Kermadec Islands - images of corals,
gorgonians and anemones
By Dr J Floor Anthoni, 2002
For New Zealanders, the Kermadecs are interesting
because the seascape is dominated by corals rather than large seaweeds.
This section shows the variety in coelentherates (flower animals) found
there, ranging from sun-loving corals to gorgonians and anemones. The reef-building
corals live in symbiosis with algal cells within their skins. These microscopic
plants produce food from sunlight, by which the polyp's wastes are recycled.
Many of the corals found at the Kermadecs are leathery to fleshy rather
The reef building corals are unique in the animal world because they
embody within their tissues, a single-celled alga (zooxanthella),
which means that animal and plant have been united inside the same body
This quirk of nature allows them to short-circuit the food chain in favour
of an extremely efficient metabolism. For them the food chain goes direct
from sunlight + nutrients = algal growth = energy and proteins for living.
The circle is closed by the algae recycling the animal's wastes, assited
by bacteria. This most efficient food cycle allows the coral polyp to live
in clear waters with low nutrient concentrations and even less zooplankton.
In the process, it is also capable of excreting massive limestone skeletons,
the basis of coral reefs. But in order for such hermatypic corals to thrive,
they need sunlight, and plenty of it. As a result, they are found close
to the surface, where their hard limestone skeletons protect them from
destruction by waves. To make it all work, the coral animal needs to be
translucent to allow all light to reach the algae, and the algae have plant-like
colours, which makes the world of hermatypic corals look rather drab yellow,
brown and greenish.
Note that we have recently discovered
the principle of symbiotic decomposition which makes a mysterious
biomass, named slush by us, available as a source of nutrients,
carbondioxide and hydrogen ions. By nurturing decomposing bacteria on their
skins, corals can thus be extremely productive in apparently clear 'desert'
seas, devoid of nutrients and plankton. Read more about these discoveries
in the DDA section.
At the Kermadecs, one also finds strange fleshy corals, which also appear
to need sunlight, but less of it than the hermatypic corals. For instance,
one does not find them deeper than 30m, where normal plant growth would
also stagnate. These do not build hard skeletons.
Under overhangs, and in caves, and deeper than 40m, one finds a colourful
world of ahermatypic corals, that grow slowly without sunlight. For them
the food chain is one step longer, and far less efficient since they need
to expend energy for catching their food: sunlight + nutrients = phytoplankton
= 10% zooplankton = energy + proteins for living - energy for catching.
Catching the zooplankton means that there is substantially less food for
them. Still, they are capable of building horny skeletons (gorgonians)
or even limestone ones (cup corals and tube corals).
Note that the corals found here, live at the cold margin of the coral
kingdom, and are therefore unsuccessful in accumulating coral reefs as
they do in the warmer tropics. The number of species decreases rapidly
in the 200km from Raoul (about 40 species) to l'Esperance Rock (about 10
species). Their density also decreases spectacularly, partly also because
much soft rock is found further south.
We regret not being able to show closeups of the reef building corals,
since expedition time would not allow for this. Many corals have symbiotic
relationships with other animals such as small lobsters, shrimps, clams
and more. Closeups would have revealed such interesting details. Also many
corals remain nameless, until we are able to identify them. For coral closeups,
visit the underwater world of Niue
which has about 200 coral species.
f031028: About 2-6m deep, one finds a kaleidoscope of hard
corals, separated by red and green algae. On closer inspection there are
only few species. Their forms can be described as 'cauliflower' and 'broccoli',
since similarly, they try to pack the most life into the least space, while
absorbing the most light. Notice their typical drab colours.
f031422: A hard rock composed of basalt, is scantily clad
in plate corals. In the middle foreground a large patch of bare grey rock,
where likely a plate coral once grew. Notice how these corals expand outward
over existing growth, without actually attaching to the rock. They are
thus relatively easily removed by strong waves.
f032000: A large plate coral sits like a cow pat on a bare
soft volcanic rock. In the centre it grows upward in what looks like nipples.
