An illustrated account of the survival skills of marine organisms
during tropical cyclones
By Dr J Floor Anthoni
Cyclone Heta has been extraordinarily damaging
to the buildings high above the sea and also underwater much life has been
eradicated, but there are notable survivors. Continuing business as usual,
they take possession of new territories while bolstering their numbers.
It gives diving in Niue a totally new perspective, seldom found anywhere
in the world. This article looks at the Niuean situation and why some organisms
survived so well. The storm's main survivors are those that can hide inside
hollows and crevices in the barren rock. Of these, the sea urchins are
entirely impregnable, but there are others too.
introduction: how did I get interested
in Niue and why? Where is Niue, by the way?
where the cyclone struck: cyclones ravage
the protected side, whereas the wild side of the island is relatively better
is a very small island of about 250 square kilometre lying North-East of
New Zealand, 10 degrees past the date meridian and 19 degrees south of
the Equator (170ºW, 19ºS). One can fit Niue in Lake Taupo twice,
with room to spare, as it is oval shaped about 20 by 15 km. On the way
to Niue by sea (2400km), one would pass the Kermadec Islands (800-1000km)
which form NZ's northernmost outcrop. In May 2002 I visited these islands
and made some ecological discoveries that, when correct, should apply equally
to Niue, another isolated island amidst an ocean desert. The map shows
Niue amongst Pacific island groups, and its Exclusive Economic Zone (EEZ)
of 390,000km2 in yellow.
Apart from a desire to dive in clear warm water, Niue attracted me because
of its (presumed) simple ecology and resemblance to the Kermadec Islands.
We had to postpone our trip planned for September 2003, and then came Cyclone
Heta (January 2004), which should have extinguished my desire since it
destroyed so much of Niue both above and under water. This however, made
the visit even more attractive for me, because where on Earth could one
study the (natural) degradation by the largest cyclone ever measured on
an island that does not have any unnatural degradation from runoff, sewage
or industrial pollution (but some from fishing)?
Niue suited me to a tee, because where could one find a remote tropical
island with incredibly friendly and civilised people, educated the New
Zealand way and all speaking English? It is a place where you won't need
to lock your car because it is entirely safe. No traffic lights, traffic
pile-ups, Warrants Of Fitness, OSH, speed traps and drunk-driving blitzes.
It's a place where everyone greets everyone else (and perhaps also knows
everyone else), where police and customs make jokes and smile, where nobody
hassles you for your money, where you can go about your business in your
own way, where you don't find DO NOT signs, no speed limits (but everyone
drives 50-60km/h) and other niceties of our civilisation which we deem
But it is also a poor country, with precious few resources, a top heavy
government bureaucracy and lots of aid money that does not want to cling
to this island. It suffers from hemorrhaging its own people to Auckland
where 20,000 live as only 1100 remain on the island. Translate this to
mean that living on Niue has tremendous disadvantages. Many houses (4 out
of 5?) stand empty and derelict as mementos of more populated times.
But for the tourist it means warm but not hot days, with an annual average
temperature of 24.7ºC, a cool breeze during the day and balmy evenings.
The water is pleasant but not warm, between 25 and 27ºC (although
this has never been measured over time). You need a wet suit for diving,
but you can swim pleasantly for an hour or more in winter (but watch that
sun burn). Saturdays and Sundays the island appears deserted as people
have traditional things to do. The Saturday is for working the land or
for communal working-bees while the Sunday is strictly obeyed for rest,
and people go to church twice or more. Boats are not allowed to go out
and there is no diving or fishing.
Diving in Niue is an experience because of its very clear water with
visibility between 30 and 60m, and usually 50m. It gives new meaning to
snorkelling when, from the surface, details can be identified at 30m deep.
The environment is as expected poor in both biodiversity and fish densities
but that is made good by extensive caves in shallow water and a community
of survivors, each having some special adaptation in order to survive in
these austere and exposed conditions. That is what this story is about
- diving with the survivors.
f044719: a diver hangs motionless between surface an bottom,
experiencing the extreme clarity of the water.
f045120: snorkelling is a new experience, being able to distinguish
a sea cucumber at 30m depth. This photo was taken with a normal 28mm lens
to avoid distortion.
f044805: a diver enters one of many caves honeycombing the
coralline flats surrounding the island.
f044513: snorkelling Limu Pools where cool fresh water flows
over warm salt water.
f044726: a typical dive scene near the Matavai Resort, where
most corals have been spared Heta's wrath.
f044723: an Acropora plate coral reaches for the sky,
having suffered two chips from last storm.
Where the cyclone struck Niue is oval shaped and located at the southern end of cyclone paths,
its north-western side struck by cyclones, whereas its south-eastern side
continually buffeted by deep swell powered by a moderate but constant breeze
of 10-20 knot, the trade winds. These do not blow all year round and come
to a calm in summer (Jan-Apr).
