Ecological remarks on Mastigodiaptomus nesus Bowman, 1986(Copepoda: Calanoida) in a Mexican karstic sinkhole

Mastigodiaptomus nesus(Bowman, 1986) was previously described from the West Indies, and its distribution was considered restricted to these islands. Recently it has been detected in several systems from the continental Yucatan Peninsula in Mexico. In this study, we contribute to the body size variability, abundance, distribution and vertical migration of Mastigodiaptomus nesus dwelling in a small, deep (264 m ², 47 m deep) karstic beaker-shaped sinkhole, located in this Peninsula. Here, this species is an important component of the zooplankton community during all the year. We analyzed the relationship between its abundance and concentration of nutrients (NO ⁻³), and Chlorophyll α. Differences in body size by sex (F = 67.56, p < 0.0001; n = 180), and sampled season (F = 6.59, p < 0.01; n = 180) were found. When animals from this sinkhole were compared with other localities, size difference was detected. Bigger animals were found in the West Indies, and smaller ones in continental localities. The nauplii were the most abundant development instars, followed by copepodids, and adults. In general, all developmental instars had a normal migration, with higher abundance at night time in upper layers (0.5 and 5 m), and diminishing towards daylight time. Direction of migrations seems to be towards the walls of the sinkhole (horizontal migration) instead of the deeper water layers, suggesting that these copepods are hidden in the irregular calcareous walls of the sinkhole. Abundance of total copepodid developmental stages were correlated with the Chl-α concentration. In addition, we found a highly significant difference between Chl-α concentration (F = 24.172, p < 0.001, n = 72), and NO ⁻³ concentration at all strata among seasons (F=25.46, p < 0.001, n = 60). Our results suggest that the body size of M. nesus is influenced by the nutrient concentration, and its vertical abundance and distribution may be explained by food availability.