On closer inspection, these nipples appear to be barnacles, embedded inside
the coral tissue. Where the coral is young at its outer rim, no barnacles
are found, but they establish themselves later. It suggests that the coral
did not grow over a barnacle colony but that the barnacles appeared later,
and that they may have a symbiontic relationship. For it to work, the barnacles
must be able to grow as old as the coral, which is unusual.
f031106: The shagpile coral (Hydnopora sp.) grows
like a plate coral, over the surface, but it has the habit of starting
on a pinnacle, and draping itself over the object as it grows. Who knows,
a treasure chest may be found underneath? The toadstool grouper found sleeping
here, may well have spent its youth hiding underneath the shagpile cloak,
like many small fishes do.
f030905: The long polyps of the shagpile coral resemble a
bunch of optical fibres. They are extended day and night, and are somehow
not grazed by other organisms. What is their secret?
f030907: One of the fleshy coral species grows outward and
upward from a small holdfast on the rock, thereby leaving space available
for organisms with a lower profile. It cannot resist wave action as well
as the hard corals, and grows deeper down (4-20m). This particular species
likes to live just under the zone of hard corals.
f030908: A close-up of the same coral shows its fleshy fingers
and small polyps. We had expected these corals to extend by night, like
some tropical soft corals do, but this is about their maximum. However,
they are able to shrink in averse conditions, which helps them to survive
strong wave action.
f030914: Some of the fleshy corals have peculiar shapes,
resembling desert plants.
f031103: The fruit bowl coral (Turbinaria sp.), as
we named it, stands as a solitary coral in sheltering depths of 10m or
more. It is easily damaged by an anchor chain. The age of such a coral
is unknown, but don't be surprised if this one is over 25 years old.
f031233: It is not only the shagpile coral which has long
polyps, but at least three other encrusting species have tentacles of varying
lengths. Among such tentacles, other vulnerable species may find protection.
f031323: Large fan corals like this purple gorgonian, are
rare because of their age (50 years?). Finding one as large as this, is
always the highlight of a dive.
f031501: A young purple fan coral started its life on a sponge-covered
rock wall inside a large cave. Here it will be able to compete for space
with other similarly slow growing organisms.
f031502: Immediately above the previous picture, a purple
fan coral has been raided by an invasive anemone, a yellow zoanthid, who
now dominates the entire fan. It killed the original coral polyps by just
growing over them and smothering them to death.
f031505: The ceiling of the cave is even more protected than
its side walls, reason why the most fragile organisms are found here. In
the centre a fragile orange-pink fan, surrounded by yellow tube corals
sp.). Looking like individual anemones, these very slowly build limestone
tubes in which they can withdraw. Behind the orange fan, top-centre a deceased
fan, covered in sponges and seasquirts.
f031508: The yellow gorgonian is much rarer than the others.
In the foreground of both pictures the common brown featherstar. This seastar
has specialised in catching minute planktonic organisms in its finely feathered
f031506: Brown and yellow featherstars surrounded by yellow
tube coral, and in the centre a young purple fan coral.
f031218: These green anemones (Dendrophyllia sp.)
are in fact also tube building corals, forming small 'trees'. However,
unlike the yellow variety shown left, these are found only in the light,
usually on steep rock faces, which suggests that they too have green algae
in their tissues.
f031814: A cluster of horse anemones
wandering anemones(Actinia sp.Phlyctenanthus sp.),
living close to the surface in wave-swept conditions. Top left, an unusual
triplefin can be detected, see right
f031814b: enlargement of unidentified fish on left photo
Note! The above red anemones i thought were horse anemones of the Actinia
genus but Michela Mitchell from museum.vic.gov.au put me right, noticing
these were wandering anemones of the unusual Phlyctenanthus genus.. What
an amazing power of observation!!