Pleijelius keni, Watson & Gunton & Kupriyanova, 2024

Pleijelius keni sp. nov.

Fig. 4A–E View Figure 4 urn:lsid:zoobank.org:act:75266D5C-47F3-48F8-8885-D1A89DF52FE7

Pleijelius cf. longae .— Gunton et al., 2021, SW Pacific, east coast of Australia

Pleijelius .— Georgieva et al., 2023, SW Pacific, east coast of Australia

Material examined

Holotype. AM W.51570, body nearly entire, 20 segments missing posterior-most segments, L: 2.3 mm, W: 0.7 mm.

Paratypes. AM W.55403; MAGNT W032914 (16 specimens of Pleijelius keni sp. nov. identified among a lot of Boudemos paulinae sp. nov. NHM. 240A. Most specimens fragmented, few entire specimens fell into a similar size range of 25 segments with the greatest length 3 mm. Ovigerous females: 17NE, L: 1.6 mm, W: 0.5 mm, oocytes visible from segment 7, mature eggs 0.08 mm in diameter from segments 13–17; 23E, L: 1.8 mm, W: 0.6 mm; 23E, L: 2.9 mm, W: 1.1 mm; 25E, L: 3.0 mm, W: 0.8 mm. SEM specimen, 19E, L: 1.6 mm, W: 0.5 mm).

DNA vouchers. AM W.54809.001 and NHM_240A.

Type locality. A pilot whale skeleton off Byron Bay, NSW, Australia. This is the first record for a Pleijelius species in the SW Pacific Ocean.

Description. Body elongate, segments with poorly defined segmental lines dorsally ( Fig. 4A–B View Figure 4 ); pale buff colour with no obvious pigmentation in preserved specimens. Dense, relatively short length, spinous, whitish coloured, notochaetal fascicles, originating dorso-laterally, leaving mid-dorsum bare. Prostomium rectangular to sub-trapezoidal shape, broader than long, with rounded corners. Prostomium not defined dorsally or ventrally by external segmental lines; first ventral segmental line includes segment I ( Fig. 4C View Figure 4 ). Digitiform median antenna arising at posterior-most margin of prostomium at mid-dorsal position on prostomium ( Fig. 4A–B View Figure 4 ); pair of cirriform lateral antennae inserted on anterior prostomial margin ( Fig. 4C View Figure 4 ). Palps inserted just behind former at level of ventral tentacular cirri of segment I; palps same size to slightly shorter than lateral antennae and positioned adjacent level to ventral tentacular cirri of segment I ( Fig. 4C View Figure 4 ). No discreet palphore in most specimens, sometimes faint ‘fold’ present. Eyes absent. Nuchal organs not discerned.

Large robust muscular pharynx extends to around segment VII. Mouth papillae cirrus-like with rounded bases, cirriform distally; some may be a more cushion-like shape. Mouth papillae number 10, visible when mouth everted. No jaws.

Segments I–III lacking podia and chaetae, each segment with two pairs tentacular cirri; total six pairs. Dorsal cirri on segments I-III long, slender, faintly segmented with subconical base not defined cirrophores; ventral cirri shorter than dorsal, particularly on segments I and III ( Fig. 4C View Figure 4 ). Biramous podia with notochaetae and neurochaetae start segment IV ( Fig. 4B–C View Figure 4 ) and continue down body.

Mid-body notopodia shallow mounds with fascicle of spinous notochaetae spread out in fan-like arrangement with shorter spines on inner side of whorl and longer spines in central part ( Fig. 4B View Figure 4 ). Notochaetae number 20–30, spines slightly curved, relatively short in length, shaft may have minute scattered tubercules but distal half to third with the outer margin denticulate composed of two densely close rows of rectangular-shaped teeth extending to distinctive, slightly expanded, truncate, perhaps slightly excavate, distal tip ( Fig. 4F View Figure 4 ).

Robust, medium-length dorsal cirri from segment IV, with broad bases, narrowing to cirriform tip, not extending in length beyond notochaetal fascicle. Dorsal cirri smooth not articulated, not alternating in size or position down body ( Fig. 4A–B View Figure 4 ).

Neuropodia composed of anterior elongate lobe and posterior more truncate lobe; large, rounded glands posterior to latter neuropodial lobe ( Fig. 4D View Figure 4 ). Two neuroaciculae present: very slender, pale, superior acicula present at the end of anterior pointed lobe; longer, brown, robust acicula ending mid-way in posterior neuropodial lobe. In one neuropodium of one specimen a single, simple, hookedshaped, slender acicular spine, parallel to but separate from a robust lower acicula, present and very hard to discern. Ventral cirri composed of broad rounded base and shorter, less broad distal part; almost globular–like in anterior segments becoming more elongate in posterior segments. Ventral cirri mid-body about half to two-thirds length of neuropodial lobe. Ventral cirri inserted on lower edge of ventral ramus immediately posterior to neurochaetal fascicle ( Fig. 4C– D View Figure 4 ).

