Fluorescence in Asellus aquaticus (Isopoda: Asellota): a first approach Martin Zimmer,* Sabine Geisler, Sylvia Walter and Heinz Brendelberger Zoologisches Institut – Limnologie, Christian-Albrechts-Universität zu Kiel, Olshausenstr. 40, D-24098 Kiel, Germany ABSTRACT In the freshwater isopod, Asellus aquaticus (Isopoda: Asellota), fluorescing metabolic products, stored in specialized cells, cause intraspecific variation in individual visibility. In many popu- lations, 50–80% of isopods exhibit increased visibility under natural light conditions, which increases predation risk to these individuals. Furthermore, fluorescing isopods exhibit different behaviour with respect to sheltering. These individuals would be expected to be out- competed by their non-fluorescing conspecifics. However, assortative mating of fluorescing versus non-fluorescing isopods warrants reproduction in both phenotypes. We hypothesize possible causes of the isopods’ fluorescing appearance and present results that allow the predicted consequences to individual isopods to be tested. Keywords: assortative mating, behavioural change, colour change, endoparasites, honest signal, metabolite storage, parasite-induced changes, predation risk. INTRODUCTION In the freshwater isopod, Asellus aquaticus (Isopoda: Asellota), 50–80% of individuals from different sites in the vicinity of Kiel, Germany, display yellowish dorsal stripes along both sides of their posterior body (Fig. 1a) that have not previously been described. In addition to being conspicuous under natural light, these stripes exhibit strong fluorescence of green to yellow light (550 nm < λ < 750 nm) when observed under UV (300 nm < λ < 400 nm) (Fig. 1b). Fluorescing yellow stripes on the back of a brownish animal crawling on dark ground enhance its visibility in shaded freshwater (authors’ observations under a natural light regime) and may make it more prone to predation (cf. Bakker et al., 1997). The observed high proportion of fluorescing individuals in natural populations is, therefore, surprising. Our aim here is to explain the observed intraspecific variation in coloration by discussing possible causes and by hypothesizing consequences that can be tested experimentally: 1. Fluorescence is caused, through some unknown mechanism, by parasitic endosymbionts that need to be ingested by their final hosts together with isopods that serve as prey. In * Author to whom all correspondence should be addressed. e-mail: mzimmer@zoologie.uni-kiel.de Consult the copyright statement on the inside front cover for non-commercial copying policies. Evolutionary Ecology Research, 2002, 4: 181–187 © 2002 Martin Zimmer