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Few sights are as spectacular as a school of fish swimming in close
formation. The school is such a cohesive unit that it seems to behave almost
as a single large ‘superorganism’. Yet fish by no means have a monopoly
on schooling. Research has now uncovered many cases of schooling among crustaceans
and other invertebrate animals. These creatures behave in ways that are
strongly reminiscent of fish-suggesting that similar selective pressures
have led the two groups along remarkably similar evolutionary paths.

Schools of crustaceans play an important part in the ecology of the
seas. Antarctic krill Euphausia superba, for example, assemble in the Southern
Ocean in mighty schools that can stretch for several kilometres. People
who have seen such schools from the air describe them as resembling giant,
slow moving amoebae. A cubic metre of water near the surface may hold 30
000 or more individuals, with a combined weight of some 16 kilograms. These
gregarious creatures occupy a pivotal position in Antarctic food webs, providing
abundant food for squid, fish, seals, sea birds and whales. If krill did
not school, the great baleen whales could not collect sufficient food and
the commercial krill fishery would never have arisen.

Until relatively recently, biologists were sceptical about the true
nature of invertebrate schools. Many believed that they were transient assemblages,
brought together by wind, currents and waves, or simply drawn to the same
abundant source of food. In some cases-jellyfish or salps, for example-this
view may be appropriate. Such groupings cannot properly be called schools
(social groups whose members are evenly spaced and facing the same way),
or even swarms (cohesive groupings whose members are not facing the same
way).

Yet these objections cannot be levelled at gatherings of many other,
more mobile creatures. Such schools are not simply created by currents,
because they maintain their integrity in rough seas and when attacked by
predators. It is now more common to regard these creatures as naturally
gregarious throughout their lives. Crustaceans of many kinds fall into this
category, such as mysids (shrimp-like creatures with a brood pouch), certain
decapods (shrimps and their allies), cladocerans (water fleas) and copepods
(small crustaceans common in surface waters).

Recent research by Dominic O’Brien at the University of Tasmania shows
that crustaceans arrange themselves in schools in much the same way as fish,
tadpoles or squid-or even birds in flocks. Like fish, crustaceans prefer
to swim in certain positions relative to other members of the school, avoiding
a position directly below or directly above a neighbour. The internal structure
of the school is relatively insensitive to physical disruption, but it changes
dramatically if a predator appears. This contrast demonstrates that the
structure of the school is primarily a consequence of the behaviour of its
members, not a result of external forces, such as currents or wave action.

Because schooling is an active affair, it must bring same important
benefits. At first sight, it would appear to make animals more visible and
attractive to hungry predators, but first impressions can be misleading.
When a school forms it leaves vast tracts of empty water, thereby reducing
the chances of a predator picking up its trail. Bigger groups are better
at maintaining surveillance, so they spot enemies more smartly. From a distance
same predators may interpret a school as one huge animal and give it a wide
berth. When the predator does finally attack, it quickly eats its fill and
so most members of the school make good their escape. Although the predator
will certainly eat some animals in the school, the probability that any
one particular member will become a meal is reassuringly low.

In addition to bringing these passive advantages, schooling permits
more active defence mechanisms to develop. At the first sign of danger,
the school becomes more compact and more polarised. If individuals in a
school are so close together that a predator’s senses cannot resolve them,
they may escape recognition altogether. Once the school has compacted and
polarised, it is ready to execute a range of escape manoeuvres that rival
those evolved by fish. The range of escape responses displayed by mysids
and euphausiids has recently been described by O’Brien and myself.

Some such manoeuvres are simply aimed at reducing the chance of contacting
a predator in the first place. The school may accelerate away, or simply
rely on camouflage to remain hidden. One ingenious technique, deployed by
some pelagic molluscs and copepods, is to freeze-a ploy designed to defeat
predators that hunt by detecting turbulence, or vibrations, created by their
prey.

Once a predator breaches this first tier of defence, a suite of coordinated
evasion techniques cames into play. The individuals closest to the point
of attack begin to accelerate and the school may split-although it will
reform later when the threat has subsided. At this stage crustaceans adopt
tactics that are virtually identical to those of frightened fish. One technique
is flash expansion, which involves an explosive acceleration of animals
away from the centre of the school and out of the path of the predator.
Another technique is the fountain effect, in which the school splits in
front of the attacker and reforms at its rear. Coordinated flipping of tails
may also occur-a ploy that is reminiscent of the ‘skittering’ response,
or startle acceleration, of European minnows.

