The oddest thing about warm ocean life is how many species live near the top of their heat tolerance.
Thailand’s Andaman Sea heats like a skillet. There’s no stove below—rather, sunlight glares mercilessly above, broken up by eerie rock columns into wavering oblong shadows. Every crystalline morsel of water inhales the streams of solar energy, but a whole ocean is hard to heat. And once the surface temperature exceeds about 90° F (32° C), evaporation saps heat away almost as fast as the Sun adds it. As a result, the Sun rarely warms the ocean past human body temperature. Even the most torrid tropical seas fall far short of the temperature you’d prefer in the shower. Truly hot water is rare in nature.
The most extreme exceptions are fissures deep in the ocean, where the planet’s crust runs thin. Red-hot magma courses just beneath the ocean floor, superheating the ocean water in underground channels like the coils of an old radiator. Some of that scalding fluid, steeped with sulfur and metals, leaks through the ocean floor through small gaps in the crust. The water would boil if it weren’t so deep and the pressure so high (see Chapter 4). It is in the hot, deep hydrothermal vents that ocean life has a chance to pit itself against the challenge of truly hot water.
But even in the comparatively balmy tropical oceans, or during warm days on cold shores, species can easily heat up beyond their tolerances. The oddest thing about warm ocean life is not just its heat-resistant adaptations, but also how many organisms live near the upper limits of their own physiologies. It is an odd principle of biology, but an Antarctic fish can die of heat stroke at about 43° F (6° C).1 Corals live well at 81° F but suffer at 90° F (27° C and 32° C, respectively).2 Extreme heat is almost entirely relative.
The three-person submersible Alvin silently descends through ever-darkening strata of water, on course for the absolute bottom: a hydrothermal vent 8,000 feet deep. When it arrives, powerful floodlights illuminate sulfurous chemicals pouring from holes in the sea bed. Thick black clouds belch out into the noxious water. Metal-rich deposits build up as the fluid rushes by, countless tiny flecks accreting over time into dark spires called black smokers spewing white-hot water out into the sea (see Chapter 4). It’s the early 1980s, and black smokers are a recent discovery—made by past researchers piloting this very submersible.3
As the sub slows, its floodlights illuminate a lumpy and irregular 6-foot spire pouring black clouds like a Dr. Seuss smokestack. For a dozen feet in all directions, the floor practically explodes with life: tube worms, albino crabs, pallid shrimp, and other bottom dwellers—creatures already familiar to Alvin’s crew. But as they pull in close, hovering in the still, benthic water, the scientists spy something novel on the smoker itself. Feathery red tufts adorn its lower reaches and when probed with a metal claw, they retract: they are in fact the fleshy, ornate heads of wriggling worms. Their rock burrows are piping-hot conduits for scalding water—but the worms are thriving. Alvin moves in for a closer look, and soon she’s discovered another brand-new species. It’s the Pompeii worm: the biggest heat-lover in the sea.4
Named for the submarine vessel that discovered them and an unfortunate Roman city suddenly buried in hot ash, Alvinella pompejana are found only at deep-sea hydrothermal vents. Delicate crimson plumes widen to fleshy gray bodies coated with hair-like bristles. The Pompeii worm’s head sits awash in chilly 40° F (4° C) water, just a few inches outside the vent.5 Its dense capillaries swell with dark red blood, rapidly exchanging oxygen with the water.6 The tail, inches nearer the vent, may as well be on another planet. Subject to high and unpredictable temperatures, it’s able to withstand water hotter than 120° F (50° C).7
Most animals do not live anywhere close to these temperatures. There is a thermal hot springs crustacean that lives about this hot, and some desert ants that live at 110° F (43° C).8 But Pompeii worms hold the record for the hottest animal. They live so deep and in such unusual environments that scientists have only recently been able to secure a living specimen. Kept at 130° F (54° C), they die. So their hardiness, living on hydrothermal tubes where the interior parts of the worm tube reaches nearly 180° F (82° C), remains a mystery. Maybe Pompeii worms rapidly circulate fluids through their hot tails and cool tentacles, like a natural heat pump. Maybe the hot parts of the tail are adapted for high heat and the cold parts of the head are adapted for the cold. Maybe incubating the heads at 130° F is fatal, but the tails could take it. It would be an odd animal that had to build cells on one of its ends to withstand the hottest temperatures the ocean offers, and an inch away, had to build cells that functioned in the cold of the deep sea.
