introduction Most fish species have excellent eyesight, even though many are colour
blind. It allows them to do everything they need to survive, in daylight.
Thus they can find their way around, often covering vast territories, and
they can find one another. Most also find their food by eyesight. Plankton
feeders have their eyes right in front of their heads, which gives them,
like humans a good appreciation of depth. Many species can also distinguish
colours, especially those living in shallow water where colour matters.
During darkness there is nothing else for them to do but sleep.
Most fish are covered in transparent scales, which lets the colour of
their skins shine through. It allows them to change their appearance. Some
use it to distinguish young from adults or males from females. Some use
it by way of body language to signal what mood they are in. Yet others
use it to camouflage themselves. Fish who can change their colours easily,
often also have 'pyjamas', the coloration they assume when sleeping.
Finding a safe place to sleep can be a nightmare, because at night a
number of predatory stalkers are active, like conger and moray eels, scorpionfishes
and others like octopus, predating crabs and even conchs (snails). So most
fish never really sleep, but are alert enough for making a quick escape.
Fish do not have eye lids, so they cannot close their eyes. But judging
by their sluggishness around midnight, they must enter a form of reduced
consciousness. Perhaps they also dream, something we may never find out.
Some parrotfish species in the tropics excrete a cocoon of slime which
may deter predators, or just cleanse their skins. Many species are so drowsy
when sleeping, that they can gently be held and returned to their bed.
(We prefer to catch fish for our aquariums at night, and do so with a tiny
hand-held scoop net that we gently place over the sleeping fish. This happens
in the bright light of a diver's torch. It does matter what time one dives,
as fish become more dopey later at night.)
Fishes of the open ocean sleep in schools, but predators able to hunt
by night, such as dolphins using sonar, can still locate and attack them.
When sleeping, fishes do not like to be thrown around by waves, so they
find a tranquil spot inside a cave, behind a sheltering wall, and so on.
Pelagic fish that sleep in the open ocean, descend to where waves are no
Reef fishes that depend for their entire lives on the rocky substrate,
would die if they were swept away by currents. So they sleep close to the
rock in sheltered places like cracks, where waves and currents cannot reach.
Some semi-pelagic nocturnal fish are able to find their way home again,
even when the reef where they sleep is far out of sight.
the night shift Many fish species are active by night. They are able to hunt with their
eyes or by their sense of smell. The night shift hides during the day in
caves and cracks or they burrow into the sand. The vulnerable plankton
eaters usually have big sensitive eyes, and they are good at intercepting
fast prey, such as fast swimming rag worms that come out at night, rising
to the surface where they feed on dead plankton.
In the open ocean large numbers of small fish and squid rise towards
the surface by night, only to disappear into dark depths by day. Sardines
and anchovy descend all the way to the bottom to sleep in the sand by day,
only to emerge and begin their slow ascent at night towards the surface.
During this ascent they have to vent compressed air from their float bladders,
and they can do this through a canal that leads to their mouths. Watching
a school of sardines or pilchards ascend in early evening, swimming circles
through their curtain of bubbles, is truly amazing.
Compared to tropical seas, New Zealand has only few nocturnal marine
species, and even octopus that are mainly nocturnal in the tropics, are
not normally active by night in NZ. In the tropical coral reefs, the overwhelming
nocturnal majority is made up of echinoderms, particularly sea urchins.
They appear from tiny holes in the coral, with spines seemingly far too
long to hide there. Feather stars and other stars appear, and sea cucumbers
emerge from the sand in large numbers.
f034531: crayfish or spiny lobsters (Jasus edwardsii)
are nocturnal and seldom found in the open by day. Their eyesight is rather
poor, but even so they have a good idea of where they are, as they forage
far away from their sleeping dens. Crayfish have a very good sense of smell
and home in on a dying or dead animal, from far away, which also makes
them easy to trap in lobster pots.
f052127: the yellowfoot paua (Haliotis australis)
comes out in the late evening. It grazes the leaves of large seaweeds,
often straying far from home. Yet every morning it finds its way back to
its sleeping spot in a narrow crack. Other abalone species also become
active at night, and home back to their sleeping spots in the morning.
f031117: these needle urchins (Diadema palmeri) hide
under overhangs near the sandy bottom by day, but forage at night. Some
are found at the Poor Knights islands, but this photo is from the Kermadec
f046607: featherstars like this one, are thought to be attached
permanently to where they are found. But this one walks fast by flailing
its arms, pulling and pushing itself along at dusk and dawn, some 3-5m
from where it sleeps. Niue.
f034522: the bigeye (Pempheris adspersa) with its
light-sensitive eyes, is truly nocturnal, hunting zooplankton at night.
