Steve Haddock remembers every detail about his first ocean encounter with a comb jelly. The open water was a bottomless deep blue. The animal, about the size of a tennis ball, shimmered with bioluminescence. “It was just cruising along like a hover craft,” says Haddock, a marine biologist at Monterey Bay Aquarium Research Institute in Moss Landing, Calif. “Comb jellies are more alien than any aliens people imagine,” he says.
Start with their appearance: The marine animals resemble translucent balloons rigged with flashing, colored lights. Some species glow. When startled, some flash electric blue. Vertical rows, or combs, made of hundreds of iridescent, hairlike cilia run the lengths of their globular bodies (thus the name comb jellies). In some species the cilia are 2 millimeters long — 200 times the length of cilia in other animals — and they beat in coordinated waves, propelling the jellies forward, backward and diagonally in search of prey.
It’s not just their appearance that is wondrous: Slice a comb jelly embryo in two and you get two half-adults that can fertilize themselves to give birth to a perfectly whole offspring. Some can reproduce while they’re still larvae. Though jellies lack eyes, Haddock and his colleagues have discovered proteins that comb jellies use to sense light. Comparative biologists like to joke that on the eighth day, God created comb jellies.
Comb jellies are gelatinous like jellyfish, but the similarity ends there. In body plan, jellyfish resemble the largely sessile, almost plantlike sea anemones, corals and other cnidarians: a group that dates back at least 550 million years. While jellyfish and other cnidarians have nerve cells that form a loose network in their bodies, comb jellies have a more sophisticated nervous system with a rudimentary brain and cellular connections called synapses that are also found in flies, humans and most other animals.
Yet, detailed looks at the genomes of two species of comb jellies suggest, surprisingly, that they are the more primitive animals, and not the jellyfish, sea anemones or corals, as has long been thought. It’s even possible that the sophisticated comb jelly lineage may have evolved before the brainless, gutless, muscle-less sea sponges.