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DANGER is all part of the job for deep-sea biologists. They risk
claustrophobia, attack by swordfish or worse when they take the plunge in a tiny
submersible. They could simply vanish in mid-ocean one dark night, as they work
on the slippery deck of a violently pitching ship. But in the sheltered haven of
a scientific conference, no one expects more than a light mauling by a
scientific rival. So Dave Dixon, a marine biologist from the Plymouth Marine
Laboratory, was a little taken aback when, during a meeting to discuss
hydrothermal vents, a tall, blonde Scotswoman made a beeline for him, wielding
her handbag with intent.

Moya Crawford, managing director of the salvage firm Deep Water Recovery and
Exploration, released the catch and pulled out a plastic bag. What, she demanded
to know, were these strange things her salvage crew had hauled up from a wreck
off the coast of Spain? One of the specimens was recognisably a coral. But the
stringy brown things? At first glance, Dixon thought they were probably not
animals at all. Then he took a closer look. They could, he thought, just
possibly be the remains of vestimentiferan worms.

These creatures are the giant worms that form forests around hydrothermal
vents and cold seeps, growing taller than an adult human. Vents are hot spots in
the ocean floor where superheated water loaded with minerals and metals gushes
from cracks in the Earth’s crust. No less remarkable are the cold seeps, leaky
spots where petroleum, methane and other hydrocarbons flow out.

For ecologists, both are marvellous places, oases of luxuriant growth in the
comparative desert of the deep ocean floor. Much of their fascination lies in
the fact that, unlike most life on Earth, the animals that live there do not
depend on the power of plants to harness energy from the Sun and
photosynthesise. With no light reaching the depths where vents and seeps form,
these communities acquire the energy they need with the help of symbiotic
bacteria that can feed off highly poisonous hydrogen sulphide or methane
emerging from the seafloor.

This apart, these self-supporting “islands” on the ocean floor pose an
enormous puzzle. Both hot vents and cold seeps are temporary. Hot vents open and
close with the vagaries of plate tectonics. They might be active for tens or
even hundreds of years, but eventually they will shut down. Cold seeps, which
are often found in regions with little tectonic activity, tend to last longer.
But in both cases, when they become extinct and stop generating a supply of
life-giving chemicals, the animals clustered around them die. The mystery is
that when new vents and seeps open up, sometimes hundreds of kilometres from any
other, the forests of worms and dense beds of clams and mussels spring up as if
from nowhere. How do they do it?

With such a teasing puzzle to solve, Dixon took more than a passing interest
in the contents of Crawford’s handbag. Crawford’s salvage crew was not working
near any known hot vent or a cold seep, so where had these worms come from? DNA
analysis of the badly rotted specimens revealed that they were
Lamellibrachia worms. The nearest known colonies were in the Gulf of
Mexico. Yet these worms were found clinging to a bale of sisal retrieved from
the wreck of the François Vieljeux, a French ship which sank in 1250
metres of water off the coast of northwest Spain in 1979.

It soon became clear why they were there. The François Vieljeux was
carrying a valuable cargo of copper and zinc, which explains Crawford’s interest
in it. But the ship was also carrying sisal, tins of pineapple, coffee beans and
sunflower seeds. After the wreck settled on the seafloor, the seeds and beans
began to rot. And as they rotted they began to give off hydrogen sulphide,
creating conditions every bit as attractive to a vestimentiferan worm as those
at a cold seep.

That settled the why. But it did little to explain how the worms had
journeyed across the ocean to take up residence. “We have never found these
worms on this side of the Atlantic before,” says Dixon. And there are no other
populations known in the Atlantic. “Maybe they travelled more than 6000
kilometres from the Gulf of Mexico.” It only requires a few of the countless
larvae a worm produces to make it to a new site and found a new colony.

The idea of a single transatlantic leap evaporated when the salvors tore open
another of the ship’s holds and discovered thousands more gently waving worms.
Dixon thinks it is inconceivable that so many worm larvae could have made it
across the ocean and conveniently stumbled across the wreck. “You might
contemplate a few reaching the wreck but the discovery of thousands of them
suggests that they must have come from somewhere much nearer,” he says. Nor does
he think this wormy forest is the fruit of prolific reproduction by a few lucky
colonists. Further DNA analysis of the animals from the wreck supports this
view, showing that there are small genetic differences between these worms and
those from the Gulf of Mexico. “So they didn’t come directly from the Gulf,”
says Dixon. He now believes that there must be as yet undiscovered seeps around
the continental margins of Europe.