It is one of the most common fishes in the warmer waters of NZ, but not
usually noticed because it hides during the day.
f046403: in the tropical coral reefs a large number of beautiful
shells like this cowry, are nocturnal. Notice its tufted mantle that normally
covers its shiny orange shell completely. Many cowrie shells have mantles
resembling the environment where they live. Niue.
f019909: a northern conger eel (Conger wilsoni) searching
for small prey and carrion. Notice its pointed snout.
f019220: the common conger eel (Conger verreauxi)
has very muscled jaws with which it snaps its prey stone dead. I then takes
the prey home to eat inside its den.
f040017: a young red cod (Pseudophycis bachus) cautiously
hunting by night.
f017017: the rock cod (Lotella rhacinus) is very shy,
even while hunting at night.
f029303: some moray eels hunt exclusively at night, others
both by night and by day like this grey moray eel (Gymnothorax nubilus).
f029112: the northern scorpionfish (Scorpaena cardinalis)
has excellent eyesight and hunts mainly by day. However, it can also see
well during moonlit nights, when it is also alert. It even makes use of
a diver's light to suddenly snap a fish that was disturbed by a diver.
These fish still ambush prey, even when there's no room left in their tummies.
finding a bunk A god spot to sleep is always in short supply, but some fish are very
good at finding a bunk to sleep on/in, and one can admire their inventiveness.
f016915: red moki (Cheilodactylus spectabilis) gather
to sleep inside 'moki holes', little caves that provide good shelter. Inside
the cave each has a ledge to sleep on.
f051102: an old red moki sleeping on a ledge (a grey sponge)
by a crack where it fitted inside when it was much smaller. Fish often
stay with their bunks, even when they have outgrown them.
f031235: a morse code leatherjacket (Thamnaconus analis)
lies flat on its side under a hard coral. It was holding on to a tuft of
red seaweed on right but let go of it while the photo was taken. Its name
refers to the blue spot by its anus. Kermadec Islands.
f043604: an Ambon pufferfish (Ambon toby) (Canthigaster
amboinensis) has found a suitable crack in the concrete wall of a wharf,
and locked itself in place by slightly puffing up its body. Niue.
f045037: a black-spotted pufferfish has wedged itself
into a suitable dent in the coral, by slightly inflating its body. (Arothron
f045028: some fish are rather dopey when they sleep and can
be handled with care, like this black-spotted pufferfish which is very
shy by day. Niue.
f046904: a sleeping star-eye parrotfish (Calotomis carolinus)
with black spotted pyjamas has wedged itself in a crack. Some parrotfishes
excrete a protective cocoon of slime. Niue.
f047913: a large (70cm) sleeping redlipped parrotfish (Scarus
ubroviolaceus) fits precisely in this dent in the rock where it braces
itself with its hard back fins. Niue.
f043612: a convict surgeonfish (Acanthurus triostegus)
is bedding down, ready to change colour for the night. Niue.
f043718: a convict surgeonfish in pyjamas, shows blotches
on its sides (Acanthurus triostegus). It has wedged itself securely
inside a narrow crevice. Niue.
f028914: a scarlet wrasse (Pseudolabrus miles) has
made this grey sponge its bedroom, keeping it open wide enough to allow
f018616: a baby leatherjacket (Parika scaber) only
4cm long, is holding on to the hairs of a nesting mussel, in order not
to drift away.
f021720: sea horses (Hippocampus abdominalis) often
live in places with strong tidal currents. Fortunately they can hold on
to objects with their prehensile tails. Left the male and right the female.
They hunt using eyesight, thus sleep by night.
pyjamas It is not known why fish change colour while sleeping. Perhaps they
relax their bodies and their skins, and the colours and patterns that then
appear, are their basic colours and patterns which they alter during the
day. But there are enough surprises.
f041807: a school of pink maomao (Caprodon longimanus)
and one coming inquisitively closer. By day they are soft pink in colour
with white bellies.
f048407: a young but mature pink maomao in pyjamas. Notice
how its skin has darkened, become blotchy, and its belly darkened as well.
f048417: an old pink maomao looks rather scruffy when it
sleeps with random blotches and an orange belly.
f048432: snapper (Sparus auratus) in dark pyjamas at night.
f020621: a mature male two-spot demoiselle (Chromis dispilus)
about to bed down. It still has its blue colour and white tail, but its
two white spots are already disappearing.
f019316: a male two-spot demoiselle has become evenly black
while sleeping in its little hole in the rock, while stinging jewel anemones
and cup corals guard the entrance.
f020421: a butterfly perch (Caesioperca lepidoptera)
is normally pale pink with a black spot and pepper spots all over, but
this one is already discolouring for a nap during the day.
f020031: when sleeping, a butterfly perch becomes dark brown
with its black spot just showing through.
f039918: this butterfly perch was found sleeping with red
blotchy pyjamas, which we had never seen before. It appears able to mimic
the colours around it.
f039806: a parore (Girella tricuspidata) in pyjamas,
taking a nap during the day. Its dark-olive colour and white blotches are
also its night pyjamas.