Neurochaetal fascicle composed of superior long shafted falcigers with longer blades more directed upwards, number <5; mid and posterior group of unidentate falcigers with medium to shorter length serrate blades, number 12–15 ( Fig. 4D– E View Figure 4 ). Superior-most neurochaetae with 1–2 simple neurochaetae positioned above the superior slender acicula, slightly curved with blade-like distal tip bearing tiny spinelets on concave edge ( Fig. 4D View Figure 4 ); simple neurochaetae may present in other fascicle positions. Neurochaetae with bidentate shafts. Internal cameration absent in notochaetae and neurochaetae.

Nearly all specimens missing posterior-most segments. Entire pygidium comprising two robust, long, dorso-lateral anal cirri and a single anal cirrus originating ventrally, about quarter the length of lateral anal cirri ( Fig. 4A View Figure 4 ).

Reproductive morphology. Gametes were observed in a nearly entire individual of 17 segments, only missing the posterior end. Large, rounded swollen ventral pads are evident from segment IV. Smaller rounded, swollen structures interpreted as external genital organs are positioned ventrally; they bulge out in between the inferior edge of the neuropodia and ventral cirri and are just visible from segment VII and very clearly present from segments IX, X ( Fig. 4D View Figure 4 ). These appear glandular in function and contain multiple, very small, rounded, dark ‘blackberry’ shapes indicative of sperm. Smaller oocytes were visible through the ventral body wall with larger oocytes (0.04 mm in diameter) present from segment XII to segment XVII. No external penes or copulatory stylets observed.

Etymology. Pleijelius keni sp. nov. is named in honour of the first author’s father, Kenneth Watson, who imbued his daughter with a love of nature and scientific curiosity. His ashes are spread at sea offshore from Byron Bay in the vicinity of the whale fall and the abundant life cycle it sustains.

Diagnosis. Notochaetae with two rows of densely serrate margins positioned close together; serrations are rectangular in shape.

Diagnostic remarks. The morphology of Pleijelius keni sp. nov. and P. longae is similar, including length and number of segments: P. keni sp. nov. ~ 3 mm length, 25E versus P. longae length 3.6 mm, 26E. Pleijelius keni sp. nov. has a rectangular, narrower body and less dense notochaetal fascicles in comparison with P. longae which has chaetigers 4–9 broader than preceding ones (Salazar-Villejo & Orensanz, 2006, fig. 3A; Shimabukuro et al., 2020, fig. 5.3C, colour photo of a live specimen). Pleijelius keni sp. nov. and P. longae both have 10 pharyngeal papillae; however, their shape is more cirriform in the former and broad, rounded, and mound-like in the latter.

The obvious morphological difference between the two species is notochaetal shape and ornamentation. Notochaetae of Pleijelius keni sp. nov. have the distal half to third with two rows of serrate margins densely close together; serrations are of a distinctive rectangular shape. The notochaetal distal tip is slightly expanded, truncate, with a slight concave depression ( Fig. 4F View Figure 4 ). Pleijelius longae possesses smooth notochaetal capillaries with a row of 2–4 tiny distal denticles, extending to blunt rounded tip (Salazar-Villejo & Orensanz, 2006, fig. 3D).

Two neuroaciculae, one slender and one robust, are found in examined Pleijelius keni sp. nov. and in published figures of Pleijelius longae . Rarely a hooked acicula spine is present additionally to the two neuroaciculae in Pleijelius keni sp. nov.; a robust single neuroacicula, which in some podia ‘doubles over’ forming a hook, is illustrated for P. longae (Salazar-Villejo & Orensanz, 2006, fig. 5B).

An entire pygidium of Pleijelius keni sp. nov. has two long, robust dorso-lateral anal cirri and a single anal cirrus originating ventrally; the latter is poorly visible as most specimens have missing or damaged pygidia. Salazar-Villejo and Orensanz (2006, fig. 2A, 3 A–B) describe the pygidia of Pleijelius longae with two pairs of long, robust anal cirri and no ventral anal structure. Their figures show a damaged posterior end, but a single pair of dorso-lateral anal cirri is visible along with typically compressed posterior-most cirrose segments.

In one specimen of Pleijelius keni sp. nov., contraction, possibly due to SEM drying, has led to the position of the dorsal cirri being much more dorsally orientated compared to the notochaetal fascicle ( Fig. 4A View Figure 4 ) observed in specimens from the same sample. These other specimens have the notochaetal fascicle visible in dorsal view and partially covering the dorsal cirri, as shown in notopodium of segment IV, on the right side ( Fig. 4B View Figure 4 ).