Nobody knows the ultimate etymology of the word “ocean.” We get it from the Greek, where it designated the name of the god Okeanos, the fabled “river of the world,” sometimes depicted as a serpent swallowing its own tail. The old Titan was as prolific as he was randy, begetting 3,000 rivers, the “neat-ankled daughters of Ocean,” as the ancient poet Hesiod put it. Centuries later, Virgil would still call Okeanos the “pater rerum,” “the father of everything.” It seems right that these all-fathering waters, where life first began, should remain as mysterious in name as they are in actuality.

Geographers recognize five official oceans. But of course, there are as many oceans as there are imaginations quickened by them. The “wine-dark sea” of Homer seems an utterly different domain from the pitching element where an oil rig drills. These, along with the vistas conjured up by painters and composers, explorers and speculators, are human oceans; they are sustained by myth and dream as well as the profit motive. But what of that domain as experienced by its actual inhabitants? The crab, the anemone, and the octopus know oceans we can scarcely imagine.

This might seem a silly, or at least an idle, question. But in his latest book, “Sensuous Seas: Tales of a Marine Biologist” (Princeton University Press; 281 pages; $24.95), Eugene H. Kaplan, the Axinn Distinguished Professor of Conservation and Ecology at Hofstra University, provides the beginnings of an answer, and his disclosures, drawn from a lifetime of research and teaching, are as fascinating as they are, all too frequently, appalling. When Hart Crane wrote “The bottom of the sea is cruel,” he was moved by a vague premonition. Mr. Kaplan knows that cruelty close-up; indeed, he rather revels in it. As he confesses, he has a distinct penchant for the lurid in nature; accordingly, scientific exactitude mingles with jovial ghoulishness on almost every page. Given the tales he has to tell, the tone seems just right.

Mr. Kaplan’s seas are definitely sensuous, though hardly in the Club Med mode. Lounging on some tropical

beach, with a parasoled daiquiri close to hand, it’s difficult to imagine that just beneath that lazy sun and those glittering waves, a watery abattoir is in full swing.And when marine creatures aren’t devouring each other, or being devoured, they’re immersed in mating rituals of fantastic complexity. Thus, the lowly squid engages in a veritable ballet of blushes. As Mr. Kaplan describes it:

The male squid approaches the female and jealously guards her from his competition, all the while undulating his fins and darting to-and-fro. Flaming bands of red and lavender pulsate down his tubular body. As the mating dance becomes more frenzied, his color darkens to purple, then blanches; he becomes almost invisible. Accepting his charms, the blushing female turns pink, then lavender, waves of color rippling across her body. At the moment culminating the mating dance, he removes a sperm packet from his spermatophoric gland with his fourth right arm and places it into the mantle cavity of the female. She will save it for later use to fertilize each egg when it is produced.

After this Technicolor foreplay and strangely demure consummation, another male may appear who routs the blushing lover and then sets about siphoning out his rival’s sperm packet to replace it with his own. Mr. Kaplan explains the process in the usual “survival of the fittest” terms; and yet, such Darwinian pieties sound oddly inadequate. If this isn’t sheer passion — even with a “fourth right arm” — then I don’t know what is.

This is but one of the elaborate sexual antics that go on beneath the waves. Mr. Kaplan is an unabashed voyeur of such shenanigans, which he describes in loving detail, ranging from the hermaphroditism of the sea hare to the transvestite disguises of the parrotfish. (Of the latter, a pumped up “supermale” in sequinned scales, he exclaims, in booming italics, “The supermale is a fishy transvestite!”) He delights too in the startling fact. Thus, it turns out that the longest penis in the animal kingdom, at least in proportion to body size, belongs not to the whale or some other behemoth, but to the humble barnacle; this “magnificent demonstration of masculinity,” as Mr. Kaplan reverently describes it, is needed so that the male barnacle, glued to a rock, can deposit its sperm into an adjacent barnacle. Of course, both barnacles are hermaphrodites, which adds a dash of piquancy to the enterprise. In the wonderful line drawings by Sandy Chichester Rivkin and Susan L. Kaplan which illustrate each tale, and which are far superior to photographs, the transfer occurs amid a tangle of delicate tentacles; it looks about as sexy as jets refuelling in midair and yet, who knows what barnacle bliss may be transpiring deep within those stony shells?

The cruelty of the sea floor couldn’t be better represented by the creature Mr. Kaplan terms “the most disgusting barnacle of all.” This is the “kentrogon” or post-larval stage, of the Sacculina barnacle, a free-swimming parasite that latches on to hapless crabs and injects its own stem cells into their “watery bluish blood.” The injected cells multiply like a cancer, eventually destroying all the crab’s organs except those that keep it alive. It is an “obscene inner monster,” in Mr. Kaplan’s words, feeding, breeding, and proliferating in the living husk of its host.

If I have any criticism of this compelling book — in almost equal measure awe-inspiring and repellent — it is that it is too episodic; of course, the author subtitles it “tales” and it contains many funny as well as scary anecdotes drawn from Mr. Kaplan’s farflung expeditions. But the book is probably better suited to judicious dipping than to sustained immersion. I read it at one go and will probably never contemplate fried calamari, a former favourite, in quite the same way again, nor will I be dabbling my bare toes in murky surf for years to come. But it did set me thinking, with some puzzlement, as to why we nonscientists always revert to the old myths when we contemplate the ocean. Here are enough new and wondrous findings to generate dozens of poems, scores of fables; still, we cling to our origins like those shore-hugging crabs never fully at home in either element. The myths are inexhaustible but so, too, are the oceans. We easily concoct fantasies about life on other planets and yet, “the river of the world” in all its swarming strangeness still baffles our imaginings.