title: Accacoelium contortum (Trematoda: Accacoeliidae) a trematode living as a monogenean: morphological and pathological implications.
abstract: BACKGROUND
  Accacoelium contortum (Rudolphi, 1819) Monticelli, 1893 is a frequent but poorly known trematode found on gills, pharynx and digestive tract of the ocean sunfish Mola mola (L.). Although the morphology of A. contortum agrees with that of a typical endoparasitic trematode, with two relatively small suckers and no large holdfasts, this parasite is normally ectoparasitic. The main objective of this paper is to explore this peculiar host-parasite relationship.
  
  
  METHODS
  A total of 106 ocean sunfish were examined for the presence of A. contortum. The oropharyngeal chamber (gills and pharynx) and the digestive tract were analysed. As the previous descriptions of this species seem to be based on contracted specimens, for the morphological study the parasites were killed using two methods: with hot 70% ethanol (with relaxed bodies) and with 70% ethanol at room temperature (with contracted bodies). For histological studies, samples from fresh fish with parasitised left gills, pharynx and digestive tract were fixed in buffered 10% formalin. For molecular studies the 18S, 28S and ITS-2 sequences were provided and compared with the available data in GenBank®.
  
  
  RESULTS
  New information on the morphology of A. contortum and on the parasite-related response and pathological alterations in the host are given. New diagnostic traits for some structures are provided: e.g. tegumental papillae of the forebody with apical digitiform swellings and mouth surrounded by a circum-oral crown of simple papillae. The length of the ventral sucker peduncle and the position of the vitellarium were found to be associated with the contraction degree of the specimen. Immature individuals of this species are described for the first time. An intense proliferative inflammatory response of host gill and pharynx epithelium at the host-parasite interface was detected and parasites became partially covered by overgrowths of host tissues.
  
  
  CONCLUSIONS
  The induction of prominent histological alterations associated with A. contortum seems to be an adaptation to the external environment, an unusual location for trematodes.
authors.affiliation: Cavanilles Institute of Biodiversity and Evolutionary Biology, Marine Zoology, Science Park, University of Valencia, Catedrático José Beltrán, 2, 46980, Paterna, Valencia, Spain. baraja@uv.es.
authors.forename: Ana Elena
authors.initials: AE
authors.lastname: Ahuir-Baraja
authors.affiliation: Fish Diseases Diagnostic Service. Departament de Biologia Animal, de Biologia Vegetal i d'Ecologia, Facultat de Veterinaria, Universitat Autònoma de Barcelona, 08193, Barcelona, Cerdanyola, Spain. francesc.padros@uab.cat.
authors.forename: Francesc
authors.initials: F
authors.lastname: Padrós
authors.affiliation: Cavanilles Institute of Biodiversity and Evolutionary Biology, Marine Zoology, Science Park, University of Valencia, Catedrático José Beltrán, 2, 46980, Paterna, Valencia, Spain. J.Francisco.Palacios@uv.es.
authors.forename: Jose Francisco
authors.initials: JF
authors.lastname: Palacios-Abella
authors.affiliation: Cavanilles Institute of Biodiversity and Evolutionary Biology, Marine Zoology, Science Park, University of Valencia, Catedrático José Beltrán, 2, 46980, Paterna, Valencia, Spain. toni.raga@uv.es.
authors.forename: Juan Antonio
authors.initials: JA
authors.lastname: Raga
authors.affiliation: Cavanilles Institute of Biodiversity and Evolutionary Biology, Marine Zoology, Science Park, University of Valencia, Catedrático José Beltrán, 2, 46980, Paterna, Valencia, Spain. Francisco.E.Montero@uv.es.
authors.forename: Francisco Esteban
authors.initials: FE
authors.lastname: Montero
mesh_terms: D000818:Animals; D016000:Cluster Analysis; D018509:DNA, Helminth / Q000737:chemistry / Q000235:genetics; D004275:DNA, Ribosomal / Q000737:chemistry / Q000235:genetics; D021903:DNA, Ribosomal Spacer / Q000737:chemistry / Q000235:genetics; D005393:Fish Diseases / Q000469:parasitology* / Q000473:pathology*; D041981:Gastrointestinal Tract / Q000469:parasitology; D005880:Gills / Q000469:parasitology; D006651:Histocytochemistry; D008853:Microscopy; D008969:Molecular Sequence Data; D010473:Perciformes / Q000469:parasitology*; D010614:Pharynx / Q000469:parasitology; D010802:Phylogeny; D012337:RNA, Ribosomal, 18S / Q000235:genetics; D012339:RNA, Ribosomal, 28S / Q000235:genetics; D017422:Sequence Analysis, DNA; D014200:Trematoda / Q000033:anatomy & histology / Q000145:classification* / Q000235:genetics / Q000302:isolation & purification*; D014201:Trematode Infections / Q000469:parasitology / Q000473:pathology / Q000662:veterinary*
chemical_list: D018509:DNA, Helminth; D004275:DNA, Ribosomal; D021903:DNA, Ribosomal Spacer; D012337:RNA, Ribosomal, 18S; D012339:RNA, Ribosomal, 28S