For this reason, molecular biologists took interest in Alvinella’s resilience from the moment the Pompeii worms were discovered. Their proteins and other cellular building bocks are some of the most heat resistant known in the animal world.9 They could have any number of uses in both science and industry, so the race is on to gene-sequence both Pompeii worms and the similarly heat-resistant symbiotic bacteria that feed them.10 Stout Alvin, still in service decades later, trucks down to the deep and gathers them into specialized high-pressure tanks in continued efforts to bring them to the lab.11 Until we can regularly observe a living animal in a lab environment, most of the mysteries of the Pompeii worm from the ocean’s burning pit will remain an enigma.
The heat of the deep-sea vents, pumped out of Earth’s bones, is a flickering flame in a vast night. The majority of the deep sea is dark and cold—ocean temperatures in the deep hover around 40° F (4° C).12 The ocean-floor vents are tiny and rare, lined up along deep cracks in the ocean floor like rest stops on a dark desert highway, with many miles between them. Though they are extremely hot, vents don’t cast heat far; even a few feet from a vigorous black smoker, the temperature drops from 650° F to 40° F (340° C to 4° C). Creep toward the smoker and get cooked; wander a few feet away, and the torpor of the dark and the cold settles in.
How does a vent animal tell where the black smoker is in a black ocean, where the only light might be brief bursts of bioluminescence? A person can feel a campfire’s heat with eyes closed, and edge closer while keeping herself safe. But water absorbs heat, and heat receptors do not work well in water.13 Enter the rift shrimp, a blind animal that sees hot water.
Rimicaris exoculata (literally, “rift-shrimp without eyes”) is a cocktail shrimp–sized crustacean found exclusively near the smokers of hydrothermal vents. Between 2 and 3 inches long, covered in a transparent carapace, the rift shrimp spends its whole adult life at the edge of death, dancing across black smoker chimneys. Strong chitin toe-tips tear away gritty deposits from the smoker walls. The shrimp slurps down the sulfide-processing bacteria, and grinds the rest into a fine powder.14 In addition, the shrimp have a whole community of bacteria living in their enlarged gills,15 busily processing the hydrogen sulfide in hot seawater, like the symbionts in the hydrothermal tube worms (see Chapter 4). Either way, staying near the smoker is crucial, but the shrimp can’t see it, lacking proper eyes.
Deep-sea biologist Cindy van Dover and her colleagues noticed two broad symmetrical patches of pigment on the back of these shrimp carrying heavy concentrations of rhodopsin: the same light-capturing pigment we have in our own eyes.16 The patches have thin corneas and sensitive retinas, and optic nerves that connect directly with the back of the brain.17 The patches are basically eyes—but not on the head. And the light that the shrimp sees with is not the light of the Sun—Rimicaris sees the eerie glow of the red-hot water.
When heated sufficiently, almost any substance radiates light in the low-frequency infrared spectrum. The exact frequency of light depends on the temperature—more heat, shorter wavelength and higher frequency. Our Sun emits yellow light because of its enormous temperature: 11,000° F (6,100° C). The heating elements of a red giant star, or a toaster, are cooler and emit more reddish light. The water of the deep-sea vents, at 650° F (340° C), emits a light at the very lowest end of red—just bright enough for the rift shrimp’s eye patches to absorb.
Rimicaris has evolved the unique ability to actually perceive that light. The broad patches of rhodopsin on the shrimp’s back are thought to increase the animal’s ability to perceive dim light sources. There would be no room for such broad patches on the narrow eyestalks of most crustaceans. A reflective backing lies under the patches, acting like the reflective surfaces in a cat’s eye to bounce what little light there is back up, should any fail to be absorbed the first time it passes through the rhodopsin patch.