The worms may not have crossed a whole ocean, but they must still have come a
long way. Genetic studies of animals from vents in the Pacific suggest that
larvae might travel hundreds of kilometres in one step, and make longer
migrations over several generations. If this is true, then vent animals need to
find suitable “stepping stones” across the seas, places where a few of their
number can form a temporary colony before sending offspring out in search of
newly opened seeps and vents. Wrecks carrying cargoes that create artificial
seeps are rare. But there may be other options. Dead whales, for example. Craig
Smith of the University of Hawaii has found many vent-loving clams, mussels and
limpets living on the carcasses of whales. The fats in the rotting whale bones
provide bacteria with the sulphides they need.

But even if larvae take refuge on the hulks of whales or ships, they still
have to leap from one stepping stone to the next. Such incredible journeys run
counter to conventional biological wisdom. Until now, the generally accepted
view has been that if a marine creature wants to send its offspring far and wide
it must have a larva that lives in the upper layers of the
ocean—preferably the top 50 metres, where light penetrates and microscopic
plants grow. Here there is not only a bountiful supply of food of a suitable
size for larvae, but also vigorous currents ideal for transporting them. This is
the case for most shallow-water animals: they almost invariably produce
plankton-eating, or planktotrophic, larvae that inhabit surface waters. But
deep-sea larvae are born hundreds or thousands of metres below this vast larder
and travelator. Getting to the top would be tough, if not impossible.

Half a century ago, the Danish biologist Gunnar Thorson maintained that if
animals living on the deep-sea floor produced small planktotrophic larvae, they
would have to swim so far to get to the sunlit upper waters that they would
starve before they got there. He reasoned that bottom-dwelling animals would
either nurture their embryos in or under their bodies until they could look
after themselves, or that the mothers must provide eggs with enough yolk to last
until the larvae settled. These nonfeeding larvae, known as lecithotrophic
(yolk-eating) larvae, would have a finite food supply and so were assumed to
stay close to home.

If this were true, how could vent species possibly travel so far? And for
that matter, how is it that some deep-sea species are found all around the
world’s oceans? To answer such questions, biologists need to know what the
larvae of deep-sea animals are like and how they behave. “Larvae are crucial for
explaining the distribution of deep-sea organisms,” says Craig Young, a larval
ecologist at Harbor Branch Oceanographic Institution in Fort Pierce, Florida.
His pioneering studies of the early stages of some of these deep-sea animals are
proving that many of the old assumptions are wildly wrong.

Since 1985, Young and his colleagues have reared the larvae of 23 species of
invertebrates from the deep-sea floor. Most of the animals were
echinoderms—sea urchins and starfish—but they also included a few
barnacles, a snail and a sipunculid worm. So far, none of Young’s larvae has
reached the stage at which it would normally settle and metamorphose into an
adult. “Even so, we can use the larvae to find out some interesting things,”
says Young. “For the first time it is possible to run physiological experiments
in the lab that allow us to answer some fundamental questions about the biology
of deep-sea species.”

For a start, most of these animals do not behave as Thorson expected. Most of
the deep-sea urchins and starfish Young has studied turned out to produce
typical plankton-eating larvae. Calculations based on the amount of energy in
the egg, the larva’s metabolic rate and the speed at which it swims, show that
some larvae are quite capable of reaching the surface. “It looks as if some
larvae should be able to do it,” says Young. Other species have larvae with
normal swimming and feeding behaviour, but which cannot reach the plankton
larder or cannot cope with the warm temperatures of the upper ocean. This does
not mean they are destined to starve, however. Experiments showed that they
quite happily eat bacteria, the most abundant organisms in the deep sea. With
this alternative supply of food, planktotrophic larvae in deep waters can
probably migrate long distances without going anywhere near the surface.

These studies show clearly that some deep-sea animals don’t stick to
Thorson’s rules. Could the same be true of the giant tube worms? Until last
autumn no one had ever seen the embryos and early larvae of a tube worm, let
alone tracked them on their travels. Then Young and a group of colleagues
succeeded in rearing some of these monster worms in the laboratory. Young’s
test-tube babies have sent a thrill through the world of deep-sea biologists.
“People are very excited about the tube worms,” he says. “There is a lot of
speculation about how they colonise these ephemeral vents. Once we have the
larvae we can find out if they ride the plumes of hot water or go to the surface
and drift around—or what.”

