Branchiosyllis exilis ( Gravier, 1900 )

Aguado, Teresa, Martín, Guillermo San & Ten, Harry A., 2008, Syllidae (Annelida: Polychaeta) from Indonesia collected by the Siboga (1899 – 1900) and Snellius II (1984) expeditions, Zootaxa 1673, pp. 1-48 : 6-10

Branchiosyllis exilis ( Gravier, 1900)

Figs 1 , 2

? Syllis (Typosyllis) exilis Gravier, 1900: 160 , figs 28–30. Branchiosyllis exilis .— Westheide, 1974: 60 –64, fig. 26.

Material examined. Holotype MNHN 143, Djibouti, Gulf of Aden; 1 spec. ZMA V.Pol. 1985.03, Indonesia, 8º30´S 119º7.5´E, 73 m, Siboga Expedition, Sta. 310, 12 Febr. 1900; 1 spec. ZMA V.Pol. 1985.05, Indonesia, Irian Jaya, Aru Isl., Pearl Banks, anchorage off Pulu Jedan, trawl, dredge and divers, sand and shells, 13m, Siboga Expedition, Sta. 273, 23/ 26 Dec. 1899; 4 spec. ZMA V.Pol. 1991, Indonesia, Irian Jaya, Aru Isl., Pearl Banks, anchorage off Pulu Jedan, trawl, dredge and divers, sand and shells, 13m, Siboga Expedition, Sta. 273, 23/ 26 Dec. 1899; 2 spec. ZMA V.Pol. 2038.03, Indonesia, Laboean, 8º44.5´S 116º2.5´E, 18–27 m, Siboga Expedition, Sta. 19, 19 March 1899; 34 spec. (1 spec. mounted for SEM) ZMA V.Pol. 2038.01, Indonesia, Lesser Sunda Isl., Postillon Isl., anchorage of Pulu Sarassa, dredge, coral, 36 m, Siboga Expedition, Sta. 43, 4/ 5 March 1899. (All ZMA specimens previously identified as Syllis exilis ).

Comparative material examined. Branchiosyllis fuscosuturata (Augener, 1922) . 2 syntypes, ZMB Verm. 6598; 1 syntype ZMH V- 9770, Caribbean Sea, Tortugas, S.W. Channel and Bird Key Reef.

Branchiosyllis cirropunctata (Michel, 1909) . 2 spec. AM W30091, Australia, Western Australia: Goss Passage, Beacon Isl., 28°25' 30"S 113°47'E, dead plates of Acropora sp., covered in coralline algae, 8 m, 19 May 1994; 1 spec. AM W30092, NE entrance to Goss Passage, Beacon Isl., 28°27'54"S 113°46' 42"E, dead plate-like Acropora sp., covered in coralline algae, 8 m, 25 May 1994 (all identified by San Martín et al., submitted).

