Microhabitat use, population densities, and size distributions of sulfur cave-dwellingPoecilia mexicana

Abstract

The Cueva del Azufre in Tabasco, Mexico, is a nutrient-rich cave and its inhabitants need to cope with high levels of dissolved hydrogen sulfide and extreme hypoxia. One of the successful colonizers of this cave is the poeciliid fish Poecilia mexicana, which has received considerable attention as a model organism to examine evolutionary adaptations to extreme environmental conditions. Nonetheless, basic ecological data on the endemic cave molly population are still missing; here we aim to provide data on population densities, size class compositions and use of different microhabitats. We found high overall densities in the cave and highest densities at the middle part of the cave with more than 200 individuals per square meter. These sites have lower H2S concentrations compared to the inner parts where most large sulfide sources are located, but they are annually exposed to a religious harvesting ceremony of local Zoque people called La Pesca. We found a marked shift in size/age compositions towards an overabundance of smaller, juvenile fish at those sites. We discuss these findings in relation to several environmental gradients within the cave (i.e., differences in toxicity and lighting conditions), but we also tentatively argue that the annual fish harvest during a religious ceremony (La Pesca) locally diminishes competition (and possibly, cannibalism by large adults), which is followed by a phase of overcompensation of fish densities.

Insights into the Social Behavior of Surface and Cave-Dwelling Fish (Poecilia mexicana) in Light and Darkness through the Use of a Biomimetic Robot

“…The size of the replica (SL: 35 mm) was derived from the mean standard length of all test fish (ranging between 28.77 and 49.03 mm). The replica was situated 0.5 cm below the water surface in accordance with the closeto-surface swimming behavior observed for both populations in the wild (Jourdan et al, 2014). We programmed RoboFish to an average speed of 10 cm/s (maximum speed of 27 cm/s).…”

Genome‐scale data reveal that endemic Poecilia populations from small sulphidic springs display no evidence of inbreeding

“…Considering the large difference in AICc between our top model and the rest of the models, we focused on the model with the lowest AICc. The parameters from ∂a∂i were converted to numbers of individuals or years by assuming a mutation rate of 6.6 × 10 −8 mutations per site per generation (Recknagel, Elmer, & Meyer, ) and a generation time of 6 months (Jourdan et al., ; Riesch et al., ). The mutation rate we used was based on mutation rates in cichlids (Recknagel et al., ); the actual mutation rate for the populations in our study may vary, which could lead to differences in calculated divergence times and effective population sizes.…”

Social networks in the presence and absence of visual cues

“…By comparison shoaling behaviour seems to have disappeared completely in our cave mollies and it is estimated that ancestral surface mollies entered this cave approximately 100 ka ago [10]. The consequences of this complete lack of shoaling behaviour (despite the very high densities of fish, in some chamber more than 200 individuals per square metre [43]) for the foraging ecology and mating behaviour are largely unknown and present interesting opportunities for future research. It is known from lab-reared populations of cave mollies that they show shoaling behaviour under daylight conditions and are only asocial under low-light conditions (in contrast to surface mollies which maintain some shoaling behaviour under low-light conditions too, [11]).…”