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Strange science explains why some animals are cool with cannibalism

An ancient way of life may explain a larger, morbid trend.

Walnut-shaped, transparent, and seemingly rave-ready, the warty comb jelly is a peculiar invertebrate with a 500-million-year old history. Today, the western Atlantic native populates oceans around the globe, invading new seas and out-competing local species.

A study published Thursday in Communications Biology presents a new theory for why warty comb jellies have been so successful, and for so long.

In order to survive harsh winters, warty comb jellies eat their own larvae, the study reports. This unsavory answer sheds light on how cannibalism has shaped some of the world's oldest creatures.

The finding also provides some explanation as to why more than 1,500 species use some type of cannibalism to survive. It's been recorded among squirrels, fish, dragonfly larvae, and chimps, among others. Because the ancestry of comb jellies goes back so far, the scientists propose that it's possible that cannibalism is a "basic, unifying feature across the animal kingdom."

This team observed that warty comb jellies give birth to a "bloom" of offspring in late summer. As winter rolls around, bringing with it a dearth of nutrients, young jelly populations typically suffer — but not the adults.

Those trends were puzzling: Why would an animal produce a burst of young late in the year if the larvae were unlikely to survive winter? The new findings clear things up: Those babies are for breakfast.

"This finding fills an important gap in knowledge as to how an invasive species exhibiting boom-and-bust behavior is able to survive long periods of nutrient scarcity," the team writes.

Evidence of cannibalism: The red arrows point to larvae within the auricles of an adult.Jamileh Javidpour/University of Southern Denmark

The boom-and-bust — The warty comb jelly strategy begins with this burst of larvae. Initially, the larvae help the species out-compete other fish and fish larvae by gobbling up all of the plankton and copepods in the area. Then, as all the species start to struggle to find food in the winter, the older watery comb jellies turn on the larvae, and gobble them up.

The shift to cannibalization in harsher conditions allows adult jellies to continue growing. It also explains why comb jelly populations fluctuate throughout the year.

Comb jellies are far from the only animal that eats their young when times get tough: Cannibalism is particularly common in water ecosystems, the study says. The practice tends to align with food scarcity, and in general, "juveniles are eaten more frequently than adults."

What's curious about the warty comb jelly is that adults and juveniles aren't competing for prey. Instead, they tag-team the decimation of prey populations to compete against other plankton and copepod-eaters.

That setup seems to have driven the spread of warty comb jellies across the oceans.

The research team also makes a case for studying the comb jellies not as individuals, but as "an intergenerational, multicellular organism."

"Since larvae cannot survive winters in the species’ northernmost habitats, our study suggests that the primary purpose of M. leidyi larvae is to gather and store energy and nutrients for adults," the study authors write.

Comb jellies are wonderfully weird — The mysterious invertebrates swim through the ocean with the help of cilia — also called "combs."

Warty comb jellies, Mnemiopsis leidyi, resemble a jellyfish that someone sketched on a Lite-Brite. Despite their appearance, they're in a totally separate phylum from jellyfish.

Perhaps even more unusual than the jellies' larva-eating strategy is its transient anus. The warty comb jelly's anus only appears when it has to go — then it disappears again when business is done. This puts warty comb jellies in a category of its own, somewhere between animals that have just one hole (for eating and defecating) and those with two.

With more than 500 million years of evolution behind them, warty comb jellies have had plenty of time to develop their peculiar evolutionary advantages — to the point where the strangest thing about these creatures, somehow, isn't the idea that it eats its young.

Abstract: The proliferation of invasive marine species is often explained by a lack of predators and opportunistic life history traits. For the invasive comb jelly Mnemiopsis leidyi, it has remained unclear how this now widely distributed species is able to overcome long periods of low food availability, particularly in their northernmost exotic habitats in Eurasia. Based on both field and laboratory evidence, we show that adult comb jellies in the western Baltic Sea continue building up their nutrient reserves after emptying the prey field through a shift to cannibalizing their own larvae. We argue, that by creating massive late summer blooms, the population can efficiently empty the prey field, outcompete intraguild competitors, and use the bloom events to build nutrient reserves for critical periods of prey scarcity. Our finding that cannibalism makes a species with typical opportunistic traits more resilient to environmental fluctuations is important for devising more effective conservation strategies.
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