RESUMEN
The degree of dietary specialization has a fundamental impact on the ecological function and interactions of suspension feeders. While niche differentiation by food particle size is common among obligate suspension feeders, its role is not evident in facultative ones. In this study, we aimed at providing new insights on the matter by focusing on sympatric mysid species. As mysids use different mechanisms for raptorial and filter-feeding, they represent a more adequate model system than for example, the more extensively studied copepods. We made morphological measurements on the 4 coexisting invasive Ponto-Caspian mysid species to determine the areas and mesh sizes of their filters. We also quantified their clearance rates on the microalga Cryptomonas sp. in a laboratory experiment to reveal how morphological differences manifest in their overall filtering capacity. We found relatively small but consistent differences in the primary filter area among the species, indicating that morphological constraints due to the enclosed position of the setae might limit the possibility for differentiation. The primary filter mesh sizes were small in all 4 species (0.69-2.73 µm) with moderate but consistent intraspecific differences, suggesting that the benefit of being able to capture small particles might outweigh the pressure for differentiation. The observed clearance rates were in accordance with the morphological characteristics of the species, highlighting that auxiliary filters (present in one of the species, Limnomysis benedeni) are needed to increase filtering capacity considerably. Our study confirmed that food particle size can contribute to the niche differentiation of facultative filter feeders, but also indicated that they can tolerate a higher overlap than obligate ones. The observed differences were related to the habitat preferences and predatory potentials of the species, suggesting that complementarity among the different niche axes might further facilitate their coexistence.
RESUMEN
Suspension feeders play pivotal roles in the nutrient cycling of almost all aquatic ecosystems. Since sufficiently large differences in the filter mesh size (FMS) can lead to different food web positions, the inter- and intraspecific variability of this trait might be of community-level importance. The aim of this study was to quantify the range of FMS variation within the three invasive Ponto-Caspian Chelicorophium species based on a large material representing various conditions (1,224 specimens from 40 samples across Central Europe), characterize the components of variation within populations, identify the main factors determining intraspecific differences, and reveal how intraspecific variation affects the FMS overlaps among species. The FMS of the most widespread invader, C. curvispinum, varied within the broadest range (between 2.34-8.28 µm, compared to 2.51-5.97 µm in C. robustum and 1.08-3.23 µm in C. sowinskyi); nevertheless, the contribution of intraspecific plasticity to the invasion success of the species is not evident based on the present study. The within-individual variability of FMS increased with the individual mean of the trait and decreased with body size; however, it showed little differences among samples. The among-individual variation within samples could be partitioned into components related to body size (ontogenetic niche shift/differences among cohorts) and sex (ecological sexual dimorphism) as well as a seemingly random component (individual specialization), varying widely in extent and relative contributions. The FMS of C. curvispinum was significantly larger in the presence of C. sowinskyi than in allopatry, likely reflecting character displacement; however, it did not show further increase when C. robustum was also present. Similar differences could not be observed in C. sowinskyi. The FMS ranges of C. curvispinum and C. robustum never overlapped with that of C. sowinskyi in co-occurrence despite the considerable intraspecific differences among sites, suggesting that their interaction can be seen as a clear case of niche differentiation by food particle size. On the contrary, the strong overlaps observed between C. curvispinum and C. robustum indicate that other factors might play the primary role in their coexistence. The studied species appear to be suitable model organisms for identifying the drivers and mechanisms of FMS variability.
RESUMEN
Groundwater is an extreme environment due to its absence of light, resource scarcity and highly fragmentary nature. Successful groundwater colonizers underwent major evolutionary changes and exhibit eye and pigment loss (troglomorphies). Consequently, their chances of dispersal and survival in the well-connected surface waters are greatly decreased, resulting in significant endemism. The West Palaearctic subterranean amphipod genus Niphargus comprises hundreds of narrowly endemic and troglomorphic species. Nevertheless, a few are known to occur in surface waters, two of which, N. hrabei and N. valachicus, have extremely large ranges that even exceed those of many surface-water amphipods. We tested if this pattern results from a secondary colonization of the relatively well-connected epigean environment, and whether this ecological shift promoted the large-scale dispersal of these species. Results showed that despite their ecological and zoogeographic similarities, N. hrabei and N. valachicus are not closely related and independently colonized surface waters. Their phylogeographic patterns indicate Middle to Late Pleistocene dispersal episodes throughout the Danube lowlands, and relatively modest yet significant genetic differentiation among populations. Clustering based on morphology revealed that the two species are phenotypically closer to each other than they are to most other epigean congeners. We presume that the ecological shift to surface environments was facilitated by their ability to thrive in hypoxic waters where rheophilic competitors from the family Gammaridae cannot survive. In conclusion, our results indicate that adaptation to groundwater is not a one-way evolutionary path and that troglomorphic species can occasionally recolonize and widely disperse in surface waters.