Hidden Catfish of Mexico’s Biosphere Reserve Reveal Surprising Reproductive Secrets

Deep in the river canyons of the Tehuacán-Cuicatlán Biosphere Reserve in Oaxaca, Mexico, two nocturnal freshwater catfishes have been quietly going about their lives with almost no scientific scrutiny. Now, a new study published in Neotropical Biology and Conservation has pulled back the curtain on the Pale catfish (Rhamdia guatemalensis) and the Filespin chulín (Rhamdia laticauda), delivering the first detailed look at where these fish live within the reserve and how their reproductive biology works. The findings matter far beyond academic curiosity: these catfishes are predators of aquatic insects, prey for larger vertebrates, food for local communities, and candidates for aquaculture, yet until now their basic life histories in this region remained largely undocumented.

The research team, working across two field campaigns in 2005-2006 and 2015-2016, sampled twenty sites within the Río Quiotepec and Río Salado sub-basins of the Papaloapan River basin using electrofishing, a technique that stuns fish momentarily so they can be collected unharmed for measurement. Specimens were preserved in 70-90% ethanol and deposited in the Collection of Continental Fish at CIIDIR Unidad Oaxaca IPN, creating a permanent, vouchered record that future researchers can verify. In total, 302 catfishes were examined: 97 Pale catfish with a mean standard length of 94.74 mm and 205 Filespin chulín averaging 87.32 mm.

The distribution results paint a picture of restricted, habitat-specific occupancy. The Pale catfish was recorded in eight municipalities of the Oaxaca portion of the reserve, between 537 and 1031 meters above sea level, while the Filespin chulín appeared in five municipalities across a similar elevation band of 524 to 1031 meters. Both species share the same two sub-basins, suggesting their ranges within the reserve are narrow and tightly tied to particular habitats. Field observations revealed a clear ecological split: the Pale catfish favors pools and slow-flowing rivers with submerged vegetation and organic debris such as algae, trunks, leaves and branches, whereas the Filespin chulín is associated with rocky riffles and faster stream currents. This substrate partitioning likely reduces direct competition between the two species even where they co-occur.

Ecological specialization of this kind carries conservation weight. Previous dietary studies in the same area have shown that these catfishes feed heavily on insects, including mayflies, stoneflies and megalopterans, groups whose larvae are notoriously sensitive to water contamination. In effect, the catfishes function as indicators of stream health: if pollution degrades the insect communities they depend on, the fish themselves will decline. Because both species occupy only a small fraction of the 31 municipalities that make up the Oaxaca section of the reserve, any localized habitat degradation could disproportionately affect their populations, making continued monitoring a priority.

On the reproductive front, the study produced several firsts for these populations. The overall sex ratio of both species did not deviate significantly from the expected 1:1 ratio, based on chi-square tests. This finding is notable because earlier work elsewhere in Mexico told a different story: female-biased ratios of 2.44:1 and 2.63:1 were reported for the Pale catfish in Tabasco and Chiapas, respectively, while a male-biased ratio had been documented for the Filespin chulín in Chiapas. The authors suggest that such discrepancies among studies can arise from a combination of external factors, including geographic location, season, and fishing methods, as well as internal factors such as feeding habits, physiological state, mortality rates, and the greater reproductive effort of females.

Perhaps the most striking result concerns sexual size dimorphism. In both species, females were consistently larger than males in standard length and body mass. The sexual dimorphism index, calculated as the ratio of mean female to mean male measurements, reached 1.21 for standard length and 1.7 for body mass in the Pale catfish, and 1.07 and 1.26 respectively in the Filespin chulín. Length frequency distributions differed significantly between the sexes in both species, with a greater proportion of females falling in the 110-150 mm range for the Pale catfish and the 80-120 mm range for the Filespin chulín. This female-biased pattern aligns with what has been documented in the well-studied R. quelen, but it remains poorly documented across the family Heptapteridae more broadly, making these observations valuable comparative data points.

