Revision of the sixgill sawsharks, genus Pliotrema (Chondrichthyes, Pristiophoriformes), with descriptions of two new species and a redescription of P. warreni Regan

Abstract

Recent sampling efforts in Madagascar and Zanzibar, as well as examinations of six-gilled sawsharks in several museum collections provided evidence for a complex of species within Pliotrema warreni Regan. The present manuscript contains a redescription of P. warreni involving the syntypes and additional material, as well as formal descriptions of two new species of Pliotrema Regan. All specimens of both new species were found in the western Indian Ocean. Individuals of the first new species, hereafter referred to as P. kajae sp. nov., were identified originating from Madagascar and the Mascarene Ridge. Specimens of the second new species, hereafter referred to as P. annae sp. nov., were only found off Zanzibar. Pliotrema kajae sp. nov. appears to inhabit upper insular slopes and submarine ridges at depths of 214-320 m, P. annae sp. nov. so far is only known from shallow waters (20-35 m). Both new species differ from P. warreni in a number of characteristics including the known distribution range and fresh coloration. Taxonomical differences include barbels that are situated approximately half way from rostral tip to mouth, with prebarbel length equidistant from barbel origin to symphysis of the upper jaw in P. kajae sp. nov. and P. annae sp. nov. (vs. about two thirds way from rostral tip to mouth, with prebarbel length about twice the distance from barbel origin to symphysis of upper jaw in P. warreni) and rostra that are clearly and slightly constricted between barbel origin and nostrils, respectively (vs. rostrum not constricted). Pliotrema kajae sp. nov. differs from P. annae sp. nov. in a longer snout, more numerous large lateral rostral teeth and upper jaw tooth rows, jaw teeth with (vs. without) sharp basal folds, and coloration, particularly pale to light brown (vs. medium to dark brown) dorsal coloration with (vs. without) two indistinct yellowish stripes. A revised diagnosis of Pliotrema and a key to the species are provided.

The Sawshark Redemption: Current knowledge and future directions for sawsharks (Pristiophoridae)

“…Presently, the morphology of the claspers has only been documented (to varying degrees) for Kaja's sixgill sawshark (Pliotrema kajae, Pristiophoridae), P. cirratus, P. japonicus and P. nancyae (Huber, 1901;Leigh-Sharpe, 1922;Moreira & de Carvalho, 2019;Weigmann et al, 2014Weigmann et al, , 2020 but no quantitative comparisons between species have been made.…”

“…schroederi and P. nancyae, have tricuspidate denticles (Springer & Bullis, 1960;Weigmann et al, 2014), as do all species in the Pliotrema genus (Weigmann et al, 2020). It is unclear whether these differences in denticle morphology relate to habitat or lifestyle but could be explored further in future taxonomic studies on sawsharks.…”

“…As a possible means of accommodating the rostrum, sawsharks exhibit several cranial and spinal modifications. For example, residing at either side of the foramen magnum (hole at the base of the skull for the spinal cord to pass through) are well‐developed, crescent‐shaped occipital condyles that act as an articulation point for the skull and vertebral column (von Bonde, 1933; Hoffmann, 1912; Jaekel, 1890; Mollen, van Bakel, & Jagt, 2016; Weigmann et al., 2020). In some species, these condyles are joined at the base [e.g.…”

“…Globally, there are currently 10 recognized species of sawshark from two genera: seven from Pristiophorus and three from Pliotrema (Ebert & Wilms, 2013; Weigmann, 2016; Weigmann et al., 2020). The genera can be distinguished by the number of gill slit pairs (five in Pristiophorus and six in Pliotrema ) and the presence or absence of serrations on the rostral teeth (non‐serrated in Pristiophorus and serrated in Pliotrema ).…”

“…The genera can be distinguished by the number of gill slit pairs (five in Pristiophorus and six in Pliotrema ) and the presence or absence of serrations on the rostral teeth (non‐serrated in Pristiophorus and serrated in Pliotrema ). What we know of these species is new: half of these sawshark species have been formally described in the past 12 years (Ebert & Cailliet, 2011; Ebert & Wilms, 2013; Weigmann et al., 2020; Yearsley, Last, & White, 2008).…”