Nactus multicarinatus (Gunther, 1872)

Nactus multicarinatus View in CoL group

This low-divergence clade of medium-sized species is widespread across the Pacific Ocean and also includes scattered populations from along the northern coast of New Guinea. In addition to the bisexual populations we targeted here, mitochondrial data indicate the widespread parthenogen N. pelagicus is also nested within this clade ( Fig. S1 View Fig , Heinicke et al., 2010).

Our sampling from Sudest and Rossell islands included type material of N. modicus , so we can unambiguously match lineage and name in this case. This name is associated with one of the more genetically divergent lineages in the N. multicarinatus group ( Figs. 2 View Fig and 3 View Fig ) and is well circumscribed geographically so we continue to recognise it as a distinct species. Our sampling from Misima includes specimens that are morphologically similar to N. fredkrausi (although not from the type locality), and we consider our material to represent this species, while noting a need for further sampling across Misima to confirm this.

We were also able to sample material from type series of at least three other species described by Zug (2020) that are nested within the N. multicarinatus group; however, all pose systematic issues. Samples from across northern New Guinea were provisionally assigned to N. allenallisoni because they include the type of this species, but this form shows only shallow divergence from other members of the clade ( p -distances typically <3%), suggesting it is not a distinct taxon.

The name N. robertfisheri was applied to all bisexual Nactus populations from New Britain through to Bougainville by Zug (2020); however, we found populations from these areas to be potentially paraphyletic with respect to some other named taxa ( N. multicarinatus ). Some samples from New Britain (including the type specimen) are more closely related to samples from across the Solomon Islands and Vanuatu than to other geographically proximate populations on New Britain. Additional divergent lineages from within the range of this putative taxon were also present on Bougainville. This means that differentiation of this taxon from N. multicarinatus (to which it was not compared in the original description) appears lacking. In all cases, divergence values among lineages were not particularly high.

Finally, Nactus alotau from around the type locality and including the type specimen clearly sits inside the broader complex of bisexuals comprising of ‘ N. multicarinatus ’ and ‘ N. robertfisheri ’.

Based on the above findings, the nomina N. allenallisoni , N. alotau and N. robertfisheri all fall within a strongly supported clade that shows extensive but shallow structure and includes samples of N. multicarinatus from around the type locality in Vanuatu. Based on these data, we synonymise all these names with N. multicarinatus , a name that applies to bisexual populations in the N. multicarinatus group occurring across the vast area extending from the northern coast of New Guinea into the outer low islands of the EPCT (Kiriwina and Yanaba) and throughout the islands of the outer Melanesian arcs (including cf. multicarinatus lineages 3 through to 8).

Lineages in the N. multicarinatus group from the Foja Mountains ( cf. multicarinatus 1) and Woodlark ( cf. multicarinatus 2) are more divergent, and we do not consider these to represent N. multicarinatus at this time. The OTU cf. multicarinatus 1 is of uncertain taxonomic placement— based on locality and slightly upland habitat, it could be N. grevifer or N. rainerguentheri ; however, for now, we refrain from referring it to any described species as the morphological characters provided by Zug and examination of the specimen do not provide us with any confidence as to its specific identity.