Abstract
Gyrocotylideans are evolutionary ancient parasitic flatworms, and like their hosts—a relict group of holocephalan fishes (Chimaeriformes)—they are considered to be “living fossils” of a vanished past. However, the species diversity, host associations and biogeography of these most basal tapeworms are poorly known. Herein, we provide evidence of a conspicuous contrast between the genetic and morphological data based on an examination of newly collected and properly processed Gyrocotyle specimens (hologenophores) isolated from holocephalans off Taiwan and Argentina. Our molecular data, inferred from three genes (COI, 28S rRNA, 18S rRNA), showed unexpected genetic interrelationships among isolates of the genus Gyrocotyle, because each of the four genotypes from Taiwan clustered with isolates of distinct gyrocotylideans from the North Atlantic. Three genotypes of Gyrocotyle from Taiwan were morphologically almost indistinguishable from each other but represented distinct genetic lineages; a single specimen of Gyrocotyle sp. genotype 4 exhibited a clear genetic and morphological distinctness, though its formal description as a new species would be premature. Additionally, specimens of Gyrocotyle rugosa Diesing, 1850, from the type host Callorhinchus callorynchus from Argentina, provided the first genetic data on the type species of the genus and enabled us to characterise it, which is necessary for future taxonomic studies. The finding of some specimens of Gyrocotyle sp. genotype 3 in Chimaera phantasma, and another one in C. cf. argiloba, together with the putative conspecificity of an unidentified gyrocotylidean from Callorhinchus milii off Australia and G. rugosa from C. callorynchus off Argentina, represent evidence that one gyrocotylidean species may parasitise more than one holocephalan host species. Existing taxonomic problems and conflicts between morphological and molecular data on species of Gyrocotyle can only be resolved if hologenophores from type hosts and localities of nominal taxa are properly characterised genetically and morphologically.
Highlights
The order Gyrocotylidea Poche, 1926, represents an enigmatic and the most basal extant group of tapeworms, with a unique monozoic body and a posteriorly located funnel-shaped adhesive o rgan[1,2]
Most of the species have been described from the North A tlantic[5,6,7,8,9,10], whereas a few taxa have been reported from the southern seas[5,11,12,13], including the type species Gyrocotyle rugosa Diesing, 1850
17 specimens of Gyrocotyle spp. from Taiwan were genotyped and four distinct genotypes were identified based on partial sequences of the c oxidase subunit 1 (COI), 28S rRNA and 18S rRNA genes
Summary
The order Gyrocotylidea Poche, 1926, represents an enigmatic and the most basal extant group of tapeworms, with a unique monozoic body and a posteriorly located funnel-shaped adhesive o rgan[1,2]. The only gyrocotylidean recorded in east Asian waters was found in Chimaera phantasma Jordan & Snyder from the coast of Japan and was identified as Gyrocotyle fimbriata Watson, 1911, in a brief report of Ichihara[14] and in two unidentified species of Hydrolagus and Harriotta off Indonesia[3]. The former finding needs verification, because G. fimbriata was originally described as a parasite of Hydrolagus colliei (Lay & Bennett) from the west coast of the United States of A merica[15]. M anter[24] and Bandoni & B rooks[16] questioned the strict host specificity of gyrocotylideans and reported G. urna and G. fimbriata as parasites of more than one holocephalan species
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