The evolutionary logic behind larger females is straightforward: bigger females can produce more eggs and achieve higher spawning success, a phenomenon known as fecundity selection. Smaller males, by contrast, may channel their energy into behaviors such as territorial defense rather than body growth. The authors also note that sex-specific dietary differences, previously documented for the Pale catfish, could further shape dimorphic patterns by influencing growth rates and how energy is allocated to reproduction. They propose that future studies apply geometric morphometrics to test for sexual shape differences, which could reveal subtler dimensions of dimorphism invisible to simple length and weight measurements.

Size at first sexual maturity, a critical parameter for setting minimum capture sizes in any future fishery, was estimated using generalized linear models fitted to binomial maturity data. Half of the Pale catfish population reaches maturity at 97.6 mm standard length, with 95% mature by 128.2 mm. The Filespin chulín matures earlier and smaller: 79 mm for the 50% threshold and 111 mm for the 95% threshold. The Pale catfish estimate fits neatly within the 80-100 mm range reported from other Mexican populations, but the Filespin chulín value is notably higher than the 64.5 mm previously recorded in Chiapas, illustrating how the same species can mature at different sizes across its geographic range depending on temperature, photoperiod, food availability, and the size range of specimens examined.

Fecundity estimates revealed impressive reproductive output. Female Pale catfish carried between 2,403 and 25,077 eggs, averaging 9,647 eggs per female, while Filespin chulín females averaged 1,457 eggs. Fecundity was significantly and positively correlated with ovary weight in both species, with Spearman rank correlation coefficients of 0.95 for the Pale catfish and 0.68 for the Filespin chulín. The authors caution, however, that because fecundity was estimated gravimetrically from ovary subsamples, part of this correlation may reflect methodological dependence between the variables, and the small sample sizes of 13 and 11 spawning-ready females warrant careful interpretation. Across the genus, fecundity clearly scales with body size: the larger R. quelen has been recorded producing between 9,120 and 69,440 eggs in Brazilian rivers.

Taken together, the results suggest these catfishes follow what ecologists classify as a seasonal life-history strategy: females larger than males, little or no sexual coloration differences, high fecundity, and an insectivorous diet that lets them exploit the most abundant trophic resources available as conditions change through the year. This strategy typically involves synchronizing reproduction with periods of highest precipitation, when food is plentiful and nursery habitats expand. The authors acknowledge limitations, including the pooled analysis of both sexes for maturity estimates and the absence of monthly sampling to pinpoint the spawning season, and they call for year-round monitoring to determine reproductive timing, spawning type, and egg size. For now, the study provides an essential baseline: as habitat degradation and invasive fish species continue to pressure Mexican freshwater ecosystems, knowing where these catfishes live, how big they get, when they mature, and how many eggs they produce is the foundation upon which any effective conservation or fisheries management plan for the Tehuacán-Cuicatlán Biosphere Reserve must be built.

Subject of Research: Distribution and reproductive biology of two Rhamdia catfish species in a Mexican Biosphere Reserve

Article Title: Distribution and first insight into the reproductive biology of two species of the genus Rhamdia ( Siluriformes , Heptapteridae ) in a Biosphere Reserve

Article References: Julián-Caballero, C. C., Cruz-Arenas, E., Martínez-Ramírez, E., Cruz-Ruiz, G. I., & Gómez-Ugalde, R. M. (2026). Distribution and first insight into the reproductive biology of two species of the genus Rhamdia (Siluriformes, Heptapteridae) in a Biosphere Reserve. Neotropical Biology and Conservation, 21(2), 131-148. https://doi.org/10.3897/neotropical.21.e187352

Image Credits: AI Generated

DOI: 10.3897/neotropical.21.e187352

Keywords: Rhamdia, catfish, Tehuacán-Cuicatlán Biosphere Reserve, Oaxaca, reproductive biology, sexual size dimorphism, fecundity, freshwater fish, conservation, sex ratio, size at first maturity, Neotropical