Yamaguticestus squali Caira & Bueno & Jensen 2021

YAMAGUTICESTUS SQUALI (YAMAGUTI, 1952) COMB. NOV.

BASIONYM: PHYLLOBOTHRIUM SQUALI YAMAGUTI, 1952

(FIG. 5A–E)

The following details of the surface features on the scolex of this species, based on examination of a specimen with SEM collected from the type host near the type locality, expand the original description of this species by Yamaguti (1952) and the redescription based on the holotype by Vasileva et al. (2002).

Anterior-most regions of bothridia densely covered with capilliform filitriches (Fig. 5B). Distal surfaces of loculus densely covered with gongylate columnar spinitriches and capilliform spinitriches (Fig. 5C); distal surfaces of apical sucker not observed. Proximal bothridial surfaces densely covered with capilliform filitriches (Fig. 5D). Cephalic peduncle lacking. Neck (Fig. 5E) and strobila with capilliform filitriches arranged in wide scutes.

Synonyms: Phyllobothrium squali Yamaguti, 1952; Crossobothrium squali (Yamaguti, 1952) Williams, 1968.

Type host: Pacific spiny dogfish, Squalus suckleyi (Girard, 1855), (Squaliformes: Squalidae de Blainville).

Additional hosts: None.

Type locality: Pacific Ocean, off Onahama, Hukusiima Prefecture, Japan.

Additional localities: Sea of Japan, off Oga City, Akita Prefecture, Japan (39°46′55.8″N, 139°51′49.2″E) (JN- 67); eastern Pacific Ocean, off Bamfield, Vancouver Island, Canada (48°50′7.9152″N, 125°08′7.7208″W).

Site of infection: Spiral intestine.

Material examined: One specimen examined with SEM collected from a shark collected off the west coast of Japan.

Sequence data: GenBank accession MW 419975, hologenophore (BAM 5-wP9) LRP no. 8674.

Remarks: By erecting the genus Yamaguticestus, we have established a more appropriate home for the species formerly referred to as Phyllobothrium squali. The transfer of this species from Phyllobothrium, as Yamaguticestus squali, resolves the issue of the nonmonophyly of Phyllobothrium that has been raised by a number of previous authors (e.g. Ruhnke, 2011; Caira et al., 2014). However, issues surrounding the identity of Y. squali remain. It was originally described by Yamaguti (1952) from a host identified as the Pacific spiny dogfish (Squalus suckleyi as Squalus suckleyii) off the eastern coast of Japan. Vasileva et al. (2002) subsequently provided a thorough redescription of this species based on examination of the holotype, in which they included illustrations of the scolex and details of the terminal genitalia for the first time. However, this species has also been reported from sharks identified as the piked dogfish (Squalus acanthias) from a variety of other localities globally, including the north-eastern Atlantic Ocean in the Bay of Biscay off Concarneau, France (Euzet, 1959) and the Irish Sea (McCullough & Fairweather, 1983; McCullough et al., 1986), the western Atlantic Ocean off Rhode Island, USA (Pickering & Caira, 2012; Ruhnke & Workman, 2013; Caira et al., 2014) and the Black Sea (Vasileva et al., 2002). Given the relatively strict degree of host specificity seen in most groups of elasmobranch-hosted cestodes (Caira & Jensen, 2014), reports from two different host species would normally have warranted closer scrutiny. However, the situation was confounded by the fact that Squalus suckleyi has been considered a junior synonym of Squalus acanthias for decades (see Compagno, 1984), and this synonymy has been embraced by many of those working with P. squali previously. For example, Vasileva et al. (2002) listed Squalus acanthias as the type host of P. squali, and Pickering & Caira (2012) referred to the cestodes of Squalus acanthias off Rhode Island as P. squali because Squalus acanthias was the accepted identity of the type host of this cestode species at that time. The relatively recent application of molecular methods to help inform elasmobranch identifications has led to a more careful assessment of the identities and distributions of species of Squalus Linnaeus, 1758 globally (Ebert et al., 2010). One of the results of that work was the resurrection of the name Squalus suckleyi for the species that occurs in the northern Pacific Ocean and is both molecularly and morphologically distinct from Squalus acanthias, which is now considered to be restricted to the Atlantic Ocean and the southern portions of the Pacific Ocean.

Thisrevisedhosttaxonomyhasprofoundimplications for the taxonomy of P. squali. The type host of P. squali is Squalus suckleyi, but the shark specimens reported to host this cestode species off Rhode Island, France and Ireland and in the Mediterranean and Black Seas are Squalus acanthias. This causes us to revisit the question of the conspecificity of the cestodes reported from Squalus suckleyi and Squalus acanthias. Although Vasileva et al. (2002) found their worms from the Black Sea (and thus from Squalus acanthias) generally to be consistent with the morphology of the holotype of P. squali (from Squalus suckleyi), they reported the worms from the Black Sea to be substantially larger than the holotype from Squalus suckleyi off Japan (i.e. 214–603 vs. 141 mm). Also interesting is the fact that the bothridia of the worm identified as P. squali taken from Squalus acanthias in the Irish Sea and examined with SEM by McCullough & Fairweather (1983: fig. 9) are more folded than those of the specimen of P. squali from Squalus suckleyi off Japan examined here (Fig. 5A). These differences led us to begin to question the conspecificity of material from these two host species and thus to advocate that the concept of P. squali be limited to information taken from specimens parasitizing Squalus suckleyi in the northern Pacific Ocean. More detailed comparisons between that material and specimens collected from Squalus acanthias in localities throughout the Atlantic Ocean and its adjacent water bodies are required to assess whether specimens from the two host species and their associated localities are conspecific. Until that time, specimens from Squalus acanthias, including those of Ruhnke & Workman (2013) and Caira et al. (2014) for which sequence data were generated, should be referred to as Yamaguticestus cf. squali.

As noted by Vasileva et al. (2002), the material from the velvet belly shark Etmopterus spinax (Linnaeus, 1758), which Euzet (1959) identified as Crossobothrium squali, differs from Y. squali in a number of respects. We believe this material is likely to represent an undescribed species of Yamaguticestus, the formal description of which requires examination of additional material.