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Sea snakes are the most abundant reptiles on Earth. More venomous than
cobras and capable of spending three hours underwater at a stretch, they
are superbly adapted to life in the warm seas. With the onset of global
warming, they could find their habitat expanding. One consequence could
be the arrival of Pelamis platurus, the yellow-bellied sea snake, along
the shores of Western Europe.

Most of the world’s 47 species of sea snake live in coastal or estuarine
waters of Southeast Asia and Northern Australia, but Pelamis is truly oceanic.
It is found in the Indian and Pacific Oceans, from the Cape of Good Hope
to the Gulf of Panama. Apparently only the land barrier of the Isthmus of
Panama – and a certain reluctance to brave the cold waters around Cape Horn
or the Cape of Good Hope – prevents this cosmopolitan reptile from invading
the Atlantic Ocean.

Yellow-bellied sea snakes make their most dramatic appearances at drift
lines – slicks of floating debris, weed and sticks which are brought together
by surface currents. Drift lines attract fish, providing rich pickings for
snakes. In the Gulf of Panama, drift lines several kilometres long, containing
hundreds of floating snakes, are a regular feature. These aggregations sometimes
reach huge proportions. In 1932, millions of Astrotia stokesii, a relative
of Pelamis, congregated in the Strait of Malacca, off Malaysia, forming
a line of snakes 3 metres wide and 100 kilometres long.

Pelamis is a relatively small snake, rarely more than 80 centimetres
long and weighing less than 200 grams. Its distinctive livery of black and
yellow acts as an unmistakable warning to potential predators. Pelamis’s
venom, delivered through fangs just 1.5 millimetres long, is extraordinarily
potent. The estimated lethal dose of cobra venom. Yet death or serious injury
due to Pelamis is rather rare. Out of the water, the snake is completely
helpless, writhing ineffectually and unable to strike. In Southeast Asia,
sea snakes of other species still kill numbers of fishermen every year,
but treatment with modern anti-venom serum can prevent death if it is delivered
in time.

Pelamis subsists on a diet of fish, but it does not actively seek its
prey. Instead it lies quietly at the sea’s surface and waits for small fish
to be attracted to it – as they are to any floating object. Vision does
not seem to be important in detecting prey, which is mainly sensed by vibration
and secondarily by odour or taste. Pelamis can swim backwards and forwards
with equal facility to adjust its position for the strike. When suitably
placed, it strikes with a sideways motion of the head. It invariably swallows
its prey headfirst.

Pelamis lives out its whole life cycle in the open ocean. Oscar Vallarino
of ANCON, the Panamanian Nature Conservation Organisation, has observed
snakes copulating and giving birth to live young at sea. A brood may contain
up to seven baby snakes about 20 centimetres in length. There is some uncertainty
about the gestation period, but there appears to be an annual breeding season.

Males become sexually mature at a length of 50 centimetres, females
when slightly larger. It is difficult to deduce the age of a sea snake from
its length, but researchers believe that Pelamis becomes sexually active
at one or two years of age.

Any object floating in the sea tends to acquire growths such as algae
or barnacles. Pelamis deals with the problem in a remarkable fashion. It
ties an overhand knot in its body and runs the knot along from one end to
ther other, cleaning the skin in the process. This ability to tie itself
in knots also comes in useful during moulting.

On land, snakes shed their old skin by rubbing up against solid surfaces.
Deprived of this ancient technique, a sea snake uses the knotting action
to free the old skin.

In common with all oceanic animals, Pelamis has no experience of solid
surfaces. Like certain ocean-going fishes, which cannot adapt to life in
an aquarium and knock themselves to death against the walls, Pelamis needs
special treatment in captivity. In a standard aquarium, its snout gets injured
and turns septic. Researchers at the Smithsonian Tropical Research Institute,
Panama have installed special soft linings to their tanks with a view to
increasing the survival of captive snakes.

Yellow-bellied sea snakes are generally regarded as surface dwellers,
but recent research has altered that picture. Ira Rubinoff, working with
Jorge Motta and Jeffrey Graham at the Smithsonian Tropical Research Institute,
monitored the diving behaviour of individual snakes by equipping them with
pressure sensitive acoustic transmitters. The team followed the snakes’
progress by pursuing them in a boat carrying ultrasonic receiving gear.
The research showed that snakes spend an average of 87 per cent of their
time below the surface. Time at the surface between dives could be as little
as one second. The longest dive observed by the team lasted 213 minutes
and one snake descended to a depth of 50 metres.

