Oddity, predation risk and social decisions in aquatic invertebrates

Group living is widespread across animal taxa, incurring benefits such as increased
foraging efficiency or an enhanced chance of surviving a predator's attack. The
chances of escaping a predator are often lower for odd‐looking individuals, as these
are detected at a higher rate than uniform looking group members. While this “oddity
effect” shall operate in animals differing in any given phenotype, including colour,
size or species identity, it has been experimentally tested mainly for odd‐coloured
individuals. We examined the oddity effect and swarming preferences in two differently
sized species of freshwater planktonic crustaceans (large Daphnia magna and
small Daphnia pulex). We experimentally investigated whether odd individuals in a
swarm of heterospecific Daphnia were more vulnerable to predation by a common
predator, the three‐spined stickleback (Gasterosteus aculeatus). Furthermore,
Daphnia's swarming preference was tested by giving individuals the opportunity to
choose between conspecific/heterospecific odour and a neutral control. In contrast
to the predictions of the oddity effect, odd individuals were not always preyed on
earlier; instead three‐spined stickleback preferentially predated large, more nutritious
individuals. Daphnia of both species reacted towards the perception of con‐ and
heterospecifics odours. While D. pulex generally avoided the smell of other daphnids,
D. magna avoided conspecifics but tended to prefer heterospecifics over the neutral
control. These findings provide new insights into swarming strategies and social preference
of an invertebrate and how this behaviour can influence predation risk.