Baby sea stars may look innocent and adorable, but they’re teensy little cannibals and eat their own siblings for their own survival, according to a new study. Two researchers discovered this behavior among baby Forbes’ sea stars (Asterias forbesi) by accident. They were originally trying to understand how baby sea stars reacted when introduced to ferocious crab predators in the lab. “But they all started eating each other before we even introduced the crabs. So we had to scrap that experiment,” Jon Allen, associate professor in William & Mary’s Department of Biology, said in a statement. So Allen and his team shifted gears to observe this previously unknown phenomenon among the baby sea stars.
…While such behavior was unknown in this species, cannibalism is not uncommon in the animal kingdom, with more than 1,300 species (including humans) documented to exhibit it, according to the statement. And the researchers think that cannibalism is likely to be even more widespread among small animals, including juveniles.
From the New York Times in 2017 (content warning, refers to human cannibalism): Until relatively recently, the party line among scientists was that cannibalism occurred in only a few species in the wild, like black widow spiders and praying mantises. Cannibalism, researchers felt, was an aberrant behavior resulting from a lack of alternative forms of nutrition or the stresses associated with captive conditions.
But over the decades, evidence has been gathering for an alternative view. Cannibalism, it turns out, occurs in hundreds of species, perhaps thousands. The behavior varies in frequency between major animal groups — nonexistent in some, common in others. It varies from species to species and even within the same species, depending on local environmental conditions.
…Gary Polis, an ecologist at the University of California, Davis, came up with a list of cannibalism-related rules for invertebrates. Immature animals are consumed more often than adults, he found, and many species do not recognize individuals of their own kind (especially eggs and immature stages) as anything other than food.
He noted that cannibalism was more common in females than in males, and that as alternative forms of nutrition decrease in availability, incidents of cannibalism will increase. Lastly, in a given population, cannibalism is often directly related to the degree of overcrowding.
Life forms in which one sibling may dine on another, as mentioned above, include black widow spiders (“Our results showed significant levels of variation in cannibalistic propensity among 26 sibships, with some families cannibalizing full sibs within 2 d and other families waiting 3 wk before resorting to cannibalism”), as well as the small octopus Robsonella fontaniana, the usually vegetarian hippopotamus, and mud blister worms, which also seem fascinating in other ways.