The sea snake’s underwater performance relies on some ingenious physiological
machinery. There is a single lung, which runs from head to tail and which,
when filled with air at the surface, occupies about 10 per cent of the body’s
volume. This, taken together with the reserves of oxygen in the blood, would
allow the snake to survive underwater for just 17 minutes. Yet a fifth of
all dives last for more than one hour. To investigage how snakes can spend
so much time under water, Rubinoff and Motta joined Graham at the Scripps
Institution of Oceanography in La Jolla, California, where they put snakes
through a series of trials in a tank 10 metres deep. They found that the
snake uses its lung not only to store oxygen but also to control buoyancy.

Prior to a deep dive, the snake overinflates the lung, possibly to as
much as 20 per cent of body volume. The dive itself is composed of four
phases. Phase one is a descent at about 5 metres per minute. At this stage
the snake must swim vigorously to overcome the positive buoyancy of the
lung. As it descends, the pressure rises, so the volume of the lung decreases
in accordance with Boyle’s law. At 10 metres where the total pressure is
twice atmospheric pressure, the lung’s volume has declined to half its volume
at the surface. In phase two of the dive there is a ‘bounce ascent’, during
which the snake bobs up at a rate of 1.7 metres per minute until it reaches
the depth at which it has no tendency either to sink or rise. Most of the
remainder of the dive – phase three – is gradual ascent at 0.11 metres per
minute. The dive ends with a rapid ascent at 3 to 4 metres per minute.

Snakes are aware of the depth they are aiming for and they fill the
lung accordingly. However the tests in the tank showed that correct inflation
demands some practice; naive snakes often made mistakes. Once the snake
is in phase three of the dive – hovering in mid-water – its need for energy
is at a minimum. This maximises the time for which it can dive on a lungful
of air. As oxygen is consumed, the snake gradually becomes less buoyant.
It responds by swimming slowly upwards, decreasing the pressure on its lung,
which expands and increases its buoyancy, allowing it to hover once more.

Yet the overfilled lung and this economical use of oxygen cannot explain
the duration of the longest dives. Pelamis makes up the shortfall by absorbing
oxygen through the skin from the surrounding water. So effective is this
ploy that it provides nearly a third of the submerged snake’s requirement
for oxygen.

The heart and circulation of the sea snake also play an important role.
In common with most reptiles, the heart of the sea snake has a single, imperfectly
divided, ventricle. This ‘hole in the heart’ condition, often described
as primitive compared to that of the mammalian heart, is of immense use
to the sea snake. In mammals blood circulates around the body, back to the
heart, then to the lungs to pick up oxygen before returning to the heart
and embarking on another circuit. If sea snakes had this type of circulation,
the lung would quickly lose its oxygen. Instead the snake forces blood to
by-pass the lung – eking out precious supplies of air – and sends it to
capillaries beneath the skin where it picks up oxygen from the water and
loses carbon dioxide.

This arrangement also allows the snake to dump nitrogen from its blood
via the skin into the sea. If the snake could not do this, nitrogen from
the air in its lung would dissolve in the blood in large amounts. When it
surfaced, the nitrogen would come out of solution and form bubbles in the
blood, resulting in the condition known as the bends, which can be lethal
to human divers. The rapidity with which the snake ascends at the end of
its dive confirms that it does not need to spend time on the decompression
stops familiar to human divers.

Hidden depths

The fact that sea snakes spend 87 per cent of their time underwater
raises same important questions. Ecologists have traditionally estimated
the size of the sea snake population by counting floating animals. Such
censuses presumably underestimate the true abundance of Pelamis, which may
well be the most numerous species of reptile on Earth.

Another question concerns the purpose of diving. If Pelamis feeds at
the surface, why does it spend so much time underwater? One answer might
be to avoid certain predators – although most are effectively deterred by
its warning colours. Nevertheless recent observations, by Vallarino and
Paul Weldon, show that a significant porportion of snakes in the Gulf of
Chiriqui, Panama, bear wounds or scars. Older snakes carry more scars –
a state of affairs that suggests that wounding could be a regular occurrence.
Brushes with the propellers of outboard motors, or attacks by birds, could
be to blame. Yellow-bellied sea snakes show no great desire to escape if
confronted while floating at the surface. They are not fast swimmers and
can be scooped up in a hand net without difficulty.

Another reason for spending long periods beneath the surface could be
the danger of floating on a stormy sea. Snakes reposing at the surface in
rough weather would be tossed about by the waves and would tend to suffer
injuries to the spine. I suggest that snakes keep away from the surface
in all but calm weather in order to avoid mechanical damage.

Tracking experiments have revealed an important seasonal change in diving
habits. In the wet season, Pelamis takes longer, deeper dives than in the
dry season. The difference in depth is particularly striking. Snakes descend
to a maximum depth of 50 metres in the wet season, but only to a maximum
of 20 metres in the dry season. The explanation hinges on the snakes’ avoidance
of cold water. The dry season (December to April) brings cold water near
to the surface, forcing the snakes to forego the deeper expeditions characteristic
of the wet season.

