Euchonoides, Magalhães & Bailey-Brock & Tovar-Hernández, 2020

Magalhães, Wagner F., Bailey-Brock, Julie H. & Tovar-Hernández, María Ana, 2020, An abundant new genus and species of fan worms (Polychaeta: Sabellidae) from Hawaii, Zootaxa 4763 (1), pp. 85-98 : 87-90

Euchonoides n. gen.

Type species. Euchonoides moeone n. sp., herein designated, by monotypy.

Diagnosis. Body short with a reduced number of abdominal chaetigers (8–10). Three pairs of radioles. Radiolar skeleton present in branchial lobes, radioles and pinnules. Each radiolar skeleton with two longitudinal rows of cells from radiole base to third proximal pair of pinnules, remainder of each radiole and all pinnular skeletons with single rows of cells. Pinnules unpaired, alternating (snowflake arrangement). Basal membrane absent; radiolar flanges absent; radiolar eyes absent. Dorsal lips without radiolar appendage. Ventral lips absent. Dorsal pinnular appendages absent. Ventral pinnular appendages present. Parallel lamellae absent. Ventral sacs absent. Radiolar lobes fused along dorsal midline; without dorsal or ventral basal flanges. Anterior peristomial ring distinctive, but no visible annulation between anterior and posterior peristomial ring. Peristomial eyes present. Peristomial vascular loops absent. Posterior peristomial ring collar incised ventrally; separated dorsally by distinct gap. Collar chaetae fascicles protruding from a short lobe, narrowly hooded chaetae. Glandular ridge on thoracic chaetiger 2 present. Superior thoracic notochaetae elongate, narrowly hooded. Inferior thoracic notochaetae includes bayonet and broadly hooded chaetae. Thoracic neuropodial uncini acicular, handles long, hoods absent, a large tooth above the main fang followed by a series of small ones. Companion chaetae absent. Thoracic chaetiger 3 always enlarged (2–3 times longer than wide). Ventral thoracic shields not differentiated. Neuropodial abdominal fascicles with elongate, narrowly hooded chaetae. Anterior abdominal uncini with square breasts, handle absent, and rasp-shaped dentition. Belt (clitellum-like) on third abdominal chaetiger present. Pre-pygidial depression composed by three chaetigers, with lateral wings. Uncini from pre-pygidial depression similar to those from anterior abdomen. Pygidium without anal cirrus. Pygidial eyes absent.

Etymology. The genus name is a free combination of Euchone and the Latin sufix -oides, in relation to the similarities between the new genus with the genus Euchone .

Gender. Neuter.

rina bicincta ( Ozolinsh, 1988) .

& Rouse, 2007).

Remarks. The new genus is proposed based on a combination of distinctive characters (see Table 2) but especially by the presence of an oblique, broad belt on the third abdominal chaetiger ( Figs 1 A–B; 3A) instead of the typical glandular ridges. This abdominal structure is not morphologically similar to the usual, glandular ridge on thoracic chaetiger 2 or to the glandular abdominal ridges in other Chone , “Chiade” or Dialychone species ( Tovar-Hernández 2008). These typical glandular ridges are whitish and occupies only the external half of the epithelium; these are composed of strongly differentiated acidophil glandular cells, tubular-shaped with granulose secretions such as described by Tovar-Hernández & Sosa-Rodríguez (2006). The abdominal belt of E. moeone n. sp. is conspicuous (visible without staining), but histological sections show that it is not a differentiated epithe-

lium from the rest, it is only a thicker epithelium as the clitellum of clitellates, with markedly basophil glands.

Reproductive features associated to the abdominal segment 3 such as presence of modified chaetae, genital pores or markings were not found, but germ cells and oocytes in different phases of development were found in abdominal chaetigers 1–3.