The map shows how
Niue has two distinctly different sides, one buffeted by mild trade winds,
the other sheltered most of the time, but occasionally struck by the worst
a cyclone can offer. Heta struck in January 2004, Ofa in 1990 and one before
that in 1979. The map also shows estimated damage from Heta by the width
of the red contour. Due to the shape of the sea bottom at Alofi and because
its coast faced the storm head-on, most of the damage occurred here. Going
north, the coast veers off the storm, but then again facing the full brunt
between Makefu and Uluvehi. But because the coast runs steeper without
a platform, waves must have bounced their energy back to sea rather than
dissipating it on the shore. Some damage was caused between Tepa point
and Avatele, but most of the damage there originated from earlier cyclones.
Cyclones are just large storms which have the habit of laying bare an
underwater zone or barren. In New Zealand such storms remove seaweeds,
barrens, which are then maintained and widened by grazers
of which the sea urchin and Cooks turban shell are the most voracious.
Likewise, regular cyclones cause barrens, roughly corresponding to the
red zone on the map. After removal of proud corals, the barrens in Niue
really look bare, but on closer inspection, produce a rich supply of fast
growing algae. Corals do not easily re-conquer these barrens because of
the many biting teeth here and because corals grow rather slow in this
cool tropical sea with very little nourishment. Not surprisingly, most
of the marine organisms here are grazers, from snails to sea urchins to
grazing fish. They truly benefit from cyclones and they are the true survivors.
But grazing alone is not a sufficient recipe for survival.
f044916: the exposed side of a coral bommy shows up to 50%
live coral cover but also a large proportion of coral-building algae. Near
f044918: the exposed side of same coral bommy shows only
about 5% live coral. It is a barren, still growing in size due to the coralline
algae that survive storms better.
f043705: on closer inspection, the barrens are very productive,
supporting armies of grazers like these nocturnal snails, nocturnal sea
urchins and daytime fish.
The large scrape marks are from parrot fishes; the thinner
ones from surgeon fishes and the crisscross marks from sea urchins.
f043801: seaweeds survive the chaos by 'turfing', growing
old and reproducing asexually by means of runners. In this picture
a green seaweed and several red seaweeds colonise the area by turfing it
over with runners. Many cover themselves in coral sand as a protection
against sand-blasting and over-enthusiastic grazers.
f043512: this nocturnal sea urchin has a strong homing instinct,
enabling it to venture far from its daytime hideout. This homing knowledge
is critical to its survival.
f043513: this robust grazing snail has a very small
opening to protect it from crab attacks. Through the gap it extends a wide
and strong foot, enabling it to graze on wave-swept platforms and to weather
f043607: Niue has a surprisingly large number of sea
urchin species, nearly all well hidden during the day. This bar-spined
Diadema species folds its long spines into a cylinder as it hides away
in a narrow slit or tunnel. (Diadema setosum)
f043526: this beautiful large soft-shelled shore crab
has long and strong legs, enabling it to scoot over the coral flats and
through rock pools. It can also stay out of the water for some time
to weather storms. It feeds mainly on algae. (Grapsus
f044913: by burrowing into the limestone coral rock,
these needle urchins are protected against the worst conditions. They are
usually found where indeed the environment is pummelled with loose debris.
f045103: most of the fish in Niue are grazers, swarming the
reefs like these surgeonfishes, grazing the shallow platforms during high
tide and visiting rock pools. Here is a school of speckled surgeonfish
The marine environment With barren zones extending to over 30m deep off Alofi, what would
the other side look like? In September the trade winds are well established
and diving there amounts to suicide. But between Tepa Pt (pronounce: sepa)
and Limufuafua Pt at the island's very southern end, the coast curves away
from the tradewind tumult, offering a chance for a dive. This dive should
show us what the protected wild coast looks like under water, and thus
what the marine environment looks like without regular destruction from
It became a memorable dive with minimal gear to reduce friction, only a
pony tank and no buoyancy compensator. All the same, the swell moved me
over distances of 7-15m across a razor sharp reef, ready to shred suit
and skin. The only way to take photos was to time the position of the shot
with the return of the water's swing, quickly planting the tripod and taking
a photo. I didn't expect them to look any good, but the 13mm fish eye lens
was quite forgiving.
f220511: It's a calm day and the sea does not show white
caps. Yet the swell, animated by long periods of winds blowing from one
direction, breaks and spills over the coral flats, making diving here impossible.
f044715: the marine environment of the wild side is rich
in corals and desirable seafood like this large Tridacna giant clam (Tridacna
maxima), showing its outside folds undamaged by storms. Yet even here
finds many barren patches formed by coralline algae, an important reef
builder, if not THE most important reef builder.