The final stages of the attack evoke a third tier of defence. This is
‘do or die’ response, which undermines the cohesion of the school. In crustaceans,
escape usually involves a series of leaps brought about by vigorous flips
of the tail. Although the direction of travel tends to be rather unpredictable,
tail flips often propel the animal out of the path of an oncoming predator.

Different situations demand different defences. Some large predators,
notably the baleen whales, threaten the entire school, so the best defence
is to avoid detection altogether. Should an attack be mounted, the best
policy is to reduce the density of the school at the point of attack. Evolution
has witnessed an arms race between krill and the baleen whales. The whales’
intention is to herd the prey into a small area, so as to maximise their
catch; the prey, once capture is imminent, must sacrifice the structure
of the school and present the least attractive target possible. To exploit
krill, the whales have had to evolve a broad array of attack behaviour.
Bill Hamner of the University of California in Los Angeles has described
how humpback whales herd their prey by, surrounding them with bubbles and
rapidly skimming the surface.

Smaller predators preying on the margins of the school present different
problems. They are more likely to be greeted by dazzling and confusing displays
of synchronised swimming. The response used in any particular case depends
on the size of the predator and its proximity to the school. This flexibility
is again highly reminiscent of fish schools.

Protection from predators is not the only benefit brought by schooling.
For fish and birds, group living simplifies the task of finding food. A
group member that stumbles on a rich source of food betrays its find by
its behaviour, attracting its fellows, who then join in the feast. Crustaceans
may behave in similar ways. The particles that they filter from the water
are patchily distributed and many crustaceans are expert at following scent
trails. It is likely that crustaceans in schools monitor their companions’
behaviour so as to cash in on their good fortune.

Yet opinions are divided over whether the individuals in a crustacean
school capture more food as a result of group living than they could alone.
Competition in a school can be intense, prompting the evolution of complex
feeding behaviour. Makoto Omori at the Tokyo University of Fisheries and
Hamner in Los Angeles have studied the technique of certain mysids, which
capture a particle of food and then circle around to the back of the school
before beginning to feed. This behaviour might reduce the risk that competitors
will see the food or taste it in the feeder’s slipstream.

Life in a school may be competitive, but it also creates social opportunities.
Same biologists have observed schools of sexually mature krill, or mysids,
and concluded that the animals have assembled solely for sexual purposes.
Although group living does remove the need for crustaceans to search for
mates, this cannot be the sole reason for schooling. Many crustaceans come
together for life and researchers have uncovered numerous instances of single
sex schools and schools containing only immature animals.

In any case, schooling may not invariably enhance fertility. As numbers
rise and as food becomes locally scarce, females may find it harder to produce
a full clutch of eggs. In very overcrowded conditions another effect could
came into play. Robert Clutter, of the Institute of Marine Biology at the
University of Hawaii, describes how immature and mature stages of certain
mysids, for example, live in separate schools, but a shortage of suitable
living space may force the schools to overlap. The adults may then begin
to dine on the youngsters. Antarctic krill have found a way of circumventing
similar problems. Their eggs quickly sink into the depths immediately after
laying-an arrangement that may have evolved to reduce the risk of cannibalism.

As these cases show, there are costs and benefits associated with schooling.
Rather than concentrating on the synchrony and homogeneity of the school,
Tony Pitcher of the University of Wales in Bangor puts forward the view
that fish schools are an assemblage of selfish individuals, who are constantly
weighing up these costs and benefits. Protection from predators, for example,
demands as large a school as possible, but beyond a certain point, competition
for food, or reproductive dangers, might prompt a reassessment. Newcomers
would then be discouraged from joining the school. Such considerations may
explain the behaviour of certain mysids, which come together in large clusters
containing both schools and swarms. Numbers within the various subgroups
are constantly changing. It is as if the mysids are for ever considering
their position, weighing up the profits to be gained and losses to be incurred
by belonging to any particular subgroup-and acting accordingly.

The circumstances of the moment seem to dictate the optimum size of
a school. Yet the urge to remain in a group is so strong that if the optimum
size is exceeded, supernumerary animals will simply join another school.
Individuals that elect to go it alone are an open invitation to predators.

David Ritz lectures in zoology at the University of Tasmania.