The Bryozoan Cauloramphus magnus (Cheilostomata: Calloporidae) in Northern Japan Includes Multiple, Co-occurring Cryptic Species.

The cheilostome bryozoan Cauloramphus magnus is common in the rocky intertidal habitat from southeastern Alaska to northern Japan. We examined its phylogeography by analyzing 576 bp of the mitochondrial COI (cox1) gene sequenced for 298 colonies from 16 localities in northern Japan. A maximum-likelihood phylogeny detected three main clades (A, B, C). Clades A and B occurred throughout the study area but differed in frequency, haplotype diversity, and haplotype distribution; each resolved into three divergent subclades (AI–III, BI–III). Clade A shared none among 15 haplotypes between the Pacific and Sea of Japan sides of Hokkaido. In contrast, Clade B (29 haplotypes) was thrice as common as Clade A among samples, with haplotype B28 common on both sides. Divergent Clade C (nine haplotypes) was detected only at Rumoi. K2P divergences of 12.3–28.3% among Clades A–C suggest these are distinct biological species, a conclusion supported by different inferred evolutionary histories. A bPTP species delimitation analysis indicated nine phylogenetic species among the sequences included in our phylogeny (AI–III, BI–III, C, and one specimen each from Alaska and the Commander Islands), with K2P divergences of 3.9–6.5% among subclades in A or B. Statistical and principal components analyses suggested weak morphological differentiation between Clades A + B and C, although overlapping ranges of measurements preclude identification to clade; these three clades are morphologically cryptic. For taxonomy, we suggest retaining the name C. magnus for lineages within this species complex across its range, followed by a clade designation, if known.