Genetic diversity and cryptic speciation of the deep sea chaetognath Caecosagitta macrocephala (Fowler, 1904)

We investigated genetic diversity and structure of the deep sea chaetognath Caecosagitta macrocephala collected in the western North Pacific (Sagami Bay) and eastern Central to South Atlantic. All of the 52 specimens analyzed had unique haplotypes in their mitochondrial cytochrome c oxidase subunit I (mtCOI) gene sequences. Four distinct lineages of the mtCOI gene sequences (mtA, mtB, mtC, and mtD) were revealed by phylogenetic analysis with robust statistical support. The specimens collected from the Atlantic Ocean comprised three of the lineages (mtA, mtB, and mtD). All specimens of the remaining lineage (mtC) were obtained from Sagami Bay. The outgroup node was placed between the mtA lineage and lineages mtB, mtC, and mtD. Two specimens from each of the four lineages were randomly selected and the nuclear ribosomal internal transcribed spacer 1 (nITS1) region sequenced, resulting in ten forms, two of which were shared by all eight individuals. Phylogenetic relationships estimated from these sequences further supported the independence and reproductive isolation of the mtA individuals from the other lineages, while no phylogenetic structure was found in the mtB, mtC, and mtD lineages. These results indicate the presence of at least two cryptic species in C. macrocephala. Interestingly, these cryptic species were collected primarily from different depth layers (meso- and bathypelagic), suggesting speciation of the bathypelagic species from a mesopelagic precursor.