f027332: the tarakihi (Nemadactylus macropterus) is
silver by day, with a black neck band behind its head.
f035327: a tarakihi in pyjamas is almost unrecognisable,
as it has dark commando spots, and its tell-tale black neck band almost
f052701: the scarlet wrasse (Pseudolabrus miles) can
have several day-time costumes, and females look quite different too. This
is a male in normal active clothing.
f020135: a male scarlet wrasse in one of its pyjamas.
f043422: an orange wrasse (Pseudolabrus luculentus)
by day can have various costumes, also marking the difference between males
and females. This is a male in one of its daytime costumes. On left the
f043323: a male orange wrasse in pyjamas, tucked away in
its sleeping den. Because so many fish tuck themselves deep inside cracks,
they are impossible to take photos of.
burrowing Some fishes, particularly wrasses, burrow in the sand. Wrasses are
the kinds of small fish with a stout body, that row with their breast fins
rather than using their tail fins. They are usually gaudily coloured, changing
into pyjamas as well. Wrasses are born female and change into male later
in life. They form small social groups of females and juveniles, guarded
over by one large male. Such groups are called harems.
At dusk, the male takes a position by a suitable spot for burrowing,
usually a patch of coarse sand in a sheltered place under a rock. One after
another the juveniles and then the females slide into the sand, heads first.
Once they are all safely under ground, the male follows. In some species
the male stays above ground, as if guarding the sleeping patch. It is very
unusual for divers to witness this behaviour, because the fish do not like
being watched while burrowing.
f010314: a male spotty (Pseudolabrus celidotus) has
carelessly burrowed in the sand.
f035802: a male spotty in pyjamas. All its male colorations
have disappeared, and it looks like a female now, including the black spot
and yellow fins.
f010309: a female spotty has nestled herself safely in the
tangled branches of the tangle weed (flexible weed, Carpophyllum flexuosum).
f048319: a male spotty by day in its male coloration. Note
that the round female spot has disappeared to make room for a black spot
on its back. Note also the banded saddle straps and bluish anal fins.
f028705: the male Sandager's wrasse (Coris sandageri)
has gaudy colours compared with the juveniles and females in the background.
They do not change colour while sleeping, and neither do they change colour
during the day.
feeling well We've discovered that some fishes migrate to shallow water to benefit
from the warmth above the thermocline, a boundary between warm surface
water and cooler water below. It is not certain that fish can feel warmth,
but when their bodies warm up, their metabolism increases, and this could
bring a sense of wellbeing. Thus fish in warm water grow faster and produce
egg mass more quickly and also perhaps more offspring. Such warm water
migrations happen on a daily basis in summer when there is a marked difference
between the warm surface and the colder deep. Such fish forage in the cold
deep, and sleep in the warm shallows.
f041232: a school of mature snapper (Pagrus auratus)
(±6 years) milling around before sleeping in the shallow warm water
of 2-4m deep, usually hidden among kelp plants. Poor Knights, summer.
f012728: a pregnant female longtailed stingray (Dasyatis
tethidis) basking and snoozing in the warm shallows. Stingrays are
active at night. Stingrays enjoy internal fertilisation, and bear their
evening spawning Some fish species do not spawn massively in a single event, but do
so in smaller batches. While they are busy foraging during the day, their
minds are focused on finding food, but when evening falls, the fish relax
and before bedding down for the night, it becomes time to spawn.
f034704: just before bedding down in summer, these red pigfish
unimaculatus) changed into their spawning colours with a star pattern
around the eyes. The male is guiding the female to the surface where they
spawn. The ceremony is repeated a few times.
f020129: female pigfish sleeping in pyjamas. Notice that
the black stripes and spots have gone and her white has become orange.
She will bed down deep inside a crevice.
daytime nap Many fish species are taking a nap during the day. Once they have fed,
solitary fishes have little else to do than take a nap. Plankton feeders
take a break when the tide is turning, and the current stops. Some fish
do not expend much energy, and they need to feed only once every few days,
so it is common to see fish resting during the day.
f041125: demoiselles and young blue maomao share a peaceful
moment underneath a protruding stalked kelp. Food was plentiful today and
their bellies are full, besides, the tide is turning and the nutritious
currents have ceased for a few hours.
f041521: young blue maomao (Scorpis violaceus) taking
a nap underneath a protective rock. They seek the shallows for warmth and
protection but fear attack from sea birds.
f012133: an old pink maomao (Caprodon longimanus)
taking a nap in a vertical position along a vertical cliff face.
f048822: a group of goatfish (Upeneichthys lineatus)
resting with their snouts into the current. Notice how they can colour
their skins. When fully at rest, the red colours prevail.
f001935: four parore (Girella tricuspidata) returning
to their sleeping den during the day. Notice the many ledges, as each is
owned by a different fish. This sheltered pool can accommodate some 50
fish, each sleeping on its own bunk.