Deuterodon Heterostomus

''Deuterodon heterostomus'' is a small species of characin endemic to a large river system in southeast Brazil. It was originally the only member of the genus ''Probolodus'', but ''Probolodus'' is now considered obsolete, synonymized with ''Deuterodon''. Much like other members of ''Deuterodon'', and like some fish in related genera, ''D. heterostomus'' is a small, silvery fish with fins in some combination of red and clear. It has a defined humeral spot that can be used for identification.

Its diet was not well-known until a 1970 study that classified it as a voracious lepidophage, preying on other fish species for their scales. Most of its food is sourced from related fishes such as ''Psalidodon fasciatus'', a cryptic species to which ''D. heterostomus'' bears a strong visual resemblance. This resemblance has been proposed to be a form of mimicry, allowing ''D. heterostomus'' to easily approach its prey.

Taxonomy

When originally described by German-American ichthyologist Carl H. Eigenmann in 1911, ''D. heterostomus'' went by the name ''Probolodus heterostomus''. It was known as such for most of its existence as a species, and can still be found listed under the genus ''Probolodus'' in some databases, such as GBIF and the Encyclopedia of Life. However, the genera ''Probolodus'' and ''Myxiops'' were synonymized with ''Deuterodon'' in a 2020 study that closely examined the phylogeny of ''Astyanax'' and related groups, such as ''Psalidodon'', ''Andromakhe'', ''Makunaima'', and ''Deuterodon'' itself. This updated classification is reflected in databases including FishBase, the World Register of Marine Species, and Eschmeyer's Catalog of Fishes.

Even before both species were reassigned to the same genus, ''D. heterostomus'' was known to be closely related to the then-monotypic ''Myxiops aphos'' (now ''Deuterodon aphos'') and ''Deuterodon iguape''.

Etymology

The genus name "Deuterodon" originates in Greek. Its roots are "deuteros", which means "second" or "repeated", and "odon", which means "tooth"; this is in reference to the similarity of all teeth in the lower jaw. The species name "heterostomus" is also from Greek, where "hetero" means "different" and "stomus" means "hole" or "mouth"; the original description noted peculiarity in the way the teeth point outwards.

Description

''Deuterodon heterostomus'' reaches a maximum of roughly SL (standard length, without the tail fin included). Most larger specimens are between and SL. The body is somewhat compressed and deep; this is more evident in smaller specimens, and body depth can vary between populations, though there is significant overlap between those located near each other. There are 45–56 scales in the lateral line, with 8–10 scale rows above it and 7–10 rows below. There are 11 total rays in the dorsal fin, 26–34 rays in the anal fin (rarely 35 or 36), 12–14 rays in the pectoral fin, 6–9 rays in the ventral fin (most often 8), and 19 rays in the caudal fin.

As early as the time of its description, ''D. heterostomus'' was noted to closely resemble ''Astyanax fasciatus'', a cryptic species now known as '' Psalidodon fasciatus''. Features include a silver base color and reddish fins, with the exception of the ventral and pectoral fins, which are clear. There is a large humeral spot, vertically elongated, that can be anywhere above the region between the 3rd and 5th scale of the lateral line. There is another spot on the caudal peduncle that continues onto the fin-rays in younger specimens; the ray pigmentation fades somewhat in adults.

Sexual dimorphism

Females are the larger of the two sexes, averaging longer than males when fully grown. Some larger males develop backwards-facing hooks on select rays of the pelvic fin when mature.

Distribution and ecology

''Deuterodon heterostomus'' can be found in the coastal river basins of southeastern Brazil. More specifically, it is known to occur in tributaries of the rio Paraíba do Sul basin. The region generally has a neutral water pH, ranging from 6 to 8, and various sites tend to have similar water temperatures, ranging from .

Diet

''Deuterodon heterostomus'' is an omnivore, but most of its diet consists of the scales of other fish - a habit known as lepidophagy. This was first observed in a study of stomach contents from 1970; in 50% of examined specimens therein, scales were the only food item found in the stomach. Other specimens had eaten small seeds, microcrustaceans, and insects (plus insect larvae). Ichthyologist Ivan Sazima proposed in 1977 that the strong resemblance between ''D. heterostomus'' and ''P. fasciatus'' is a form of aggressive mimicry; because ''D. heterostomus'' looks like ''P. fasciatus'', specimens of the latter are more likely to let the former get close, which would result in an easy source of scales for food.

Conservation status

''Deuterodon heterostomus'' has received an evaluation of Least Concern from the IUCN. It is a widespread species throughout its range, which indicates stability. It does, however, live in a region under ecological pressure from sources including pollution, drought, and municipal use.

References

{{commonscat
Fish described in 1911
Taxa named by Carl H. Eigenmann
heterostomus
Freshwater fish of Brazil
Stygichthyini
Taxobox binomials not recognized by IUCN