Biotodomella gen. nov. (Monogenoidea: Dactylogyridae) is proposed to accommodate Biotodomella mirospinata sp. nov., found on the gills of Biotodoma cupido (Heckel, 1840) (“cara bonita” or “green-streaked eartheater”), a freshwater cichlid from the Peruvian Amazon. The new genus and species differ from other Neotropical dactylogyrids in having the hooks of each hook pair well-differentiated from the remaining pairs, shafts varying in robustness, weakly sclerotized expansions at the proximal end of shaft, and a dorsal anchor with grooved shaft, bifid point. This study represents the first record of a monogenoid from a species of Biotodoma Eigenmann & Kennedy, 1903.

Biotodoma, Cichlidae, Dactylogyridae, Monogenoidea, taxonomy

During a survey of monogenoid parasites from the freshwaters of the Peruvian Amazon, 20 specimens of a new genus and species were collected from the cichlid Biotodoma cupido (Heckel, 1840) (“cara bonita” or “greenstreaked eartheater”). This fish species can reach up to 9.7 cm and is distributed in the Amazon River basin in Peru, Bolivia, Brazil, and in the Essequibo River in Guyana (Reis et al. 2003). To date, there are no records of monogenoids parasitizing a species of Biotodoma Eigenmann & Kennedy, 1903. The new genus and species of Monogenoidea are unique among the Neotropical Dacylogyridae in having pairs of hooks conspicuously distinct from each other and a bifid point in the dorsal anchor.

Twenty fish specimens were collected in March 2018 by local fishermen in the “Shiruy Caño” in the Nanay River, in Iquitos, Peru. Symbiotypes (Brooks 1993) are deposited in the Colección ictiologica del Instituto de Investigaciones de la Amazonia Peruana (CIIAP) number IIAP-CIIAP 1003. Fish were euthanized, gill arches were removed and placed in vials containing heated water (68 °C). Each vial was shaken vigorously, and 96% ethanol was added to the final concentration of 75–80%. In the laboratory, the content of each vial was examined under a dissecting microscope. Helminths were removed from the gills or sediment using dissection needles. Some specimens were stained with Gomori’s trichrome (Humason 1979) and mounted in Dammar’s gum, to determine internal morphology, while others were mounted in Hoyer’s mounting medium (Humason 1979), for the study of sclerotised structures.

Illustrations were prepared with the aid of a drawing tube and a micro-projector attached to an Olympus BX 50 microscope (both phase contrast and DIC). Measurements were made following the procedures of Mizelle and Klucka (1953). Lengths of curved or bent structures (anchors, bars, MCO, and accessory piece) represent the straight-line distances between extremes. All measurements are in micrometres and are presented as the average followed by the range in parentheses. The type-specimens are deposited in the Collection of the “Instituto Nacional de Pesquisas da Amazônia” (INPA), Manaus, Brazil.

The phylogenetic relationships of B. mirospinata sp. nov. within the Dactylogyridae are not clear due to the uniqueness of its diagnostic features. The definition of its sister-group relationships among the Dactylogyridae will most likely require the integration of molecular and morphological characters. However, some features on the single species of Biotodomella are shared with species of Gussevia, Sciadicleithrum, and Parasciadicleithrum. Among the shared features are the dextral vaginal aperture (as in species of Gussevia and Sciadicleithrum), a long and slender dorsal bar (as in Gussevia spp.), and clockwise MCO (as in most species of these genera). This indicates that species of these genera represent descendants of a single ancestral marine or Neotropical freshwater species.

As suggested by Matschiner (2019) and references thereof, the ancestral of all Neotropical cichlids probably dispersed through the Atlantic Ocean long time after splitting of the Gondwana continent (app. 140-100 Ma according to McLoughlin 2001), around 75-60 Ma. For Pariselle et al. (2011), because of the dispersion of Cichlidae through the marine environment, freshwater parasites are likely lost and the fish lineage that reaches the new freshwater environs either bring parasite acquired during dispersion (marine lineages of parasites) or are colonized by local freshwater Monogenoidea.

Hence, there is a great possibility that the community of monogenoids in each geographic area derive from distinct processes resulting in a mosaic structure (Hoberg and Brooks 2013). Support for this expectation is provided by the phylogenetic analysis of Mendoza-Palmero et al. (2017), suggesting the existence of, at least two distinct and distant lineages composing the community of Monogenoidea from Neotropical Cichlidae.

When Mendoza-Palmero et al. (2017) suggested that Parasciadicleithrum and Sciadicleithrum represent independent evolutionary lineages in their phylogeny, they did not offer any evolutionary explanation for the obvious morphological similarity among species of these two genera. Their proposal for the new genus is, therefore, “... based mainly on the results of the molecular phylogenetic analyses” (Mendoza-Palmero et al. 2017). The lack of morphological diagnostic features (e.g. synapomorphies) creates a fragile taxon. This is especially true when new taxonomic groups are justified on phylogenetic hypotheses based on a single species (as for Parasciadicleithrum) since the analysis provides no test for the monophyletic nature of the proposed taxon (a monotypic taxon is monophyletic by definition). Accumulation of erroneous taxonomic decisions generates unstable classifications, as in the case of the Neotropical Dactylogyridae. Ideally, the proposal of new taxa should be based on a reasonable sample of subordinate species or robust morphological diagnostic features (as in the present proposal). Mistakes can be made even following these guidelines, but classifications evolve and there are formal protocols for that (Ride et al. 1999). When there are fewer accumulated mistakes the classification is more stable.

We thank the project Acuicultura of the Instituto de Investigaciones de la Amazonía Peruana for their assistance in the field study. The study was partially supported by a grant of the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Brazil. WAB is a research associate of CNPq, Brazil.