Idanthyrsus mikeli

Description. Holotype complete (ECOSUR- 252) mature male. Body colorless, 12 mm long, 2 mm wide, with three parathoracic segments, 34 abdominal segments, caudal peduncle 3 mm long (Fig. 3). Opercular crown and opercular stalk completely divided into two lobes (Fig. 3 C). Opercular disc oblique, with two rows of amber paleae; outer row with 19 paleae per lobe, inner row with 13 paleae per lobe. Outer paleae denticulate, 10 – 14 pairs of lateral denticles, curved on convex margin, straight or slightly curved on concave side of paleae (Fig. 4 A). Base of the blade with transverse thecae (Fig. 4 B). Blade distally curved at an angle of 150 ° – 155 °. Inner paleae cylindrical, straight, slowly tapering towards the blunt tip; blade with transverse compact thecae (Fig. 4 C). Opercular peduncle colorless (Fig. 3 A – C). One pair of amber nuchal hooks, slightly recurved, with long limbation, tip blunt (Figs. 3 C; 4 J). Tentacles and palps colorless. Median ridge as long as the opercular stalk, with marginal brown eyespots. Median organ small, conical, colorless, without eyespots (Fig. 3 D). Building organ with small brownish spots. Thorax without notochaetae. Chaetiger 1 with a pair of neuropodia bearing capillary chaetae. Chaetiger 2 with capillary neurochaetae, two pairs of small cirri, and a pair of branchiae. Parathorax colorless with three segments, all with paired branchiae (Fig. 3 E). Notopodia with six lanceolate chaetae, seven capillary chaetae (Fig. 4 D). Neurochaetae lanceolate and capillaries, thinner than notochaetae (Fig. 4 E). Abdominal segments colorless, with a pair of branchiae of decreasing size towards posterior segments; absent in the last five segments. Neuropodia with verticillate chaetae (Fig. 4 F). Notopodia with a series of uncini with 7 – 8 transverse rows of teeth (Fig. 4 G). Abdominal segment eight with longer neurochaetae (Fig. 3 F). Caudal peduncle cylindrical, colorless, translucent (Fig. 3 A). Variation. Body 8 – 14 mm in total length, 1.2 – 2 mm wide, 25 – 34 abdominal segments, caudal peduncle 2 – 3 mm. Opercular crown made of 11 – 19 outer paleae per lobe (commonly 17 – 18 paleae), and 7 – 18 inner paleae per lobe (regularly 13 – 15 paleae). Almost all specimens had one pair of hooks, except for two that had two hooks on the left side.

Figures 3 A – F; 4 A – J LSID: urn: lsid: zoobank. org: act: 09483 D 75 - FE 35 - 4 D 78 - 8 C 51 - B 2104692 E 305

Remarks. Idanthyrsus mikeli n. sp. resembles I. cretus Chamberlin, 1919 b, from Taboguilla Island, Panama, in having denticulate outer paleae with distally curved blades. However, these species differ in the morphology of the opercular paleae and nuchal hooks. Idanthyrsus mikeli n. sp. has outer paleae with broad transverse thecae in the basal region and 10 – 18 lateral denticles of similar size (Fig. 4 A – B), whereas I. cretus has outer paleae with thinner, less elevated, and compact thecae and 16 – 32 pairs of lateral denticles with finer denticles on one margin. The margins of the inner paleae are also different, being slightly annulated in I. mikeli n. sp. (Fig. 4 C, I) and completely smooth in I. cretus. Besides, I. mikeli n. sp. has blunt nuchal hooks with a long limbation that almost reaches the tip (Fig. 4 J), whereas in I. cretus nuchal hooks are sharp with short limbation that does not reach the convex region. Idanthyrsus mikeli n. sp. was found in the same aggregation as I. bastidai n. sp. Both species are distinguished mainly by the morphology of the outer paleae, presenting distally curved blades and short, curved marginal denticles in I. mikeli n. sp. (Fig. 4 A, H) and nearly straight blades with long, slender, straight marginal spines in I. bastidai n. sp. (Fig. 2 A – B).

Distribution. Turpialito, Cumaná, Venezuela at 1.5 m depth. Associated with Millepora corals. Specimens in the same aggregation as I. bastidai n. sp.

Etymology. This species is named after Dr. Ildelfonso Liñero-Arana (Mikel), who collected the type specimens. The name is based on his nickname ‘ Mikel’. The species name is a noun in the genitive case (ICZN 1999, Art. 31.1.2).

Material examined. Venezuela. Holotype (ECOSUR- 252), Cumaná Turpialito, on Millepora, 1.5 m, February 22, 2002, coll. I. Liñero-Arana. Nine paratypes (ECOSUR- 253), Cumaná, Turpialito, same data as for holotype.