Caruncle in Megalomma Johansson, 1925 (Polychaeta: Sabellidae) and the description of a new species from the Eastern Tropical Pacific Marı ´a Ana Tovar-Herna ´ ndez a,b * and Sergio I. Salazar-Vallejo a a Departamento de Ecologı ´a Acua ´tica, El Colegio de la Frontera Sur, Unidad Chetumal, Chetumal, Quintana Roo, Mexico; b Present address: Laboratorio de Invertebrados Bento ´nicos, Instituto de Ciencias del Mar y Limnologı ´a, Unidad Acade ´mica Mazatla ´n, Universidad Nacional Auto ´noma de Me ´xico, Mazatla ´n, Sinaloa, Mexico (Received 14 January 2008; final version received 18 April 2008) The presence of a caruncle in the polychaete family Sabellidae is documented. It is placed dorsally above the mouth, between the dorsal lips, supported with hyaline cartilage constituted by a homogeneous eosinophilic matrix and it is innervated directly from the cerebral ganglion. The caruncle surface has four longitudinal ciliated bands; the cilia from the lateral bands are hypertrophied, wider distally, tongue-like, forming a ciliated curtain or membranella. The genus Megalomma is emended based on the presence of a caruncle, a long dorsalmost radiolar pair, a high number of radiolar skeletal cells, and the presence of interramal eyespots. We describe a new species from Acapulco in the Tropical Eastern Pacific – Megalomma carunculata sp. nov. It has a caruncle, subtriangular ventral lappets, well developed dorsal pockets, compound eyes in all radioles, and interramal eyespots in the thoracic and abdominal segments. Keywords: compound eyes; interramal eyespots; new species; sabellids; sense organs Introduction Nuchal organs are paired epidermal chemosensory structures commonly present in polychaetes (Verger-Bocquet 1992; Purschke 1997). They are often situated dorsally or dorsolaterally at the posterior edge of the prostomium where they can be recognized as densely ciliated areas. These cilia are usually present in pits or grooves, although they could be restricted to ciliated surfaces, and even extend posteriorly on one or several segments as in some spionid genera. In the hesionids, the nuchal ciliated surfaces might fuse mid-dorsally but in most families they are present as separate lateral organs. There are some polychaete families where the nuchal organs are placed in an eversible pouch, as in Capitellidae or Opheliidae, while some members of other families, such as Syllidae or Typhloscolecidae, have paired rigid structures that might represent hypertrophied nuchal organs and which have been called epaulettes in the former family (Purschke 2005). Another modification is present in Poecilochaetidae, where there is a trifid organ collectively regarded as the nuchal organ (Rouse and Pleijel 2001); the lateral tines might be homologous with the hypertrophied nuchal organs, while the median tine would be a caruncle. *Corresponding author. Email: maria_ana_tovar@yahoo.com Journal of Natural History Vol. 42, Nos. 29–30, August 2008, 1951–1973 ISSN 0022-2933 print/ISSN 1464-5262 online # 2008 Taylor & Francis DOI: 10.1080/00222930802140186 http://www.informaworld.com Downloaded By: [UNAM] At: 14:29 18 August 2008