Comprehensive Description

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Hemiodontichthys Bleeker, 1862 ZBK .

Type species: Hemiodon acipenserinus Kner, 1853 ZBK .

Lectotype : NMW 46139 , Brazil , State of Mato Grosso , Rio Guapore drainage .

Gender: masculine.

This monotypic genus is widely distributed in the Amazon basin and the Essequibo, Oyapock, and Paraguay River drainages. Hemiodontichthys ZBK is a sand dweller that lives partially buried in the substrate, its cryptic coloration providing efficient protection. As with other representatives of the Loricariichthys ZBK group, mature males develop hypertrophied lips for brooding eggs. Eggs are laid in a mass and held by the male in the fold made by its lips (pers. obs.). This taxon is often compared with morphologically similar Reganella depressa(Kner, 1853) . Isbrücker & Nijssen (1974b) and Isbrücker (1979, 1981a) characterized these two genera without discussing any relationship between them or to other taxa. Isbrücker (1979) created the subtribes Reganellina and Hemiodontichthyina to accomodate these two genera. However a comparison is made in Isbrücker (1979) between Hemiodontichthyina and Loricariichthyina with reference to abdominal cover and sexual dimorphism, comparable in both subtribes. Rapp Py-Daniel (1997) considers Hemiodontichthys ZBK to be the sister genus of Reganella ZBK on the basis of osteological data. However, the similar external morphology of these two taxa could be interpreted as an evolutionary convergence, as they occupy the same ecological niche. Considering morphological data given in the key, particularly the mouth structure and abdominal cover, Hemiodontichthys ZBK is assigned herein to the Loricariichthys ZBK group. Molecular data (Montoya-Burgos et al. 1998) tend to support this relationship by placing Hemiodontichthys ZBK as sister to Loricariichthys ZBK . We assign Reganella ZBK to the Pseudohemiodon ZBK group on the basis of its mouth shape, the presence of vestigial fringed barbels, its strongly depressed body, and the characteristics of its abdominal cover made of little plates without particular organization and extending toward the lower lip margin. This kind of organization in the abdominal plating is never observed within the Loricariichthys ZBK group. Thus, the appearance of a rostrum and the loss of maxillary teeth could have evolved independently in different lineages subjected to similar environmental constraints. Given its broad geographic range and variation in morphometric features, Hemiodontichthys acipenserinus could comprise a species complex. Isbrücker & Nijssen (1974b) reported that populations from the Amazonian region tend to be more slender than those from the Paraguay and Guaporé Rivers.

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