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

Abstract

Bosmina (Cladocera) populations, especially within the subgenus Eubosmina, show a variety of phenotypes that exhibit large differences in body size and shape and antennule length. In some populations, the morphological traits also vary during the season, with the most extreme forms occurring in periods with high densities of certain invertebrate predators. However, while temporal phenotypic variation in other cladocerans, as in the family Daphnidae, has been shown to be an adaptation to reduce the risk of predation by invertebrate 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 Bosmina forms (subgenus Eubosmina), differing in morphology from each other, which are found in lakes together with the predator Leptodora kindtii (Cladocera). Bosmina (E.) longispina has a relatively low and elongated carapace with a caudal mucro, and short antennule, B. (E.) coregoni gibbera has a higher 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 Leptodora in Petri dishes. In a second experiment, we observed directly the escape efficiency of B. longispina, B. gibbera and B. retro extensa, and the handling time of Leptodora. The two Bosmina forms with more extreme morphological features had a lower death rate and higher escape efficiency than B. longispina. Prey that escaped did so, in most cases, within 5 min. Predator handling time was correlated to predator body length and antennule length of the prey. The results suggests that Bosmina species with extreme morphological traits may be less vulnerable to invertebrate predators.

Changes to zooplankton community structure following colonization of a small lake by Leptodora kindti

“…Late summer populations of Ceriodaphnia and Bosmina that overlap temporally with Leptodora were noticeably absent only 1 yr after the introduction. Hellsten and Stenson (1995) also observed a rapid disappearance of Bosmina longispina in a Swedish lake when Leptodora populations were dense. The rapid shift in size structure and near extirpation of several small-bodied cladoceran species following the introduction of Leptodora is comparable to that observed when Chaoborus colonizes a lake.…”

“…With an increase in Leptodora abundance, Bosmina became larger-bodied and the population shifted from short-featured (mucro, antennule length) to long-featured morphs. Bosmina morphs with long features have lower death rates and higher escape efficiencies than short-featured morphs (Hellsten et al 1999).…”

Trait- and size-based descriptions of trophic links in freshwater food webs: current status and perspectives

“…burrows in benthic species and larval cases in caddisflies. Immobilization and thanatosis are efficient against predators that rely on movement stimuli (Pritchard, 1965) and may also allow prey to confuse the predator and subsequently escape (Hellsten et al, 1999;Gyssels and Stoks, 2005;Scarton et al, 2009). Behavioural responses may vary between and within taxa; recent research has linked intraspecific variation in prey vulnerability and defensive behaviour to behavioural syndromes (Sih et al, 2004).…”

Morphological variation of the predatory cladoceran Leptodora kindtii in relation to prey characteristics

“…Induced defences in cladoceran prey strongly influence the effectiveness of Leptodora attacks: "defended" forms of Bosmina spp. (Hellsten et al 1999) and Daphnia cucullata (Agrawal et al 1999) were shown to be much less vulnerable than the ones without defences, thus adaptive responses of Leptodora for changing prey morphology were expected.…”