Fish eyes

recognise fish by their eyes

By Dr J Floor Anthoni (2003)
www.seafriends.org.nz/photolib/eyes.htm

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the eyes have it

The eye is the most wonderful part of our bodies. Interestingly, it was already fully developed hundreds of millions of years ago, reason why most creatures have very good eyesight, and their eyes are similar to ours. The eyes of the more primitive creatures, such as reptiles and birds, are very noticeably different from ours. Even those of cats, which are mammals just like us, differ because they have different pupils.
Considering that all reptiles, amphibians, birds and mammals originally stemmed from fishes in the ocean, it is surprising that fish eyes are so well developed, and that they are so similar to ours. But fish have larger eyes, in order to see better in the darkness of the sea. The fishes found in depths of 100-200m have indeed very large eyes, which you can admire in the fish shop. But even at the depths visited by divers, the light has diminished to only a fraction of its intensity above water. The eyes of the sea creatures living there, thus had to adapt to low light conditions, which means that their eyes are comparatively larger than those of land animals.
If we try to see with our eyes submerged, we'll notice that everything looks blurred. That is because the lens of our eye is a water lens, which works only because there is air outside our eyes. In the water it does not work very well. So fish eyes have a different substance as their lens, and it is highly curved. Because the outside of our eyes needs to be moistened frequently, we have eye lids. These also allow us to close our eyes. But fish lack such a mechanism. It is therefore surprising that they are able to keep their eyes so clean, and free from attack by the likes of sea lice. Some fish have indeed very beautiful eyes.

f021328: This john dory (Zeus faber) was photographed by night. It is a hunter who depends entirely on its eyesight. Flat as a plate, he can make himself invisible by simply looking at you. Because he swims by vibrating his fins, it seems as if he does not move at all. But his biggest surprise lies in the final attack, in which he extends his mouth suddenly far forward. Notice his very curved eye lens.

f017712: A john dory has turned itself flat on its side and changed its colours to resemble an innocent stone. However, its prey, young jack mackerels, seen in the top right corner, have recognised him, keeping their distance.

f016731: john dory mouth

f016731: this john dory died in a trawl net, the pressure of which caused internal bleeding. His extremely extendable mouth is shown by pulling it outward. The close-up of his eye shows further internal bleeding.

f019612: porcupine fish (Allomycterus jaculiferus) are very endearing because of their large, beautiful eyes. These fish have a bulky shape, while they swim with the two fins near their tails, which makes them rather slow. But they have excellent defences, being poisonous to eat, and being able to inflate themselves with water. Their prickles also help a little.

f019609: porcupine fish have very good frontal vision, as this photo shows, but they are also able to roll their eyes backward for a complete rear view. Notice how each eye has an outer lens and also a very curved inner lens. Many fish have dark circles around their eye balls, which helps to prevent glare.

f031210: The Lord Howe coralfish (Amphichaetodon howensis) is one of many whose skin patterns run through their eyes by way of disruptive coloration. This fish is found only in a very restricted range in the subtropical South Pacific. Once mature, it finds a partner, and the two stay loyal to each other until one dies.

Like other coral fishes, it finds food in between the complicated structures of coral. Having a pointed snout then serves to advantage. With its eyes set very close to its mouth, it can even find those creatures that have camouflaged themselves well. Notice how its breast fins have become small, and that its belly fins have taken some of their function.

f030108: Young snappers (Pagrus auratus) have beautiful shiny pink bodies, covered in blue spots. Their fins also look blue when the light falls on it from the right angle (from above and behind). Above their eyes they carry a bright iridescent blue spot. Notice how these spots disappear as they grow older (see right).

f030211: This large snapper has no fear of divers because it lives in a marine reserve where it is fully protected. Divers respect these fish which are often older than expected. This snapper is about twenty years old. f030119: large snapper

f030119: a large snapper of about 30 years old.

f031709: The black spotted grouper (Epinephelus damelii) which can still be found in the remote Kermadec Islands, grows very large and very old, up to four times the weight of an adult person, and over 100 years. They are intelligent fishes, and a joy to encounter. Once they know you, they love to be played with and to be stroked. This one is a large female. On the back of its eyeballs, the grouper has a number of smaller fake eyes, see right-hand enlargement. It is not known why.

