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May 16, 2012

One brush with comb jellies is not enough: They’re back!

Comb_jelly_blogComb jellies are back on view in the Jellies special exhibit. A new species to the exhibit, Mnemiopsis leidyi, is native to the temperate waters along the coasts of North and South America—our combs were collected off New England—but it has found its way by ship ballast water into the Black, Caspian and Mediterranean Seas. While combs are present in coastal waters throughout the year, says special exhibits collection manager Mark Schick, “good concentrations are sporadic, so availability is, too.”

A few things to note about comb jellies:

They don’t have the trailing, stinging tentacles that characterize so many jelly species.

They do have rows of comblike structures running lengthwise on their bell-shaped bodies.

They can eat 10 times their weight in zooplankton and other prey a day. True, these animals are microweights, but that’s still an impressive feat of gourmandism.

And where they put all that food is, at first glance, a mystery, because they are as transparent as the water they float through.

Most of the jellies you see in the Jellies special exhibit are cnidarians (nye-DAIR-ee-uhnz, with a silent c), members of the phylum Cnidaria, a diverse group of mainly marine invertebrates that snare prey and snap at predators with toxin-shooting stinging cells. Biologists refer to cnidarian jellies, like the moons and nettles, as “true” jellies.

But “jellies” is a generous generic term for any gelatinous marine invertebrate, not just the cnidarian clan.

Comb jellies are ctenophores (TEN-uh-forz), members of another phylum. (A phylum is a vast taxonomic grouping, following kingdom; humans are in the phylum Chordata, animals with backbones.) Ctenophores by definition have comb rows, eight of them, that consist of closely spaced cilia, like tines of a tiny comb, that beat in a wave pattern from the top to the bottom of the bell.

Get your face up to the glass of the comb jelly display to see these cilia in action. They can beat so fast that they blur, but they propel the jellies in a smooth, stately motion—in contrast, say, to the herky-jerky bounce of the blue blubber jellies. If that’s not fascinating enough, as a comb jelly pivots, light passing between the cilia is diffracted into an iridescent rainbow. Comb jellies are also faintly bioluminescent—they glow dimly in the dark.

For all their ethereal appearance, comb jellies are voracious predators, consuming the eggs and larvae of invertebrates and fishes as well as juvenile fishes, sea jellies and other ctenophores. Keep watching these jellies and you’ll see that their bodies are divided into two large lobes that can open and close. These are oral lobes lined with sticky cells. Smaller prey is captured as a comb jelly pumps water into its body cavity. Some comb jellies can also go after larger prey (including M. leidyi) by engulfing it with their outstretched oral lobes. As to the question of where the food goes, digestion begins as soon as something enters the mouth. Food is broken down a little more in stomach tissue, then distributed throughout the comb jelly’s body by a network of canals where digestion is completed and nutrients are absorbed by the jelly’s cells. Some of these canals run under each comb row.

Turnabout is fair prey: Comb jellies are a favorite meal of another group of ctenophores, and they also figure in the diets of cnidarian jellies, fishes and sea turtles.

Karen Furnweger, web editor

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