Squalus clarkae sp. nov., a new dogfish shark from the Northwest Atlantic and Gulf of Mexico, with comments on the Squalus mitsukurii species complex

Abstract

Sharks of the genus Squalus have slow reproductive rates coupled with low genetic diversity, as is typical of deep-water sharks, making this group slow to rebound from depletion due to overfishing. The number of species within Squalus has been expanding recently due to increased attention on taxonomic revision, and a growing research focus on little-known deep-water sharks in general. Here we use genetics and morphology to describe a new species of dogfish shark, Squalus clarkae sp. nov. from the Gulf of Mexico (GoM) which replaces Squalus mitsukurii in this region, and place it in the context of congeners from the Atlantic and elsewhere. Previously, S. clarkae sp. nov. was considered a part of the Squalus mitsukurii species complex, a group of closely related but distinct species. We sequenced the mitochondrial cytochrome oxidase I and the NADH Dehydrogenase II gene of S. mitsukurii from the type location in Japan, S. clarkae sp. nov. from the GoM, as well as three closely related species (S. cubensis, S. blainville, and S. megalops) and S. cf. mitsukurii from Brazil. Squalus clarkae sp. nov. is genetically distinct from other species with significant statistical support (>98.6% bootstrap support/posterior probability), and 2.8% divergent from S. mitsukurii in the type location of Japan. Morphological estimates also revealed differences between S. clarkae sp. nov., S. mitsukurii, and other Atlantic Squalus species, with S. clarkae sp. nov. exhibiting a longer body, smaller interorbital space, shorter caudal fin, and a differently-proportioned first dorsal fin. In general, dogfish sharks in the Atlantic and GoM are characterized by similar but distinct morphology, significant genetic variation, and small species ranges.

Description of a new deep-water dogfish shark from Hawaii, with comments on the Squalus mitsukurii species complex in the West Pacific

“…Additional specimens and samples were collected from bottom fish surveys from Maui to Lisianski Atoll, spanning nearly 2,000 km of the Hawaiian Archipelago. Because the 2010 genetic study showed extremely low diversity, we deemed 5–10 specimens adequate for taxonomic evaluation, a number on par with other revisions in this genus (Ward et al 2007; Pfleger et al 2018). To that end, five whole mature females and three whole mature male S.cf.mitsukurii from Oahu were retained as voucher specimens for morphometric and genetic analyses.…”

“…Recent years have shown that DNA sequencing in conjunction with morphological analyses is an effective approach for elucidating the alpha taxonomy of deep-water sharks (Avise 2004; Ward et al 2005; Last et al 2007c; Ebert et al 2010; Veríssimo et al 2014; Pfleger et al 2018). Findings from several studies have shown a mix of low genetic distances between well-established morphological species, and deep genetic splits between animals identified as conspecifics (Daly-Engel et al 2010; Veríssimo et al 2017; Daly-Engel et al 2018; Pfleger et al 2018; Daly-Engel et al submitted). …”

“…Complicating identification issues is the fact that deep-water sharks, whose cold environment results in low metabolic rates relative to other elasmobranchs, may undergo an overall slower rate of molecular evolution compared with shallow coastal species (Martin et al 1992; Martin and Palumbi 1993). As a result, a number of investigators have found the more-rapidly evolving ND2 gene to be an effective genetic marker for estimating both inter- and intraspecific variation in dogfish sharks (Veríssimo et al 2010; Naylor et al 2012; Veríssimo et al 2017; Pfleger et al 2018). …”

“…Such complexes are not unusual in nature, having been observed across a variety of phyla from insects (Perring 2001) and nematodes (Chilton et al 1995) to bony fishes (Barluenga and Meyer 2004), and have been shown to harbor “cryptic” diversity not always apparent from morphology alone (Daly-Engel et al Submitted). Taxonomic reevaluation of Squalus in the Indo-Pacific and elsewhere has revealed many undescribed species that were historically lumped together under a single name (Ward et al 2007; Naylor et al 2012; Viana et al 2016; Pfleger et al 2018): although the relatively shallow spiny dogfish Squalusacanthias Linnaeus comprises just one wide-ranging species apart from the North Pacific (Ebert et al 2010; Veríssimo et al 2010), we now know that both the Shortspine dogfish shark Squalusmitsukurii Jordan & Snyder and the shortnose dogfish shark Squalusmegalops Macleay comprise global species complexes (Veríssimo et al 2010; Naylor et al 2012; Veríssimo et al 2017; Pfleger et al 2018; Daly-Engel et al Submitted). …”

“…Re-examination of local S.mitsukurii stocks has revealed many new species, including four from the West Pacific alone: S.formosus White & Iglesias (2011), S.chloroculus Last, White, & Motomura (Last et al 2007b), S.montalbani Whitley (Last et al 2007b), and S.griffini Phillips (Duffy and Last 2007). Other revisions of S.mitsukurii have been done in the Atlantic using either genetic tools or morphological characters (Viana et al 2016; Veríssimo et al 2017), though not both (but see Pfleger et al 2018). …”