Depth Perception – Deep-reef Fish in the Aquarium Trade

When most people think about where some of their favorite aquarium fish come from, they usually think of brightly colored corals and fish darting about in the sunlight. If you ask them how fish are collected, they might picture collectors snorkelling or skin-diving to the reefs to hand-collect the fish that will soon appear in their local fish stores. But, for a large number of fish in the aquarium trade, this is just not a reality. Sure, plenty of fish come from shallow reefs close to the surface, but many also come from deep, dark reefs over 100 feet below the surface where light doesn’t penetrate and even some of the colors of the fish themselves don’t, for all practical purposes, exist.

The “deep reefs” exist on the leeward side of islands (the side opposite from where the current is directed at the island), on the sides of cliffs and canyons udnerwater, and farther down the reef slopes than the traditional coral reefs popular on calendars and postcards. Instead of bright corals that rely on sunlight and photosynthesis for most of their food, the corals on the deep reefs are mostly filter feeders and get their nutrition from the water column itself. Fish on these reefs are used to dimmer lighting and instead of orientating themselves to the surface, most swim by or “perch” on whatever caves or ledges or sufaces are around them (even “upside-down” by our right-side-up standards). Most of these fish, you might notice, can tend to have a great deal of red in their coloration. As light filters through the water, colors created by a higher wavelength (starting with red on the goold old ROY G. BIV spectrum we all learned in school) are filtered out first. Since these colors don’t make it down to the deep reefs, a red fish would appear grey or black compared to its surroundings.

At these depths, the water pressure is also tremendous. At the surface, the atmospheric pressure is usually about 14.7 psi (pounds per square inch). At 50 feet down, the pressure is about 37 psi, and at a 100 foot depth, the pressure increases to almost 60 psi. This creates a problem not only with collecting the fish since special diving equipment is needed to even get down that deep and only for short periods of time, but also with bringing the fish up to the surface. Unless the fish (or the diver, for that matter) are brought up slowly so they can acclimate to the lower pressure, the decreased water pressure around them can cause their bodies to expand and even rupture. It is much the same as when we fly somewhere on an airplane – even in a pressurized cabin, you can still feel your ears popping as the pressure changes.

Some of the rare, expensive fish you may see in books and in your local fish store are rare and expensive because they are deep-water fish. It costs more for collectors to get to them and get them to the surface, the risk is greater in getting them to acclimate to our 14.7 psi environment, and they are just plan harder to find in the pitch-black, cold, deep, dangerous areas they live. Some of these fish are pygmy angels like the Multicolor Pygmy Angel, Manybanded Angel, or (the Holy Grail) Peppermint Angelfish; wrasses like the Rhomboid Wrasse, Mystery Wrasse, or Lined Fairy Wrasse; butterflies like the Bank’s Butterfly or Tinker’s Butterfly; many anthias like the Blotchy Anthias or Redbar Anthias; basses like the Wrasse Bass or Cave Bass and many, many other fish. Even fish that can be found in shallow waters like Firefish Gobies, Chalk Basses, Longnose Hawkfish and some damsels can be found on deep reefs as well.

So, the next time you look at fish in the tanks of your local fish store, put aside those images of snorkelling around some tropical retreat and seeing schools of them swimming around below you. That fish may have travelled a longer distance and went through a lot more than you may think to make it into the aquarium!

Thanks, until next time,

Eileen

Nepthaea coral image referenced from wikipedia and originally posted by Nhobgood