Idanthyrsus nesos, Hutchings & Capa & Peart, 2012

Hutchings, Pat, Capa, María & Peart, Rachael, 2012, 3306, Zootaxa 3306, pp. 1-60 : 13-18

Idanthyrsus nesos n. sp.

Figures 1C–D, 6A, 7, 8, Table 1

Material examined. HOLOTYPE: 1, WAM V7850 *, Dolly Beach , Christmas Island, Indian Ocean, 10°31'S 105°41'E, 27–28 xi. 1969, intertidal reef and pools, 18.5 mm in length, 1 mm in width, 33 chaetigers . PARATYPES: 12, WAM V7851 *, same locality as holotype , 8–20 mm in length without cauda, 1 mm in width, 20–29 chaetigers; same locality as holotype , AM W37778 (mounted for SEM on two pins) .

Description. Holotype thin ( Fig. 1C), body with slightly pigmented cauda, abdominal segments dorsally ridged, pigmented and glandular, ventrum with scattered pigmentation. Operculum with elongate, partially fused lobes; with distal end sloped posteriorly (oblique to longitudinal axis). Outer paleae, 24 pairs ( Figs 1D, 7A, 8A, B), with cylindrical shaft and ornamented thecae as compact rings with irregular edges, restricted to proximal third of length of paleae ( Fig. 8C) and flattened straight blades with alternate, mostly straight sharp-tipped denticles, increasing in length along shaft towards tip ( Fig. 8C). Inner row with 15 pairs of cylindrical paleae tapering evenly to acute tip ( Fig. 8D); ornamented distally with compact narrow ring thecae ( Fig. 8D). Outer paleae longer than inner paleae ( Fig. 8B). Opercular papillae, five pairs, reduced, stout, inserted external to outer row of paleae on each lobe ( Fig. 8B). One pair of stout and flattened nuchal spines present, strongly recurved tips ( Figs 1D, 7C, 8B), and concave margins with wide limbation. Tentacular filaments compound, arranged in 13 horizontal rows ( Fig. 7A, B). Median ridge and median organ present at dorsal junction of opercular lobes, with eyespots present on their margins. Palps about one fifth of length of opercular lobes. Segment 1 (chaetiger 1) with rounded lobe-shaped neuropodia ( Fig. 7A) on either side of U-shaped buccal organ, with fine capillary neurochaetae, flattened, with finely denticulated margins, widest at base and tapering towards fine tip ( Fig. 8E). Segment 2 (chaetiger 2) with two pairs of poorly developed triangular lobes with blunt tips, dorsal one larger than ventral, connecting branchiae to neuropodia ( Fig. 1D), and with similar chaetae as those of chaetiger 1. Neuropodia of chaetiger 1 and 2 not vertically aligned, those of chaetiger 2 situated more laterally ( Fig. 1D). Twenty one pairs of dorsal branchiae present from chaetiger 2, absent from posterior abdominal segments. Branchiae with narrow base and weakly ridged, tapering gently to rounded tip, decreasing in size posteriorly. Segments 3–5 (parathoracic) with two types of notochaetae arranged transversely, five, large, flattened, lanceolate with frayed tips, with upturned, curved margins ( Fig. 8F, G); and thinner chaetae with straight tips, theca with expanded margins, interspersed with lanceolate ( Fig. 8F, G). Notochaetae appear to be highly retractile ( Fig. 8F). Segments 3–5 (parathoracic) with two tiers of lanceolate neurochaetae; seven, thick and seven thinner ones with finely denticulated margins. Neurochaetae less robust than notochaetae. Abdominal region with 27 chaetigers. Notopodia as transverse tori, with number of uncini per torus decreasing posteriorly. Notopodial tori becoming increasingly erect and decreasing in width posteriorly. Each uncinus with two vertical rows of teeth, each row with seven to eight teeth ( Fig. 8I). Neuropodia with capillaries becoming longer posteriorly, with thin and flattened blades ornamented with thecal laminar extensions distally forming oblique rows with elongate frayed tips ( Fig. 8H), similar on all abdominal chaetigers. Cauda, smooth, equivalent in length to 15 chaetigers ( Fig. 7D).

