | DOI: 10.4319/lo.2012.57.4.1057
ABSTRACT: Calanus glacialis and Calanus finmarchicus dominate the zooplankton community in the Arctic and North Atlantic, respectively. The vast zone of sympatry between these species, the potential for overlap between reproductive seasons, and the evidence for intermediate values of discriminant traits suggest that these species hybridize. We genotyped 684 individuals from 14 Arctic to Atlantic stations using one mitochondrial (16S) and 10 nuclear loci (microsatellites). Strong genetic differentiation between parental species confidently identified hybrids in areas of sympatry. Hybrid frequency was highly variable among stations and did not covary with mean annual sea surface temperature. In the St. Lawrence Estuary, parental and hybrid genotypes were nonrandomly distributed between depth layers (300-100 m and 100-0 m) and across sampling dates, and hybrids seemed more frequent in July than in May and September. Overall, the bimodal frequency distribution of parental and hybrid genotypes suggests that reproductive barriers limit gene flow between these species. The opportunity for interbreeding is more likely restricted by differences in species reproductive phenology than by dispersal. Hybridization also affects prosome length, a morphological trait widely used to discriminate Calanus species, and thus possibly contributes to species misidentification. Highly introgressed individuals indicate that hybrids are fertile and reproduce, suggesting that hybrid fitness could affect estimates and models of these species population dynamics. This is the first evidence for interspecific hybridization between marine zooplankton species, but similar cases could be uncovered using nuclear genetic markers in groups of closely related and morphologically similar marine zooplankton species.