Genetic investigation of algae-boring amphipods in the genus Ceinina (Crustacea: Amphipoda) in relation to their habitat utilization and species identity

The selection of seaweed as a nesting substrate by amphipods that bore into seaweed thalli may affect seaweed growth and community structure. Ceinina japonica (Amphipoda: Eophliantidae) burrows into the thalli of various species of seaweed to construct a nest. At the study site along the coast of the Oshika Peninsula, Japan, it nests primarily in Phaeophyceae (brown algae) of orders Fucales and Laminariales. There, the sporophytes of Undaria pinnatifida (Laminariales: Family Alariaceae) disappear at the end of June; while the bases of adjacent Sargassum siliquastrum (Fucales: Family Sargassaceae) remain throughout the year. It was considered that, towards the end of June, C. japonica migrates from U. pinnatifida to S. siliquastrum. To test this hypothesis, adjacent thalli of U. pinnatifida and S. siliquastrum were collected and amphipod specimens obtained from them were analyzed genetically using MIG-seq-based genome-wide SNPs. Ceinina specimens taken from Eisenia bicyclis (Laminariales: Family Lessoniaceae) were also included. The analysis revealed that individuals inhabiting S. siliquastrum (S-type) and those inhabiting U. pinnatifida (U-type) were genetically distinct despite being collected from adjacent thalli at the same locality. Morphological differences between the two types were also observed. It is provisionally concluded that the U-type is C. japonica and the S-type population may represent a cryptic species. Individuals inhabiting E. bicyclis formed a monophyletic group with C. japonica, suggesting that, at the study site, the genuine C. japonica does not migrate between U. pinnatifida and S. siliquastrum, but it may migrate between the two species of Laminariales.