Stable isotope analysis of dietary differences of two sympatric Baltic mysis species

摘要

Two mysis species groups occur sympatrically in the open-sea areas of the northern Baltic Sea. The northern distribution of Mysis mixta is at the shallow transition zone between the Bothnian Sea and the Bothn-ian Bay, whereas Mysis relicta occurs throughout the northern Baltic Sea region (SALEMAA et al. 1990). Gut content analyses have indicated that both of these species are omnivorous, feeding mostly on copepods, cladocerans, rotifers, detritus and algae in both pelagic and benthic habitats (VIHERLUOTO et al. 2000, GROSSNICKLE 1982). Although the two species occupy similar habitats, some dietary differences have been observed. In the Baltic Sea, M. mixta has been proposed to be predominantly carnivorous (VIHERLU-OTO et al. 2000, HANSSON et al. 1990, RUDSTAM & HANSSON 1990, RUDSTAM et al. 1986) in contrast to M. relicta, which has been shown to rely more on phytoplankton (VIHERLUOTO et al. 2000). However, to our knowledge only one study has extensively investigated dietary differences of these two species. VIHERLUOTO et al. (2000) found that gut contents of M. mixta and M. relicta overlapped by 75%. The main difference was due to M. mixta ingesting more zoo-plankton and pelagic material than M. relicta. They also found that the diet of M mixta shifted gradually from sedimented phytoplankton to a more pelagic and carnivorous diet during growth.