Tuesday, April 28, 2020

[Invertebrate • 2020] Euchonoides moeone • An Abundant New Genus and Species of Fan Worms (Polychaeta: Sabellidae) from Hawaii

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

A new genus and species of Hawaiian sabellid polychaete, Euchonoides moeone n. gen. et n. sp. is described. This new species has consistently been one of the most abundant polychaetes collected in Mamala Bay, Hawaii, reaching densities of 141,046 ind. m-2 and representing up to 30.6% of all polychaetes collected in that region. The species has a small body (~2 mm length) with a reduced number of abdominal chaetigers (8–10), and is distinguished by the presence of the following features: 1) three pairs of radioles; 2) radiolar skeleton with two longitudinal rows of cells from radiole base to third proximal pair of pinnules, remainder of each radiole skeleton with single rows of cells; 3) pinnules unpaired, alternating (snowflake arrangement); 4) thoracic uncini acicular with a large tooth above the main fang followed by a series of small ones; 5) thoracic chaetiger 3 enlarged; 6) a wide belt on third abdominal chaetiger; and 7) a pre-pygidial depression composed of three chaetigers, with lateral wings, among a combination of several others features. Histological sections have shown that the abdominal belt seems to be a clitellum-like structure where oogenesis takes place. The new genus is compared with other plesiomorphic genera sharing similar morphological features. Patterns of abundance of the new species are presented for the past 27 years in Mamala Bay.

Keywords: Polychaeta, Euchonoides, Euchone, clitellum-like segment, histology, sewage outfall, abundance

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.

Figure 2. Euchonoides moeone n. sp.
 A, anterior end, ventral view, stained with methyl green; B, posterior end, dorso-lateral view, stained with Methyl Green; C, peristomium in ventral view showing brownish eyespots; D, dissected radiolar crown stained with Shirlastain A. 
Abbreviations: ar, abdominal ridge; ch, chaetiger; dl, dorsal lip; pyg, pygidium; R, radiole; vra, ventral radiolar appendage

Family Sabellidae Latreille, 1825

Euchonoides n. gen. 

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.

Euchonoides moeone n. sp. 

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.

Figure 3. SEM of Euchonoides moeone n. sp. A, complete specimen in lateral view, inset showing glandular belt on third abdominal chaetiger; B, radiolar crown; C, anterior peristomial ring and posterior peristomial ring collar in dorsal view; D, posterior peristomial ring collar in ventro-lateral view; E, posterior end with pygidium, ventro-lateral view.

Figure 5. Longitudinal, histological sections of  Euchonoides moeone n. sp. 
A, body, regenerating thorax; B, base of radiolar crown and collar, lateral view; C, same, frontal view; D, pre-pygidial depression and pygidium; E–F, belt on third abdominal chaetiger, G, sexual abdominal segments; H, J, details of glandular epithelium of belt of third abdominal chaetiger; I, detail of a mature oocyte and follicle cells. In A–B, D and I, the section plane is shown, where A refers to the anterior region, P to the posterior region, d dorsal zone and v ventral zone. Black arrows in A, E–G, I–J points to glandular belt on third abdominal chaetiger.
Abbreviations: A1: abdominal chaetiger 1, A2: abdominal chaetiger 2, A3: abdominal chaetiger 3.

 Wagner F. Magalhães, Julie H. Bailey-Brock and María Ana Tovar-Hernández. 2020. An Abundant New Genus and Species of Fan Worms (Polychaeta: Sabellidae) from Hawaii. Zootaxa. 4763(1); 85–98. DOI: 10.11646/zootaxa.4763.1.7