The Ocean of Life

by Callum Roberts

  Soon after North America slammed into South America and the Isthmus of Panama blocked the exchange of water, animals, and plants between the tropical Atlantic and eastern Pacific, a wave of extinctions convulsed the Caribbean and wiped out three quarters of all coral species and many other forms of life. Evolution has since gone its own way in each region and the two oceans now have few species in common. The result is that Caribbean reefs look different from those everywhere else. One feature that distinguishes them is their great abundance of sea fans.

  Sea fans are corals with a flexible horny skeleton over which is stretched a thin layer of tissue sprinkled with tiny polyps. These polyps are like miniature sea anemones. By day they are mostly retracted, but at night they expand to catch drifting food with a crown of stinging tentacles. As their name implies, sea fans are flat and fan-shaped, with a mazelike filigree of minor branches stretched between upright ribs. They are almost ubiquitous on Caribbean reefs, their supple forms bending to the movement of wave and current. One day in 1995, on a dive in Puerto Rico, I came across a sea fan that didn’t look well. The tissue between its upright ribs was blotched with dark purple stains and several ribs stood bare where parts had died. It was the first of many such sea fans I saw.

  Soon after I got home I was contacted by another scientist who had spotted sick sea fans on reefs in Curaçao, some five hundred miles to the south. At the time I was based at the University of the Virgin Islands in St. Thomas, and he was keen to know if I had come across anything like it. By year’s end, reports from other countries confirmed that the Caribbean was in the midst of a full-scale epidemic. It would reduce the population of sea fans by as much as 75 percent in the next few years.1 So what was killing these sea fans and why did it spread so quickly? The pathogen turned out to be a land-living fungus, and so attention swung rapidly to the red soil bleeding onto reefs from deforested island landscapes. But others thought the source could have come from somewhere far more unexpected—Africa.

  In their idle moments, shuttle astronauts sometimes contemplated the Earth’s great convulsions as they unfold in graceful slow motion far below. The fury of a hurricane is reduced to a twist of bright cloud; the flood of a mighty river a delicate discoloration of blue sea; dust whipped by desert winds a fan of brown streaks above the ocean. It is hard at first to accept that a Saharan storm might affect Caribbean reefs, but jet streams of air far above the sea carry millions of tons of African dust across the Atlantic every year. When I lived in St. Thomas sometimes the rain would fall in dirty red drops that left a thin layer of Africa over everything it touched. Incredible as it may seem, the island of Bermuda owes its red soils to thousands of years of such atmospheric dustfall. Charles Darwin remarked on African dust near the Cape Verde Islands when he stopped there on the Beagle:

  The dust falls in such quantities as to dirty everything on board, and to hurt people’s eyes; vessels even have run on shore owing to the obscurity of the atmosphere. It has often fallen on ships when several hundred, and even more than a thousand miles from the coast of Africa, and at points sixteen hundred miles distant in a north and south direction.2

  As Darwin soon discovered, this windblown dust carried the spores of land plants and fungi. Although it is difficult to prove conclusively, some scientists believe that African dust fall was responsible for infecting Caribbean sea fans with the fungus that started the epidemic.3

  Sea fans are not the only organisms to have been felled by disease on Caribbean reefs. In the 1980s, waves of disease wiped out nearly all the staghorn and elkhorn corals, both signature species for the Caribbean; they were ubiquitous in underwater pictures and resort brochures from the 1950s to the 1970s. Elkhorn coral formed dense arbors of robust branches, as thick as human limbs, whose tips broke the surface at lowest tides. As the bottom fell away to the seaward of these ramparts, they blended into bushy thickets of the more delicate staghorn coral, whose branches sheltered flamboyant troupes of shimmering fish. The cause of the elkhorn epidemic was later discovered to be a human gut bacterium, carried in sewage effluent.4 This outbreak was one of a collection of pathogens that has contributed to the loss of 80 percent of living coral cover in the Caribbean since 1977.5 Another disease destroyed almost every long-spined sea urchin in the Caribbean. Waving forests of sea urchin spines blackened the bottom before the epidemic, and often obscured the corals below. Their loss left reefs vulnerable to rampant seaweed overgrowth.

