Linking metacercariae and adults of Microphallus basodactylophallus (Digenea: Microphallidae), based on larval stages from ctenophores and adult parasites from aquatic birds found in Mexico

Members of the genus Microphallus Ward, 1901, are endoparasites mainly of birds and mammals distributed worldwide. Unencysted metacercariae of Microphallus sp., were collected from the mesoglea of ctenophores of the genus Pleurobrachia Fleming; adult digeneans were recovered from the intestines of Eudocimus albus Linnaeus (Threskiornithidae) and Buteogallus urubitinga Gmelin (Accipitridae), in four locations from southeastern Mexico. Adult specimens were identified as M. basodactylophallus (Bridgman, 1969) based on the following features: body pyriform entirely covered by minute spines, prepharynx short, oesophagus very long, caeca short and widely divergent, testes slightly symmetrical and excretory vesicle short and V-shaped. Sequences from D1–D3 domain of the large subunit of ribosomal DNA (LSU) were generated, aligned, and compared with those of congeneric species available in GenBank. Phylogenetic analyses indicated that the metacercariae and adults formed a clade together with an isolate identified as M. basodactylophallus from Florida, USA (GenBank: AY220628). The intraspecific genetic divergence among isolates was low ranged from 0.0% to 0.6%, allowing the link between the two stages of the life cycle. We observed phenotypic plasticity in the morphological traits of M. basodactylophallus adults in definitive hosts (mammals and birds) throughout the distribution, which ranged from the USA to southeastern Mexico. Finally, the unencysted metacercariae identified as M. basodactylophallus represent the first report of a microphallid in ctenophores.