It’s All in the (Jellyfish) Family

A variety of jellyfish
They may not all look the same, but cnidarians all have cells that are specially designed for stinging. (Allen Collins, Smithsonian)

With their huge number of venomous stinging cells, jellyfish aren’t very cuddly. A few have been observed engaging in social feeding behavior, but for the most part, they’re loners. However, jellyfish do have families, just like everyone else. They’re related to other jellyfish, of course, but they also have more distant cousins that are different kinds of animals, in the same way humans are related to apes. Who are these relatives?

Dr. Cheryl Ames, a marine biologist who works with jellyfish at the Smithsonian National Museum of Natural History, asks people this question all the time. Sometimes her audience has no answer. “They just think jellyfish are a blob, with these stinging tentacles hanging down,” Dr. Ames explains. “And so it might as well be related to a plastic bag.” (Confusing the two is a mistake that sea turtles sometimes also make when feeding!) But often, people will guess that jellyfish are related to cephalopods—octopuses or squids—since they all have tentacles. It’s not a bad guess. But it’s incorrect.

Sorting out the family tree of all life on earth is called systematics, and its basic rule is that biological relatives share traits with each other—like tentacles. But, as Dr. Ames points out, the differences in this case overwhelm the similarities. “In an octopus,” she says, “[the tentacle] is made of meat that you can eat; it’s very hardy muscle.” In a jellyfish, the tentacles are more like hollow straws. For a shared trait to be relevant in classifying organisms into groups, it has to have been passed down from a shared common ancestor. The tentacles of cephalopods and jellyfish don’t qualify: they’re dissimilar because they evolved independently. So they don’t provide any clues about genetic relationships. Besides, there are other significant differences between these two tentacled creatures. Cephalopods have three layers of tissue while jellyfish only have two, and two openings to their digestive tracts while jellyfish only have one. Cephalopods also have large brains and other organs that jellyfish lack.

Preview Model of a lion’s mane jellyfish on display at the Smithsonian National Museum of Natural History. This jellyfish, a member of the phylum Cnidaria, can have tentacles up to 190 feet long (58 meters) and a bell almost 7 feet (2 meters) in diameter.

Model of a lion’s mane jellyfish on display at the Smithsonian National Museum of Natural History. This jellyfish, a member of the phylum Cnidaria, can have tentacles up to 190 feet long (58 meters) and a bell almost 7 feet (2 meters) in diameter.

(Allen Collins, Smithsonian Institution)

In fact, jellyfish aren’t closely related to cephalopods at all (and neither are they closely related to comb jellies, another gelatinous sea-going creature). Their closest cousins include corals and anemones. “Corals, anemones, things we call hydroids, sea pens, and jellyfish,” lists Dr. Ames. They all belong to the phylum Cnidaria (pronounced ny – DARE – ee - a). A phylum is one of the large groupings used in taxonomical classification to refer to a group of creatures that has evolved from a common ancestor. For example, humans belong to the phylum Chordata, which includes all the organisms with backbones as well as others with backbone-related structures.

Cnidaria is a big and somewhat disparate group. “Thirteen thousand species are known in that phylum,” Dr. Ames says, including the recently classified group of microscopic parasites called myxozoans. “You’re looking at something small and parasitic that never develops into a jellyfish, and then in the same big group you have something humongous and free-swimming like the lion’s mane [a jellyfish whose tentacles can reach lengths of 190 feet].”

It may seem odd that two such different creatures could be invited to the same family reunion. It’s hard to imagine what myxozoans and the lion’s mane jellyfish could have in common with each other, much less with hard-skeletoned stationary corals or flowery-looking anemones.

But, in fact, all these animals do share one trait—and it’s the trait for which jellyfish are perhaps best known. Cnidarians all have certain cells that are specially designed for stinging. Through these cells, which contain the coiled up harpoon-like nematocysts, scientists relate jellyfish to corals, anemones, and the seemingly plant-like hydra, which shoots out stings to bring in its prey. “That became the unifying character, despite so much difference in form and function and sexual reproduction and all of those other things,” summarizes Dr. Ames. “They were unified by these unique cells.” And, lo and behold, if you look closely, you can find these cells in the worm-like myxozoans too although with very different functionality, possibly due to their parasitic lifestyle. 

The ability to attack other creatures with a venomous sting may seem like a nasty trait to share with your family members. It’s certainly no guarantee you’ll get along: Some kinds of jellyfish use their sting to kill and eat other species of jellyfish. But nematocysts have many virtues. They come in very handy for cnidarians trying to protect themselves or grab a meal. In jellyfish, the special nematocyst-containing cells fill the cylindrical tentacles, arranged in outward-facing rings of venom capsules paired with miniature harpoons ready to strike. Depending on the species, nematocysts might fill the jellyfish’s gut to help sting its prey, or protectively cover the embryos that the jellyfish releases. Indeed, Dr. Ames has spent a lot of time studying a particular species of jellyfish called Copula sivicksi, which is named for having mating habits that are unusual for a jellyfish. Not just the embryos but “the gonads are packed with nematocysts,” she says. “It’s like a protection against being eaten.” Copula sivicksi takes extra care to protect its reproductive organs and the embryos that will one day be offspring. So there’s at least one cnidarian out there that uses its dangerous stinging cells to look out for its family.

August 2017