  As I mentioned in an earlier chapter, green turtles around the world have in recent decades blossomed grotesque disfiguring viral tumors that bulge from armpits and groin, face and throat. Badly afflicted animals even have tumors that overgrow their eyes and mouth. This is just what you can see. They often have more tumors in their stomachs, throats, lungs, kidneys—pretty much anywhere there is soft tissue. The disease is caused by a papilloma virus and was first reported in the 1930s, but it has reached epidemic proportions since the 1990s in densely populated areas such as Hawaii, Florida, and Barbados.6 Since then it has been seen in other turtle species, such as the critically endangered leatherback, but green turtles remain worst affected. This disease, as I said, appears to be linked to tumor- promoting chemicals released by harmful algal blooms, which in turn are worsened by coastal pollution.

  Other marine habitats have also experienced disease epidemics. A wasting disease has destroyed sea grass meadows from the Gulf of Maine to Florida since the 1930s. Wild and cultivated oysters in Europe succumbed to disease epidemics in the early to mid–twentieth century. In the 1980s, harbor and gray seals in the North Sea were decimated by a distemper virus similar to one carried by dogs, and pilchards in Australia were wiped out by a virus in the 1990s.

  Diseases certainly seem more visible today than in the past, but is that just because there are more people looking now? The problem is that we lack a baseline from the distant past against which to measure the prevalence of diseases and parasites today. Although there were reports in the nineteenth century of disease outbreaks in commercial sponges and oysters that destroyed fisheries, mostly people didn’t get overexcited about marine diseases, because they couldn’t see them. If birds fall from the sky, dying dogs litter the streets, or stands of forest trees wither they are obvious and worthy of attention. But animals and plants in the sea can sicken and die by the millions unnoticed.

  Jessica Ward, from Cornell University, and Kevin Lafferty, from the U.S. Geological Survey in California, made an attempt to build a disease baseline of sorts by trawling through all the reports made of marine diseases since 1970.7 The total number of reports of anything to do with marine life has climbed steadily since then. There are more scientists at work today and more are interested in the sea. So they corrected for this increase by finding the proportion of reports about disease in any given year. This increased over time for turtles, corals, mammals, urchins, and mollusks. There was no upward or downward trend for sharks and rays, sea grasses, or crabs, lobsters, and prawns. Fish appeared less afflicted by disease today than in the past. Of course, this approach has its problems. Science is as amenable to fashion as any human endeavor, and people get excited about different subjects as time passes. Professors train students in the subjects of their own passions, and those students often pursue the same interests in their own careers. But the results have a ring of truth that chimes with experience for those of us who have dived the oceans over this period. We see a lot more pestilence and plague today than we did when we first jumped in the water.

  I have said a lot about the downside of parasites and diseases so far, but we must remember that they are a natural and important part of the way the world works. Their presence indicates that an ecosystem is complex and functioning well enough for them to make a living, particularly where parasites or diseases have complex life cycles, perhaps with multiple hosts. One of the richest places Kevin Lafferty has seen for reef fish parasites is the near pristine Palmyra Atoll in the mid-Pacific, where he discovered a particular abundance of parasites that use
sharks to complete their life cycles.8 He thinks that having parasites in the system is akin to having top predators, in the sense that they help maintain checks and balances so that dominant species don’t overwhelm their rivals. In other words, while we dismay over coral disease, to a certain degree these diseases probably help maintain the coral diversity that fascinates us.

