Euchonoides moeone Magalhães & Bailey-Brock & Tovar-Hernández 2020, n. sp.

Euchonoides moeone n. sp.

( Figs 1–6)

Euchone sp. B: Nelson et al. (1991), Bailey-Brock et al. (2001; 2002), Ambrose et al. (2017 and previous years).

Material examined. Holotype: Mamala Bay, Oahu, Hawaii, Sta. HB 3 R2, 21°17’ 02.8’’ N, 158°01’ 39.5’’ W, Jan/2016, 70 m, sediment composed primarily of fine and medium grain sand (BPBM R-3914). Paratypes: same locality, station and date as holotype (20, BPBM R-3915; 10, UFBA 1582).

Description

Body short, rounded in cross section (Fig. 1 A–B). Holotype total thorax-abdomen length 1.67 mm (radiolar crown 0.48 mm), maximum width 0.1 mm. Paratypes ranging from 1.8–2.1 mm long (radiolar crown 0.3–0.45), maximum width 0.1 mm.

Three pairs of radioles (Figs 1D; 2D), each with 6–12 pinnules, with snowflake (unpaired, alternating pinnules) distribution; radiolar rachis slightly thicker than pinnules (Fig. 1D). Basal pinnules short, then increasing abruptly in size towards last third of radiolar length, most of pinnules terminate at approximately same height distally (Figs 1 A–B, D; 2D). Radiolar tips filiform, as long as one quarter of radiolar length (Fig. 2D). Dorsal lips digitiform, as long as ¼ of the radiolar crown length, without mid-rib (radiolar appendage) (Figs 1D; 2D). Ventral lips absent. A pair of ventral radiolar appendages as long as radiolar length (shorter in some paratypes) (Figs 1D; 2D). Inter-radiolar membrane and radiolar lateral flanges absent (Fig. 3 B–C). Each radiolar skeleton with two longitudinal rows of cells, in cross section, from radiole base to third proximal pair of pinnules, remainder of each radiole with single rows of cells (Fig. 5 B–C). Radiolar eyes absent.

Anterior peristomial ring exposed partially on lateral sides (Fig. 3B, D), fully exposed dorsally (Fig. 3C). A pair of brown peristomial eyes (Figs 1 A–B; 2C). Posterior peristomial ring collar well-developed with entire smooth margins (Figs 1 A–C; 3C–D). Ventral margin of collar longer than dorsal, with deep mid-incision forming rounded ventral lappets (Figs 1 B–C; 3D). Lateral collar margins diagonal (oblique) (Figs 1A; 3B, D). Dorsal collar margins fused to faecal groove, forming a narrow gap (Fig. 3C). Collar chaetiger with 4–6 elongate, narrowly hooded chaetae of equal length (Fig. 3C). Ventral shield of collar rectangular (Fig. 2A), with a transverse patch of cilia (Figs 1C; 3D).

Thorax composed of seven chaetigers in holotype (paratypes with 3–7 chaetigers). Glandular ridge homogeneously narrow on chaetiger 2 (Figs 1C), light brown in color, when fixed. Thoracic segments increasing in length from chaetigers 1–3, chaetiger 3 always longest, 2–3 times longer than wide (Figs 1A, B; 3A). Ventral glandular shields not differentiated (Figs 1 A–B; 3A). Notopodia: superior group of chaetae with a row of two elongate, narrowly hooded chaetae; inferior group with one row of two bayonet chaetae (Figs 1E; 4A), and one row of broadly hooded chaetae (Figs 1F; 4B). Neuropodia: acicular uncini numbering 3–4 per torus, handles longer than 5 times the length of crest, hoods absent (Fig. 1G), a large tooth above the main fang (Fig. 4C), followed by a series of small ones, dentition covers half of the main fang length (Fig. 4C).

Abdomen with nine chaetigers in holotype, posterior three forming pre-pygidial depression (8–10 chaetigers in paratypes, all paratypes with pre-pygidial depression composed of three segments). Third abdominal chaetiger (A3) with unusual, broad belt (clitellum-like), readily distinct and present in all specimens (Figs 1 A–B; 3A); dorsally entire, ventrally interrupted by faecal groove; composed of wide glandular, columnar epithelium (Fig. 5A, E–H, J). Neuropodia with 3–6 elongated, narrowly hooded chaetae, slightly longer on posterior-end chaetigers. Anterior abdominal notopodia with 4–6 uncini per torus with square breasts, handle absent, and at least nine rows of small, equal sized teeth covering most than 3/4 of main fang length (rasp-shaped dentition) (Figs 1H; 4D). Pre-pygidial depression with raised membranous lateral flanges (Figs 1B, J; 3E; 6D); tori shorter than anterior abdominal segments, and with decreasing number of uncini (4, 3, 2, respectively); uncini not distinct in shape than rest of anterior abdominal uncini (Fig. 1I). Pygidium enlarged, bluntly rounded, lacking pygidial eyespots and pygidial cirrus (Figs 1J, K; 3E; 5D).

