Morphological similarity and ecological overlap in two rotifer species.

Carmen Gabaldón,Javier Montero-Pau,Manuel Serra,María José Carmona

doi:10.1371/journal.pone.0057087

Abstract

Co-occurrence of cryptic species raises theoretically relevant questions regarding their coexistence and ecological similarity. Given their great morphological similitude and close phylogenetic relationship (i.e., niche retention), these species will have similar ecological requirements and are expected to have strong competitive interactions. This raises the problem of finding the mechanisms that may explain the coexistence of cryptic species and challenges the conventional view of coexistence based on niche differentiation. The cryptic species complex of the rotifer Brachionus plicatilis is an excellent model to study these questions and to test hypotheses regarding ecological differentiation. Rotifer species within this complex are filtering zooplankters commonly found inhabiting the same ponds across the Iberian Peninsula and exhibit an extremely similar morphology—some of them being even virtually identical. Here, we explore whether subtle differences in body size and morphology translate into ecological differentiation by comparing two extremely morphologically similar species belonging to this complex: B. plicatilis and B. manjavacas. We focus on three key ecological features related to body size: (1) functional response, expressed by clearance rates; (2) tolerance to starvation, measured by growth and reproduction; and (3) vulnerability to copepod predation, measured by the number of preyed upon neonates. No major differences between B. plicatilis and B. manjavacas were found in the response to these features. Our results demonstrate the existence of a substantial niche overlap, suggesting that the subtle size differences between these two cryptic species are not sufficient to explain their coexistence. This lack of evidence for ecological differentiation in the studied biotic niche features is in agreement with the phylogenetic limiting similarity hypothesis but requires a mechanistic explanation of the coexistence of these species not based on differentiation related to biotic niche axes.

Highlights

In the last decade, molecular approaches have revealed great biological diversity in the form of cryptic species [1,2,3]
Functional response The log concentration of both T. suecica and N. atomus decreased linearly (R2.0.68) with increasing incubation feeding time for both rotifer species (Fig. 1), indicating that the experimental food concentrations were below the incipient limiting level (ILL) [58], the threshold food concentration up to which clearance rates remain constant
The power analysis demonstrated that, given our data variance, the difference between the average clearance rates of the two rotifer species would be need to be 30% to detect a statistically significant difference at the 5% significance level for rotifers fed T. suecica, whereas a 15% difference would be required when using N. atomus as food. Both B. plicatilis and B. manjavacas were three times more efficient when feeding on T. suecica than on N. atomus

Summary

Introduction

Molecular approaches have revealed great biological diversity in the form of cryptic species [1,2,3] Cooccurrence of these species is common [4] and raises important questions, especially in terms of their coexistence and ecological similarity. There are alternative processes not based on biotic niche axis differentiation that could explain the co-occurrence of ecologically similar species, such as density-dependent life-history adjustments [11,12] or those invoked by neutral models [13]. In cryptic species, this differentiation either does not exist or is subtle. The study of co-occurring cryptic species can illuminate the existing mechanisms, aid in the discovery of new ones, and offer the opportunity to experimentally quantify concepts such as limiting similarity

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Conclusion