Reinstatement ofPhascolosoma(Phascolosoma)variansKeferstein, 1865 (Sipuncula: Phascolosomatidae) based on morphological and molecular data

Abstract

Phascolosoma (P.) varians, a sipunculan species known from the Greater Caribbean, was designated as a synonym of Phascolosoma (P.) nigrescens, which was originally described from Fiji. Their synonymy was primarily based upon an interpretation that these two species were morphologically indistinguishable. After its designation as a synonym, no further detailed analyses of morphological or molecular characteristics were performed to corroborate the assumed widespread distribution of Phascolosoma (P.) nigrescens. In this study, Phascolosoma (P.) varians is redescribed, and notable differences between this species and its proposed senior synonym are presented. These two species differ in the shape of their hooks, the spatial attachment of nephridia to the body wall, and the morphology of the contractile vessel. Additionally, there is high genetic divergence between nucleotide sequences within their respective cytochrome c oxidase subunit 1 (COI) genes, which supports the morphological data. Herein, the synonymy of Phascolosoma (P.) varians with Phascolosoma (P.) nigrescens is rejected due to morphological and molecular differences. Furthermore, the assumed widespread distribution of Phascolosoma (P.) nigrescens is still considered as questionable.

First records and two new species of sipunculans (Sipuncula) from the Southern Mexican Pacific

“…The idea that most marine invertebrates have wide distributions has been questioned recently (Hutchings & Kupriyanova 2018). Regarding sipunculans, there are many studies rejecting these wide distributions, supported by careful morphological and molecular investigations (Staton & Rice 1999;Kawauchi & Giribet 2010, 2014Schulze et al 2012;Johnson et al 2016;Silva-Morales et al 2019;Silva-Morales 2020).…”

How Many Sipunculan Species Are Hiding in Our Oceans?

“…In addition to P. varians, Cutler [19] lists 12 other junior synonyms for P. nigrescens. However, Silva-Morales [21] recently provided morphological and molecular evidence to support the reinstatement of P. varians. The three Caribbean specimens included in their study diverged from the specimen from New Caledonia (the closest available sequence to Fiji, the type locality for P. nigrescens) by an average K2P distance of 24% for COI.…”

“…They proposed that P. nigrescens (from the Indo-Pacific) and P. varians (from the Caribbean) can be distinguished by their hook morphology (rounded secondary tooth in P. varians; square in P. nigrescens), attachment of the nephridia to the body wall (partially in P. varians; fully in P. nigrescens) and the shape of the contractile vessel (simple in P. varians; with lateral sacs in P. nigrescens). Furthermore, Silva-Morales' molecular analyses suggest that P. nigrescens might consist of at least five additional species, from Spain, Australia, New Caledonia, Israel and South Africa, respectively [21].…”

“…Since 1994, 12 additional species and one new subspecies have been described, and one previously synonymized species has been reinstated [21]. Based on morphological characters, Kawauchi and Rice [22] described two new species of Nephasoma from the western Atlantic.…”