Pseudexogone losinensis, Plathong & Plathong & Dean, 2024

Plathong, Jintana, Plathong, Sakanan & Dean, Harlan K., 2024, New species of Glyphohesione and Pseudexogone (Annelida, Pilargidae) from the Gulf of Thailand, Zootaxa 5428 (2), pp. 235-252 : 245-247

Pseudexogone losinensis sp. nov.

Figs 8–11

Material examined. Thailand, Gulf of Thailand. 54 specimens , Losin Island (7°09'N, 101°59'E), coll. Marine Ecosearch Management Co., Ltd., SCUBA diving, sand mixed with dead coral rubbles. Holotype: PSUZC-POL-0456, Sta. LO2-4 , 23 Aug. 2022, 32 m. Paratypes: 53 specimens; Sta. LO1 (same as holotype) , 23 Aug. 2022; 25.5 m: PSUZC-POL-0457 (1 spec.), Sta. LO1-1 ; PSUZC-POL-0458 (3 specs.), Sta. LO1-3 ; PSUZC-POL-0459–0461 (12 specs., 3 specs. on SEM stub), Sta. LO2 (same as holotype) , 23 Aug. 2022, 27–32 m; PSUZC-POL-0462–0464 (14 specs., 2 specs. on SEM stub), Sta. LO3 (same as holotype) , 23 Aug. 2022, 29 m; PSUZC-POL-0465 (3 specs.), Sta. LO4 (same as holotype) , 24 Aug. 2022, 29 m; PSUZC-POL-0466 (2 specs.), Sta. LO5 (same as holotype) , 22 Aug. 2023, 31 m; PSUZC-POL-0467 (2 specs.), Sta. LO6 , 23 Aug. 2023, 28.5 m; PSUZC-POL-0468 (4 specs.), Sta. LO7 (same as holotype) , 23 Aug. 2023, PSUZC-POL-0469 (4 specs.), Sta. LO8 (same as holotype) , 23 Aug. 2023, 35.6 m; PSUZC-POL-0470 (8 specs., 3 specs. on SEM stub), Sta. LO9 (same as holotype) , 24 Aug. 2023, 31 m.

Additional material. Southern Gulf of Thailand, Pattani Province. 86 specimens : 12 Jul. 2011, One specimen, Sta. PN 02 (7°00'N, 101°23'E), 18 m, sand mixed with shells. Losin Island (same as holotype) ; 23 Aug. 2022, Sta. LO2 (11 specs.), Sta. LO3 (18 specs.) ; 24 Aug. 2022, Sta. LO4 (3 specs.) ; 22 Aug. 2023, Sta. LO5 (2 specs.) ; 23 Aug. 2023, Sta. LO6 (6 specs.) ; Sta. LO7 (4 specs.) ; Sta. LO8 (4 specs.) ; 24 Aug. 2023, Sta. LO9 (38 specs.) .

Diagnosis. Pseudexogone with eyespots; bidentate notospines from chaetiger 6; furcate chaetae from chaetiger 2; median and posterior chaetigers lacking furcate chaetae; blade of larger tine very curved, expanded, tip of smaller tine with a distal excavation. Short limbate and denticulate capillary chaetae with double rows of teeth; tip of short limbate chaeta unidentate, acute.

Description. Holotype largest complete specimen, 9.9 mm long 0.2 mm wide, 63 chaetigers. Paratypes, complete specimens 5.5–9.8 mm long, 0.14–0.20 mm wide about 41–60 chaetigers. Body annulate, transparent, slightly wider anteriorly, tapering posteriorly ( Fig. 8A ).

Prostomium bilobed, longer than wide. Eyespots, small, dark brown, semicircular, close to lateral antennae. Three cirriform antennae; median antenna longer than laterals, located on posterior margin of the prostomium; lateral antennae located anterior to eyespots ( Figs 8A–B , 9A–E, 11A–B).

Palps bilobed, anteriorly rounded; pair of digitiform ventro-lateral palpal papillae present, shorter than antennae, located half way along the palp length (Figs 9A–E, 11A–B). Brain lobes extended posteriorly to chaetiger 4.

Peristomium biannulate, first ring laterally reduced; tentacular cirri ventro-laterally placed, dorsal tentacular cirrus slightly longer than ventral ones (Figs 9A–E, 11A–B).

Parapodia uniramous in chaetigers 1–5, thereafter biramous. Notopodial bidentate sigmoid spines present from chaetiger 6 to the posterior end. Each notopodia with a single notospine (Figs 9A, 10AB, F, 12A, E); in anterior chaetigers notospine shorter than posterior ones ( Fig 10A, F ). Dorsal cirri larger and longer than ventral cirri ( Fig. 10A ).