From this relatively pristine environment to one of the favourite dive
spots near the Matavai Resort, shows the gradual procession to the barrens
f044927: the corals near the Matavai Resort show variety
and damage, recent and from storms long past. It shows broken corals amongst
the living, and in rubble fields.
f044721: down from 25m depth, the plate corals look untarnished
as they dominate space, being able to grow much deeper still, due to the
very clear water. It is here where the barrens stop and the deep reef begins.
f044933: Acropora corals grow relatively fast like
this kind of stagshorn coral. They survive by being hard and strong, yet
growing relatively fast, designed to dominate space first. On right two
f044934: one of the survival strategies of some corals is
not being brittle but leathery, like this coral with long polyps resembling
an anemone. (Sarcophyton elegans)
The Alofi scene is that of extensive rubble fields and barren rocks,
with coral-grown ridges descending from 15m to 30m. These suffered most
damage because they are located out of reach of debris and damage by normal
storms. The lesson is that large storms nibble away at the edges untouched
by lesser storms, causing large damage to those corals that managed to
grow there inbetween. Such large storms widen the grazed barrens, which
may or may not be repopulated with corals, depending on the voracity of
existing grazers and how these survived the storm.
f044601: barren and broken rocks, yet providing ample
food for an army of grazers. In the background an extensive rubble field.
f044603: barren and broken rock and bent steel beams are
now but mementos of the storm's brute force.
The survivors and their strategies We were lucky to photograph many of Niue's common animals, being able
to show how each has a strategy for survival. By means of photographs we
like to tell their stories here.
f043511: the sea snake is an active hunter by day but sleeps
by night. It lives happily on land in case of averse conditions in the
sea. These poisonous snakes have intimate knowledge of their ranges, knowing
where to leave the sea inside underwater caves, where they also lay their
large eggs. This sea snake is sleeping in a protective cavity in the coral
f220922: a very effective survival strategy is to leave the
water. These land crabs live on land for most of their lives, but return
their eggs to the sea to complete their life cycles. This large crab was
encountered by night on the coral-sealed road. It uses one leg like a blind
man's cane. (Discoplax longipes)
f043535: this most amazing rock skipping blenny leaves the
water for long times in the day, being able to move fast over the rocks
and over the water's surface. It shelters in small cracks and hollows in
f043630: the banded coral shrimp (Stenopus hispidus)
surprised us most because it survived inside rockpools and shallow water,
even though being so fragile. It shows just how much shelter can be found
inside coral cavities. Being a cleaner shrimp, fish bring its food towards
it, so it does not need to expose itself in search of food.
f043906: this large armoured pineapple sea cucumber survived
by being armoured on its back and having thousands of strong tube feet
to hold itself in place. It too has a good knowledge of where it lives.
f044213: the armour plates of the pineapple sea cucumber
resemble the scales of a pineapple. This sea cucumber feels hard and sharp
to the touch. Its strategy is to find a sheltered spot, suck on hard to
bare rock, and survive being pummelled by debris.
f043928 (left) and f043929 above: the slipper coral is a
wandering 'mushroom' coral which does not attach itself to the coral rock.
It is claimed that it can support itself on the tentacles extending from
its hollow foot. By night it shows long polyps. Its strategy is to be moved
by waves, such that it always lies on top of other corals, and cannot easily
be outgrown by them. (Herpolitha sp)
f044104: just before sunset there were no feather stars on
the reefs, yet before we began our night dive, they were all there, occupying
strategic commanding views. Their survival strategy is their homing instinct
and being able to walk distances of 2-10m.
f044106: if anyone told me that feather stars crawl on all
their arms, I would not have believed it. Yet here it is. This one pulled
with one half of its arms, pushing with the others, developing a speed
of over 5cm per second! Fact is often stranger than fiction.
f044135: these colourful and dainty christmas tree fanworms
live inside tubes encapsulated by growing Porites coral. They can
withdraw inside their tubes, closing the hatch with the caps shown here.
f044133: this strange snail lives its entire life in a horn
cemented inside a Porites coral. It can filter plankton or throw
a sticky catch net as shown here, to catch food. Later it will pull the
net in and gobble it up, including any caught food.
Survival does not only depend on weathering the worst of storms, but
also on being bale to live in the barren habitats made by these storms.
