Cryptic speciation and latitudinal genetic differentiation in sipunculan-commensal bivalves with exceptionally large larval shells: the Planktomya amamiensis species complex (Galeommatidae)

Cryptic speciation is common among marine bivalves; however, it remains largely unexplored in symbiotic bivalve groups. Planktomya amamiensis (Galeommatidae) is a commensal bivalve that lives on the body surface of sipunculan worms and is characterized by its exceptionally large prodissoconch II (PII). In this study, we investigated the cryptic species diversity within the Pl. amamiensis complex in the Japanese Archipelago based on morphological, ecological, and genetic data. Our molecular assessment based on the COI and ITS2 genes indicates that Pl. amamiensis in Japan is genetically separated into three distinct species ( Planktomya spp. 1–3). Planktomya spp. 1 and 3 were both collected from the subtropical Amami Islands in the Ryukyu Archipelago, southern Japan, and partly shared the same Siphonosoma species as hosts, whereas Planktomya sp. 2 was collected with different Siphonosoma species from the temperate Kii Peninsula on Honshu Island, Japan. Planktomya sp. 2 was genetically close to Planktomya sp. 1, showing 9.6% K2P divergence in COI. Although Planktomya spp. 1–3 share similar shell morphology and gross anatomy, Planktomya sp. 2 is distinguished by its slightly more elongated shells and larger PII (> 1 mm). Based on these diagnostic characters, its temperate distribution, and genetic distinctiveness in both mitochondrial and nuclear markers, this taxon is described here as Planktomya kiiensis sp. nov. Our study suggests that (i) latitudinal temperature differences and associated host shifts may play an important role in the speciation of commensal bivalves, and that (ii) an integrative approach is essential for accurately assessing species diversity of Galeommatidae.