Divergence in Diel Vertical Migration of Mysis: Individual Choice or Fixed Behavior?

Mysids are mid-trophic level omnivores that undergo diel vertical migration (DVM) and can exert strong effects on aquatic community composition and function. Mysids are omnivorous and therefore capable of foraging in both benthic and pelagic habitats, providing the opportunity to be flexible in their habitat choice. Divergent DVM behavior in Mysis populations, whereby some portion of the population remains on the bottom at night and/or remains suspended during the day, appears to be common. This may be a plastic (e.g., hunger-satiation, body condition, reproductive stage) or fixed behavior. To evaluate, we used stable isotopes to test for differences in the foraging ecology of Mysis collected on the bottom and in the pelagic zone at night. If DVM is a plastic behavior, there should be no differences in the δ¹³C and δ¹⁵N signatures between the two sample groups. If the divergent patterns represent fixed behaviors, there should be significant differences in δ¹³C and δ¹⁵N values between Mysis collected on the bottom (benthic-based diet) and in the pelagic zone (benthic- and pelagic-based diet). We tested these hypotheses using Mysids collected from Lake Champlain in spring and summer of 2012. If results suggest DVM is a plastic behavior, then future work should focus on the conditions under which individual Mysis choose to migrate. Alternatively, if results suggest DVM is a fixed behavior, future work should focus on processes such as adaptation to migrating or non-migrating behavior and genetic differentiation. In either case, the omnivorous nature of Mysis provides a complementary template on which to examine DVM behavior of mid-trophic level organisms compared to more widely-studied obligate planktivores.