Morphologische Plastizität

The aim of this study was to reveal factors affiliated with
morphological plasticity in Daphnia cucullata. This planktonic
crustacean shows a seasonal variation in body shape that is
commonly known as cyclomorphosis. Here I show that both
predator-released chemicals and water-soluble “alarm” signals from
wounded Daphnia can induce certain morphological features in
Daphnia cucullata. Specifically, the inductive effect of the
“alarm” signals is minute compared to the predator-exposed animals.
Moreover, laboratory-generated small-scale turbulence was
identified to be a proximate factor in cyclomorphosis. This special
kind of turbulence is compared to turbulence evoked by the movement
of other organisms. A distinct protective effect of the
morphological traits against invertebrate predation was revealed as
the likely ultimate factor of cyclomorphosis. Furthermore, a unique
method of ultrasonic microscopy reveals “hidden” morphological
plasticity in daphnids. The armor of predator-exposed animals is
much stronger than that of those not exposed to predators. I
developed a new SEM-preparation method to get detailed insight in
ontogenesis in Daphnia. I detected morphological defenses in
predator-chemical-induced Daphnia cucullata already in the last
embryonic stage. Induction experiments on transgenerational effects
in Daphnia cucullata show that defended mothers release offspring
that are better defended than the descendants of unthreatened
parents. Furthermore, daphnids could asses their actual predation
risk by taking both predator and prey density into account.
Thereafter the degree of a defense is adjusted to the individual
hazard, thus saving unnecessary costs associated with the
development of a particular defense. The combined results of this
study indicate that cyclomorphosis in Daphnia cucullata has evolved
as an adaptation to variable environments.