Young’s vestimentiferans, Lamellibrachia and Escarpia, came
from a methane seep at a depth of around 600 metres in the Gulf of Mexico. Adult
Lamellibrachia can grow 2 metres tall, forming great bushes, while the
coiled tubes of Escarpia can be a metre long. Young and his team, Elsa
Vazquez and Anna Metaxas, and deep-sea biologist Paul Tyler from the University
of Southampton, plucked the worms from the seep during a dive in the Harbor
Branch submersible Johnson-Sea-Link II. Back aboard ship, they opened them up,
extracted ripe eggs and sperm, and mixed them together. The fertilised eggs were
cultured at their normal temperature of around 9 °C. They began to divide
after 14 hours, dividing again every few hours until, after three days, they
were recognisable larvae with a covering of hair-like cilia to propel them
through the water.

All too soon the ship returned to port. “We packed them in coolers and drove
like crazy from Louisiana to the lab,” says Young. Despite the rough and ready
travel arrangements, the larvae continued to grow. Although they survived only
three weeks, this was long enough to reveal some interesting features of the
tube worm’s early life and fit together a few pieces in the puzzle of how they
might get from seep to seep.

For the first week of life, the embryos floated upwards. But after eight days
they lost their buoyancy and headed back down into deeper water. Soon after, the
Lamellibrachia young began to stretch and show the first signs of a
worm-like shape. When the larvae died, at 21 days old, they still showed no sign
of developing a mouth or the hair-like structures seen on settling juvenile
larvae. This suggests they still had some way to go before reaching the end of
their footloose life. “We don’t know how long the larvae stay in the water
column,” says Young. But with more than three weeks as a highly mobile, swimming
larva, these worms have enormous potential to travel long distances, he
says.

Unlike some of his other deep-sea larvae, Young’s vestimentiferans come
supplied with yolk and so have no need to find food. This, according to
Thorson’s dogma, should limit their horizons and keep them close to home. But
some biologists no longer believe that a big supply of yolk hampers dispersal.
“The old view that you had to have a planktonic-feeding larva to disperse well
has been thrown out,” says Tyler. “After all, a larva that needs to feed could
starve before it finds a place to settle. And some larvae with good supplies of
yolk live for a very long time.”

Young has also found examples of yolk-eating larvae that can reach the upper
ocean. The larvae of two species of deep-sea urchin, Phormosoma
placenta and Araeosoma fenestratum, for example, could reach the
surface in two to four days. An animal is unlikely to risk entering the surface
waters where they face a good chance of being eaten by predators unless there is
some advantage. In this case, the most obvious one is that transport is
faster.

The vestimentiferan larvae are unlikely to reach the surface waters, however.
A few back-of-the-envelope calculations based on the speed at which the larvae
swim suggest that it would take them 30 days to reach the upper waters. But
after only a week, the larvae lose their buoyancy and start to swim downwards.
“So my suspicion is that they disperse in deep water,” says Young. The fact that
they are mobile for so long, however, means that they can probably get high
enough to escape the immediate area of a seep and reach faster-moving waters
just above the seafloor.

Although water generally moves much faster near the surface, there are
currents only a few tens of metres above the seabed that can shift larvae quite
swiftly. For example, at a depth of 490 metres around the Bahamas there are
currents of 18 centimetres a second. A month travelling at a steady 18
centimetres a second could transport the larvae around 500 kilometres before
they settled.

Larval biologists may soon get a chance to fit more pieces into the puzzle,
as part of a new research programme called LARVE. The five-year programme, which
is expected to be funded by the US National Science Foundation, will focus on
one well-studied field of hot vents at 9° North on a stretch of mid-ocean
ridge called the East Pacific Rise, where vents open and close regularly. Larval
biologists will collect eggs and sperm, and rear larvae to find out how long
they live and whether they need to find food for their journey to a new vent. To
help them trace the movements of the animals, physical oceanographers will work
out where the plumes of superheated water that gush from vents go. Whether
“riding the plumes” really is a feasible strategy for a worm or a clam seeking a
new home, biologists do not yet know. But they are getting closer to
understanding how those strange brown remains came to be in Crawford’s
handbag.