A similar abdominal belt was reported for Amphicorina bicincta ( Ozolinsh, 1988) and on another undescribed Amphicorina species from Chukchi Sea (Leslie Harris pers. com.). These three species are similar and the differences are presented on remarks for the new species and on Table 1. In her review of Oriopsis, Giangrande et al. (1999: 196) emphasized that A. bicincta probably belonged to a new genus. In addition to the belt on abdominal chaetiger 3; the pinnular arrangement (snowflake); the presence of three pairs of radioles and a pre-pygidial depression composed of three chaetigers, the new genus is established based also on a combination of unique features as supported above.

Among these unique set of characters, the radiolar skeleton of Euchonoides n. gen. is similar to that present in Terebrasabella Fitzhugh & Rouse, 1999 : each radiolar skeleton has two rows of cells from radiole base to third proximal pair of pinnules, and the remainder of each radiole with single rows of cells. The genera Terebrasabella , Caobangia Giard, 1893 and Amphiglena Claparède, 1864 also lack ventral lips.

A low number of radioles have been reported in few sabellids. Terebrasabella has only two pairs of radioles whereas the presence of three pairs of radioles have been reported in Desdemona ornata Banse, 1957 , some species of Amphicorina Claparède, 1864 , Euchone incolor , E. scotiarum , E. trilobata , E. hanckocki and Euchone x sensu Cochrane and the new genus here described.

The new genus is quite remarkable in having such a small number of abdominal chaetigers (8–10 chaetigers). In comparison, Terebrasabella present only three abdominal chaetigers, Amphiglena lindae Rouse & Gambi, 1997 has 26 abdominal chaetigers and Caobangia presents 25–40 abdominal chaetigers.

Enlargement of some chaetigers (not depending of contraction) has been observed in Terebrasabella where posterior abdominal segments are notoriously longer than wide. In the new genus, thoracic chaetiger 3 is also markedly longer than remainder of body segments.

Comparing with other sabellid genera having a pre-pygidial depression with lateral wings, radioles are free in Euchonoides n. gen., not joined by a basal membrane and lacks flanges (both, membrane and flanges present in Euchone restricted and Euchoneira ). Arrangement of pinnules as snowflake (unpaired, alternating pinnules) is present in Euchonoides n. gen. (feather duster= paired pinnules in Euchone restricted and Euchoneira ). Dorsal lips are digitiform without mid-rib support in Euchonoides n. gen. whereas lips are distally tapered with mid-rib in Euchone and broadly rounded without mid-rib in Euchoneira . In Euchonoides n. gen., the third thoracic chaetiger is always enlarged (versus not enlarged in Euchone and Euchoneira ). Ventral shields are well differentiated, swollen in Euchone and Euchoneira , but not differentiated in Euchonoides n. gen. Dentition of thoracic uncini of Euchonoides n. gen. is characterized by uncini with a large tooth above the main fang in contrast to the small teeth of Euchone and Euchoneira . The number of abdominal uncini is reduced in Euchonoides n. gen. (8–10) but larger in Euchone (up to 29) and Euchoneira (30). See Table 2 for additional comparisons.

Figures

FIGURE 1. Euchonoides moeone n. sp. A, complete paratype, lateral view; B, complete paratype, ventral view; C, detail of patch of cilia (pc) on posterior peristomial ring collar and glandular ridge (gr) on chaetiger 2; D, dissected radiolar crown with arrows showing elongate dorsal lips and ventral radiolar appendages; E, inferior, thoracic, short, broadly hooded chaetae; F, inferior, thoracic bayonet chaeta; G, thoracic acicular uncini; H, anterior abdominal uncinus; I, posterior abdominal uncini, frontal and lateral view, respectively; J, posterior end showing pre-pygidial segments and pygidium, ventral view; K, posterior end, dorsal view. Abbreviations: ar, abdominal ridge; ch, chaetiger; es, eyespots; fg, faecal groove; gr, glandular ridge; pc, patch of cilia; pyg, pygidium; pre-pyg, pre-pygidial chaetigers. Pinnules in A–B may be confused with radioles, there are only three pairs or radioles.