Editor's note: Image of Pleurobrachia below kindly supplied by Wim van Egmond.
This genus typifies the beauty and luminosity of the Ctenophores but has a different form to some species described below.
I was underwater in the most luminous tropical sea ever imagined. The featureless white sand floor of a small bay, thirty or forty feet below, shimmering from light refracted by wavelets at the surface, stretched off in the distance, diffusing into a kind of submarine mist. The water itself had no substance; it wasn't even there; it wasn't suspending me as I drifted through empty space. It was a surreal experience, made even more dreamlike as a vision of beauty materialized in front of me.
I know the marine world pretty well, and its inhabitants are generally no mystery. Even if I can't pin a name on some odd creature, I can assign it to a family group.
This time my mind wouldn't compute. For a moment I thought I was losing consciousness, perhaps hyperventilating with attendant flashes of light interfering with my vision. No. I remained in good shape. What was out in front was real, but it was nothing I had seen or could at first identify.
Mental files were riffled through. I looked, wondered, thought of every possibility—and then suddenly I knew. For the first time in my life I was looking at a living Cestum, a creature so diaphanous, so immaterial, so beautiful, it could hardly be of our world. Better in the company of angels.
Cestum. Long before acquiring a scientific name, it was known as Venus's Girdle, and properly so, for the goddess of love deserves the most lovely of accouterments. And that provided a basis for its official naming, for "kestos" is Greek for girdle.
Unlike all others of its comb-jelly group, the phylum Ctenophora, Cestum is a meter-long, broad band of jelly held in shape by the thinnest possible layer of cells. The entire animal itself is so thin, you look right through it to see undistorted images on the other side.
A transparent, living belt of jelly by itself may not be breath-taking, but this creature is, because of what it is doing. Every comb-jelly, no matter its shape, is not a jellyfish in the familiar sense. They aren't even related to those circular, umbrella-shaped animals that pump their way through the sea. A ctenophore's place in the scheme of animal existence isn't sure, but it surely is different.
The shape of comb-jellies remains more or less constant according to their species' characteristics, although pressure can change them momentarily, after which they come back to their own "comfortable" shape, as Piglet would say in the Pooh stories. They move by eight rows of tiny comb-like bands of cilia beating in sequence in a pattern known as metachronal, or "change-time" rhythm. These synchronized beats progress along the rows in sequence, and as light strikes the little combs of hair-like cilia, they reflect prismatic colors.
This was what I was seeing in tropical Marigot Bay of St. Lucia in the Lesser Antilles. Colors were passing across my field of vision like beautiful, ethereal neon signs, with no sign of how they did so. Cestum itself was invisible. Only its ciliary colors proved it was there, alive, swimming in slow and graceful fashion through clear featureless water.
One can use the word mesmerize and hardly know what it means. But I know, and it was this single experience that taught me.
Ctenophores have always fascinated me. And they are fun for children who find their chunkier, more solid species in shallow northern waters and pick them out to throw at one another. It's not good for a ctenophore, but they have no stinging organs, so the kids are safe.
The ctenophores I know best along the Atlantic coast are attractive in their own right. Beroë has a rosy caste to its helmet-shaped body; Mnemiopsis is clear to whitish, with heavy lobes underneath. Pleurobrachia is the tiny misnamed sea gooseberry that trails long sticky feeding tentacles below. No stinging cells are present in ctenophores.
At times one of these, Mnemiopsis, occurs in such prodigious quantities in Middle Atlantic waters that it almost seems you can walk on the water made dense by their numbers. Waves are stilled, and the surface appears slightly bumpy as their little jelly-bodies are thrust up by the crowding beneath. Great sea turtles, fishes of every kind, and hopeful (but misguided) sea birds come rushing to feed on their insubstantial tissues. It makes no difference: ctenophore numbers continue seemingly unabated. And then they vanish.
To get a feeling of their mass, I once slipped into the water in the midst of a Mnemiopsis bloom, and wished I hadn't. It was impossible to swim, even to move about effectively. It was like being in a ocean-wide bowl of unresisting, living jello and I had to be hauled out by a companion in the boat, shedding comb jellies from my hair and off my body.
Ctenophores are intriguing, not only for what they are, how they navigate and reproduce, but what their place is in the animal kingdom. All sorts of theories attempt to make sense of their presence.
Some say they evolved from coelenterates, the real jellyfish that are radially symmetrical—like a wheel, pie, or umbrella. But a ctenophore is confusingly biradially symmetrical, which means each half of its body is hemicircular, or hemispherical, and mirrors the other half. (One side of Cestum's ribbon-like body mirrors the other side.) This arrangement doesn't make sense from the original jellyfish's point of view.
Others say ctenophores came from bilaterially symmetrical creatures, like flatworms (and you and me), and still others say, no, they came first and gave rise to flatworms in the early days.
Discussions of this sort, that often give rise to heated arguments among scientists promulgating their pet theories, are futile. We can theorize all we want, and will never know—unless as yet undiscovered DNA relationships are found that link ctenophores with one group or another.
There are only about 50 species of ctenophores, and all live only in the sea. As animals go, they have never been a wildly popular model. Besides the utterly unique system of rows of ciliary comb plates, each species has a remarkable sense organ, a statocyst, or organ of balance and equilibrium. Toward the top of the body, the eight rows of comb plates combine into four, and these terminate at the top into four little projections that support a tiny cluster of mineral grains known as a statolith.
Imagine balancing a golf ball upright with two fingers on one hand, and two fingers on the other. If you tilt your body or your hands, the weight of the golf ball is going to press more on one or two fingers than the others, and you will feel the difference. Without using your eyes, you can bring the arrangement back into balance until each finger feels the same amount of pressure as the others.
This is how a ctenophore detects body position. If it tilts, one or two "fingers" send impulses down their respective rows of comb plates encouraging the cilia to beat more vigorously, thereby returning the animal it is normal upright position. This goes on constantly, for swimming and ocean currents tip the little animal one way or another, yet it always tries to come back to the vertical.
At least that is what all ctenophores do except that marvelous Cestum, which has little difficulty keeping its band-shaped body in a horizontal position. If you gently move it (as I did), it uses the same technique to get back to where it belongs, stretching horizontally beneath the surface.
There is much more to tell about these strange, rather simple creatures, so I suggest you check your encyclopedia or appropriate websites to learn about these harmless beauties that share the world with us.Comments to the author William H. Amos are welcomed.
© 2004 William H Amos
Note: For readers interested in the derivation of scientific names, here's an explanation of those mentioned (all are from the Greek):
Cestum (girdle)Ctenophora (comb-bearer) Beroë (a nymph, daughter of the god Oceanus) Mnemiopsis (one with a memory—meaning able to return to its upright position) Pleurobrachia ("arms" or tentacles to the side)
Micscape article - Comb
jellies by Wim van Egmond.