The limit of 20 metres is significant: it is the depth at which the
sea temperature drops to 18 °C. Research has shown that Pelamis will
not voluntarily enter water whose temperature is below this critical level.
This limitation both curtails its diving activities and restricts its geographical
distribution.

In the Eastern Pacific, upwelling cold water off Peru and California
locks the species into tropical latitudes. In the South Atlantic, the Benguela
upwelling off Namibia provides an effective barrier to ingress of sea snakes
from the Indian Ocean. The world’s thriving population of Pelamis floats
as it were in a basin of warm water, surrounded by inhospitable cold. If
sea temperatures and ocean currents change, then Pelamis could break out
of its present straitjacket. An increase in its range could be one of the
first indicators of large-scale changes resulting from global warming.

* * *

The Panama connection, a route to disaster?

The absence of Pelamis and its allies from the Atlantic is a consequence
of the short evolutionary history of modern sea snakes. Geologists have
found fossils of marine snakes in Europe, North America, South America and
Africa, but these represent an ancient lineage which became extinct about
35 million years ago. Today’s sea snakes are descended instead from the
cobras, which did not themselves appear until the Miocene, less than 25
million years ago. Pelamis itself presumably reached the Eastern Pacific
less than three million years ago – after the Isthmus of Panama rose from
the sea between North and South America. This accident of history has made
it possible for the Atlantic to be a snake-free ocean.

The distribution of sea snakes has been studied in detail for 20 years
by researchers at the Smithsonian Tropical Research Institute, Panama, under
the direction of Ira Rubinoff. Rubinoff, together with Jeffrey Graham and
Max Hecht, has found that sea temperature also limits the geographical range
of Pelamis. Tests on captive snakes show that they succumb if exposed to
the relatively mild temperature of 11 °C. As far as the geography of
the species is concerned, a more significant temperature seems to be 18
°C, the temperature below which feeding ceases. Pelamis is absent from
regions of the Indian and Pacific Oceans in which the sea temperature is
below this critical value of 18 °C.

Such sensitivity prevents Pelamis from migrating into the Atlantic via
Cape Horn or the Cape of Good Hope. However, Pelamis could conceivably colonise
the Atlantic through a new, or refurbished, Panama Canal. The Caribbean
Sea and the Sargasso Sea would make eminently suitable living quarters.
Snakes might then be transported across the ocean by the Gulf Stream, reaching
the English Channel in the summer months.

The present Panama Canal is a fresh water canal, with a series of locks,
so it presents an insurmountable barrier to marine organisms. However, the
1960s saw several proposals for the construction of a canal at sea level,
which would have resulted in free passage of marine life between the oceans.
Similar migrations have taken place via the Suez Canal, allowing fish from
the Red Sea to colonise the Eastern Mediterranean.

Such movements are called Lessepian migrations, after Ferdinand de Lesseps,
who designed the Suez Canal and was responsible for pioneering work on the
Panama Canal. Although the Suez canal has not caused a major ecological
catastrophe (there are no sea snakes in the Red Sea), biologists foresaw
that sea snakes could prove troublesome if a new Panama Canal were ever
built.

Some years ago, Rubinoff and his colleague Chaim Kropach showed that
Atlantic and Pacific predators react differently to yellow-bellied sea snakes.
In the Pacific, would-be predators are much warier and, even when hungry,
cannot be persuaded to eat dead Pelamis. Atlantic predators, on the other
hand, are initially wary, but hunger will drive them to attack live snakes,
often with fatal results. The tests yielded a startling insight into the
sort of consequences that might accompany any invasion of the Atlantic by
Pelamis. One Atlantic snapper fish consumed 22 snakes in 31 days before
succumbing to snakebite.

The prospects of building a new canal at sea level have now receded.
(One proposal was linked to the US ‘Plowshare’ programme for the peaceful
use of nuclear explosives. This scheme, now discredited, failed to consider
the fate of the cubic kilometres of material thrown into the air by nuclear
explosions.) The capacity of the existing canal has been increased by dredging,
installation of navigational aids and provision of lighting. As a result,
ships can negotiate the locks at night. Further improvements may follow,
such as the creation of a deeper or wider channel. Ecologists are concerned
that an enlarged canal may eventually need regular infusions of sea water.
If so, the Panama Canal could present less of a barrier to marine plants
and animals, sea snakes included.

Dr I G (Monty) Priede teaches zoology at the University of Aberdeen
and was a visiting fellow at the Smithsonian Tropical Research Institute.