develop; these are sometimes bifid. Biology Reproduction Body weight and standard length are positively correlated with the relative fecundity of specimens; in other words, large individuals are more likely to be capable of reproduction. Spawning appears to be year-round in most of its range, but can be seasonally restricted depending on the specific site. No matter the case, peak reproductive activity appears to be during warm and wet periods. For instance, populations of A. aeneus in the Champotón river mate the most during the stretch from April to July, and if water quality is poor, they can respond by transferring energy from the liver to increase gamete production, allowing year-round spawning. Trophic role and bioindicator status Astyanax aeneus is considered a keystone nutrient recycler, partially because of its varied appetite, and this specifically applies to phosphorus, which can limit the health of a given ecosystem (especially in neotropical streams like the ones it inhabits). Furthermore, it is considered a good bioindicator of trace metals in the ecosystems of the Yucatán Peninsula, because of how the metals accumulate in the body. Stress levels measured by certain physiological responses to poor water conditions also make A. aeneus a good indicator of water quality in environments affected by various factors. One example is the Champotón river in southwestern Mexico, which is poorly studied but is under pressure from natural phenomena (like hurricanes) and anthropogenic hazards (like agriculture). Anthropogenic hazards also include pesticides; cholinesterase activity in A. aeneus could possibly be used as a biomarker of pesticide exposure, but field research is inconclusive despite notable laboratory results. Taxonomy When originally named, A. aeneus was designated Tetragonopterus aeneus by Albert Günther in 1860. In 1921, Carl H. Eigenmann reassigned Tetragonopterus aeneus to the genus Astyanax, which is where it currently belongs, but as a subspecies of Astyanax fasciatus, labeled Astyanax fasciatus aeneus. This is now considered an obsolete designation, and A. aeneus is recognized as its own species within Astyanax. In 1990, various specimens of A. aeneus were designated a new species, Astyanax armandoi, but the morphological characteristics said to warrant differentiation were found to be in line with standard inter- and intraspecies variation of A. aeneus, so the name Astyanax armandoi is now considered a junior synonym. Further synonyms include Tetragonopterus humilis and Tetragonopterus microphthalmus, both given by Günther in 1864. Given its widespread and hardy nature, various species were once grouped together with