A group of nocturnal fish known as "Big-eyes" is found throughout the world in tropical waters and belong to the family Priacanthidae. As the name suggests, these animals have distictively large eyes. One species, the Short Big-eye (Pristigenys alta), is commonly found in the West Indies and Gulf of Mexico, but is also known to stray north with the Gulf Stream current as far as Cape Cod, Massachusetts.
As I described in my pictoral essay on tropical strays, many of the species of fish found along the southeast coast of the United States have eggs and larval stages that are pelagic. Each summer some of these planktonic fish eggs are trapped in the northern flowing Gulf Stream and find their way to New England. These fish will live for a few months until late October when the water gets too cold for them to survive. Although the ones found in my area only get to be an inch or two long before they die, adults in their native habitat may reach a foot or so in length.
The newly hatched P. alta remains
planktonic for several weeks before
settling down onto a coral reef.
Big-eyes are strictly nocturnal fish which spend their days hiding beneath rock ledges and coming out at night to feed. This behavior makes them easy to catch if you know where to look for them. During the day the fish can literally be picked up with bare hands as it sleeps. This species prefers shallow water when it is young, but adults can move down to five hundred feet or more as they grow.
There are two physical characteristics that give away its nocturnal habits. One is obviously the enormous eyes. These allow the fish to see at night using just the faint light of the moon. You’ll notice the very large pupils in the photo at the top. You’ll also notice that the image appears to be not very well lit. This is because to get a good profile shot of a big-eye you can’t use a flash. How many times have you seen pet photos with those evil-looking glowing gold eyes? The reason for this is that many nocturnal animals, including cats and big-eyes, have a membrane that lines the back of the eye lens. This membrane, known as a tapetum lucidum, acts like a mirror in order to reflect light back through the retina. This essentially gives the retina a second chance to catch the light and thus improves the animal’s night vision.
The second nocturnal characteristic is the red color, which is bright red in the young and fades to orange as the fish ages. Many nocturnal and deep-water marine animals are red as part of their protective coloration. This may seem counterintuitive since a brilliant red fish would stand out starkly and seem to be easy prey. This may be true in shallow water and during the day, but not at night.
As sunlight travels through water it is slowly absorbed until at around 600 feet it is mostly gone, (and then we enter The Twilight Zone). White light is made up of a spectrum of colors ranging from red to blue and these various colors are absorbed by the water at different rates. As it happens, the color with the longest wavelength, red, is the first to disappear, therefore a red fish is virtually invisible in the dark as it will appear black to other fish.
Big-eyes are fierce predators on other fish. The large, upturned mouth is used to ambush animals from below. The hunting behavior is fascinating to watch as the fish hovers just out of sight beneath its rock ledge, watching carefully for another fish to come close. When its victim is chosen the big-eye will slowly descend beneath it and shoot straight up through the water to capture the prey from below. It is so specialized to hunt this way that it is not able to eat anything that is swimming below it. This can be seen in captivity when food is dropped into the water. If the big-eye misses the food with its upward strike and the food sinks to the bottom it will go uneaten by the fish. Although these are pretty smart fish and can be taught to look for food on the bottom. This does take quite a bit of patience on the aquarist’s part.
The prey can be surprisingly large and still be consumed. Rows of tiny, but very sharp, teeth line the mouth (In fact, the scientific name comes from combining the Greek words meaning "Saw" and "face", possibly referring to these teeth). These help to secure the struggling prey and keep it from escaping. As it swallows the food, teeth in the throat help to push the prey down into the stomach. And the stomach and belly skin is actually able to stretch to accomodate food that would normally be too large for a fish of its size.
Below is a movie I made of one of my one-year-old short big-eyes feeding on a piece of fish. Notice the behaviors I described above are exhibited in this short clip, including watching for food from beneath a ledge (in this case a brick structure I built for him) and the little ambushing move it makes as the food is placed in the water.
Click the image below
to watch it.
Other diaries in this series can be found here.