  But something has clearly changed. What is responsible for the rise of illness in the sea? For outbreaks of disease or parasites to occur you need a source of virulent pathogens, a population of susceptible individuals, and a means of transferring infection from one to another. The many ways in which we are changing the oceans offer fertile opportunities for pathogens to become established and spread. People who are stressed tend to be less healthy than their more relaxed colleagues, because stress compromises our immune systems. In an analogous way, multiple stresses leave animals and plants more susceptible to illness. So illnesses endemic to a region and those inadvertently brought in by people will often result in an outbreak. Disease outbreaks illustrate the intersecting effects of many different drivers that increase susceptibility. They show how cumulative stresses sum together to compromise life in the sea. The epidemic that wiped out Caribbean long-spined sea urchins began in Panama, close to the canal that connects the Atlantic and Pacific. Although marine species cannot pass through the canal unaided, because of its freshwater lakes, they can hitch rides on ships. Ballast water not only carries invasive species, it carries disease. When diseases find susceptible animals in abundance, they can cause catastrophic epidemics. As Jared Diamond pointed out convincingly in his book Guns, Germs, and Steel, disease was a greater force than weapons in the suppression of indigenous peoples as Europeans colonized the world. Epidemics of influenza, smallpox, and cholera swept through tribes, islands, and entire continents, destroying societies and cultures along the way. Every time contact was made, whether hostile or in friendship, the result was massacre by disease. In some places disease left only one survivor for every ten people originally present.9 The Caribbean island of Hispaniola (today’s Haiti and Dominican Republic) lost 240,000 of its 300,000 inhabitants within twenty years of Columbus’s arrival, probably to smallpox.

  Diseases had such profound effects on people in the New World and the Pacific Islands because they had never encountered them before and had no immunity. They spread like the wind and felled almost everyone they touched. Much the same is true for life at sea. The ancient disconnect between the Caribbean and the Pacific means that long-spined sea urchins in each ocean developed their own diseases. Pass one to another and the result is an epidemic on a massive scale.

  The wave of death from bacterial infection that engulfed long-spined sea urchins in the 1980s sped round the Caribbean so fast that it was over almost before anyone noticed, the disease having run out of hosts. A later effort to trace the timing of this outbreak suggested it swept across eighteen hundred to three thousand miles of reefs in a year.10 Similarly, a herpes virus that infected Australian pilchards in 1995 raced around Australia and New Zealand at six thousand miles per year. It was probably brought in with unquarantined shipments of assorted frozen bycatch fish destined to feed captive southern bluefin tuna being grown for Japanese markets. Anyone who has been to Australia and been doused with insecticides on arrival will be surprised that such shipments are still let into Australia without any precautions.

  Epidemics in the sea seem to spread faster than almost any on land. The only outbreaks that have come close are myxomatosis and calicivirus in rabbits and West Nile virus in birds. All these are carried by flying insects that quickly spread the virus through susceptible populations. There are fewer barriers to dispersal at sea than on land—no mountain ranges, deserts, or inland seas to halt the progress of outbreaks. Ocean currents can move pathogens hundreds or thousands of miles in a matter of days or weeks. But epidemics like the ones that attacked long-spined sea urchins or Australian pilchards also spread against the current, which is puzzling until you remember the sea-surface microlayer. This thin skin over the sea can be churned into an aerosol by storm winds, together with any floating particles, and whipped long distances across the ocean surface regardless of the direction of underlying currents. Entire shoals floated belly up in miles-long rafts during the peak of the pilchard epidemic, so it is easy to see how viruses could have found their way into sea spray.

  Across the world, populations of many species of marine mammals have struggled back from the brink over the last fifty years or more. Historic hunting of whales, dolphins, porpoises, and seals pushed some to the edge of extinction long ago—animals like the Guadeloupe fur seal in the southwestern United States and Mexico, or the Mediterranean monk seal. Others did succumb and are no longer with us, like Steller’s sea cow from the North Pacific and the Caribbean monk seal. Porpoise meat was greatly admired in places like the Baltic Sea. The name itself derives from the French porc poisson, or pig fish. In England it was served at banquets and declared a “royal fish” in the early fourteenth century, presumably to protect stocks from overexploitation by common folk. These animals and others like them were among the first to benefit in the twentieth century from conservation efforts in the sea.

