Megalomma fauchaldi, Giangrande, Adriana, Licciano, Margherita & Gambi, Maria Cristina, 2007

Giangrande, Adriana, Licciano, Margherita & Gambi, Maria Cristina, 2007, A collection of Sabellidae (Polychaeta) from Carrie Bow Cay (Belize, western Caribbean Sea) with the description of two new species, Zootaxa 1650, pp. 41-53 : 46-47

Megalomma fauchaldi sp. nov.

Figure 2

Material examined. Type material: Holotype, on Thalassia at 3 m, on lagoon side of CBC, collected in November 2005, MNCN 16.01/11018 (sample CBC-KF 1012). Paratypes, one specimen from the same locality USNM 1102753; two specimens from the same locality PCZL.

Description. Holotype specimen complete with 8 thoracic and 58 abdominal chaetigers. Body length 20 mm, width 3 mm. Crown length 7 mm; 14–15 pairs of radioles in each lobe with three brown bands. Subterminal compound eyes on seven pairs of radioles. Dorsalmost radioles bearing larger eyes and small tip beyond the eye spot ( Fig. 2 D). The others with smaller eyes and longer tips ( Fig. 2 E). Dorsal lips long with broad base, with brown pigment ( Fig. 2 A), radiolar appendage present, long, 1/4 of length of radioles. Ventral lips with parallel lamellae. Dorsal margin of collar fused to faecal groove and forming two very deep pockets, clearly visible. Base of collar dorsally completely enveloping first chaetiger and extending to second chaetiger ( Fig. 2 A, B). Collar slightly higher ventrally with poorly developed lappets ( Fig. 2 C). Ventral shield of first chaetiger quadrangular, following shields rectangular ( Fig. 2 C). Chaetiger 3 with four superior thoracic notochaetae elongate narrowly hooded. Inferior thoracic notochaetae broadly-hooded, resembling paleate chaetae and arranged in two rows, 10 present in chaetiger 3 ( Fig. 2 F). Thoracic neuropodia widely separated from ventral shield ( Fig. 2 C), neurochaetae avicular with well developed breast ( Fig. 2 G), up to 20 uncini on chaetiger 1, and 14 on last thoracic neuropodia. Companion chaetae with roughly symmetrical tips, with teardropshaped membrane ( Fig. 2 H). Abdominal neurochaetae arranged in two rows, anterior row with elongate, narrowly hooded chaetae and modified, elongate, narrowly-hooded chaetae in posterior row, about 7 per row. Abdominal uncini avicular with manubrium shorter than present on thoracic uncini, about 14 present per torus on mid abdominal segments. Tube encrusted with shell and sand.

Remarks. The number of chaetae and neurochaetal uncini is variable along the body of all material examined. The number of superior thoracic chaetae ranges from 4 to 7, the number of inferior thoracic chaetae from 3 to 10, the number of thoracic uncini from 14 to 20, and the number of abdominal chaetae from 5 to 7, in each row.

Megalomma species are usually placed into 5 artificial groups on the basis of whether the mid-dorsal collar margins are fused to the faecal groove or are unattached, whether the dorso-lateral collar margin forms pockets or not, and the distribution and abundance of eyes on the radioles ( Knight-Jones, 1997). Although artificial, these groups have been useful for species comparison ( Nishi, 1998; Fitzhugh, 2002, 2003). The newly described species belongs to the group “1A”, which is defined by the dorsal margin of the collar being fused to the faecal grove, the presence of dorso-lateral pockets, and the occurrence of eyes on most radioles. Among the sabellid species previously listed from the Carribean (Tovar - Hernández and Salazar-Vallejo, 2006) including M. vesiculosum (Montagu, 1815) , M. lobiferum (Ehlers, 1887) , M. pacificum Johansson, 1927 , M. heterops Perkins, 1984 , M. bioculatum (Ehlers, 1887) , M. pigmentum (Reish, 1963) , and M. perkinsi Tovar- Hernández and Salazar-Vallejo, 2006, only M. vesiculosum , M. lobiferum and M. pacificum belong to this group. Megalomma fauchaldi n. sp., can be distinguished from M. lobiferum by lacking the “caruncle”, a distinctive feature which is present also in M. pigmentum , and in other two species from the Caribbean area, previously described as M. pigmentum by Perkins (1984), and Megalomma sp. 2 (Tovar-Hernández and Salazar- Vallejo, 2006), but probably belonging to different species for the moment indicated as Megalomma sp. 1 and Megalomma sp 2. Megalomma fauchaldi n. sp., differs from M. pacificum , and M. vesiculosum , whose presence within the Caribbean region needs confirmation, especially with regards to the presence of the broadlyhooded inferior thoracic chaetae. This is particularly relevant for records of M. vesiculosum from the Caribbean, following the recent revision of Mediterranean species which limits the distribution of this taxon to high latitudes (Giangrande and Licciano, in press). Finally, the collar features of M. fauchaldi n. sp. clearly distinguish this taxon from all the other worldwide distributed species belonging to group “1A”, as it has chaetae of similar shape to those present in M. fauchaldi n. sp., M. claparedei (Gravier, 1908) from the Red Sea, M. circumspectum (Moore, 1923) from California, M. multioculatum Fitzhugh, 2002 from Thailand and M. suspiciens (Ehlers, 1904) from New Zealand. The shape of the collar of M. fauchaldi , with the ventral pockets extending to the second chaetiger, appears unique within Megalomma . Among the species of the group “1A”, a similar feature is also present in M circumspectum (Moore, 1923) and M. lanigera (Grube, 1846) in which, however, the pocket base does not reach the second chaetiger.

Type locality: Carrie Bow, Belize.

Etymology: This species is named in honour of Kristian Fauchald as a tribute of his major contribution to polychaete systematics, his kindness and long friendship with us, and facilitating our visit the CBC.

Figures

FIGURE 2. Megalomma fauchaldi sp. nov. A) anterior end dorsal view from the holotype, with branchial crown open to show the dorsal lips, B) anterior end dorsal view from paratype, C) anterior end ventral view from the holotype, D) tip of the dorsalmost radiole encircled by the eye, E) tip of one of the following radioles, F) inferior thoracic notochaetae from the third chaetigers, G) thoracic uncinus, H) companion chaeta. dl = dorsal lip