Owenia dichotoma, Parapar, Julio & Moreira, Juan, 2015

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, Zootaxa 4019 (1), pp. 604-620 : 611-612

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.

Figures

FIGURE 1. Map of the study area showing collecting sites of Galathowenia and Owenia species described or reported in this work. In (B − D) arrows facing to cross-shelf sampling sites: EI = Eagle Island; NDI = North Direction Island. Dots refer to material collected during LI Workshop held in August 2013; stars refer to AM material; white star refers to type locality of O. picta n. sp.

FIGURE 2. World map showing species of Galathowenia and Owenia described (grey circles) or reported (white circles) in East Asia, Australasia and nearby regions. 1. Grube (1878); 2. Augener (1922); 3. Augener (1926); 4. Caullery (1944); 5. Kirkegaard (1956); 6. Gallardo (1967); 7. Gibbs (1971); 8. Vohra (1972 in Tan & Chou 1993); 9. Hutchings & Rainer (1979); 10. Hartmann-Schröder (1979, 1980, 1981); 11. Hutchings & Murray (1984); 12. Imajima & Morita (1987), 13. Koh & Bhaud (2001); 14. Koh & Bhaud (2003); 15. Ford & Hutchings (2005); 16. Martin et al. (2006); 17. Capa et al. (2012); 18. Hutchings et al. (2014); 19. This study.

FIGURE 6. Owenia dichotoma n. sp. (A – B. holotype AM W. 45857; C – D. paratype AM W. 424). A. Anterior end, right dorsolateral view; B. Detail of the branchial crown and u 1, ventral view; C. Detail of the branchial crown and u 1, ventral view; D. Detail of the branchial crown, dorsal view.

FIGURE 7. Anterior end in ventral view, of species of Owenia described or reported in the South East Asia and Australia, all redrawn from original; between brackets the collecting locality (A and B) or type locality (C – F). Organized alphabetically and by date. A. O. fusiformis (in Caullery 1944); B. O. fusiformis (in Imajima & Morita 1987); C. O. gomsoni (redrawn from a SEM micrograph in Koh & Bhaud 2001); D. O. australis (in Ford & Hutchings 2005); E. O. bassensis (in Ford & Hutchings 2005); F. O. mirrawa (in Ford & Hutchings 2005). O. petersenae Koh & Bhaud, 2003 was not included because it could not be redrawn from original pictures.