Idanthyrsus bastidai

Description. Holotype complete (ECOSUR- 250), mature female. Body colorless, 11 mm long, 2 mm wide, with three parathoracic segments, 28 abdominal segments, caudal peduncle 3 mm long (Fig. 1). Opercular crown and opercular stalk completely divided into two lobes (Fig. 1 A – B). Opercular disc oblique, two rows of light amber paleae; outer row with 25 paleae per lobe, inner row with ~ 18 paleae per lobe. Outer paleae straight, marginal spines long, thin, progressively larger distally; 20 – 30 pairs of lateral spines. Paleal anterior region with compact thecae with irregular margins. Blade slightly inclined at an angle of 170 ° (Fig. 2 A – B). Inner paleae cylindrical, slightly inclined, tapered; blade with transverse thecae (Fig. 2 C); tip smooth, blunt (Fig. 2 D). Opercular peduncle colorless (Fig. 1 A – C). Two amber nuchal hooks on the left lobe, one hook on the right (Fig. 1 B). Nuchal hooks falcate, with long limbation, tip sharp (Fig. 2 H). Tentacles and palps colorless. Median ridge as long as opercular stalk, with marginal brown eyespots. Median organ small, conical, colorless, without eyespots (Fig. 1 E). Building organ with small brownish spots. Thorax without notochaetae. Chaetiger 1 with a pair of neuropodia bearing capillary, pinnate chaetae. Chaetiger 2 with capillary neurochaetae, a pair of lateral small cirri, and branchiae. Parathorax colorless with three segments, all with paired branchiae (Fig. 1 C – D). Notopodia with six lanceolate and seven pectinate chaetae. Neurochaetae lanceolate and capillaries, thinner than notochaetae. Abdominal segments colorless, with a pair of branchiae decreasing in size towards segment 16. Neuropodia with verticillate chaetae (Fig. 2 E). Notopodia with a series of uncini with 6 – 7 transverse rows of teeth (Fig. 2 F). Abdominal segment eight with longer neurochaetae (Fig. 1 F). Caudal peduncle cylindrical, colorless (Fig. 1 A). Variation. Body 4 – 12 mm in total length, 0.8 – 2 mm wide, 18 – 33 abdominal segments, caudal peduncle 1 – 4 mm. Opercular crown made of 10 – 33 outer paleae per lobe, and 8 – 19 inner paleae per lobe. Almost all specimens with one pair of hooks (Fig. 2 H), except for five bearing an extra hook on one side, including the holotype. One paratype specimen with fragments of the tube made of coarse sand (Fig. 2 G).

Figures 1 A – F; 2 A – G LSID: urn: lsid: zoobank. org: act: 2 D 4701 B 1 - FC 70 - 4 B 65 - 8 CCB- 456 DC 980120 D

Remarks. Idanthyrsus bastidai n. sp. closely resembles Idanthyrsus sp. fide Chávez-López (2019) from Guerrero, Mexico. Both species have outer paleae with long, thin, straight spines; however, Idanthyrsus sp. has completely straight outer paleae and blunt nuchal hooks (Chávez-López 2019: Fig. 9 C, F) with limbation shorter than that in I. bastidai n. sp. that is characterized by slightly bent outer paleae (Fig. 2 A – B) and sharp nuchal hooks (Fig. 2 H). The coloration pattern also differs in both species, while Idanthyrsus sp. has the opercular peduncle and thorax strongly pigmented (Chávez-López 2019), I. bastidai n. sp. lacks apparent pigmentation (Fig. 1 A – C).

Distribution. Turpialito, Cumaná, Venezuela at 1.5 m depth. Associated with Millepora corals.

Etymology. This species is named after Dr. J. Rolando Bastida Zavala, for being a great mentor and teaching me about the world of polychaetes. The species name is a noun in the genitive case (ICZN 1999, Art. 31.1.2).

Material examined. Venezuela. Holotype (ECOSUR- 250), Cumaná Turpialito, on Millepora, 1.5 m, February 22, 2002, coll. I. Liñero-Arana). Thirty-eight paratypes (ECOSUR- 251), Cumaná, Turpialito, same data as for the holotype.