Variations. The paratypes, including sexually mature female specimens, vary from 8–20 mm in length without cauda, no variation in width, with 20–29 segments, 12–24 pairs of outer paleae, 10–15 pairs of inner paleae; 4–9 pairs of opercular papillae; 9–19 pairs of dorsal branchiae; and 15–24 abdominal chaetigers. These variations can be attributed to size of specimens, with larger animals tending to have more paleae, papillae, uncinial teeth and abdominal chaetigers than smaller ones.

Remarks. Idanthyrsus nesos n. sp. is the only species described in the genus with the following combination of characters (see Table 1): presence of one pair of limbated nuchal hooks (shared with I. bihamatus ( Caullery, 1944) , I. cretus Chamberlin, 1919 , and presumably I. bicornis ( Schmarda, 1861) , I. kornickeri Kirtley, 1994 , I. luciae ( Rochebrune, 1882) , I. manningi Kirtley, 1994 , and I. valentenei Kirtley, 1994 ), presence of two lateral lobes on segment 2 (shared with I. bihamatus ) and presence of fine denticles (20–25) along the edges of the outer paleae, increasing in length towards the tip. Idanthyrsus nesos n. sp., was found at Christmas Island, an isolated location on the Indian Ocean and can be distinguished from I. australiensis , the most common species on the mainland, by the number and shape of nuchal spines (one pair of flattened hooks with large limbation in I. nesos n. sp., and 2–3 pairs of cylindrical hooks lacking limbations in I. australiensis ) and the number of pairs of lateral lobes on segment 2 (two in I. nesos n. sp., and three in I. australiensis ) and by the distribution of branchiae (restricted to anterior abdominal segments in I. nesos n. sp., and present on all abdominal segments in I. australiensis ).

As summarised in Table 1, other species reported from the Indo-Pacific differ from I. nesos n. sp., in one or more of the diagnostic features, although it should be noted that information is missing for many species. For example, I. albigens ( Ehlers, 1908) , described from deepwater off Diego Garcia,has two pairs of nuchal hooks whereas I. nesos n. sp., has one pair of these spines. Idanthyrsus bicornis ( Schmarda, 1861) , described from intertidal areas of Sri Lanka, has outer paleae with more than 30 denticles with an asymmetrical arrangement on both margins and with maximum length at mid paleae, (see Fig. 6.4. 1 in Kirtley 1994), in contrast to the 20–25 alternate denticles increasing in length towards the tip present in I. nesos n. sp. Idanthyrsus bihamatus ( Caullery, 1944) , known only from deep water in Indonesia, has outer paleae with denticles similar in size or slightly longer at mid length of paleae while in I. nesos n. sp., denticles increasing in length towards the tip (see Table 1 for comparison of other features).

Idanthyrsus bicornis ( Schmarda, 1861) , described from intertidal areas of Sri Lanka, has outer paleae with more than 30 denticles which increase in size towards the tip and arising at 45˚ (see Fig. 6.4. 1 in Kirtley 1994, not illustrated by Schmarda and also no other diagnostic characters are given) which differs from the new species and I. bihamatus ( Caullery, 1944) known only from deep water in Indonesia which has outer paleae with smooth shafts and numerous denticles of uniform size along shaft and arising at 45˚ whereas I. nesos n. sp., has straight denticles increasing in length along the strongly denticulate shaft, although they share many other characters (see Table 1). Idanthyrsus nesos n. sp., differs from I. okinawaensis Nishi & Kirtley, 1999 , described from shallow waters in Japan, in the number of pairs of opercular papillae, 30 in I. okinawaensis and less than ten in the new species,and the number and shape of nuchal hooks, two pairs and without a limbation in I. okinawaensis and one pair of limbated hooks in I. nesos n. sp.

Distribution. Christmas Island, Indian Ocean.

Habitat. Intertidal, found in small colonies.

Etymology. The specific name nesos is a Greek word for island and refers to the isolated type locality of this species, Christmas Island.