Can extreme morphology in Bosmina reduce predation risk from Leptodora? Anexperimental test

Bosmina (Cladocera) populations, especially within the subgenus Eubosmina, show a variety of phenotypes that exhibit large differences in body size an d shape and antennule length. In some populations, the morphological traits also vary during the season, with the most extreme forms occurring in peri ods with high densities of certain invertebrate predators. However, while t emporal phenotypic variation in other cladocerans, as in the family Daphnid ae, has been shown to be an adaptation to reduce the risk of predation by i nvertebrate predators, the reason for such changes in Bosmina is much less clear. We examined whether certain morphological traits in Bosmina species could act as a defence against invertebrate predators. We tested three Bosm ina forms (subgenus Eubosmina), differing in morphology from each other, wh ich are found in lakes together with the predator Leptodora a kindtii (Clad ocera). Bosmina (E.) longispina has a relatively low and elongated carapace with a caudal mucro, and short antennule, B. (E.) coregoni gibbera has a h igher and more protruding carapace without caudal mucro, and a much longer antennule. Finally, B. (E.) coregoni retro extensa has a carapace like that of B. longispina but with no caudal mucro and a much longer antennule. In one experiment, B. longispina and B. gibbera were exposed for 12 h to Lepto dora in Petri dishes. In a second experiment, we observed directly the esca pe efficiency of B. longispina, B. gibbera and B. retro extensa, and the ha ndling time of Leptodora. The two Bosmina forms with more extreme morpholog ical features had a lower death rate and higher escape efficiency than B. l ongispina. Prey that escaped did so, in most cases, within 5 min. Predator handling time was correlated to predator body length and antennule length o f the prey. The results suggests that Bosmina species with extreme morpholo gical traits may be less vulnerable to invertebrate predators.