Dispersal and colonization of Rotifera in the Chihuahuan Desert: A mesocosm experiment

Background/Question/Methods Scattered across the Chihuahuan Desert are a variety of catch basins that serve as habitat for zooplankton. These vary from tiny ephemeral huecos and seeps to large permanent reservoirs and rivers. In this arid region, where there is little to none intra-site connectivity among basins, rotifer diapause stages must disperse among sites through agencies other than hydrochory; in monogononts these stages are diapausing embryos (resting eggs), and in bdelloids they are xerosomes (anhydrobiotic adults). What are the relative roles of aeolian transport and zoochory as dispersal agents for rotifers in deserts? We investigated this question using three methods: mesocosms, insect surveys, and rehydration of dry sediments. Six covered (0.63 cm²) mesh) mesocosms (68 L of MBL) were set out in an isolated Chihuahuan Desert site. Samples were taken weekly (over 9 weeks) and examined for zooplankton. Dry pond sediments and aeolian dust were rehydrated and examined for several weeks for zooplankton. Insects (n = 360) were rinsed in MBL and the rinsates were incubated and examined for zooplankton. Results/Conclusions Algae and protists were present in mesocosms after one week whereas rotifers colonized by week 2. Species richness was variable between samplings and among mesocosms. Ten rotifer taxa (7 families) were found representing ca. 19% of the recorded 52 species and 18 families found at the site. No microcrustaceans were seen during the experiment. All rotifers colonizing the mesocosms had been previously found in temporary ponds in the area including the newly described Epiphanes chihuahuanensis. Of rotifers collected from the mesocosms, six also had been found in filled ponds and six had been hatched from dried sediments taken at the study site. These species richness levels rival those of two published mesocosm studies that ran much longer (several months or years). Rehydrated pond sediments and aeolian dust yielded algae, ciliates, rotifers (both monogononts and bdelloids), ostracodes, branchiopods, copepods, and nematodes. However, the species list represented only a small portion of the species inventory assembled from extant communities. None of the insect washes yielded rotifers, although algae and ciliates were recovered. In summary, we found that aeolian transport played a main role in dispersing rotifers, while zoochory via insects did not appear important. This work is part of a larger study to survey selected meso- and micro-invertebrates of a wide array of springs and ephemeral waters of the Chihuahuan Desert, both USA and México.