Exogone naidinoides Westheide 1974

Exogone naidinoides Westheide, 1974

(Figures 2–4)

Exogone naidinoides Westheide, 1974: 301 –305, figs 50–51e–f; Russell 1991: 57 –59, fig. 3; San Martín & Bone 2001.

Exogone (Sylline) naidinoides. San Martín 1991: 737, fig. 7a–f; Capa et al. 2001: 623; Ruíz-Ramírez & Salazar-Vallejo 2001: 128, fig. 4 (66–76); San Martín 2005: 146 –147, fig. 93; Fukuda 2010: 149 –151, fig. 44.

Material examined. Project ‘ BIOTA ’: State of São Paulo: Ubatuba, Praia de Picinguaba (23º22'S 44º50'W), on algae: 5 specimens (MZUSP 1015), coll. 0 8 June 2001. Caraguatatuba, Praia de Martim de Sá (23°38'S 45°23'W), on Sargassum sp.: 43 specimens (MZUSP 1014), coll. 16 March 2001. São Sebastião, Praia da Baleia (23°47'S 45°40'W), on Sargassum sp.: 5 specimens (MZUSP 1012), coll. 10 April 2001. Project ‘BioPol-NE ’: State of Paraíba, Baía da Traição, Praia do Farol (06º41'S 34º55'W), intertidal: 27 specimens (MZUSP 1343), coll. 0 9 August 2010. Rio Tinto, Barra de Mamanguape (06º45'S 34º55'W), intertidal: 9 specimens (MZUSP 1342), coll. 11 August 2010. João Pessoa, Cabo Branco (07°08'S 34°47'W), intertidal: 30 specimens (MZUSP 1339), coll. 0 2 February 2010; Recife do Picãozinho (07°4.243'S 34°49.291'W), intertidal: 4 specimens (MZUSP 1341), coll. 15 September 2012. Conde, Praia de Jacumã (07º14'S 34º47'W), intertidal: 1 specimen (MZUSP 2082), coll. 29 January 2010; Praia de Carapibus (07º17'S 34º48'W), intertidal: 1 specimen, coll. 10 February 2009. State of Pernambuco, Ilha de Itamaracá, Ponta do Jaguaribe (7°44'S 34°49'W), intertidal: 1 specimen (MZUSP 2081), coll. 11 December 2012.

Additional material examined. Exogone naidinoides. Galápagos Islands, Santa Cruz – holotype (ZMH P- 13611), coll. & det. W. Westheide, 1972. Cuba, Punta del Francés – Isla de Pinos, in dead coral, 1 m, coll. & det. G. San Martín, 1 specimen (MNCN 16.01/631).

Description. Small species, longest specimens ca. 2.5 mm long and 0.1 mm wide, with up to 23 chaetigers. Palps narrower than anterior chaetigers, distally rounded, totally fused (Figs 2 A, 3A–B) with conspicuous line of fusion (Fig. 2 A). Prostomium pentagonal to ovate, slightly shorter than palps, with two pairs of eyes in trapezoidal arrangement, anterior eyespots absent; antennae inserted in nearly transverse row, close to each other, anterior to anterior pair of eyes, median antenna slightly posterior and longer, reaching tip of palps (Figs 2 A, 3A–C). Nuchal organs as pair of ventrolateral ciliated pits between prostomium and peristomium, close to peristomial cirri, visible only under SEM (Fig. 3 B–C, H). Peristomium shorter than following chaetigers, sometimes covering posterior part of prostomium, including posterior eyes; peristomial cirri short, papilliform (Figs 2 A, 3C, H). Dorsal cirri ovate (Figs 2 A, 3A, C, 4C, F), slightly larger than peristomial cirri, absent on chaetiger 2 (Figs 2 A, 3A). Ventral cirri similar to dorsal cirri but smaller (Fig. 3 B). Parapodial lobes conical. Anterior parapodia with 3–7 bayonet chaetae each; chaetae subdistally spinulated, blades 4–10 µm long (Figs 2 B, 3D–F); after chaetiger 4–5, only 1 bayonet chaeta per parapodium, blades 5–13 µm long, and 2–3 chaetae with short nail-like blades fused to shafts, with crown of spines surrounding tip of shafts (Figs 2 C, 4A, D). Dorsal simple chaetae present on all chaetigers, with acute tip (Figs 2 D–E, 3I, 4D), progressively thicker posteriorward; ventral simple chaetae present only on mid- and posterior body chaetigers, sigmoid, bidentade, subdistal tooth larger (Figs 2 F, 4E). Single acicula per parapodium throughout, thick, subdistally enlarged, with tip protruding from parapodial lobes on mid- and posterior body chaetigers (Figs 2 G–H, 4A–B, D). Pygidium with thin, elongate pair of anal cirri (Fig. 4 F). Pharynx through 2–3 segments, tooth at anterior border; proventricle extending for 1–2.5 chaetigers (Fig. 2 A), with 13–16 rows of muscle-cells.

Remarks. The position of the nuchal organs and the presence of protruding aciculae are recorded for the first time for this species. The nuchal organs are in an unusual ventrolateral position, rather than in the dorsolateral position commonly observed in other syllids (Fig. 3 A–C, G–H).

Exogone aquadulcensis Pascual, Nuñez & San Martín, 1996, described from Tenerife, Canary Islands, Spain, is the species most similar to E. naidinoides. It differs by having thicker antennae but of about the same length; in addition, ventral simple chaetae are unidentate in E. aquadulcensis and bidentate in E. naidinoides. The sympatric species E. simplex is less common than E. naidinoides; both have palps that are distally rounded and totally fused with a conspicuous line of fusion, dorsal cirri absent on chaetiger 2, solitary acicula per parapodium throughout, similar lengths of pharynx and proventricle, as well as a similar number of proventricular rows of muscle cells. In contrast, E. simplex has chaetae lacking blades throughout, shorter antennae, and papilliform dorsal cirri, while E. naidinoides has 3–7 bayonet-shaped chaetae on anterior chaetigers and, after chaetigers 4–5, a single bayonet chaeta per parapodium and 2–3 chaetae with short nail-like blades fused to the shafts, digitiform antennae almost reaching the tip of the palps, and dorsal cirri longer than those of E. simplex.

Brazilian specimens have lateral antennae that are proportionally longer than those of specimens from Australia (San Martín 2005) and Galápagos (Westheide 1974), and the bayonet chaetae of chaetigers 1–3 are longer. The specimen from Cuba has up to six bayonet chaetae in each parapodium of chaetigers 1–3, with blades ca. 4 µm long; from chaetiger 4 onwards, each parapodium bears a single bayonet chaeta with a blade 5–8 µm long and 2–3 chaetae with blades fused to shafts; that specimen has blades of the bayonet chaetae relatively shorter than those of the Brazilian specimens.

Although specimens from Brazil do not match perfectly the available descriptions for this species, the differences are very subtle and include characters that are visible only under SEM and therefore cannot be compared with populations from other localities, as they have not been studied under SEM. Future molecular studies may reveal whether this is one case of a complex of sibling species.

Type locality. Ecuador, Galápagos Islands (Pacific Ocean).

Distribution. Atlantic Ocean: Canary Islands, Cuba, Mexico, Belize, Panamá, Venezuela and Brazil (Paraíba, Pernambuco and São Paulo). Pacific Ocean: Galápagos Islands. Indian Ocean: Australia (Western Australia). From the intertidal zone to ca. 12 m deep.