Trigonostomum tillicum, Van Steenkiste & Leander, 2018

TRIGONOSTOMUM TILLICUM SP. NOV.

( FIG. 1)

Etymology: The species epithet refers to the word ‘tillicum’ which means ‘people, family, tribe and relatives’ in the Chinook jargon of the Pacific Northwest.

Type locality: Clover Point , Victoria, British Columbia, Canada (48°24 ′ 12 ″ N, 123°21 ′ 03 ″ W), algae in rocky lower intertidal (02/09/2015; 03/03/2016) GoogleMaps .

Type material: Twelve whole mounts, one of which is designated as the holotype ( SMNH Type-8918); the others are paratypes ( BBM MI4021 – MI4031 ) .

Other material: Observations on about 20 live animals. 18S rRNA (GenBank accession # MF321753 View Materials ), 28S rRNA (GenBank accession # MF321762 View Materials ) .

Diagnosis: Species of Trigonostomum with 80- to 92- μ m-long stylet. Stylet composed of a stylet proper that makes a 90° proximal turn, and a mantle consisting of two heteromorph plates with a terminal hook. Distal end of the stylet proper pointed. Bursal appendage 230–300 μ m long, consisting of a coiled tube that distally splits into six finer tubes. Finer tubes swollen to vesicles just before the distal end.

Description: Live animals between 1.1 and 1.9 mm long. With lenticular eyes of which the reniform pigment zone is often divided into two parts. General appearance typical of species of Trigonostomum with a ciliated epidermis full of rhabdites, adenal rhabdite tracks in the caudal part on both sides of the body, an apical tuft of sensory bristles, a rostral integumental invagination (‘proboscis’) and a forwardly inclined pharynx with a long prepharyngeal tube in the first body half.

Paired gonads behind the pharynx. Large oval testes and seminal vesicles. The latter join and proximally enter the globular prostate vesicle in the rear end of the body. Two types of prostate secretion, a coarse-grained and a fine-grained one, fill the prostate vesicle. Extracapsular parts of the coarse-grained prostate glands were observed in live animals. The sclerotized parts of the male copulatory organ are nearly identical to the one in T. vanmecheleni . The stylet measures 80–92 μ m (x = 84 μ m; n = 11; non-axial: 68–78 μ m) and consist of (1) a 74- to 80- μ m-long (x = 76 μ m; n = 11) and 4- to 6- μ m-wide (x = 5 μ m; n = 11) stylet proper that is proximally curved over 90° and distally straight with a pointed tip; and (2) a half-open mantle consisting of two heteromorph plates with hooked tips that surround the straight part of the stylet on each side ( Fig. 1B–D). The proximal part of the stylet proper is funnel-shaped and connects to the plates through a system of ridges that fringe the proximal rim of the funnel. The larger plate is 43–51 μ m long (x = 47 μ m; n = 11) and 10–12 μ m wide (x = 11 μ m; n = 11), while the more slender plate is 38–45 μ m long (( x = 42 μ m; n = 11) and 2–5 μ m wide (x = 3 μ m; n = 11). The proximal bases of the plates resemble a panhandle. On one side the plate bases connect to the stylet base and on the other side they connect to each other to encompass the stylet proper. The distal part of the plates ends in a hook pointing away from the stylet base.

The vitellaria extend dorsally on both sides of the body from behind the pharynx to the ovaries. The large female bursa is elongated and anteriorly provided with a sclerotized bursal appendage. This sclerotized appendage is a coiled, 230- to 300- μ m-long tube (x = 261 μ m; n = 10) that is funnel-shaped proximally and distally splits into six slender tubes with a swollen vesicle-like part just before the distal end ( Fig. 1E–G). In some specimens, these six tubes seem to be grouped in two units of three tubes for the first proximal part. Although difficult to observe in the squeezed whole mounts, the appendage seems to make two coils from the proximal funnel to the distal tubes.

Discussion: This species clearly belongs to the genus Trigonostomum because of the rostral integumental invagination (‘proboscis’), the forwardly inclined pharynx, and the construction of the male and female genital system in general and of the stylet and bursal appendage in particular. The stylet is typical for the species group 1B (stylet with a proximal turn of 90–180°) as defined by Willems et al. (2004b). Furthermore, based on the morphology of the sclerotized structures, this species is almost identical to T. vanmecheleni from the Italian Adriatic. The latter species belongs to the T. lilliei species group as designated and discussed by Artois et al. (2013) which now comprises four morphologically similar species: T. lilliei (von Graff, 1911b) Meixner, 1924 ; T. prytherchi Kepner et al., 1941 ; T. vanmecheleni and T. tillicum sp. nov.

A thorough re-examination of the type material of T. vanmecheleni revealed the stylets to be almost identical to these of the new species from British Columbia. The only notable difference is the size of the stylet, which is about 1/3 larger in T. tillicum sp. nov. Artois et al. (2013) describe a spur-like structure on the base of the funnel-like proximal opening of the stylet proper in T. vanmecheleni , which we think corresponds to a protruding ridge on the funnel rim as also observed in T. tillicum sp. nov. The bursal appendages of both species form a coiled tube that splits into six smaller tubes, but is twice as long in T. tillicum sp. nov. as in T. vanmecheleni . In addition, the six smaller tubes consistently display vesicle-like enlargements in T. tillicum sp. nov., which are absent in T. vanmecheleni .

Based on the above-mentioned differences and the disjunct geographical distribution, we assign the specimens from British Columbia to a new species. In doing so, we follow Artois et al. (2013) who argue that the morphology of the bursal appendage is a good diagnostic feature within the T. lilliei group. It is nevertheless clear that all species of this group are very closely related, which raises interesting questions on the biogeography of this group given the disjunct localities of these species in the Northwestern Atlantic ( T. lilliei , T. prytherchi ), Southwestern Atlantic ( T. lilliei ), Adriatic ( T. vanmecheleni ), Southwestern Pacific ( T. prytherchi ) and Northeastern Pacific ( T. tillicum sp. nov.). A thorough integrative taxonomic analysis including molecular data from the previously described species will be necessary in the future for a better understanding of species boundaries and diagnostic characters within this group.