Megalomma inflata, Capa & Murray, 2009

Capa, María & Murray, Anna, 2009, Review of the Genus Megalomma (Polychaeta: Sabellidae) in Australia with Description of Three New Species, New Records and Notes on Certain Features with Phylogenetic Implications, Records of the Australian Museum 61 (2), pp. 201-224 : 217

Megalomma inflata n.sp.

Figs 4G,H , 5D , 11

Material examined. New South Wales. HOLOTYPE AM W35503, southeast of Bate Bay , 34°04'S 151°13'E, coll. by The Ecology Lab for RMI/ Pioneer Project, 15 Jan. 1990, 45– 50 m, Stn RMI C26 . PARATYPES: AM W35504 (1 spec.), east of Bass Point , 34°36'S 150°54'E, 25 Jun.–26 Jul. 1990, 45– 50 m, Stn RMI BP51 ; AM W35505 (2 spec.), east of Bass Point , 34°36'S 150°54'E, 3–18 Jan. 1991, 65– 70 m, Stn RMI 4/236 ; AM W35506 (1 spec.), east of Bass Point , 34°36'S 150°54'E, 25 Jun.–26 Jul. 1990, 45– 50 m, Stn BP 43 ; AM W23057 (1 spec. on SEM stub), east of Bass Point , 34°36'S 150°54'E, coll. by The Ecology Lab for RMI/ Pioneer Project, 1 Feb. 1990, 25– 50 m. AM W35753 (1 spec.), south of Batemans Bay, south side of Burrewarra Point , 35°50'16"S 150°14'05"E, 28 Mar. 2004, 17 m, from algal and bryozoan (cf. Amathia ) turf on vertical rock face, coll. Australian Museum party, Stn NSW 2631 .

Additional material. Megalomma quadrioculatum , holotype ZMUH PE 1303 , Aripu Coral Reef , Sri Lanka, coll. Willey, 1905 .

Diagnosis. The synapomorphy of this species is the presence of an inflated peristomium protruding from collar. Moreover, M. inflata n.sp. is characterized by a combination of characters: dorsalmost radioles with large subdistal compound eyes and occasionally one or two small pairs in following radioles, radiolar skeleton with around 10 cells in transverse section, dorsal margins of collar fused to the faecal groove forming broad and U-shaped pockets on each side, caruncle absent, and thoracic inferior chaetae with tips progressively tapering (type B).

Description. Holotype complete and in good condition measuring 22 mm long and 2 mm wide, with eight thoracic chaetigers and 37 abdominal chaetigers. Crown longer than thorax, with 14 pairs of radioles arranged in two semicircular lobes. External margin of radioles quadrangular, lateral flanges absent ( Fig. 11A ), skeleton with 8–10 cells in transverse section ( Fig. 5D ). Tip of radioles digitiform and shorter than pinnules ( Fig. 11A ). Three pairs of dorsalmost radioles with a subdistal compound eye and an extra fourth eye on the right side, with distinct ommatidia, eye-size diminishing ventrally; dorsalmost eyes spiral, almost surrounding radioles. Dorsal lips with radiolar appendages shorter than two thoracic chaetigers and no visible pinnular appendages. Ventral lips rounded and well developed. Ventral sacs present. Caruncle absent. Short smooth keel present. Posterior peristomial collar with rounded dorsal margins fused to faecal groove, dorsal collar lappets absent ( Fig. 11C ); collar pockets present, low due to deep incision (dorsal peristomium inflated and dorsal collar difficult to discern) ( Fig. 11C ); lateral margins of collar smooth, ventral lappets prominent, pointed and not overlapping ( Fig. 11B,C ). Ventral shields quadrangular, separated from the neuropodial tori, all similar in width ( Fig. 11B ). First ventral shield longer than the rest, with m-shaped anterior margin although not conspicuous. First chaetiger with superior and inferior elongate narrowly hooded chaetae, superior longer than inferior ( Fig. 11D ). Rest of thoracic chaetigers with elongate narrowly hooded superior chaetae and broadly hooded inferior notochaetae with tips progressively tapering (type B) ( Fig. 11E,G ). Neuropodial tori slightly diminishing in width posteriorly. Thoracic uncini with several rows of small teeth, similar in size, above main fang ( Fig. 11H ); uncinus with well developed breast, handle twice length of the distance from breast to main fang ( Fig. 4G ). Companion chaetae with asymmetrical membrane and teeth covering about half the length ( Fig. 11I ). Neuropodia of anterior abdominal chaetigers with narrowly hooded chaetae ( Fig. 11F ). Notopodia with uncini similar to thoracic uncini ( Fig. 11J ) but with shorter handles ( Fig. 4H ). Pygidium trilobed from ventral view, as a rounded papilla from dorsal view, with scattered eyespots laterally. Tube made of a thin and flexible layer of mucus covered by a mixture of fine sediment and mud.