The rhodopsin absorbs the light and conveys some sort of image directly to the brain. The glow is too feeble to form a true image, but rhodopsin can create a sense of proximity to heat. Although completely blind to light we’d call visible, his strange eyespots are perfectly built to perceive the only danger that really matters to him. Perceiving the vent’s horizon by its own ember-glow light keeps him safe while mandibles skitter away at brittle sediment. Put a pillowcase over your head and walk into a lit room; you can perceive the light and shadows, but not see much else. Likewise, the rift shrimp perceive heat waves only vaguely.18 They survive like the fictional blind samurai Zatoichi, tapping the ground with a walking stick while snatching arrows from the air.19
Picture a secluded stretch of Samoan coast. Classic palm trees break up the sandy beach’s blinding white, and behind them loom craggy volcanic hills. Warm, placid lagoon water laps at your ankles. A few hundred yards out in the water, tall rollers sputter their lives out in torrents of white foam. The submerged barriers sapping their strength can be seen from the beach: dark, amorphous forms. It’s a coral reef, built over millennia into massive offshore walls. Coral polyps—tiny flower-like animals that clone themselves to form the living tissue of a coral—spread with an industrious passion unrivaled in the sea, every day secreting thin undercoats of enduring limestone.20
Over countless years, those microscopic films pile up into structures capable of feeding and sheltering thousands of other species. And even when the coral animals are dead and gone, the built-up limestone remains. The hard coral head cast up on a tropical beach is made of this limestone. Australia’s Great Barrier Reef, one of the world’s most spectacular natural wonders, is made of this limestone. Millions of years in the making, it is the only natural biological structure visible from space. The end result of industrious coral construction is the closest thing to a structured city you’ll see beneath the waves.
Live reefs are magnificent to behold. Stunning blues and yellows and pinks and greens flash across the reef: fish, urchins, shrimp, and snails with a huge variety of body shapes and lifestyles. Worms huddle in their secret tubes, poking feathery heads out to troll the currents. Spires and walls of corals gradually rise from the ocean floor until the city hosts millions of creatures.21 Even the white-sand beaches on tropical shores are mostly coral, ground to flecks over years by relentless waves and gnawing fish.
But for all their towering accomplishments, corals are disturbingly fragile. A temporary rise in temperature of only a few degrees can set off a major mortality event, wiping out whole swaths of polyps. Large cyclic heat waves in the Pacific, called El Niño weather, can devastate them. In 1998, this phenomenon killed up to 90% of the live polyps on some reefs.22 In the past century, global warming from atmospheric carbon build-up has raised water temperature over a degree (Fahrenheit) in the tropics. That doesn’t sound like much, but it spells trouble for such sensitive and foundational animals.
Corals feed themselves mostly by farming photosynthetic single-celled algae named Symbiodinium. Living inside the coral polyps’ own body cells, they need plenty of sunlight and warm water to be productive. Thus, corals must live close to the surface in waters clear of cloudy sediment. These conditions are rarely found far from the equator, so that is where the overwhelming balance of corals live.23 There is in fact a set distance from the equator—named to honor Charles Darwin’s studies of coral reefs—beyond which corals can no longer build substantial reefs. The Darwin Point is located at about the latitude of Midway Atoll in the North Pacific, and just south of the Cook Islands in the South Pacific.24
Corals polyps can tolerate high temperatures, but their photosynthetic thralls are another story. Excess heat interferes with the algae’s photosynthesis. The huge power of tropical sunlight, captured by the algae, is typically converted into high-energy electrons. At high temperatures, Symbiodinium leak these high-energy electrons like a pot boiling over. They bond into a nasty form of oxygen called reactive oxygen species. It’s a toxin, poisoning the coral and forcing a response. The colony has only one option: expel the irritant.25 The coral does this through the earliest evolutionary form of “cutting off your nose to spite your face”—by killing off its own cells. Sputtering algae are jettisoned wholesale into the sea. Most of the time the corals die and begin to crumble. This is called coral bleaching.
In a profound discovery that is now used daily by reef health predictors, Paul Jokiel at the University of Hawaii’s Institute of Marine Biology mapped the temperatures at which corals in different areas began to bleach. Along with his colleague Steve Coles, he noticed an oddly regular pattern: across the tropics, corals began to bleach at about 2–4° F hotter than the average annual peak.26 Just a few weeks spent above that threshold triggered bleaching across hundreds of miles.
