Flexible habitat selection and interactive habitat segregation in the marine congeners Idotea baltica and Idotea emarginata (Crustacea, Isopoda)

Abstract

Habitat segregation among competing species is widespread yet very little is know how this is achieved in practice. In a case study, we examined short-term effects of conspecific and congeneric density on habitat selection in two competing marine isopod species, Idotea emarginata and Idotea baltica. Under semi-natural conditions in large outdoor cylindrical tanks (4 m high; volume 5.5 m 3 ), animal groups of different size and composition had the choice between a set of relevant habitat samples (surface-floating seaweed, the water column, seaweed on the bottom). Habitat selection in both I. baltica and I. emarginata proved to be largely independent of conspecific density (level of intraspecific competition). In single-species treatments, both species showed a similar and stable pattern of distribution, with a clear preference for seaweed on the bottom. In mixed-species treatments (MST), however, the species were largely separated by habitat. While the distribution of I. emarginata was completely unaffected by the mere presence of interspecific competitors, habitat selection of I. baltica changed notably when I. emarginata was present. The habitat use patterns observed in MST conformed to those realized in geographical areas where the two species overlap in distribution: I. emarginata is dominant among decaying seaweed on the sea floor, and I. baltica is the dominant species among surface-floating seaweed. Our findings suggest that habitat segregation between the two species is essentially interactive, resulting from rapid decision-making of I. baltica with respect to habitat selection. The underlying mechanism is discussed. I. emarginata is highly superior to I. baltica in interference competition and rapidly eliminates the latter from one-habitat systems which do not allow I. baltica to escape from this interaction. In more natural, heterogeneous environments, however, I. baltica seems to be able to coexist with the superior competitor due to its broader habitat niche, flexibility in habitat selection, and a behavioural disposition to avoid normally preferred habitats when these are occupied by I. emarginata.

Grazing Impact Of, and Indirect Interactions Between Mesograzers Associated With Kelp (Laminaria Digitata)1

“…This finding suggests that the indirect positive effect of L. vincta grazing may be persistently beneficial for mesograzers that are not associated with L. digitata . I. emarginata is classified as a good swimmer (Ingolfsson and Agnarsson 2003) that is able to move between its preferred habitat—that is, decaying kelp on the seafloor (Franke et al. 2007)—and attached specimens of kelp.…”

“…1990). The second species of isopod used in this study, I. emarginata , prefers decaying seaweeds on the seafloor as a habitat (Franke et al. 2007) but is an active swimmer (Ingolfsson and Agnarsson 2003) and can move between its preferred habitat and the Laminaria zone.…”

Herbivore-induced defence response in the brown seaweedFucus vesiculosus(Phaeophyceae): temporal pattern and gene expression

“…The isopod Idotea baltica is a littoral mesograzer species (Orav-Kotta & Kotta, 2004), but is also abundant among drift Fucus and Ascophyllum thalli (Franke et al, 2007). All isopods used for this study were taken from an I. baltica culture fed with A. nodosum and customary fish food and maintained in a constant temperature room at 15°C in an aerated 200 l flow-through tank with a 12/12 h light/dark cycle.…”

Distribution of four isopods (Peracarida: Crustacea) in shallow waters of the Bizerte Lagoon (Tunisia, SW Mediterranean Sea)

“…Kotta et al (2000) proposed that the decline of I. chelipes probably reflects the decreasing cover for benthic vegetation in the Gulf of Riga since other phytophilous species have declined. Ingólfsson and Agnarsson (2003), studying dispersal and movements of isopods and amphipods during high tide, and Franke et al (2007) in their work about habitat selection and habitat segregation in I. balthica and I. emarginata, showed that I. balthica was the dominant species among surface-floating algae.…”