Variation. The paratypes range in size from 10–25 mm in length. The number of pairs of radioles varies from eight to ten. They possess one to two pairs of radiolar eyes (although the holotype possesses 3 pairs plus an extra eye on one side). One paratype (from AM W35505) also possesses dark, slightly inflated spots on the inner subdistal sides of most of the rest of the radioles (excluding the ventralmost three pairs) which could be incipient eyes. However, ommatidia could not be detected, and these therefore may just be pigmented areas.

Colour pattern. Pigment occurs at the base of radioles and in a few transverse irregular bands in radioles and pinnules. Body unpigmented.

Etymology. The name of this species is related to the conspicuously inflated peristomium. This feature is unique among other Megalomma species.

Remarks. This new species displays similar features to another species described in the Indo-Pacific area, M. cinctum from Taiwan —the fusion of dorsal collar margins to the faecal groove, the presence of dorsolateral pockets and the distribution of compound eyes in 1–3 dorsalmost pairs of radioles (this is if the spots found in one specimen are not considered as eyes). However M. inflata n.sp. is distinguished from M. cinctum by the conspicuous inflation of the peristomium shown in all the specimens, the deep U-shaped dorsolateral incisions of the collar (absent in M. cinctum ), the presence of well-developed ventral sacs (absent in M. cinctum ) and the narrowly hooded shape of the inferior thoracic chaetae (or at least slender and thin), but broadly hooded (type A) in M. cinctum .

There are species described from several localities around the world that share with this new species some features such as the development and shape of collar dorsal margins, collar margins fused to the faecal groove, the presence of pockets, eyes present on one to three dorsalmost pairs of radioles, and the absence of a caruncle. These are M. coloratum ( Chamberlin, 1919) from California, M. modestum ( de Quatrefages, 1866) from Peru, M. roulei ( Gravier, 1908) and M. splendidum ( Moore, 1905) . None of these species have an inflated peristomium, nor are their collar pockets as broad and U-shaped as M. inflata n.sp. Another species described from the Indian Ocean and which shares some features with M. inflata n.sp. is M. quadrioculatum ( Willey, 1905) but examination of the holotype for this review revealed the presence of a large caruncle, which is absent in the new species.

Figures

Figure 4. Drawings to scale of thoracic and abdominal uncini. Megalomma phyllisae n.sp.:

Figure 5. Drawings to scale of radioles sections, at the base: (A) Megalomma

Figure 11. SEM of Megalomma inflata n.sp., AMW23057; (A) tip of radioles and dorsalmost compound subdistal eyes; (B) base of radiolar crown and anterior segments, lateral view; (C) margins of collar, dorsolateral view; (D) chaetae, first thoracic segment; (E) chaetae, anterior thoracic segment; (F) inferior thoracic chaetae, posterior thoracic segment; (G) abdominal chaetae; (H) thoracic uncini; (I) companion chaetae, same chaetiger; (J) anterior abdominal uncini.