This data set was used by scientists from the National Oceanographic and Atmospheric Administration to devise a disarmingly simple metric: degree heating weeks. If temperatures persist 1° C (about 2° F) above bleaching temperature27 for 1 week, the index goes up by 1.0. Two degrees (C) for a week adds 2.0, as does 1° C for 2 weeks. This simple index maps the danger to reefs from local warm spells, alerting locals and the world when a bleaching event is in progress. Nearly 90% of the reefs in Thailand suffered from bleaching in 2010, with up to 20% of total Thai corals left dead.28 At this writing, there are not many patches of unusually warm water around the planet. The biggest one squats harmlessly north of Hawaii, just past the Darwin Point.29
The left coral branch from Ofu, Samoa, has bleached but the one on the right has not. They are the same species treated the same way, but the right coral was growing in a warmer Ofu pool and has higher heat tolerance. Photograph by Dan Griffin–GG Films.
On the island of Ofu in American Samoa, the Sun rises at about 6 a.m. on the day before Christmas. It’s a bright summer day in the southern hemisphere. Safe from the ocean swells behind a wall of coral, a lagoon sparkles in the sunrise like liquid cobalt. At dawn, the water temperature is already 84° F (29° C): a coral nirvana. By noon, fueled by the summer Sun and stilled by low tide, the lagoon reaches 95° F (35° C). The corals simmer for more than 3 hours at temperatures well past their normal tolerance. It’s long past dusk when finally they cool below 90° F (32° C). Santa Claus is well on his way across the Pacific.
The corals of the Ofu lagoon should be dead from their daily bake, and yet they thrive. When their heat endurance was estimated in experimental hot-water tanks, Ofu corals were among the toughest ever tested.30 New research shows that the hot-water pulses of the daily tides have sparked them to develop heat resistance. Twenty-four-hour exposure to 35° C heat would have killed them all long ago, but 3-hour stretches are bearable. In the classic film The Princess Bride, the hero has trained himself to resist poison in a similar way: gradual exposure, day by day, until once-lethal doses grow laughable.
Taking a page from human biomedical research, coral researchers measured how individual coral colonies use their genes during stress. Three days of heating activates a battery of 250 different stress genes in the typical coral. In the Ofu lagoon, the corals keep about 60 of these “heat genes” operating at high capacity all the time. Some of these corals seem to be born with these guardian genes turned on, but others only turn them on when moved by scientists to the reef’s hottest region. Some never activate the crucial genes; these colonies simply die. The cumulative result is a small band of survivors thriving in a small backreef lagoon a quarter mile across, growing in the intense sun and heat. Though they are the Pacfic’s toughest known corals, human interference still pressures them. Overfishing leads to choking algae growth, a landfill leaks heavy metals into the lagoon, and some on the island look to improve commerce by extending their tiny airstrip directly out over the reef, right where these corals live.
Luckily, these reefs are partly protected and deeply appreciated by the local villages, which are caught between their conservationist impulses and the realities of economic development. We still have time to figure out the survival secrets of the mighty coral polyp, to help the villages of Ofu protect their reefs, and to see whether their coral’s survival skills can be duplicated.
Nestled near the Great Rift Valley of Africa, where some of the planet’s tectonic plates come together, lies an unlikely place full of corals. The Red Sea is deep, plunging down into a 7,000-foot trough, but the shallow edges of this desert ocean are ringed with spectacular reef formations.31 The water is extremely warm. Fueled by the murderous Middle Eastern Sun and the surrounding desert, the average summer water temperature is 86–88° F (30–31° C), high enough to bleach most Pacific corals.32
From the air, the Red Sea coast transitions from white sandy beaches, to sapphire blue water, and finally to a convoluted brown crust like a million muddy cul-de-sacs. Red Sea corals form “fringing reefs,” growing out in broad sheets toward the steep shelf of the coastline.33 Charles Darwin himself took an interest in the Red Sea’s corals, describing their distinctive fringe formations. But some coral structures made no sense to him: they stood far apart from the land, seeming to describe the curves of coastlines that didn’t exist. Others were isolated columns of ancient limestone, thrust up from the ocean floor in irregular spires teeming with colorful fish. Darwin was at a loss to explain them:
in the Red Sea, and within some parts of the East Indian Archipelago (if the imperfect charts of the latter can be trusted), there are many scattered reefs, of small size, represented in the chart by mere dots, which rise out of deep water: these cannot be arranged under either of the three classes: in the Red Sea, however, some of these little reefs, from their position, seem once to have formed parts of a continuous barrier.34
As it turns out, these formations were lingering evidence of a turbulent and unusual history. The Great Rift Valley is in a state of constant geologic change as its three plates struggle to pull away from one another.35 The competing forces make the Valley a giant seismic anomaly: a never-ending war between three drifting continents that raises and lowers the level of the Red Sea. At this moment, the level of the Red Sea is at a historical high: about 150 feet higher than the half-million-year average. The spires of coral rock and offshore reefs that Darwin puzzled over are flooded now but were in a normal position along the coast during those older epochs.
