Owenia dichotoma Parapar & Moreira, 2015, n. sp.
Creators: name: Parapar, Julio
name: Moreira, Juan
Publisher: name: Zenodo
Year: 2015
Subjects: Biodiversity
Taxonomy
Animalia
Annelida
Polychaeta
Sabellida
Oweniidae
Owenia
Owenia dichotoma

description: Owenia dichotoma n. sp.
(Figs 1 C, 2, 6, 7)
Material examined. Holotype: AM W.45857, fine sediment, 12 m. Paratypes: AM W.424, 2 m; AM W.45858, sand, 9 m; AM W.45859, fine sand, 12 m; AM W.45861, medium sand, 3 m; AM W.47387, median abdominal fragment on SEM stub.
Description. Based on holotype. Body elongated; 16 mm long, 1.0 mm wide, incomplete, with 9 chaetigers. Branchial crown very short; provided with four pairs of tentacles, each tentacle with larger ramification close to crown base thus appearing as if eight pairs of short tentacles present (Fig. 6 A); second dichotomy very close to distal end. Total length of crown about 1/3 of thorax length (C/T ratio ≈ 0.33). A pair of very small eye spots located at the base of the crown (Fig. 6 B). Junction between thorax and tentacle crown marked by a distinct, straight collar, closely aligned with body wall, slightly overlapping base of crown. Crown and thorax of similar width. RLTS: 1–2–1. Thorax with three pairs of poorly developed notopodia with capillary chaetae. Notochaetae with shafts composed of densely packed scales. Thoracic neuropodia absent. Width of each abdominal segment decreasing posteriorly, being posterior segments short and compact. Abdominal notopodia poorly developed with capillary chaetae similar to thoracic notochaetae. Abdominal notochaetae arranged dorsally. Neuropodia consisting of wide sessile podia with several rows of neurochaetal hooks; tori almost encircling body except on dorsal surface between notochaetal bundles. First abdominal segment (b1) slightly longer than total thoracic length. Neurochaetal uncini similar through, consisting of hooks with shaft ending in two teeth side by side.
Preserved animals creamy white with no pigmentation. Tube coated by quartz grains and shell fragments piled on top of one another like roof tiles.
Variations. The holotype and the paratype W.424 were the only specimens still having a well-preserved crown of tentacles (Fig. 6). This paratype is a large incomplete specimen (25 mm long, 1.2 mm wide, with 9 chaetigers) which shows more irregular ramifications on branchial crown (Fig. 6 C–D) than those of the holotype (Fig. 6 A–B). The paratype W. 45858 is wider (1.3 mm) than holotype and incomplete but still having 25 abdominal segments. On the other hand, paratype W.45861 is complete and showing the conspicuous decrease of the width of abdominal segments backwards, being posterior segments short and compact; it bears 22 abdominal segments and the pygidium is multilobed (ca. 6 lobes). Unfortunately, lack of specimens has led to use only one abdominal fragment for SEM study which allowed to study the morphology of chaetae but did not provide suitable images.
Remarks. The closest species to O. dichotoma n. sp. are O. mirrawa and O. fusiformis sensu Imajima & Morita (1987). The most remarkable character of O. dichotoma n. sp. agrees with that highlighted by Ford & Hutchings (2005) for O. mirrawa (Fig. 7 F): “the most distinguishable character is positioning of the ramifications close to the base of the tentacles, giving the impression of many pairs of tentacles, much more than the four pairs present”. However, the ratio between the total length of the branchial crown and the total thorax length (C/T ratio: 0.6) of O. mirrawa is higher than that of O. dichotoma n. sp. (C/T ratio: 0.3). Furthermore, O. mirrawa bears fewer abdominal chaetigers (11) than those of paratype W.45861 of O. dichotoma n. sp.
The new species also resembles Owenia fusiformis sensu Imajima & Morita (1987) (Fig. 7 B) in the size and shape of the branchial crown. Thus, the crown of O. fusiformis sensu Imajima & Morita (1987) shows a similar C/ T ratio and bears dichotomously branched tentacles; this species differs, however, from O. dichotoma n. sp. in having coloration both in the tentacles and in the dorsal anterior region (cfr. Fig. 2 b–c in Imajima & Morita 1987).
The Pacific species Owenia collaris Hartman, 1955, from Southern and Central California was recently redescribed and illustrated by Blake (2000); the species resembles somehow to O. dichotoma n. sp. However, the former species seems closer to O. gomsoni (Fig. 7 C) and O. mirrawa according to the size of the branchial crown (C/T ratio: 0.5; see key below). In addition, O. collaris differs in the body pigmentation and in the pattern of ramification of the branchial crown (cfr. Fig. 5.9.A in Blake 2000).
Etymology. The species name (“ dichotoma ”) refers to the dichotomous divisions of the tentacles of the branchial crown.
description: Published as part of Parapar, Julio & Moreira, Juan, 2015, The Oweniidae (Annelida; Polychaeta) from Lizard Island (Great Barrier Reef, Australia) with the description of two new species of Owenia Delle Chiaje, 1844, pp. 604-620 in Zootaxa 4019 (1) on pages 611-612, DOI: 10.11646/zootaxa.4019.1.20, http://zenodo.org/record/243370