Exogonoides joaoi, Fukuda, Marcelo Veronesi, Martín, Guillermo San, Carrerette, Orlemir & Paresque, Karla, 2016

Fukuda, Marcelo Veronesi, Martín, Guillermo San, Carrerette, Orlemir & Paresque, Karla, 2016, On a new species of the rare syllid genus Exogonoides (Annelida, Phyllodocida, Syllidae), Zootaxa 4144 (2), pp. 291-295 : 292-295

Exogonoides joaoi sp. nov.

Figures 1–2

Type material. Holotype ( MZUSP 2910 ): Atlantic Ocean, Brazil, Rio de Janeiro, Campos Basin (22°19'10"S 40°5'42"W, 400 m), coll. Campaign Hab 9, 10 Feb 2009.

Comparative material examined. Exogonoides antennata — South Africa, off Cape Province (34°02'S 23°28'E, 49 m), on sand, mud and rock: 2 specs ( BMNH 1963.1 .34, holotype; 1963.1.35, paratype), coll. J.H. Day, 29 Nov 1960, det. J.H. Day, 1963.

Description. Thin, elongate body, 14.80 mm long, 0.32 mm wide, with 145 chaetigers ( Fig. 1 A). Palps kidneyshaped, free from each other at least along distal 2/3 of their length, ventrally bent beneath prostomium ( Figs 1 B, D; 2A– C). Prostomium rounded, with pair of lateral antennae inserted close to anterior border, median antenna absent; lateral antennae ovate to slightly pyriform ( Figs 1 B, D; 2A–B); eyes not visible. Peristomium apparently partially fused to prostomium, with pair of minute, ovate peristomial cirri. Small dorsal cirri on anterior body, rounded, progressively larger, ovate, until midbody ( Fig. 2 A–B); largest dorsal cirri on midbody, ovate to pyriform ( Figs 1 E; 2D), diminishing in size towards posterior body, dorsal cirri on posterior body rounded, small, but slightly larger than those on anterior body chaetigers ( Fig. 2 D–E). Ventral cirri apparently fused to parapodial lobes. Anterior parapodia with ~7 falcigers each, midbody with 1–2, posterior parapodia with 2 falcigers each; falcigers with subdistally spinulated shafts; short, bidentate blades, distal tooth slightly larger than subdistal one; blades with inconspicuous dorso-ventral gradation in length ( Fig. 2 G, I, K), 12–8 µm long on anterior body, ~10 µm long on mid- and posterior body. From anterior body, each parapodium with at least dorsalmost chaeta simple, similar to falciger shafts but with tip slightly more rounded on anterior body ( Fig. 2 F, H), with thinner tip towards posterior body ( Fig. 2 J); each anterior and posterior parapodium with one of these simple chaetae, up to 4 simple chaetae per parapodium on midbody chaetigers, possibly secondarily simple by loss of blades. Capillary dorsal simple chaetae with different morphology apparently absent. Ventral simple chaetae present from midto posterior body, thinner than other chaetae, subdistally sigmoid, tips resembling those of falciger blades, bidentate, distal tooth larger than subdistal one ( Fig. 2 L). Anterior parapodia with up to 2 aciculae each ( Fig. 2 M), only one acicula in each midbody and posterior parapodium; aciculae almost straight, distally tapering. Body ending by ~10 achaetous segments and rounded pygidium with pair of claviform cirri ( Figs 1 A, C; 2E), cirri ~ 0.12 mm long. Pharynx straight, slightly sinuous close to proventricle, through ~5 segments, trepan apparently present on anterior margin, with teeth directed forwards but difficult to visualize and count; proventricle oblique, 0.2 mm long, through 1.5–2 segments ( Figs 1 A–B, D; 2A–C), number of muscle cell rows difficult to assess.

Remarks. The specimen herein described as E. joaoi sp. nov. could be assigned to the Exogonoides since, except for the prostomium, it is remarkably similar to E. antennata in overall body shape and morphology of chaetae, fitting in the diagnosis of this rare genus.

