SubBioLab

Research / examples of our research

Niches within a niche

Evolution of subterranean animals has mostly been regarded as gradual transformation from non-adapted or pre-adapted surface ancestors to highly adapted subterranean specialists. Only recently has it become clear that in caves, and the subterranean environment in general, a second type of evolutionary process is taking place as well. This process is divergent evolution of lineages or species that already have become cave dwellers in an earlier phase, and are diversifying and adapting to different ecological niches, all within the subterranean realm. It is the same process – known also as adaptive radiation – that takes place after a new island is colonized by the first representative of a taxon, or after a key innovation (like flight) evolves that enables the use of previously unexploited resources.

Our research has shown that in the large amphipod genus Niphargus assemblages of co-occurring species tend to be morphologically more different than if assembled by chance from the regional species pool (Fišer et al. 2012). This is an indication that competition between co-occurring species is driving them to evolve different functional morphologies to exploit different ecological niches.

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Modified from Evolution (2012), vol. 66, 3852–3865: Ecomorphological convergence of cave communities.

Further investigation revealed the relationship between morphology of species and the type of cave-microhabitats they occupy (Trontelj et al. 2012). We used two physical dimensions to describe aquatic microhabitats within a cave: (1) size of available spaces, and (2) velocity of water currents. Morphometric analyses grouped the species into four distinct ecomorph classes – a concept first used to explain the recurring pattern of morphological diversity of Anolis lizards on different Antillean islands. Niphargus ecomorphs were consistently associated with a certain type of microhabitat: small pore habitats (cave interstitial and epikarst), cave streams, and cave lakes (with two differently-sized ecomorphs).

Modified from Evolution (2012), vol. 66, 3852–3865: Ecomorphological convergence of cave communities.

Each ecomorph class (left tree; grouping by morphological similarity) has multiple in-dependent phylogenetic origins (right tree; molecular phylogeny). Individual traits are associated with microhabitat properties and thus might represent adaptations to a particular dimension of internal cave microhabitats, rather than to the subterranean environment in general. For example, long antennae and legs are typical only of low water currents. These results shed new light on the morphological diversity of subterranean animals. What appear to be different degrees of cave-adaptation could in fact be adaptations reflecting fine-level niche partitioning within caves.

Photo Credit: Gergely Balázs, the Proteus Project.

With its up to three centimeters long body and even longer antennae, Niphargus balcanicus is a representative of the extreme "daddy-longleg" ecomorph. It is a predator from deep cave lakes, found only in a few caves in Eastern Herzegovina.

Taxonomy and Systematics

In spite of intensive taxonomical work that contributed to the recognition of a number of true subterranean dwellers (i.e. troglobionts) in Slovenia and worldwide, a great part of the subterranean biota remains still unidentified and/or un(der)explored. By describing new species and their phylogenetic relationships we are providing fundamental tools for conservation and biodiversity studies.

Integrative studies of the largest European freshwater amphipod genus Niphargus showed that this chiefly subterranean group is in dire need of a comprehensive revision (Fišer et al. 2009, 2010). All previously proposed taxonomic subdivisions of the genus were rejected. In order to assist collaborative effort towards a most appropriate taxonomic solution, an interactive Niphargus-Website was created.

Our contribution to the ongoing web-revision of the genus Niphargus. All data are freely available in the DELTA (DEscription Language for TAxonomy) file

In the freshwater cave shrimp Troglocaris and the isopod crustacean Monolistra, our morphometric analyses revealed new diagnostic characters for lineages and species. Combining molecular and morphological data, we described several new species (Sket and Zakšek 2009, Prevorčnik et al. 2010, Jugovic et al. 2012).

The distribution of species in the subgenus Microlistra matches current river systems better than palaeo-hydrographically defined basins. Modified from Zoological Journal of the Linnean Society(2010), vol. 159, 1–21.

The common freshwater isopod Asellus aquaticus has invaded subterranean waters several times independently (Verovnik et al. 2005). One of these invasions gave rise to the first European subterranean species of the genus, A. kosswigi from NE Italy and W Slovenia (Verovnik et al. 2009). Asellus is becoming increasingly popular as model organism in developmental, evolutionary and ecotoxicological studies (Protas et al. 2011).

A highly polymorphic population of Asellus aquaticus showing surface and cave phenotypes from the cave Črna jama (Slovenia). Photo Credit: Boris Sket