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SILVERY trails of slime trace the path of a slow-motion rampage. These are
the telltale signs that slugs or snails have somehow found a crack in your
defences and invaded your flowerbed, greenhouse or even your home. But while
such domestic problems can usually be solved with a few strategically placed
slug pellets, the trails pose a sticky conundrum for biologists.

It’s not easy being slimy. Making mucus is an exceptionally expensive mode of
transport. And to what end? Other animals manage to creep around in a similar
fashion without producing a bed of slime. So why do the slugs and snails that
live in our gardens and on our seashores ooze mucus?

What’s becoming clear is that there are hidden benefits lurking in the
creatures’ slime. The animals recoup some of the costs of their expensive
locomotion by putting their slimy tracks to a wide range of uses, from
cultivating private gardens of algae to finding their way back home, or chasing
each other about to mate or huddle together for protection. Slime trails may
even shape the pattern of life we find on our seashores.

Mucus certainly does smooth the path of the animals that produce it, but
their mobility doesn’t depend on it. Slugs that have had their slime-producing
glands cauterised can still get about, though not as effectively as before. This
finding led researchers to suspect that slugs and snails evolved their slime to
open up new horizons—to allow them to scale sheer walls and occupy
overhangs, for example. That the animals can achieve such feats is down to the
unusual physical properties of their secretions.

Mucus is around 96 per cent water with a few salts dissolved in it, but the
rest is glycoprotein. The slime gets its special properties from the cross links
between the glycoprotein molecules, something first explored by Mark Denny of
Stanford University. Using the terrestrial slug Ariolimax columbianis,
Denny showed that mucus remains sticky under small to moderate forces, but when
a large force is applied, the cross links break apart and it becomes liquid,
reforming once the force is removed. This handy “yield and heal” property helps
the animals to get about and defy gravity.

Riding the glue wave

Slugs and snails propel themselves along with waves of contractions that
ripple along their muscular foot. Each wave starts at the animal’s tail and
travels to the head, pushing it forwards. The pressure of the leading edge of
the wave liquefies the mucus, so the wave slips through it more easily. But the
slime solidifies again as the wave passes, forming an effective glue. This cycle
acts like a ratchet, drawing the mollusc forwards and preventing it slipping
backwards. “Part of the foot is over liquid, part is over solid—which is
how an animal with only one foot can walk on glue,” says Mark Davies of the
University of Sunderland.

But this neat trick makes slugs and snails the gas guzzlers of the animal
world. Denny estimates that the cost of making slime to get about is ten times
greater than the energy other animals expend in running, swimming or even
flying. To put it another way, slugs and snails use 35 times more energy in
making mucus than they do in contracting their muscles to move themselves along,
according to Davies’s calculations. Seventy per cent of the energy that the
animals consume in food goes on making slime. “That is a huge amount,” says
Davies. “It’s nagged me for a long while—is this just a means of getting
around or is it something else?”

The first clue to slime’s hidden uses came as a result of someone not doing
the washing-up. In 1984, Valerie Connor of Central Valley Regional Water Quality
Control Board in Sacramento was studying the foraging behaviour of limpets in
the lab at the University of California in Davis. Connor was looking for a way
to track limpets in the field and had persuaded owl limpets, Lottia
gigantea, to crawl all over glass plates, hoping she might find stains that
would highlight their slimy trails. With the weekend fast approaching, and not
wanting to antagonise her colleagues by cluttering up the lab, Connor dumped the
plates in an aquarium, intending to clean them on Monday. On her return,
however, she had a surprise. “It was amazing,” she recalls, “I could see all the
trails, because they had stuff stuck to them.” Left to their own devices, green
algae had adhered to the slime trails and flourished.

This serendipitous discovery raised the possibility that the limpets might be
eating the algae growing on their trails. That’s exactly what Connor found.
What’s more, she discovered that the slime trails of the owl limpet actually
stimulated the growth of the algae that stuck to them. And when Connor looked at
the composition of the mucus, she found that it was rich in nitrogen.
Apparently, the limpets were adding fertilisers to their slime.

The advantages of growing your own food are obvious, providing you get to eat
it. Owl limpets are territorial and always return to the same place on a rock
after foraging trips, which means they can keep an eye on their algal gardens.
Even so, other animals will take advantage of the owl limpets’ horticultural
skills if they can. Connor noticed that a limpet species called Collisella
scabra sometimes hitched a ride on the owl limpet’s shell and poached its
trails. “You could see them crawl off the back end of the owl limpet and graze
their mucus trail,” she says.

Many slugs and snails are even more vulnerable to freeloaders than the owl
limpet, which is why the fertilisation strategy is far from universal. Animals
from non-territorial or aggregating species are always wandering across trails
laid down by others, making it less profitable to include fertilisers in their
secretions. Any “cheat” mutants that didn’t produce fertile slime could benefit
from the other snails’ efforts at no personal cost. Connor showed, for example,
that the trails of an aggregating limpet,Collisella digitalis, did not
boost algal growth, suggesting that they do not have fertilising additives.

“But that doesn’t mean that limpets or winkles that are not territorial won’t
be using old mucus trails as a substrate for nutrition,” says Davies. Last year,
he showed that periwinkles, Littorina littorea, grazed more heavily on
each other’s mucus trails than on alternative surfaces. In the marine
environment, slime doesn’t need any special additives to attract algae, he
points out. Seawater is full of floating plants, which cling to old mucus trails
just because they are sticky.