The two species of Exogonoides can be distinguished mainly by head features. Exogonoides antennata presents globose, rounded to ovate palps and antennae, which may appear as 'anterior lobes' ( Aguado & San Martín 2008), but are clearly homologous to the structures found in other syllids. On the other hand, E. joaoi sp. nov. has the palps with similar morphology to the usual pattern of the family, structures more dorso-ventrally flattened, in this case, also ventrally bent beneath the prostomium; furthermore, the antennae are clearly different from those of E. antennata , conspicuously smaller, with somewhat thinner tips. Also, eyes are not visible in E. joaoi sp. nov., whereas E. antennata presents 4 lensed eyes on the prostomium.

The type of E. antennata was the only specimen of this genus available for examination, and, considering the absence of the pharynx and proventricle in that material since those were dissected ( Day 1963) and, apparently, not preserved, Aguado & San Martín (2008) considered this taxon as ' Nomina dubia '. However, the morphological information provided by the examination of the holotype of E. joaoi sp. nov. allows us to confirm the inclusion of this genus within the family Syllidae , possibly more closely to the subfamily Autolytinae Langerhans, 1879 , by the palps ventrally bent beneath the prostomium, ventral cirri fused to the parapodial lobes and by the slightly sinuous pharynx, the latter two features also observed in E. antennata (description of the pharynx provided by Day 1963). The affinity with the Autolytinae was already pointed out by Day (1963), although not conclusively, since members of that subfamily present two pairs of peristomial cirri. As the position within the family was never analysed in phylogenetic studies, it would be premature to attest that Exogonoides belongs to the Autolytinae , which would further require an amendment of the diagnosis of the subfamily.

Due to the exhaustive collection efforts of the project in which the present material was found, we consider to be less likely that the presence of this single specimen was an effect of sampling deficiencies, rather considering it as a rare species, based also on the fact that the single other species of the genus was found only once.

Descriptions of new species based on singletons can be seen as inadvisable (e.g., Dayrat 2005), however, following what was suggested by other authors ( Lim et al. 2012), we consider that choosing to ignore these rare species could lead to important underestimates of the biodiversity since, apparently, the case of rare species is not, itself, rare. Another case of a singleton, Nuchalosyllis maiteae Fukuda & Nogueira, 2013 , had already been previously described based on material found in the scope of this same project ( Fukuda & Nogueira 2013). Recently, another specimen of N. maiteae was found, associated with rhodolith beds in the Fernando de Noronha Archipelago (NE Brazil), over 2000 Km north of the type locality (Santos et al. 2016).

San Martín & Aguado (2014) considered 74 genera of syllids as valid; so, Exogonoides raises to 75 the number of valid genera in the Syllidae .

Distribution. As previously mentioned, E. antennata was originally described from off Cape Province, South Africa (Indian Ocean), therefore, far from the type locality of E. joaoi sp. nov., in the South Atlantic Ocean.

The deep Atlantic waters form an efficient, meso-oceanic barrier to the dispersal of organisms between Africa and South America, however, this barrier may not be effective for some taxa. Some species of fishes or even of marine invertebrates were recorded on both sides of the Atlantic, suggesting the ability of these taxa to cross the ocean, still in larval stages or even through rafting in adults ( Briggs & Bowen 2013).

The relationship between the two known species of Exogonoides remain unresolved and an interesting topic for future research, whether their separation was derived from a more recent event or an old split, possibly even accompanying the continental drift, in which case the rarity of the group could be explained by this being a relic of past times.

Etymology. This new species is dedicated to João Miguel de Matos Nogueira (IB-USP) as an appreciation for, besides all the teachings, his friendship along the years.

Figures

FIGURE 1. Exogonoides joaoi sp. nov. (A) complete specimen, dorsal view; (B, D) anterior body, dorsal view; (C) posterior body, dorsal view; (E) midbody, dorsal view. Scale bars: A, 1 mm; B – C, 0.5 mm; D, 0.2 mm; E, 0.3 mm.

FIGURE 2. Exogonoides joaoi sp. nov. (A – B) anterior end, dorsal view; (C) anterior end, ventro-lateral view; (D) midbody chaetigers, dorsal view; (E) posterior end, dorsal view; (F) simple chaetae, anterior parapodium; (G) falcigers, anterior parapodium; (H) simple chaetae, midbody parapodium; (I) falciger, midbody parapodium; (J) simple chaetae, posterior parapodium; (K) falcigers, posterior parapodium; (L) ventral simple chaeta, posterior parapodium; (M) aciculae, anterior parapodium. Scale bars: A – E, 200 µm; F – M, 20 µm.