f031613: White-lip is the large male grouper, seen here nuzzling up to a snorkeldiver. Look at his size! He has a harem of five females who stay with him. But earlier in life, he too, was a female. These fish change sex later in life when it suits them. f041724: grouper playing with diver

f031742: a snorkeldiver descends from the surface to meet White-lip, the large black spotted grouper. Such moments are awe-inspiring.

f025120: the northern scorpionfish (Scorpaena cardinalis) hunts by sitting still and waiting until a hapless victim comes close to its mouth. Then it lurches forward, while opening its cavernous mouth. This movement also sucks the victim inside. f030522: scorpionfish hiding

f030522: Scorpionfishes are so well camouflaged that they are hard to see.

f027420: The jock stewart (Helicolenus percoides) or sea perch lives in the cooler waters of New Zealand, where the northern scorpionfish is no longer found.

f030005: the grey moray eel (Gymnothorax nubilus) has rather small eyes for its size. Moray eels hide among crevices by day, in order to hunt by night. But this moray is active by day, and can be found enjoying itself while looking for food.

f025133: The yellow moray eel (Gymnothorax prasinus) is larger than the grey moray, but not as outgoing. However, when extended a human hand, it comes out of its hole to sniff and nuzzle it, which is rather daunting, considering its impressive sharp teeth.

f006305: a young bigeye (Pempheris adspersa) shows why it is named this way. Bigeyes are nocturnal fish that hide by daylight and come out at night. This makes many people think they are uncommon. Bigeyes have a peculiar way of swimming. They hang motionless in the water, in order to be able to observe the slightest movement around themselves. Then they suddenly dart forward, not to pursue their prey but to skilfully intercept it.

f022713: A diver enters the hold of a shipwreck where thousands of bigeyes are hiding from the daylight. Blinded by the light and attempting to dodge the divers, they nervously swim around in this dark cavern, which is home to them. Yet they do not dare to swim outside. Near hideouts like this, one often finds large scorpionfish waiting till a bigeye makes a mistake. Like many other small fishes this bigeye is endemic to New Zealand.

f029324: The crested blenny (Parablennius lacticlavius) is no larger than your finger, and usually much smaller. But unlike other small fishes, it grows old, and as can be expected of old creatures, it grows wiser as well. Here it is waiting inside a narrow hole in the rock, for its visitors. Larger fish visit him because of his sharp little teeth which enable him to prise sea lice from their skins.

f029621: a side-on photo like this is difficult to obtain, because these fish are very cautious. Notice how its breast fins have developed into fingers, with which it can firmly hold onto rocks in wave-washed conditions.

f030427: the blue-eyed triplefin (Notoclinops segmentatus), also called the red-banded triplefin, shows just how extravagant the colouring can be in small fishes like triplefins. The name triplefin comes from the three fins they carry on their backs.

f019121: the specctacled triplefin (Ruanoho whero) has dark eyes and a band in between, joining them up like having 'shades' on. With these eyes it obtains a very clear perception of its environment.
Small fishes like these do not spread easily across oceans, and both shown here are home to New Zealand only (endemic).

f015700: the seahorse (Hippocampus abdominalis has a long snout with a very small mouth at its end. It can turn its eyes independently to look all around or to focus on prey in front of it. Its large gill cover behind its eyes, allows it to suck water in with a loud snap. At the same time it jerks its snout upward. With these two movements it efficiently catches free swimming shrimps and crustaceans hiding on the bottom and on plant leaves. Its large eyes allows it to see the slightest movement.

f003929: A seahorse holds on to a sunken twig in order to resist the fierce current in this current channel of an estuary. Estuaries are rich in food, particularly in small organisms creeping over the sandy bottom. When the tidal currents stop, the seahorse makes long excursions over the sand flats, but returns to his favourite hang-out before the current becomes unbearable.

f021623: seahorse at a wharf

f021623: a male seahorse in the tangling environment surrounding a commercial wharf. The human-made mess gives the seahorse many items to hang on to.

