Rhynchospio darwini, Radashevsky, Vasily I., 2015

Radashevsky, Vasily I., 2015, Spionidae (Annelida) from Lizard Island, Great Barrier Reef, Australia: the genera Aonides, Dipolydora, Polydorella, Prionospio, Pseudopolydora, Rhynchospio, and Tripolydora, Zootaxa 4019 (1), pp. 635-694 : 684-686

Rhynchospio darwini n. sp.

( Figs 33 , 34 )

Type material. Holotype: NTM W025648, Northern Territory, Fannie Bay, Darwin, Bullocky Point, 12.4356°S, 130.8323°E, intertidal, muddy sand, coll. V.I. Radashevsky, 3 Sep 2013. Paratypes: MIMB 28105 (1), same details as holotype; AM W.45506 (1), Queensland, Lizard Island, MI QLD 2408.

Adult morphology. Largest intact individual holotype 5 mm long, 0.3 mm wide for 60 chaetigers (21 anterior chaetigers of the holotype deposited at the NTM W025648, while the posterior chaetigers fixed in ethanol for a molecular analysis); smallest individual 2.5 mm long, 0.2 mm wide for 33 chaetigers (AM W.45506). Pigmentation absent on body and palps; yellow pigment present on ventral pair of pygidial cirri. Prostomium with two conical fronto-lateral horns ( Figs 33 A, B, 34A, B), bearing small knobs with short non-motile cilia. Caruncle low, indistinct, levelled in end of chaetiger 1. Occipital antenna absent. Two pairs of small red eyes arranged trapezoidally. Nuchal organs metameric; first pair of metamers on chaetiger 1 as curved ciliary bands on lateral sides of low caruncle; successive metamers as one pair of ciliary bands on posterior half of each chaetiger ending on at least chaetiger 15. Palps as long as 5–10 chaetigers, with frontal longitudinal groove lined with fine cilia, fronto-lateral compound cilia situated on sides of groove and beating towards the groove, and short transverse bands of long cilia regularly arranged on inner lateral side on distal half of each palp.

Chaetiger 1 with long capillaries and small postchaetal lamellae in both rami. Posterior notopodia with only capillaries. Prechaetal lamellae not developed. Notopodial postchaetal lamellae elongated; neuropodial lamellae short and rounded.

Sabre chaetae in neuropodia from chaetigers 11–12, 1– 3 in a tuft, alimbate, with fine granulation on distal half of shaft ( Figs 33 E, 34H).

Hooks in neuropodia from chaetigers 11–12, up to four in a series, accompanied by 1–4 alternating capillaries and inferior sabre chaetae throughout. Alternating capillaries with narrow wing in anterior hook-bearing neuropodia ( Fig. 34 E), alimbate, hair-like in posterior neuropodia. Hooks with only outer hood, tridentate, with two small upper teeth arranged in line above main fang ( Figs 33 D, E, 34F, G); uppermost tooth tiny in hooks in anterior neuropodia, prominent in hooks in posterior neuropodia.

Branchiae from chaetiger 2 through most part of body, free from notopodial postchaetal lamellae, flattened, with surfaces orientated parallel to body axis, each with two rows of short cilia running along inner surface. Afferent and efferent branchial blood vessels forming a loop and not interconnected by radial capillaries.

Nototrochs from chaetiger 1 onwards, each composed of two rows of cilia, separated from branchial ciliation by narrow gap. Nototroch on chaetiger 1 interrupted middorsally by caruncle; nototrochs on succeeding chaetigers complete transverse rows. Intersegmental transverse ciliation absent. Intersegmental longitudinal ciliation from chaetiger 2 onwards, as short bands of short cilia on dorso-lateral edges of chaetigers, extending from one chaetiger onto anterior part of successive chaetiger.

Pygidium with one pair of ventral cirri and two pairs of thinner, slightly longer dorsal cirri in all three type specimens ( Figs 33 C, 34D).

Glandular pouches in neuropodia from chaetigers 7–8.

Narrow oesophagus extending through 8–12 anterior chaetigers. Ventral buccal bulb below oesophagus extending to almost end of chaetiger 2. Gizzard-like structure in digestive tract absent.

Main dorsal blood vessel transformed into gut sinus in anterior part of midgut. Heart body absent inside main dorsal vessel. Blood red, without globules or other elements.

Nephridia from chaetiger 4 onwards.

Reproduction. Rhynchospio darwini n. sp. is hermaphroditic. The holotype has sperm in chaetigers 11–14 and small developing oocytes in paired ovaries in chaetigers 15–30. The oocytes are up to 65 µm in diameter, with thin and smooth membrane less than 1 µm thick, with germinal vesicle about 30 Μm, and single nucleolus about 10 µm in diameter. The paratype MIMB 28105 has sperm in chaetigers 11–14 and small oocytes up to 25 µm in diameter developing in ovaries from chaetiger 15.

Remarks. Rhynchospio darwini n. sp. appears similar to Rhynchospio nhatrangi Radashevsky, 2007 originally described from Nhatrang Bay, Vietnam, in having up to three pairs of cirri on the pygidium, sperm developing in chaetigers 11–14 and oocytes from chaetiger 15 onwards. The two species differ however in that adults of R. nhatrangi have falcate unidentate hooks in neuropodia of chaetigers 11–14 and tridentate hooks from chaetiger 15 onwards, whereas adults of R. darwini n. sp. have only tridentate hooks in neuropodia from chaetigers 11–12.

Rhynchospio darwini n. sp. differs from two other Australian species, R. australiana and R. glycera , by the shape of prostomium, and from R. glycera also by the presence of tridentate hooks in neuropodia from chaetigers 11–12 instead of quadridentate hooks from chaetiger 27. Rhynchospio australiana has tridentate hooks in neuropodia from chaetiger 8 but has no notochaetae in chaetiger 1 which are present in Rhynchospio darwini n. sp.

Etymology. The species is named after its type locality, Darwin, the capital and chief port of Northern Territory, Australia, and thus is in honour of the British naturalist Charles Darwin. On 9 September 1839, HMS Beagle sailed into Darwin harbour during its surveying of the northern Australia. The ship’s captain, Commander John Clements Wickham named the region “Port Darwin” in honour of his former shipmate Charles Darwin, who had sailed on the earlier second expedition of the Beagle which had ended in October 1836.

Habitat. Adults of Rhynchospio darwini n. sp. were found on sandy intertidal.

Distribution. Australia: Northern Territory and Queensland.

Figures

FIGURE 33. Rhynchospio darwini n. sp. A—MIMB 28105; B – E—AM W. 45382. A. Anterior end with palps missing, dorsal view; B. Anterior end, left lateral view; C. Posterior end, left lateral view; D, E. Bidentate hooded hooks from median neuropodia; D. Hooks in frontal view; E. Hooks in lateral view. Abbreviations: al—alternating capillary, ho—hooded hooks, sa—ventral inferior sabre chaetae. Scale bars: A – C— 50 µm; D, E— 10 µm.

FIGURE 34. Rhynchospio darwini n. sp. A—NTM W 025648; B – H—AM W. 45382. A. Anterior end in dorsal view with palps missing; B. Anterior end in left lateral view with palps missing; C. Left palp, inner lateral view; D. Posterior end, left lateral view; E – H. Neurochaetae from middle chaetiger; E. Alternating capillary chaeta with narrow wing; F. Tridentate hooded hook in lateral view; G. Same in frontal view; H. Inferior sabre chaeta.