WHAT’S THE DIFFERENCE BETWEEN JELLYFISH AND COMB JELLIES? They both come in blobby shapes and gelatinous, transparent bodies. But comb jellies – ctenophores – are entirely different from their oceanic jellyfish neighbors.
Most importantly: They don’t sting. And some of them put on fantastic light shows.
Note that I said “neighbors,” not “cousins.” The difference between jellyfish and comb jellies runs far deeper than the absence of stinging. Recent research suggests that combs evolved with astoundingly different make-ups than jellyfish – and every other form of life on our planet.
THE CTENOPHORE NAME
All comb jellies, members of Phylum Ctenophora, feature strips called comb rows evenly spaced around their bodies. Each comb row bears a band of tiny, hair-like cilia – the comb-like structures in comb jellies.
Both the common name comb jelly and the scientific name ctenophore (tĕn′ə-fôr, ignoring the silent “c,”) derive from these features. The “ten” part is taken from the Greek word for “comb,” the “phore” from “to bear.”
Ctenophores swim by operating their cilia like little oars on a galley, sometimes compared to The Wave people do at ball games. This is another important difference between jellyfish and comb jellies; jellyfish achieve locomotion by pulsating their bodies.
Another difference between jellyfish and comb jellies is that jellies tend to move with their mouths trailing, combs forage and move with the mouths forward.
- Have soft, transparent, sack-shaped bodies constructed with external and internal surfaces holding in a middle layer of gelatinous tissue.
- Are free-swimming (actually, more free-floating; they’re most of all captives of the currents).
- Are carnivorous, with prey ranging from microscopic zooplankton to, depending on size and species, small crustaceans like copepods, krill – and other ctenophores.
- Mostly capture their prey with a pair of retractable tentacles covered with sticky cells. In most species these tentacles have little side tentacles (“tentilles”). Both tentacles and tentilles are densely covered with microscopic sticky cells that capture prey with a sort of spider web effect that nails it victims like flies on flypaper.
ANOTHER DIFFERENCE BETWEEN JELLYFISH AND COMB JELLIES:
Ctenophores are capable of putting on extraordinary light shows, but it depends. They’re known for generating dramatic rainbows of colors running along their comb rows as they swim, but that’s actually the scattering of colors – light diffusion, in science-speak – as they beat their little cilia to motor along.
Most combs are bioluminescent, but it’s much more subtle, a green or blue that’s visible only in darkness. Species in the genus Pleurobrachia – including sea gooseberries – and other groups have no bioluminescence capabilities at all.
WORLDWIDE BUT LOCALIZED
There are more than 100 known species of ctenophores. They’re found worldwide and in every ocean segment – polar to tropical, coastal to open ocean, pelagic shallows to deep sea.
Although widely distributed, combs tend to be restricted to specific segments of the water column – say, shallow coastal waters versus deep ocean. A major factor: combs inhabiting coastal waters have to be more durable in order to hold up against tidal currents and wave action.
Many deep sea combs are so fragile that they can’t be captured and brought to the surface without breaking up. Our major evidence of them tends to be photographic.
Some sources suggest many more species exist but haven’t been identified. And, taxonomic (and street name) nomenclature for ctenophores appears to be fairly fluid. Hence, I haven’t rushed to I.D. creatures the way I normally would.
NOT LIKE ANYTHING ELSE
For a long time ctenophores were regarded as distant cousins of jellyfish, with globby, transparent bodies, a circular mouth at one end and external and internal surfaces sandwiching gelatinous material.
But the difference between jellyfish and comb jellies has become more delineated with research indicating, for starters, that combs’ nervous systems were developed relying on a different chemical language – a different set of molecules and genes – than any other animal. That is, not just jellyfish but every other member of the animal kingdom.
Subsequent work found the same true about combs’ muscles and other genetic traits. The implications are that:
- Ctenophores, not sponges or jellies, may be the oldest group of animals on Earth.
- Life may have evolved at least twice on our planet, the second time utilizing an entirely different and unrecognized chemical language.
Comb jellies may looks benign but they are, fact, highly effective hunters. The most dramatic example of their voraciousness followed the accidental introduction of an invasive species, Mnemiopsis leidyi, into the Black Sea in the 1980s. The invader vacuumed up zooplankton in the Sea, leaving little for native fish larvae, leading to a collapse in native fish stocks. The situation has come under better control with the appearance of a ctenophore that preys on other combs.
EXCEPTIONS. THERE ARE ALWAYS EXCEPTIONS
Two exceptions and odd fact to note in all of the above are:
- Nearly all ctenophores are free-swimming but there are a few species that are benthic – they attach themselves to some surface.
- Ctenophores don’t grow stingers but there are species in one ctenophorian genus, Haeckelia, that can sting. They don’t do it with their own nematocysts, however. They eat jellyfish and absorb their victims’ nematocysts into their own tentacles.
PRINCIPAL SOURCES: Marine Biology, Fourth Edition, Peter Castro, Michael Huber; “Ctenophores,” C. E. Mills, University of Washington; “The Phylum Ctenophora,” earthlife.net; “The hidden biology of sponges and ctenophores,” ScienceDirect; “Ctenophora,” Wikipedia.org; “Aliens in Our Midst,” Aeon Magazine.