RHAPSODY IN BLUE Porpita porpita, left, commonly known as the blue button, is easy prey for this blue dragon. Oxford Scientific

At dawn on a remote beach along Mexico’s Baja California Peninsula, in a world of pink and red desert and forests of cardón cactus, I come upon an unlikely mass stranding of Portuguese man-of-wars, or bluebottles. They’ve come ashore in the night and their blue sails are as bright and shiny as living tissue, as if the beach were strewn with thousands of excised lungs. Their tentacles lie hopelessly tangled around them, reminiscent of dissected blood vessels, giving the otherwise peaceful morning the feel of the abattoir, as if many animals have been butchered and their larger parts consumed.

Vision is the art of seeing things invisible, wrote Jonathan Swift. And sometimes the invisible is a huge, dominant, virtually omnipotent presence in our world, the feature for which our planet should have been named, and may well be named by distant intelligent beings with the means to peer farther than we can at present. The ocean is our blind spot: a deep, dark, distant, and complex realm covering 70.8 percent of Earth’s surface. We have better maps of the surface of Mars than of our own sea floor. Yet under our skin, we’re a plasma ocean, so entwined with the outer seas that we can’t easily know either ourselves or our water world.

This ocean is the largest wilderness on Earth, home to wildlife in staggering multispecies aggregations, and with a lineage of life three billion years older than anything above sea level. Its three-dimensional realm comprises 99 percent of all habitable space and is so embedded with life as to be largely composed of life, with an ounce of seawater home to as many as 30 billion microorganisms -- and counting. Honing our technological eyesight, we begin to observe what was once too small to be seen, in an exercise that mirrors infinity.

For most of our time on Earth, most of what we’ve known of the ocean has been its dead oddities on the beach. The Portuguese man-of-wars must once have seemed the leftovers of immortals, stamped with the teethmarks of Oceanus, Varuna, or Tangaroa. Modern explanations are likewise riddled with paradox, since science reveals the man-of-war to be not a jellyfish but a siphonophore, not a single body but a collection of bodies, a colony of as many as 1,000 polyps. The blue bottle bobbing at the surface, the pneumatophore, functions as an upside-down sailboat keeping the colony afloat, its inflatable sail rigged to navigate wind and waves. The nearly invisible tentacles, the dactylozooids, trail scores of feet below the boat and fish for prey with built-in stinging harpoons. The digestive polyps, the gastrozooids, cook up the catch and serve it to the pneumatophore, the dactylozooids, and the last members of the colony, the gonozooids, whose job it is to reproduce the man-of-war. Combined, the team is as integrated as a single animal. Yet absent of leadership, insight, or foresight, these strange conglomerates -- we don’t know if they function as one or many beings -- beach, often in blue fleets by the dozens or hundreds. As we might if our legs were separate entities from our heads, our stomachs and sex organs separate again.

But we are not jellyfish. And we see the alarms, the messages inside the man-of-wars’ blue bottles joining a host of other distress signals washing ashore these days from oil spills, fish kills, slain cetaceans, washed-up seabirds composed in part of lethally indigestible plastic. Some are silent sirens: the disappearing seashells and horseshoe crabs, the missing sea turtles and their eggs, the vanished egg casings of sharks and rays, the lost coral debris, the dwindling beach-spawning grunion, and the anguillid eels, those long-distance travelers that migrate thousands of miles from ocean to river and back again, and appear to be evaporating from the face of the earth.

Only of late have we learned to see the ocean’s surprising vulnerabilities. That it’s neither infinite nor inexhaustible. That it’s the beleaguered terminus of all our downstream pollutants, part of a dynamic system intensely interactive with land and atmosphere and everything we do there. Only in the past decade has science discovered the ocean to be fragile in the way only really enormous things are fragile: with resilience teetering on the brink of collapse. Yet our behavior lags far behind our understanding, and the ocean awaits our enlightened action.

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One in seven people on Earth depend on food from the sea as their primary source of protein. Yet one of the more optimistic assessments calculates that we’ve depleted up to a third of all the world’s fisheries, with 7 percent to 13 percent of stocks collapsed, perhaps never to recover. These declines happen in our lifetime: bluefin tuna, once cheap, becomes exorbitant; species once scorned become market favorites.

The ocean is Earth’s last frontier and its fish stocks are the bison we’re currently obliterating with trawlers, longliners, purse seiners, and gillnets. Modern fisheries target not only the ocean’s herbivores but also its carnivores -- the apex predators such as tuna, sharks, and billfish, the marine equivalent of wolves, mountain lions, and grizzlies. Of late, we’ve begun large-scale hunting of the field mice of the sea, the forage fish, such as sardines and anchovies, which form the ecological backbone of many marine food webs and which, as we’ve learned from examples off California, Peru, Japan, and Namibia, collapse catastrophically whenever the stresses of climate change intersect with the stresses of overfishing. Worse, we’re turning our guns on the ocean’s primary consumers, like krill, which, one or three trophic links removed, feed most everything else that lives in or makes its living from the sea, including us.

It’s not easy to calculate the magnitude of our appetites. We visit the ocean and look forward to eating the food of the sea, even those of us who would not in our wildest dreams consider eating elephants, lions, or leopards on our visit to a dry wilderness in Africa or India. Yet it’s about more than the cost of eating wildlife. It’s also about suffering. We hook, bludgeon, net, drown, and drag to death seabirds, sea turtles, seals, dolphins, and whales in the course of hunting seafood. We blindly assume that fish feel no pain -- though many scientists firmly believe otherwise -- and leave unquestioned the inhumane business of slaughtering them by the billions in the wild with truly cold-blooded detachment.

Life cycles in the sea are more complex than those of terrestrial life. Virtually all species that spend their adulthood anchored to the sea floor, such as corals, oysters, and sponges, spend their youth adrift: a two-part life history that allows them to disperse before becoming immobile. Most swimming species, such as swordfish and tuna, spend their embryo-like larval lives as translucent plankton-pickers no bigger than fingernail clippings afloat on currents. This fundamental difference in strategy between marine and terrestrial development magnifies our impacts in frightening ways. We hunt adults in one part of the ocean and destroy the nursery grounds of their larvae in another part and poison the habitat of their juveniles somewhere else while tangentially depleting the species they depend on for food at each stage.

Consider the Atlantic bluefin tuna, an endangered species on the Red List of the International Union for Conservation of Nature (IUCN). Overfishing contributes to its decline, yet pollutants inflict an insidious, often invisible toll. In 2010, tuna spawning partially converged in time and space with the Deepwater Horizon catastrophe in the Gulf of Mexico, one of its only two known breeding areas. We don’t know the effects of 206 million gallons of oil and almost two million gallons of chemical dispersant on adult fish that came to spawn that year, or will come the next, or the next. We don’t know the outcome of unprecedented levels of pollution on delicate eggs, and therefore on entire generations of an endangered species. We may not know for years, decades, or ever. Add to that ongoing debacle the other ongoing calamities in the Gulf: the disappearing wetlands; the channelization of the Mississippi River; the overfertilization of America’s breadbasket, which downriver fuels the world’s second-largest oceanic dead zone; the laying of 36,000 miles of offshore pipeline; the drilling of 52,000 offshore wells; the thousands of rigs left abandoned. Most of what undermines the Gulf has been done with little or no consideration of its waters and wildlife, in marked contrast to our developing attitudes toward the land. In this, the battered Gulf of Mexico is a microcosm of the global ocean.