Body color and methyl green staining pattern: Preserved specimens lacking pigmentation. Ventral shields only distinct with staining (Fig. 2 A–B). Methyl green stained more intensely transverse glandular bands of thorax and anterior abdomen (Fig. 2 A–B); posterior abdomen staining lightly, pygidium staining more distinctly than pre- pygidial depression (Fig. 2B); thoracic ventral region staining more intensely than respective dorsal region, especially ventral shields (Fig. 2 A–B).

Tubes: Tube composed of fine and medium grain sand particles and shell fragments bound by mucus membrane and fine layer of silt.

Reproduction: Several individuals with developing radiolar crowns and variable number of thoracic segments (3–7) were found within tubes of adults, indicating intratubular direct larval development. In these offspring, radioles are rudimentary (as small, rounded protuberances or short, digitiform radioles without pinnules) and thorax is not completely formed (few chaetigers are present and the first three thoracic segments are narrower and shorter than rest of the body). Ripe males or hermaphrodites were not found in the hundreds of examined specimens, but packages of germ cells were observed in abdominal segments A1–A3 (Fig. 5A, G). Oogenesis was observed only in abdominal segments A1–A3. Oocytes measured 37–50 μm in diameter, and few oocytes (three per specimen) were found in abdominal segments A1–A3 (Fig. 5G, I).

Remarks. Cochrane (2003) recognized a group of small-sized Euchone species with snowflake pinnular arrangement and the presence of three chaetigers in the pre-pygidial depression named by her as ‘Chiade’. This clade is composed of E. trilobata (Banse, 1957) from the Falkland Islands, E. incolor Hartman, 1965 from off New England, E. hancocki Banse, 1970 from southern California, E. scotiarum Hartman, 1978 from Antarctica, and Euchone x sensu Cochrane, 2000 from the North Sea and the Norwegian Sea. These species of ‘Chiade’ differ in the number of abdominal segments anterior to pre-pygidial depression and on the presence of glandular ridges similar to that in thoracic chaetiger 2, but also present in some abdominal chaetigers. Abdominal glandular ridges are present in E. incolor (pre-chaetal on third abdominal chaetiger), E. hancocki (post-chaetal on first abdominal chaetiger) and Euchone x (pre-chaetal on third abdominal chaetiger). Euchone trilobata do not have abdominal glandular ridges and these were not described or corroborated in E. scotiarium (Banse 1957). However, specimens from Hawaii here studied are unique among all the species of ‘Chiade’ by the presence of an oblique, broad belt on third abdominal chaetiger.

A similar abdominal belt was reported for Amphicorina bicincta (Ozolinsh, 1988) and an undescribed species of Amphicorina from Chukchi Sea (Leslie Harris pers. com.). In their review of Oriopsis, Giangrande et al. (1999: 196) emphasized that A. bicincta probably belongs to a new genus. Amphicorina bicincta and E. moeone n. sp. are similar in relation to the presence and shape of the oblique abdominal belt on third abdominal chaetiger and presence of three pairs of radioles but these are readily distinguished by the presence of a broad glandular ridge on thoracic chaetiger 2 in A. bicincta (narrow in E. moeone n. sp.) (see Table 1 for a detailed comparison).

Terebrasabella heterouncinata Fitzhugh & Rouse, 1999 has one pair of densely ciliated sperm ducts present along the posterior margin of thoracic chaetiger 8. These ducts are located ventro-laterally, just posterior to neuropodial uncini, extending dorsally and sometimes slightly anteriorly as low ridges; these ducts terminate at the lateral margins of faecal grove (Fitzhugh & Rouse 1999). Reviewing SEM images by Fitzhugh & Rouse (1999: fig. 5), there is a great external similarity between sperm ducts of Terebrasabella and the belt of the new genus located on abdominal segment 3. Both are low ridges and terminate on lateral margins of the faecal groove. To date, spermatozoa have not been observed in the new Hawaiian genus.

Etymology. The new species epithet derives from the Hawaiian language and the implied meaning of moeone in Hawaiian is ‘small worm that hides in the sand’.

Distribution. The type locality is Mamala Bay, Oahu, Hawaii at the vicinity of Barbers Point sewage outfall at 70 m. This species has also been collected at Ala Wai Canal, Kailua Bay, and Waianae on Oahu from shallow subtidal to up to 100 m in fine and medium sand.