Cannibalism of newly-metamorphosed juvenile sea stars

Cannibalism is widespread across the animal kingdom, occurring in more
than 1300 species of invertebrates and vertebrates across terrestrial and
aquatic habitats (Fox, 1975; Polis, 1981; Elgar and Crespi, 1992).
Cannibalism, defined as the consumption of all or part of a conspecific
individual, can occur during multiple stages in the life cycle of an
organism (Elgar and Crespi, 1992).  As such, cannibalism
potentially affects animal behavior, life history strategies, population
size and reproductive success (Cushing et al., 2015). Intra- and
inter-specific competition, overcrowding, lack of food, or poor quality
food are all stressors that induce cannibalistic behavior (Elgar et al.,
1992). Cannibalistic individuals should even be selected for since those
individuals would gain nutritional resources with high stoichiometric
compatibility and face decreased intraspecific competition (MacArthur et
al., 1966; Mitra & Flynn, 2005). Acquisition of high quality
nutrients and reduced competition lead to increased growth efficiency and
survivorship (Mitra & Flynn, 2005), so cannibals are more likely
to survive high conspecific recruitment years and low prey recruitment
years. As a result, cannibalistic individuals are more likely to
successfully reproduce and spread cannibalistic tendencies throughout
their population, a strategy that has recently been shown to be
evolutionarily stable (Cushing et al., 2015). While cannibalism is
therefore both predicted and observed to be widespread in nature, we have
recently observed cannibalism in an unexpected setting: among newly
metamorphosed juvenile sea stars (Figure 1; VideoS1). Cannibalism among
juveniles has been reported in Arachnids, Insects, Amphibians and Reptiles
(reviewed by Elgar and Crispi, 1992), but has only rarely been reported or
described in marine invertebrates (but see Byrne,
1996).