The hot, salty Red Sea roller coaster might seem an inhospitable place for glacially slow-building coral reefs. Yet marine species thrive there, feeding off opportunistic corals and evolving rapidly into new species. Of all the reef fish in the Red Sea, 10% are found there and nowhere else.36 Many changes are purely cosmetic. The crown butterflyfish, Chaetodon paucifasciatus, is a stunning fish with black chevrons running lengthwise down its sides and a black, yellow, and white bar running through the eye. A broad crimson patch is splashed on the abdomen, mirrored by a similar colored band on the tail.37 This species is found only in the Red Sea, but its nearest relative (according to DNA evidence) is an almost identical species in Madagascar.38 Its patches are duller, demoted from scarlet to mustard yellow.
The local corals are similar to those found elsewhere but have evolved some unique abilities. Red Sea reefs, living close to their bleaching temperatures, contain heat-resistant symbiont algae. They’re a type of Symbiodinium, the common coral symbiont, yet they’re able to keep their hosts healthy even in much warmer water than usual. How this happens is not completely known. But there’s little doubt that the Red Sea Symbiodinium gives these corals an edge during the sweltering summer heat. And across the Red Sea, the hotter the water, the more common the hot symbionts become.39
Red Sea corals seldom bleach in the normal range of summer temperatures.40 But like so many of their cousins, they have very precise thresholds for survival. Heat-resistant algae have their limits. Anne Cohen and her colleagues at Woods Hole Oceanographic Institute measured the past growth rates of corals in a CT scanner: a version of the medical equipment physicians use to measure human bone growth.
Coral biologists can measure growth rings in coral heads the way foresters read tree rings. Cohen and her colleagues found that in the 1940s (when there were two consecutive hot 90° F [32°C] summers), corals grew at just a fraction of their expected rates.41 Recent temperatures in the Red Sea are ticking up; its water warms with the rest of the globe. As Cohen’s research suggested, coral growth is already trending down. Red Sea corals may be the world’s toughest, but they’re strained to their limits.
The shoreline in north Baja is a bleached mosaic of old boats: simple skiffs with outboard motors, green and blue and pink that used to be red, all crammed with fishing gear and white gill nets like billowing clouds. The water is shallow, minestrone-warm, and bears the chromatic sheen of leaked boat fuel. Fifty yards out, the harbor is only knee-deep. At its edge, where the water turns from sandy blonde to topaz, sea turtles pop up their heads for bubbling breaths. Tall formations of white stone loom like ivory castles, guarded over by rowdy seabirds. This is the northern tip of the Gulf of California: the world’s warmest open waters.42 Their soupy productivity sustains masses of shrimp and fish, jumbo squid, summering whales, and some of the ocean’s most diverse microorganisms. They also harbor a unique species of mammal on the very edge of extinction: vaquita, “little cow,” the pygmy desert porpoise.