Not surprisingly, most fish are grazers. A most notable and colourful group
consists of surgeon fishes or tangs, named after the tang, a sharp spike
or scalpel that these fish carry on their tail stocks. It makes them almost
invulnerable to predator attacks, and this in turn enables them to grow
old, which is necessary for surviving on a small island surrounded by a
f043612: the convict tang (Acanthurus triostegus)
scours the shallows in groups, but by night sleeps on its own where it
slowly discolours, showing patches on its sides.
f043631: the speckled tang (Acanthurus guttatus) schools
by day but sleeps individually by night. It does not change colour remarkably.
f044111: the velvet tang (Acanthurus nigricans) is
a very beautiful fish, here seen before bedding down. It is common in Niue.
f044131: the Achilles tang (Acanthurus achilles) is
conspicuous by its orange spot and fine white lines.
f043737: large fish like this beautiful parrotfish asleep
at night, can swim to deeper and sheltered water during storms. Their main
threat is being fished and eaten, but parrotfishes have earned some immunity
by being poisonous at times, infected by ciguatera.
f043708: the night shift consisting of squirrelfish and soldierfish,
sleeps hidden deep inside protective caves and they survived Heta almost
entirely unscathed. A white-tipped soldierfish (Myripristis vittata)
active by night. Behind it a finelined squirrelfish (Sargocentron microstoma).
f043628: an unidentified butterfly fish, common in Niue,
is one of the surprises. What secrets is it hiding?
f045029: Niue seems like Noah's ark at times, having a little
of many different species. The black spotted puffer (Arothron nigropunctatus)
is common and easy to photograph at night.
f043919: storms rounded the shape of this large spider shell
which has lost its outer cusps, but inside, the snail is thriving and surviving.
These shells also grow old, particularly in an environment deprived of
f045034: this harmonica sea cucumber visits a large area
without even leaving home, as it is able to inflate itself from an insignificant
blob to over one metre in length. Homing then becomes easy. (Opheodesoma
f044037: large areas are mysteriously covered in 'loose'
sand which proves not to be loose at all but held firmly by turfing algae
underneath. Here the sand has been removed with some difficulty to expose
the algal mat. The cover of sand protects against being sand-blasted.
f220623: three large sea cucumbers in a small rock pool have
protected themselves with sand against being sand blasted.
Summary Diving in Niue is an unforgettable experience because of its ultra
clear warm waters. But clear waters are also poor in nutrients, resulting
in low densities of fish, while corals grow slowly for the same reason.
Niue has an additional disadvantage that it is a single, small island surrounded
by a large empty ocean. Because the larvae of most species spend some time
near the water's surface before descending onto a reef, most of them float
away into an ocean devoid of food, and no place to settle. The chance of
them 'recruiting' on the island is indeed very small. The ocean currents
around Niue are driven by south-Easterly trade winds originating from even
poorer and clearer seas that are also cooler. Thus Niue is showered with
very few new arrivals from elsewhere. As a result, the species found in
Niue must depend on their survival skills in order to thrive in sustainable
growing old: because reproduction has a low chance of success, age
should make up for this, as it creates more opportunity for a successful
nest care: rather than spawning in open water, small fish benefit
from caring for their eggs until these hatch. Many such nests are made
in the protective shelter of rock pools where plankton productivity is
higher due to higher temperatures, reflected light from coralline flats
and nutrients percolating from the land. The water inside rock pools is
also less likely to flow away on ocean currents, which enhances the chance
of recruitment (settlement).
turfing roots: because the water is comparatively cooler than in
the warm tropics, and because of the regular damage from cyclones, turfing
algae are the primary survivors. They multiply asexually by means of running
roots, so their pelagic spores are not of critical importance. Most of
the reefs are constructed by coralline algae, known as 'pink paint' that
thrives better than the corals. The pink paint encourages the growth of
fine algae that are easily removed by many grazers, from snails to sea
urchins, to grazing fish.
living from algae: since the main food around Niue is produced by
short algae, all species adapted to eating plant food, survive better and
in higher numbers. Even omnivores such as the local green crayfish, thrives
hiding and homing: the ability to hide and to home back to one's
hiding spot, is of critical importance to survival. Any night dive shows
just how many animals exhibit this behaviour, from grazing snails and cowries
to sea urchins and grazing fish.
leaving the water: leaving the water during averse conditions is
a good strategy available only to few animals. Sea snakes probably do this.
burrowing and living in tubes: the coral rock is relatively easy
to dig into because it can be dissolved by acids. Sea urchins are particularly
successful at this. Others cement their tubes to growing corals, using
the coral as protection. These include snails, fan worms and crustaceans.
being robust: some organisms can grow old while surviving the direct
impact from flying rubble. These include an awesome large armoured sea
cucumber and large snails such as Tridacna and spider cowries.
swelling and shrinking: an age-old protection mechanism is that
of swelling when times are right and shrinking when they ae not. Anemones,
soft corals, tube corals and some sea cucumbers use this method to protect
covering with sand: if one is not able to burrow, a cover of sand
suffices as protection against sand blasting or being rough-handled by
turbulent water. We've seen it used by algae and sea cucumbers. Small gobies
living in small sand pools inbetween larger rocks, also survive well.