Neuropodia of chaetiger 1 with 5 chaetae, two short limbate chaetae and three elongate denticulate capillaries. Each neuropodia with 5–6 chaetae per bundle; one furcate (in anterior chaetigers), 1–2 short limbate chaetae and 2–3 elongate denticulate capillaries ( Fig. 10A, C ). Furcate neurochaetae from chaetiger 2, lacking in median and posterior chaetigers; furcate chaeta with unequal tines, large tine strongly curved and expanded, smaller tine with excavated tip ( Figs 10E , 11C–D ). Posterior chaetigers with 3 chaetae per bundle; one pectinate and two denticulate capillaries ( Fig. 10F ). All neurochaetae pectinate, with double longitudinal rows of teeth ( Figs 10C–E , 11C ).

Oocytes present in holotype and paratypes, oocytes visible in lateral side of parapodia from anterior to posterior chaetigers, measure about 6–9 μm in diameter ( Fig. 8C ).

Pygidium with two cirriform ventrolateral anal cirri, longer than posterior dorsal cirri. Anus dorso-terminal, ciliated. Posterior end with one achaetous segment ( Figs 8D , 11F ).

Etymology. This species is named after Losin Island, a smallest Island in the Gulf of Thailand, the locality where the specimens were collected.

Habitat. Found in sandy and dead coral rubble at Losin Island.

Distribution. Only known from the type locality, Losin Island at 25.5–32 m depth, the Gulf of Thailand.

Remarks. Pseudexogone losinensis sp. nov. belongs to a group of Pseudexogone species with eyespots ( Table 2). This group includes P. backstromi Augener, 1922 , P. imajimai Salazar-Vallejo, Bailey-Brock & Dreyer, 2007 and P. williamsae Salazar-Vallejo, Bailey-Brock & Dreyer, 2007 . Pseudexogone losinensis sp. nov. differs P. backstromi by having the first notospines from chaetiger 6 and the large tine of the furcate chaeta being strongly curved; P. backstromi has the first notospines from chaetiger 7 and the large tine of the furcate chaeta straight. Moreover, the median antenna in the new species is longer than the lateral antennae whereas in P. backstromi the median antenna and lateral antennae are of similar length ( Salazar-Vallejo et al. 2007).

Pseudexogone losinensis sp. nov. differs from P. imajimai in that the prostomium is longer than wide while in P. imajimai the prostomium is wider than long. Moreover, the large tine of the furcate chaetae of the new species is strongly curved and expanded without a subdistal notch and the tip of the small tine has a distal depression. In P. imajimai the large tine of the furcate setae has a subdistal notch and the tip of the small tine lacks a pore ( Salazar-Vallejo et al. 2007). Additional, P. losinensis sp. nov. has one prepygidial achaetous segment while P. imajimai has two prepygidial achaetous segments ( Salazar-Vallejo et al. 2007).

Pseudexogone losinensis sp. nov. differs from P. williamsae in that the longer tine of the furcate chaetae of P. williamsae is uncurved with a subdistal hump. Moreover, P. losinensis sp. nov. has only a single furcate chaeta per bundle in anterior chaetigers, whereas P. williamsae has up to 2 ( Salazar-Vallejo et al. 2007).

Figures

FIGURE 8. Pseudexogone losinensis sp. nov. light photographs (A, PSUZC-POL-0467, paratype; B–D, stained with Shirlastain- A, PSUZC-POL-0456, holotype). A. Whole specimen, lateral view; B. Prostomium, show semicircular eyespots, lateral view; B. Oocytes in parapodium, lateral view; D. Posterior end, lateral view. Abbreviations: ac, anal cirrus; ma, median antenna; ns, notospine; pa, palp; py, pygidium; vc, ventral cirrus.

FIGURE 10. Pseudexogone losinensis sp. nov. (A–B, PSUZC-POL-0461; C–F, PSUZC-POL-0470) A. Anterior parapodia (chaetiger 8–9), lateral view; B. Close-up anterior notopodia of chaetiger 9, lateral view; C. Anterior neuropodia (chaetigers 4–5, start from left side), lateral view; D. Close-up short limbate chaeta with unidentate tip (mid-chaetiger), lateral view; E. Close-up furcate chaeta of chaetiger 4, lateral view; F. Posterior region, dorsal view. Abbreviations: ac, anal cirrus; dc, dorsal cirrus; denticulate cc, denticulate capillary chaeta; exc, excavated tip; fc, furcate chaeta; ne, neurochaetae; ns, notospine; slc, short limbate chaeta; vc, ventral cirrus.

FIGURE 11. Pseudexogone losinensis sp. nov. illustrate A.Anterior region, dorsal view; B. Same, ventral view; C–D. Close-up furcate chaeta, lateral view; C. Chaetiger 4; D. Chaetiger 7; E. Close-up bidentate notopodial spine, lateral view. Abbreviations: dc, dorsal cirrus; exc, excavated tip; la, lateral antenna; fc, furcate chaeta; ma, median antenna; mo, mount; ne, neurochaetae; ns, notospine; pa, palp; tc, tentacular cirri; vc, ventral cirrus; vpp, ventro-lateral palpal papillae.