Plakosyllis curvispina n. sp. (Figs. 1 A–C, 2A–F) Material examined. Holotype: complete specimen (CPICML POH–37–002), Palmitas E2B1, 16°49.420’N and 99°54.733’W, 25 May 2006, 10.5 m, coarse sand Paratypes: 8 specimens (CPICML POP–37–003), Palmitas E2B1, 16°49.420’N and 99°54.733’W, 25 May 2006, 10.5 m, coarse sand. 6 specimens (MNCN 16.01/13262), Palmitas E2B1, 16°49.420’N and 99°54.733’W, 25 May 2006, 10.5 m, coarse sand. 6 specimens (LACM-AHF POLY 2639) Palmitas E2B1, 16°49.420’N and 99°54.733’W, 25 May 2006, 10.5 m, coarse sand. 6 specimens (AM: W.37086), Palmitas E2B2, 16°49.420’N and 99°54.733’W, 25 May 2006, 10.5 m, coarse sand. Additional material. 2 specimens (CPICML PO–37–079), Palmitas E2B2, 16°49.420’N and 99°54.733’W, 25 May 2006, 10.5 m, coarse sand. 2 specimens (CPICML PO-37-079), El Jardín E3B1, 16°49.436’N and 99°54.981’W, 26 May 2006, 12 m, coarse sand. Comparative material examined. Plakosyllis brevipes. 4 specimens (MNCN 16.01/6605). Spain: Mediterranean Sea, Balearic Is, Cabo Nati, NW of Menorca, 40°43´10” N, 03°49´28” E, 31 m. Plakosyllis quadrioculata, permanent microscope slide (MNCN 16.01/6605). Cuba: Cayo Matías. Description. Holotype complete specimen 2.8 mm long, 0.3 mm wide, with 54 chaetigers. Longest specimen 4.37 mm long, 0.32 mm wide, with 61 chaetigers. Body small, strongly flattened, oval, elongated, yellowish when preserved. Prostomium wider than long, trapezoidal, with two pairs of eyes, anterior one in dorsal position and posterior one in ventral position, and one pair of small ventral eyespots, (Figs. 1 A); in a few paratypes eyespots missing or only one present on either side. Antennae globular, lateral pair slightly smaller than median antenna, all arising anteroventrally, partially or totally covered by prostomium (Figs. 1 A, 1D, 2B). Palps round to ovate, ventral, not visible dorsally, separated but very close to each other (Figs. 1 B, 2B). A pair of ventrolateral ciliated nuchal organs, between prostomium and peristomium (Fig. 2 B, indicated by arrows). Peristomium well defined, shorter than subsequent segments, with two pairs of tentacular cirri; dorsal pair similar in size to lateral antennae, slightly larger than dorsal cirri; ventral pair smaller than dorsal cirri, only visible ventrally (Figs. 1 B, 2B). Intersegmentary rows of sub-cuticular glands covered by small sediment particles, visible under SEM (Fig. 2 A, indicated by arrows), giving an amber colour on some anterior chaetigers. Dorsal cirri with oval cirrophore and distinct cirrostyle, spherical to ovate. Ventral cirri small, conical, similar in size or slightly smaller than parapodial lobes, with numerous pores, visible under SEM (Fig. 2 C, indicated by arrow). Parapodial lobes conical, acute, with abundant glandular material. With 9–11 compound chaetae per parapodium, having longer blades and spines on anterior parapodia. All falcigers with unidentate blades, slightly hooked, strong dorso-ventral gradation in blade length: 3– 4 upper falcigers having long blades (6 and 10.4 μm long blade on posterior and anterior parapodia respectively), with very long and curved spines on edge (Figs. 1 B, 2D), 6–7 median with shorter blades and shorter spines (Fig. 2 E); and ventral-most with smooth blades (4.7 and 5.14 μm long on posterior and anterior parapodia respectively) (Figs. 1 D, 2F). Slight anteroposterior gradation in length blades, shaft and blade seem to be partially fused in some falcigers (Fig. 2 E, indicated by arrow). One simple ventral chaeta in most posterior chaetigers, slender, smooth, pointed and slightly hooked. One acicula per parapodium, yellowish, stout, slightly blunt distally protruding out from parapodial lobe (Fig. 1 E). Pharynx slender, extending though five (in some paratypes six) segments, with a small and conical tooth near anterior margin (Fig. 1 A). Trepan not seen. Proventricle shorter than pharynx, cylindrical to barrel-shaped, extending through 3 segments (in some paratypes two), with 13 rows of muscle cells (Fig. 1 A) (variation in the type material is from 9 to 13). Pygidium bilobed, with two anal cirri similar to dorsal cirri. Remarks. This new species is the fourth known of the genus, in addition to Plakosyllis brevipes Hartmann- Schröder, 1956 from Atlantic European coasts, Mediterranean Sea and Australia, P. a m e r i c a n a Hartman, 1961 from southern California and P. quadrioculata Perkins, 1981 from Florida and the Caribbean Sea. This is thus the first record of the genus from the Mexican Pacific coasts. Plakosyllis curvispina n. sp. is characterized by having compound chaetae with long blades and long and distally curved spines (sometimes with shaft and blade partially fused), and by having all antennae in ventral position, partially or totally covered by the prostomium. The other three species of the genus have falcigers with short blades and short spines on margin, and only the lateral antennae are inserted in ventral position. Furthermore, P. americana has falcigers of a single kind with entirely smooth shafts. P. curvispina n. sp., has four eyes in trapezoidal arrangement, a dorsal pair and a ventral pair, and one pair of ventral eyespots, while P. americana has four dorsal eyes in a nearly transverse row and lacks eyespots. P. c u rvispina n. sp., has distally obliquely and blunt aciculae and ventral cirri similar in size or shorter than parapodial lobes, whereas both P. brevipes and P. quadrioculata have straight aciculae ending in acute tips and ventral cirri even longer than parapodial lobes. Habitat. Subtidal, in coarse sand, 10.5 to 12 m depth. Distribution. Western coast of Acapulco Bay, Southern Mexican Pacific. Etymology. The specific name refers to the curved spines of the compound chaetae.

Published as part of Salcedo, Diana L., Martín, Guillermo San & Solís-Weiss, Vivianne, 2011, Two new species of Syllidae (Annelida: Polychaeta) from the Southern Mexican Pacific, pp. 41-52 in Zootaxa 2800 on pages 43-46, DOI: 10.5281/zenodo.201624