  Over the course of decades, mammals like elephant seals, harbor seals, and sea lions have gradually repopulated their former haunts. Once again they fill breeding beaches, laze on drying rocks at low tide, or frolic in the waves. This change in fortune has cost some populations, because it has made them more susceptible to epidemics. Diseases thrive where there are many susceptible hosts. To trigger an epidemic there must be a sufficient density of vulnerable animals to sustain transmission; otherwise, a disease would fizzle after felling a handful of animals. In 1988, harbor seals began to sicken and die across northern Europe.11 After much head scratching, the culprit was finally isolated. It was a virus very close in identity and nature to canine distemper virus and was named phocine distemper virus. This disease depresses the immune system of seals. Victims become lethargic, and their eyes runny, but usually it is a secondary infection like pneumonia that kills.

  Phocine distemper swept through the harbor seal population and took a few hundred gray seals along the way. By the time it was over a year later eighteen thousand were dead, half the population. Epidemics end when disease transmission rates fall below a critical threshold. The mass death of the seals caused their population size to plummet, and transmission rates fell. Those left behind probably have some resistance so it is harder for the pathogen to find susceptible victims and the disease disappears… for a time. After the outbreak, seal numbers began to recover. By 2002, they had come back sufficiently for distemper to take hold once again. Like all pathogens, distemper is able to mutate over time into novel strains to which its host has little or no immunity. The epidemic in 2002 was a near exact replay of the one which preceded it, but this time it cut down over twenty thousand.

  The similarity of phocine distemper to canine distemper gives a clue to its possible origin. Other animals provide one of the main sources of emerging diseases. The same is true for pathogens that affect us. Ebola is one of the most feared illnesses, and it leads to near certain death through internal bleeding as blood vessels disintegrate. It is passed to people through contact with primates in Africa, perhaps animals that have been hunted for bushmeat. West Nile virus is passed to people from birds via mosquitoes, while swine flu came from pigs. About three quarters of emerging infectious diseases of people have their origins in other animals. Phocine distemper was probably passed to seals from dogs, which carry the virus without suffering harm. When an epidemic dies down, the dogs continue to harbor the illness until conditions are ripe for another outbreak. In the late 1980s, eighty thousand to one hundred thousand seals were killed in Lake Baikal, this time by the canine distemper virus itself. In Antarctica, crabeater seals were affected by a distemper virus that was probably passed to them by sled dogs.

  The much heralded recovery of sea otters in California has recentl
y been stalled by disease.12 Otters have been dying from a parasite of domestic cats, called Toxoplasma gondii. The parasite causes brain inflammation, and while it does not always kill directly, it makes sick otters much more likely to be taken by sharks. Otters living near urban centers with rivers flowing into the sea were three times more likely to be infected than those farther away, which leads some scientists to believe that the rivers carry parasites from cat feces in rainwater runoff. They think that the parasites, once in the sea, could be concentrated from the water by filter-feeding clams and mussels that the otters eat.

  Transfer of disease from land to sea is a common theme of recent outbreaks. The African dust suspected to have inoculated Caribbean sea fans with fungus is laden with other spores and microorganisms, many of which remain viable even after their high-altitude journeys. Runoff also carries soil and a mix of other pathogens to the sea. California sea lions have been infected by a soil fungus that causes the occasionally lethal valley fever in people of the southern United States and Mexico. The toxoplasmosis that culled sea otters has also recently been discovered in a spinner dolphin and beluga whales. Spread of disease agents from land to sea is hastened by land clearing, which promotes soil erosion, and by increasing human population densities in coastal areas. It seems likely that these trends will lead to many new instances of disease and parasite transfers in the future.

  As I mentioned, diseases thrive where susceptible animals or plants are common. In this time of rapid global change several forces have come together to increase the numbers vulnerable to infection. Earlier I touched on how chemical contaminants like PCBs and methyl mercury can suppress the immune system. Mass mortality of harbor seals in the Baltic raised the possibility that pollution in this highly contaminated sea had weakened their ability to fight off infection. An experiment in which one group of seals was fed herring from the Baltic and another from the less contaminated Atlantic confirmed this hunch.13 After two and a half years of dining on Baltic or Atlantic herring, the immunity of those from the Baltic group was lower, an effect blamed on PCBs, while the Atlantic group remained healthy.