f024810: this baby sand flounder has covered itself expertly with sand, leaving its bulbous eyes free to look out. Flounders are very hard to find this way.

f007802: a sand flounder (Rhombosolea plebea) swims freely over the sandy bottom, relying on its camouflage to protect it. Not only do these fish cloak themselves with sand when settling down, but they can also change the colour and patterns on their skins to match that of the bottom.

f021002: An eagle ray (Myliobatus tenuicaudatus) has burrowed itself in the sand, while at the same time digging up a deeply hidden clam. With its very strong bony jaws it then cracks the clam to remove its flesh. Rays do not use their eyes for catching prey, and these are less developed as a result.

f021005: An eagle ray flies over the sand in search of food. Notice its blunt head and antenna-like tail with a little fin on it. Stingrays have adapted to living on the bottom by becoming flat-bodied. Related to sharks, which also have cartilageous skeletons, the rays have enlarged their breast fins into wings, with which they fly like birds. The fins on the rear of their bodies are pelvic fins.

f028304: A sand octopus (Octopus gibsi) has come out of its deep den in the sand to have a better view of the photographer. These animals are related to sea slugs, but their eyes are sharp and inquisitive. Here the octopus has closed its eyelids as it squints in the photographer's modelling light. It sleeps with its eyelids closed.

f022605: a large female sand octopus is guarding her eggs inside her burrow in the sand underneath a large rock. Octopus mature in two years, mate and the female lays her eggs in strings hanging from the ceiling of her burrow. From that moment on she stops eating, never leaving her nest. By the time the eggs hatch, she dies to make room for her offspring. Male octopus live a little longer and may mate several times.

The stalk-eyed mudcrab (Hemiplax hertipes) carries its blue eyes on stalks, by way of periscope, in order to be able to spy its surroundings while submerged under the sand. Here he is seen in a defying stance, defending its territory, or just soaking up the last sun rays of the day.

Here the stalk-eyed mudcrab is seen feeding on a morsel it found hidden under the sand. Like most crabs, mud crabs have taste sensors on their legs, and taste the bottom while walking around. This crab is not common on mud flats, but prefers a sandy bottom amongst the eelgrass.

f006333: The mantis shrimp (Squilla sp.) is one of the most amazing shrimps on earth. Constructed like a swiss army knife, it bristles with all kinds of utensils: boxing gloves, catching claws, brushes, trowels, spades, knives and more. It has the most amazing eyes ever evolved, since each side consists of two grooved domes, within which two eyes move independently. The grooves run perpendicular to each other, bottom and top, which allows the animal to see depth with stunning accuracy.

f006130: the mantis shrimp is half-round in shape and very flexible, being able to turn around inside its snug round burrow. It constructs such burrows in rather loose sand, by cementing the tunnel with 'spit' using its brushes and then trowelling it smooth with its trowels. It can swim forward and backward with ease, using its flat tail fins as ailerons.
In captivity it is a most endearing animal to observe, always active, always inquisitive.

f011705: A bottlenose dolphin (Tursiops truncatus) swims by, really close. From a distance it has already observed the photographer's acoustical image, but now it is catching a glimpse with its eyes. Because dolphins have such excellent echolocation, their eyesight is rather poor.

f011708: bottlenose dolphins approaching

f011708: We will never know what dolphins see, because most of their perception consists of images, reflected by loud clicks. Nonetheless, they are often seen poking their heads out of the sea, to look around ('spy-hopping'), and each dolphin has enough visual markings to tell it apart from others. Compare these two dolphins for instance.
The bottlenose dolphin can swim large distances without food, reason why it is found in many oceans.

f031221: The splendid hawkfish lives in turbulent water where it holds itself steady with the fingers on its breast fins. Its white-red-green colours make it look splendid indeed. However, in natural light, the colour red becomes brown.

This detailed photo shows a peculiarity of the family of hawkfishes: they have brushes at the ends of their dorsal spines. It is not known what these are for, but they look cute. Although very colourful on these photos, the splendid hawkfish is very difficult to spot in its natural surroundings. Very little is known about it.