Life history, patchy distribution, and patchy taxonomy in a shallow-water invertebrate (Mollusca: Polyplacophora: Lepidopleurida)

Abstract

Things without names are difficult to rationalise, and so species that go without names are difficult to conserve or protect. This is a case study in resolving conflicts in historical taxonomy and 'real' species (identifiable and evolutionarily relevant groupings) using an approach including population genetics, natural history, and pragmatism. We report the observation that populations of a shallow-water chiton species from Washington and British Columbia demonstrate extremely high site fidelity and patchy distribution. Their limited dispersal potential and isolation could be explained by a brooding life history. This stands in direct contrast with the supposedly wide distribution of this "species", Leptochiton rugatus (Carpenter in Pilsbry, 1892) sensu lato, from the Sea of Japan to Baja California. But this lineage has previously been suggested to comprise several cryptic species. Indeed, a haplotype network analysis using 61 individual sequences of the cytochrome oxidase c subunit I gene for L. rugatus s.l. revealed four discrete clusters which correspond to different parts of the geographic range. We infer these to represent four distinct species, at least two of which are likely novel. Leptochiton rugatus sensu stricto is herein reinterpreted as restricted to California and Baja California, and the new name L. cascadiensis sp. nov. is established for the lineage with a distribution in the Cascadia coastal bioregion from the panhandle of Alaska to Oregon. There are minor morphological differences among these species in the L. rugatus species complex, but genetic data or morphological observations alone would not have been sufficient to definitively recognise these groups as species-level lineages. The observation that different species within the complex may have different life history strategies provides important support for interpreting different populations as genuinely separate species.

Leptochiton subrugatus sp. nov. (Mollusca: Polyplacophora) from low boreal waters of northern Pacific

“…High density is more unusual for lepidopleuran chitons, which usually occur in relatively low abundance. The northeast Pacific lepidopleuran Leptochiton cascadiensis occurs in dense but small and very isolated patches, and this was speculated to be correlated with brooding life history (Sigwart and Chen 2018). In the case of the majority of chitons which are not brooders, high density is associated with high resource provisioning and a strong ecological function of chiton grazing (Dethier and Duggins 1984;Littler et al 1995).…”

“…We used previously prepared shell sections to examine the presence of growth rings in cross section, to determine the approximate age of adult specimens of the new species. We further used data for the cytochrome oxidase subunit 1 (COI) fragment of mitochondrial DNA, which is commonly used as a DNA barcode, to compare specimens we attribute to the new species with other species in the same morphological grouping, based on previously published results (Sigwart et al 2011;Sigwart 2017;Sigwart and Chen 2018). Kuroda and Kinoshita 1951: 7;Huang and Xu 1964: 3;Jakovleva 1952: 55, fig.…”

“…kaasi Sirenko, 1990. Several of these species have ranges that extend to the northeast Pacific, but understanding the range and biogeography of each species and the group has been confounded by confusion over their taxonomy (Sigwart and Chen 2018).…”

“…The strong similarity in radula and girdle characteristics often led to confusion in defining species. At present, the range attributed to Leptochiton rugatus includes the whole North Pacific, but this is likely not being the true range of the Leptochiton rugatus s.s. Another important point of confusion is the species Leptochiton alascensis, which has an ambiguous type locality (Sigwart and Chen 2018); however, its inclusion here as a valid species in the NW Pacific is recognized based on morphological comparisons with the type material. The type specimens of Leptochiton alascensis represent a diagnosable distinct morphology, although its type locality and therefore its geographic range are not currently known.…”

“…revealed several significant and consistent morphological differences, which we describe here. The second author had conducted molecular studies (Sigwart et al 2011;Sigwart and Chen 2018) which demonstrate that the records of Leptochiton rugatus s.l. found off the coast of Asia are clearly different from the species living off North America.…”