Following mucus trails can provide lunch, but it has other potential benefits
too. Snails and slugs travelling over the paths laid down by others might be
able to make savings in their own mucus production. Davies’s group is working on
methods to measure trail thickness to see whether animals generate less mucus
when they forage over old trails. We know that trail followers can outpace trail
blazers, suggesting that old trails may make locomotion easier. What’s more,
snails do vary the amount of mucus according to the surface under their foot. If
it is very rough, they produce more slime, explains Davies, probably to fill in
pits and provide a continuous layer on which to move.

As well as lubricating the path for those who travel on them, old trails are
also useful for navigation. Finding your way can be a problem if you are only a
few millimetres tall, have poor eyesight, and live among towering crags that are
washed twice daily by the tide. But having sticky signposts could make the task
a lot easier. When limpets return to the same patch of rock, for example, they
retrace their steps via their own slime trails rather like following a ball of
string in a labyrinth.

Non-homing species also follow trails, except that these belong to other
members of the same species. “They could be following for food, they could be
following in order to mate, or they could be following to aggregate,” says
Davies. No one is quite sure which explanation best fits the vast clumps of
thousands of winkles on the shores of New England, for example. But the animals
certainly tap into the network of slimy pathways to find one another. And to do
this, they must be able to recognise the trail of another winkle and follow it
in the right direction.

Davies has examined slime trails under a scanning electron microscope,
hunting for signposts in their structure. “There are microscopic fragments that
seem to show the axis of the trail,” he says. But so far, he’s found no
“arrowhead” that reveals the track’s direction or “polarity”. But the trail
followers may take their cue from a chemical signal rather than structural one,
he admits. If so, these creatures can perform a remarkable feat of detection far
beyond the capabilities of our instruments.

When snails meet a trail, they seem to know which direction it is going in
almost instantly. They work out the polarity of the trail by touching it with
the tentacles on their head, which are around a centimetre apart. The difference
in the concentration of any chemical cue in the trail is likely to be minuscule
over such a small distance. What’s more, the snails can tell which way a trail
was laid even if it is a week old. “The longer the trail is there, the more of
them ignore it and the more of them go the wrong way,” says Davies.

In 1992, Davies and his colleague Steve Hawkins, now at the Centre for
Coastal Marine Science in Plymouth, measured the half-life of mucus
trails—the time it takes for half the slime to disappear from the shore.
Although mucus degrades within a day or so in the heat of tropical regions, on
temperate shores winkle mucus had a typical half-life of around 12 days. The
longest lasting mucus, with a half-life of 40 days, came from the limpet
Patella vulgata.

The impact of such long-lasting mucus on rocky shores such as those around
the British coast could be startling, according to Davies. If snails can
recognise each other’s trails, perhaps other creatures can do so too. Many
sedentary animals such as barnacles have larvae that float about among the
plankton. The distribution of such animals on the shore depends on where the
larvae land and develop into adults. “If most of the shore is covered in mucus
of varying ages and types, that may well be important in determining what
settles where,” says Davies.

Sticky slime trails that contain nutrients are a good place for algal spores
to settle, if only they can avoid being grazed by the snails. On the other hand,
Davies’s group has found that barnacle larvae avoid settling on dog whelk
mucus—the calling card of a voracious barnacle predator. Slime also
effects the distribution of snails themselves. Many species have evolved
additives that make their mucus unpalatable or too slippery for predatory snails
to handle. The predators, in turn, lace their slime with toxins to fight back.
So the little-studied coat of mucus left on temperate shores by the busy passage
of its denizens could be crucial in understanding the patterns of life found
there.

And that’s not the end of it. Along with its other vital roles, slime even
seems to promote good health. Davies has found that snails in heavily polluted
waters concentrate metals in their mucus. They can secrete as much lead, for
example, in one day as they usually contain in their entire body. It appears to
be a very efficient means of decontamination.

Doubtless, there are slimy secrets waiting to be unlocked—not least its
benefits to terrestrial animals. But explaining the well-lubricated lifestyle of
slugs and snails is now less sticky. “I was stunned when I found out how much of
their energy snails put into slime,” says Connor. “It was only after figuring
out that they used it to do almost everything that it made a little more sense
to me.”

IF there’s one thing a love-struck mollusc just can’t seem to do without,
it’s a glob of mucus. Take slugs, says Christopher Viney, a chemist at
Heriot-Watt University in Edinburgh. “Some species crawl along a branch, lower
themselves down and perform sex at the end of several feet of snotty rope.” It’s
not the romantic tryst itself that interests Viney and Nicola Cowan, but the way
the “snot” rope forms. It seems to self-assemble from the same mucus that the
slug uses to crawl around on, says Viney. The mystery is how the creature
transforms a sticky lubricant into long fibrous rods strong enough to support
the weight of a pair of passionate slugs.

Slime is just as important in the strange mating rituals of the common garden
snail. These hermaphrodite molluscs fire mucus-tipped “love darts” at each other
just before mating. They are not aiming for the heart though. Instead, Cupid’s
plan is that a smitten snail will fall for the incoming sperm. Joris Keane and
Ronald Chase at McGill University discovered that the snail’s needle-sharp
calcareous love dart is little more than a tiny hypodermic device, designed to
inject its partner with mucus laced with a mysterious pheromone. This chemical
seems to trigger muscular contractions in the recipient’s reproductive tract so
that when mating does occur, sperm released by the dart-firer is guided towards
the sacs where fertilisation takes place. Ben Crystall