Ontogenetic shifts in the migratory behavior of Chaoborus flavicans MEIGEN: field and experimental evidence

Wide range diel vertical migrations of fourth-instar Chaoborus larvae have been shown in many field and experimental studies, but little comparative i nformation is available for the pupae and first three larval instars. In th is study we monitored the midday and midnight depth distributions of differ ent instars in a stratifying lake with abundant planktivorous fish as well as in experimental chambers with and without fish kairomones. In both the f ield population exposed to natural fish predation in the lake, and experime ntal animals exposed to fish kairomones, we found that the range of migrati on changed ontogenetically in relation to size/instar-specific vulnerabilit y to predation, from a nonmigratory behavior among the 1st and 2nd instar l arvae, to an intermediate-range diel migration in the 3rd instar larvae, an d a wide-range migration in the 4th instar larvae and pupae (no experimenta l support available for pupae), that are largest and thus most vulnerable p rey to planktivorous fish. The midday depth and the midnight depth selected by the late larvae and the pupae, were also both greater than those select ed by the early larvae. Under laboratory conditions, the range of migration in the 3rd and 4th inst ar larvae from the field population in the lake was significantly smaller i n animals transferred to a medium free of fish kairomones than in those tra nsferred to a medium with fish kairomones, but the migration behavior did n ot fade away completely in 17 days, and nor did the midday depth selected b y larvae decrease to the level observed in animals grown in kairomone-free medium from the time of hatching. The reverse shift in behavior was much fa ster: when naive 4th instar larvae grown in fish kairomone-free medium were transferred to medium with fish kairomones, the midday mean depth selected by these larvae increased within 12 h, and their behavior was changed from nonmigratory to migratory in 48 h.