Description. Best preserved specimen incomplete, with regenerating posterior part, 17.5 mm long, 0.8 mm wide, with 69 segments. Longest complete specimen 14 mm long, 0.8 mm wide and 64 segments. Body cylindrical, ventrally flattened. Reddish to orange pigmentation. Prostomium rounded; four eyes in open trapezoidal arrangement. Median antenna longer than combined length of prostomium and palps, inserted between anterior eyes, with about 22–30 articles; lateral antennae inserted on anterior margin of prostomium, shorter than median antenna ( Fig. 1 A), with about 17–23 articles. Palps similar in length to prostomium or slightly longer, ventrally folded. Peristomium shorter than subsequent segments; dorsal tentacular cirri similar in length to median antenna, with about 28–38 articles, ventral ones shorter than dorsal tentacular cirri ( Fig. 1 A), with about 18–23 articles. Dorsal cirri with distinct cirrophores. Dorsal cirri of chaetiger 1 long, with 53– 58 articles; dorsal cirri of chaetigers two and three with 25–35 articles; 45–50 in chaetiger four; remaining anterior dorsal cirri varying in length but not clearly alternating ( Fig. 1 A), with 35–45 articles. Midbody dorsal cirri with about 40–50 articles, posterior cirri with 40–65. Parapodial lobes distally bilobed, prechaetal lobe shorter than postchaetal one, both digitiform ( Fig. 1 F). Ventral cirri digitiform, longer than parapodial lobes ( Fig. 1 F) and proximally inserted to anterior parapodia; more distally inserted and increasing in length towards posterior segments, twice as long as parapodia ( Fig. 1 G). Usually six to ten compound heterogomph chaetae on anterior parapodia; blades falcigerous, bidentate, with long spines on margin, dorsoventrally decreasing in length (ca. 46 µm dorsal-most, ca. 24 µm ventral-most) ( Figs 1 B, 2A, B), shafts with distal spines. Midbody chaetigers with five to eight compound chaetae, three to six bidentate with shorter spines than anterior ones (ca. 39 µm dorsal-most, ca. 25 µm ventral-most), first two most ventral chaetae unidentate with spines only at base of blade, shafts with distal tip curved ( Figs 1 C, 2C). Dorsal chaetal shafts with distal spines, shafts of most ventral chaetae smooth ( Fig. 1 C). Bidentate chaetae becoming unidentate towards posterior end ( Fig. 2 D). In posterior chaetigers, first two blades of most ventral chaetae turn out 180º, becoming claw-shaped, with short, smooth shafts ( Figs 1 E, 2E, F); remaining six to seven chaetae with distally curved shafts with small spines, and hooked unidentate blades about 30 µm long, with long spines on edge ( Fig. 1 D). Anterior parapodia each with four slender aciculae, three straight, pointed, and one distally curving; from midbody posteriorly, number of aciculae per parapodium diminishes to two on posterior parapodia, one straight and pointed, the other slightly oblique at tip, both slightly protruding from parapodia. Pharynx through about six to eight segments; pharyngeal tooth located anteriorly, surrounded by crown of ten soft papillae. Proventricle longer than pharynx, through nine segments, with about 37 muscle cell rows, and distinct mid-dorsal line ( Fig. 1 A). Pygidium small, regenerating in most specimens, with two anal cirri similar to dorsal cirri ( Fig. 1 G). Some specimens with attached acephalous stolon, small, short, with about eight to ten chaetigers. Some specimens with dark oocytes laterally inside posterior segments ( Fig. 1 G).

Remarks. Branchiosyllis exilis , originally described from Djibouti, Gulf of Aden, is characterized by the presence of claw-shaped chaetae on posterior chaetigers, the most diagnostic character of the genus, and by the absence of branchiae, even though the generic name refers to the presence of branchiae. It is a widely distributed taxon, considered circumtropical, although material from different localities exhibits slight variations in some characters. The holotype unfortunately lacks all dorsal cirri. Specimens from Indonesia largely agree with those described by Westheide (1974) from the Galapagos Islands. The latter specimens have long dorsal cirri (35–50 articles, as compared with 38–45 in Indonesia), while specimens from the Mediterranean (with approximately the same body length as those from Indonesia) identified as B. exilis (by San Martín, 2003) have shorter dorsal cirri (14–22 articles). Specimens from Indonesia have the fangs of the chaetae slightly curved in their distal part, while they are not curved in specimens from the Mediterranean, nor in the holotype. Westheide (1974) did not describe this character for specimens from the Galapagos. However, the examined types of B. fuscosuturata from the Caribbean Sea (synonymized with B. exilis by Westheide) present fangs slightly curved distally. Specimens from Indonesia present normal compound chaetae plus claw-shaped chaetae in the posterior chaetigers, while there are only claw-shaped chaetae in Mediterranean B. exilis . However, Indonesian specimens were regenerating their pygidiums and maybe such modified chaetae were present but these segments are now missing. In some of the examined Indonesian specimens, ventral cirri of posterior segments are distinctly long, twice as long as the parapodia; this feature has not been found in any description from other localities. In conclusion, there are many small differences between specimens identified as B. exilis around the world. A revision of all the material identified under this name will be necessary, as it may represent a complex of species. Branchiosyllis cirropunctata is a similar species but clearly differs in having dark spots on dorsal cirri and some details of chaetae.

Distribution. Apparently circumtropical, also present in the Mediterranean Sea.

Figures

FIGURE 1. Branchiosyllis exilis ZMA V. Pol. 2038.01. A: Anterior part, dorsal view; B: Anterior chaetae; C: Midbody chaetae; D: Posterior chaetae; E: Posterior claw-shaped chaetae; F: midbody parapodium, anterior view; G: Posterior part, dorsal view. Scale A: 0.5 cm.; B – D: 20 µm; F: 1 mm; G: 0.2 mm.

FIGURE 2. Branchiosyllis exilis ZMA V. Pol. 2038.01. SEM. A: Anterior parapodia, dorsal view; B: Anterior chaeta; C: Midbody chaetae; D: Midbody to posterior chaetae; E, F: Posterior claw-shaped chaetae; G: Posterior parapodia and ventral cirri, dorsal view; H: Posterior parapodia, ventral view.