Euchone perseyi

Euchone perseyi (Zenkewitsch, 1925), redescription

Figures 2–4

Chone Perseyi Zenkewitsch, 1925: 11–12, Figs 1 –7.

Euchone perseyi.— Tovar-Hernández 2007: 62.

Type locality: 71°30′40″N, 52°16′30″E, Belushya Guba (meaning bay of beluga or white whale Delphinapterus leucas (Pallas)), 3–8 m, Novaya Zemlya, Russia.

Material examined: Zoological Museum of Moscow State University, [PI –4839] lectotype, Station 110b, [Pol610] 7 paralectotypes, Station 110b, [Pol611] Station 108 a (1 syntype), [Pol609] Station 115 a (3 syntypes), [Pol612] Station 116 b (4 syntypes).

Redescription based on lectotype, with variation of paralectotypes in brackets.

Body excluding crown 6 mm long (3–6 mm), 0.3 mm wide (0.25–0.3 mm). Body with cylindrical trunk (Fig. 2D, 3A). Radiolar crown 2 mm long (1.8–2 mm), as long as thorax, united at base by a low palmate membrane extending up to 1/4 of the radiolar length. Three-six pairs of radioles with long filiform tips, as long as two first thoracic chaetigers (Fig. 2E), as long as a one quarter of radioles length. Long pinnules located at the last third of crown, twice longer than basal ones (Fig. 2F). Lateral flanges narrow. A pair of long ventral radiolar appendages about 1/2 the length of radiolar crown (Fig. 2F). Insertion of the crown exposed beyond collar dorsally and laterally (Fig. 2A, C–D). Dorsal collar margins oblique (Fig. 2A). Collar incised ventrally, forming two rounded ventral lappets (Fig. 2B). Ventral side of collar without remarkable glandular shield, except for a whitish area nearly rectangular, located in anterior half of segment (Fig. 2B). Lateral sides of collar are diagonal, exposing the anterior peristomial ring (Fig. 2C). Thorax composed by 8 chaetigers (Fig. 3A–B), including collar chaetiger. Glandular ridge on chaetiger 2 narrow all around, whitish (Fig. 2B–C). Ventral shields not developed. Thoracic notopodia with two rows of chaetae: superior row with 5 elongate, narrowly hooded chaetae and inferior row with 2–3 short narrowly hooded chateae (Fig. 3C–D). Thoracic neuropodia bearing 4–5 acicular uncini with long handles (>8 times the length of main fang), 4–5 rows of teeth above the main fang nearly equal in size, slightly diminishing in size towards anterior end. Dentition covering a half to 3/4 the length of main fang (Fig. 3E–F). Hoods absent.Abdomen with 14–19 chaetigers (Fig. 3A). Abdominal chaetigers with elongate, narrowly hooded chaetae (Fig. 4B), those from posterior abdomen 25% longer than those of anterior abdominal segments. Abdominal tori with 12–16 uncini in abdominal segment 1, then decreasing gradually in number in posterior segments (Fig. 4D–G). Abdominal uncini with sub-squared to rectangular breast, handles absent and dentition above the main fang as rasp-shaped plates. Dentition of uncini with series of nearly uniformly sized teeth covering 3/4 of the main fang, without noticeable variation along abdomen (Fig. 4C–G). Last six abdominal chaetigers diminishing in width to pygidium, without a pre-pygidial depression (Figs. 2G–H, 4A–B). Pygidium conical (Fig. 2G–H). Methylene blue staining pattern

Ventral shield of collar unstained (Fig. 2B). Ventral epithelium stains nearly uniformly, except for distinct inter-segmental grooves and glandular ridge on chaetiger 2 that remain unstained (Fig. 2B). Dorsally there are scattered small dots (presumably glands) deep blue stained (Fig. 2A). Laterally there are blue dots that surround the noto- and neurochaetal lobes (presumably glands) (Fig. 2C–D).

Measurements provided in original description

Body with 24–27 segments (it is suggested that number includes thoracic and abdominal segments), body length 5–7 mm (perhaps including crown) and 3–4 pairs of radioles.

About the etymology

The specific name “ perseyi ” was established in honor to the first Soviet research ship “Persey” (Zenkewitsch 1925, page 11, paragraph 4). Chone perseyi was collected while the RV Persey was in Belushya Bay during its first scientific voyage in 1923.The ship was sunk on 10 July, 1941, during a German aerial attack, in the Eina Bay (the Motovsky Bay) (Sokolov et al. 2022).

Remarks

Euchone is a paraphyletic genus composed by 35 species (Cochrane 2003, Giangrande & Licciano 2006, Licciano et al. 2009, Capa et al. 2021) and it demands a major taxonomic revision. Traditionally, Euchone was recognized by the presence of a pre-pygidial depression (anal depression) composed of the lateral wings (Banse 1972, Fauchald 1977). However, some authors demonstrated that there is strong intra- and interspecific variation in the posterior abdominal segments, as discussed below.

Cochrane (2000) emphasized the finding of an unnamed taxon (referred as “ Euchone x”) from North Sea and Norwegian Sea with a remarkable ontogenetic variation of the pre-pygidial depression, which is composed of three segments, being from poorly developed to bordered by lateral wings. In the phylogeny proposed by Cochrane (2003), small species of Euchone were plotted in a clade called “ Chiade ”, separated from Euchone.

Later, as demonstrated by Bick & Randel (2005), Euchone analis Krøyer, 1856 (type species of the genus) has remarkable ontogenetic variation in the presence and shape of the pre-pygidial depression as well as the number of abdominal chaetigers forming that depression. They found four stages of development of the pre-pygidial depression: 1) depression not developed in juveniles (specimens with 18 or 19 abdominal chaetigers), 2) depression in an early formation stage where the lateral margins of the anal depression increasingly emerge (specimens with 15 anterior abdominal chaetigers and six forming this early stage), 3) depression bordered with anterior and lateral margins (subadult stage, composed by 8 pre-pygidial segments), and 4) depression with lateral wings (adult stage, depression formed by 10–12 pre-pygidial segments). Species in the genera Euchoneira Licciano, Giangrande & Gambi, 2009, Chone Krøyer, 1856, Dialychone Claparède, 1868 and Paradialychone Tovar-Hernández, 2008 also have pre-pygidial depressions under different development stages (Tovar-Hernández 2008, Licciano et al. 2009).

Under this framework, Chone perseyi would be nested in the clade named “ Chiade ” sensu Cochrane (2003) that includes some of the small members of Euchone. As the recognition of this potential new genus demands a formal establishment, Tovar-Hernández (2007) considered Chone perseyi as a member of Euchone. Chone perseyi shares the following features with small members of Euchone (Banse 1970, Cochrane 2000, 2003): thoracic chaetigers equipped with elongate, narrowly hooded chaetae, bayonet chaetae and acicular uncini lacking hoods, posterior abdomen with elongate, narrowly hooded chaetae (25% longer than those of anterior abdominal segments) and uncini with sub-squared to rectangular breast and dentition above the main fang similar along abdomen, presence of a ventrally notched collar, forming two well developed ventral lappets, absence of glandular shields, and lack of a pre-pygidial depression. In the present study, the placement of Chone perseyi in Euchone is maintained for stability because phylogenetic analyses are needed (including some morphological features not considered before and molecular data), which may provide a better understanding of the relationships and classification to these genera.