The vaquita, Phocoena sinus, is the smallest and most endangered cetacean on the planet, a dolphin-like creature just 5 feet in length.43 Rubbery skin covers the little cow in a solid gray raincoat, but dark markings ring her eyes like a bandit’s and lend her lips the appearance of a coy smile. Vaquitas inhabit a tiny triangle of shallow water, 40 miles on each side and wedged into the Sea of Cortez’s northernmost tip. They live nowhere else in the world, in climes warmer than those inhabited by any other cetacean.44
Living in the northern Gulf of California is like being chained in a hot tub: pleasant at first, a trial over any duration. Mammals can tolerate extreme heat for short spans, but no marine mammal encounters the kind of heat the vaquita endures all its life.45 Water temperature spike high in the summer, as hot as anything in the Red Sea, and a warm-blooded mammal struggles to shed its own body heat. Small terrestrial mammals have a high surface-area-to-body-mass ratio and rarely overheat. Large mammals have contrived tactics like sweating, panting, or ear flapping to cool off. But vaquitas are caught in a bind: large enough to struggle with heat but trapped in water, where they can neither sweat nor pant.
Vaquita porpoise statue in San Felipe, Baja California. Photograph by Cheryl Butner.
To compensate, the little cows have two adaptations. First, they’ve shed their blubber. Vaquitas don’t look skinny compared to typical porpoises, but they carry far less body fat. Dropping insulation keeps heat from building up.46 Second, their fins are much larger than a typical porpoise’s. From dorsal to pectoral fin and back to the tail flukes, these oversized appendages act as radiators.
”Little cows” have tough lives, but the heat doesn’t kill them: no deaths by heat stroke or summer fever have been recorded. They seem to have adapted to this environment and specialized to it - but their triangle of hot water is so small that they are one of the rarest mammals on Earth. Their first recorded species census (they were only recognized as a species by Ken Norris and William MacFarland in 1958) posited less than 600 individuals.47 They are only one of six “true” porpoise species in the world, and their nearest relatives are the South American coastal Burmeister’s porpoise and the spectacled porpoise of the Antarctic.48 Both these species live in much colder waters. Neither lives within 1,000 miles of the vaquita. How and when did the pygmy porpoise reach the Gulf of California and evolve into a hot-water specialist?
DNA evidence suggests that vaquitas entered the Gulf of California 2–3 million years ago: a turbulent time amidst the Pleistocene Ice Ages, when sea surface temperatures varied widely. With massive glaciers locking up much of the planet’s water, and the cold of the poles reaching much farther toward the equator, the typical girdle of warm water in the tropics narrowed. Perhaps a roving pod of porpoises crossed the equator at that time, seeking greener pastures like the first ancient caribou to cross Alaska’s Bering Strait. Once in the Gulf of California, they may have been trapped by some accident of geography or climate. However it happened, the vaquitas’ survival proves it made the needed evolutionary changes. Yet those very adaptations make it difficult for them to live anywhere else.
Pompeii worms live through scalding black-smoker belches of 150° F (66° C), but a typical Antarctic fish dies of heat stroke at 43° F (6° C), what we’d call ice water.49 How can species have such different sensitivities to heat? Digging down to the level of genomes, there are lots of tricks evolution can play to adjust physiology to higher heat. For instance, the amino acids in proteins can be re-engineered to retain their shapes even at high temperature. Work on the Pompeii worm shows that wholesale evolution can take place across the genome to craft cells that function in hot-vent habitats.50 Species that control their body temperatures, such as mammals and birds, often evolve changes in size or metabolic rate. And there are many ways for an individual to adjust its internal physiology to accommodate temporary heat stress.
In the face of these mechanisms, it is perhaps a surprise that so many species are so sensitive to small increases in water temperature. Biologist Jonathan Stillman made a careful study of the heat tolerance in coastal crabs, from the hot shores of the Gulf of California to the chilly fog of Monterey.51 Depending on where a species lived, he observed heat-induced heart attacks at just a few degrees above their environments’ typical peak. Tropical crabs, the ones on the hottest shores, had less leeway than their temperate cousins. Even though they had more tolerance of average heat levels, their heat ceiling was just a few degrees above normal. A few unusually hot days could kill them.
Stillman’s discovery changes how we have to think about future warming, because it means that species that are adapted to high temperatures are not more likely to be safe when future oceans heat up. Our normal thinking would intuitively suggest, “They are already good at tolerating heat, so they should be able to survive some extra warming.” But when heat-resistant species are already living at the upper edge of their tolerances, they are no more able than other species to survive one extra degree.
