Microphthalmus hvalr, Watson & Gunton & Kupriyanova, 2024

Microphthalmus hvalr sp. nov.

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Fig. 3A–C View Figure 3

Microphthalmus sp. — Gunton et al., 2021: 48, fig. 12 A–C; Georgieva et al., 2023: 177, fig.11 A–F.

Material examined.

Holotype. AM W.55402; 25NE, L: 2.00 mm, W: 0.45 mm, large oocytes (0.12 mm in diameter) present from segments VI –XX. Body comprises anterior end of eight segments, mid- and posterior segments number XVII, including regenerated posterior-most end of five segments with intact pygidium; an additional regenerated posterior end of four segments also with intact pygidium comes off the body at an angle at segment XII.

Paratypes. AM W.54569 , NHMUK ANEA 2022.412 - 420 About NHMUK ; NHMUK ANEA 2022.434 (about 20 fragmented specimens, including 1E, 23 segments, L: 2.0 mm, W: 0.4 mm; 1 NE 15 segments, comprising anterior end of seven segments plus two mid-body sections, four segments each, with oocytes from segment 6) .

DNA vouchers. NHMUK ANEA 2022.434, AM W.54817.001.

Type locality. A pilot whale skeleton off Byron Bay, New South Wales, Australia. This is the first bathyal record for a Microphthalmus sp. in the SW Pacific.

Description. Body width similar throughout, colour whitish to pale buff with distinct pharynx ( Fig. 3A View Figure 3 ). Muscular pharynx extending to segments V-VI, with 10 broad, coneshaped pharyngeal papillae visible in paratype; jaws absent.

Prostomium semi-circular, anteriorly slightly cleft, broader than long. Prostomial appendages and body cirri faintly pseudo-articulated, cirriform to often filiform. One pair of relatively short, cirriform lateral antennae and one pair of shorter palps terminally located; slender, finger-like median antenna inserted mid-prostomium, near to posterior prostomial edge, length extends beyond prostomial edge ( Fig. 3B View Figure 3 ). Eyes absent.

Achaetous segments I–III bearing six pairs of long, cirriform tentacular cirri; tentacular ventral cirri of segment III comparatively shorter ( Fig. 3B View Figure 3 ). Dorsal and ventral cirri present from chaetigerous segment IV; dorsal cirri of segment IV shorter than those of segment V and onwards. Parapodia down body sub-biramous; notopodia with dorsal cirri, chaetae absent ( Fig. 3C View Figure 3 ). Neuropodia with pointed pre-chaetal lobe bearing slender acicula, blunt postchaetal lobe with larger, thicker acicula. Compound falcigerous neurochaetae of different lengths, number 11–15 in mid-body; slender blades with serrated edge, heterogomph shafts non-camerate.

Pygidium with one pair dorso-lateral anal cirri, length variable in paratypes, from shorter than, to as long as, to just extending past posterior edge of anal membrane; bi-lobed ventral anal lamella with a shallow medial notch and smooth margins lacking papillae ( Fig. 3A, C View Figure 3 ).

Reproductive morphology. Oocytes and sperm were observed in the same individuals. Paratypes include one individual, 15 NE, with large oocytes 0.12 mm in diameter from segment VI, parapodial seminal receptacles and dark brown, tiny, rounded ‘blackberry’ shaped structures, putatively sperm, observed in longitudinal patches from segment VII .

Etymology. The name, hvalr , is from the Old Norse which incorporates a number of terms pertaining to a whale, including whalebone, the substrate from which the new species was collected.

Diagnosis. All notochaetae absent.

Diagnostic remarks. Morphological characters used to distinguish species in Microphthalmus are primarily chaetal and pygidial structures (Westheide, 1967, 1977, 1982, 2013; Westheide & Purschke, 1992). Microphthalmus spp. are simultaneous hermaphrodites with unique male copulatory organs (Westheide, 1973, 2013), which have also been suggested as morpho-anatomical characters for species discrimination (Westheide, 2013). However, this character has not been used here as histological work was not possible.

In Australia, Microphthalmus paraberrans Hartmann-Schröder, 1982 and M. westheidei Hartmann-Schröder, 1982 have been described from subtidal habitats off Western Australia. Both possess sickle-shaped serrate notochaetae as well as pygidia with long anal cirri and anal plates composed of short, broad ventral lamella with smooth margins. M. hvalr sp. nov. has relatively short anal cirri and its ventral anal plate is composed of a rounded, smooth lamellate lobe. Thus, the anal plate of M. hvalr sp. nov. is similar to those of the Australian species except the plate is much rounder and the anal cirri are relatively shorter.

Most Microphthalmus species possess 2–12 small simple and/or pectinate notochaetae per notopodium while a subtidal commensal M. hamosus Westheide, 1982 is one of very few that possesses a single notochaeta per notopodium. The comparison of 38 Microphthalmus spp. by Westheide (2013) and 21 Microphthalmus species by Salazar-Villejo et al. (2019) shows that Microphthalmus hvalr sp. nov. lacks diagnostic characters other than the absence of all notochaetae.