Butterflyfishes Excerpt

Copperband Butterfly, Chelmon rostratus, in a large reef aquarium.

BUTTERFLYFISHES
by Daniel Knop

Butterflyfishes are among the most splendidly colored denizens of the coral reef. They look like works of art with their colorful body markings in red, orange, yellow, blue, black, and white, giving the impression that they are deliberately endeavoring to outdo one another. However, the motivation behind the splendor is not vanity but the desire to survive; the main purpose of the patterns that we humans find so attractive is to break up the fishes’ body contours to confuse predators.

Butterflyfish’s eyes are often camouflaged by their body patterns and become invisible: examples include Chaetodon ornatissimus (Ornate Butterflyfish), C. kleinii (Klein’s Butterflyfish), C. melannotus (Blackback Butterflyfish), C. ocellatus (Spotfin Butterflyfish), Forcipiger flavissimus (Yellow Longnose Butterflyfish), and many more. This reinforces the effect of the masked body form; an apparently eyeless object may well appear to be something other than a fish. This would serve to divert the interest of any piscivore to some other fish in the vicinity. In addition, some butterflyfish species have a “dummy eye.” Chaetodon capistratus (Foureye Butterflyfish), for example, has large ocelli near the tail. This may have a protective function, in that it provides misleading information about the likely direction of flight. Such eyespots are seen most frequently in juveniles. A false eye can also play a part in intraspecific communication, a function that remains relatively little known but has been studied in detail by Professor Ellen Thaler (Thaler 2003).

THRIVING IN THE STONY CORAL MAZE

Butterflyfishes almost always inhabit the shallow-water areas of the reefs, at depths of less than 60 feet (18 m). Only a very small number of species have specialized in living at greater depths. Interestingly, it is in this shallow-water zone that the vast majority of symbiotic corals are found. Of course there are zooxanthellate corals below the 65-foot (20-meter) mark, but anyone who dives to 80 or 100 feet (25 or 30 meters) on the reef will find that they are far less dense there and do not form any continuous cover. Due to the lack of light at these depths, corals grow less vigorously and attain smaller sizes than those found in shallow water. This deeper zone is the domain of those cnidarians that have specialized in capturing plankton; man-high gorgonians and huge, tree-like soft corals of the genus Dendronephthya, their “trunks” as big around as a human thigh, are found here.

Saddled Butterflyfish, Chaetodon ephippium, a large, beautiful species not suitable for the typical reef aquarium.

But most butterflyfishes need a dense labyrinth of stony-coral skeletons to feel at home and develop their species-typical behavior. Corals are indispensable as a source of food for the majority of butterflyfishes. In addition, the absence of suitable structures to hide behind, or in, would create long-term stress in any butterflyfish—it would be too visible to predators against a background of bare or algae-covered rock. The colorful and complex structure of the dense coral covering of the reef offers the eye so many targets that, quite simply, it takes longer to spot the butterfly. In addition, the reef contains innumerable crevices, overhangs, and cavities, offering the opportunity to disappear from sight in a split second with a few beats of the fins.
Living in immediate proximity to corals is apparently embedded in the genome of the majority of butterflyfish species (Thaler 2007); expecting a butterflyfish to live without corals is almost like stranding a city-dweller in the desert. This may be one of the reasons that many species have proven to be disease-prone and problematical to keep in the aquarium. Without corals, butterflyfishes feel threatened and vulnerable; this triggers a chain of physiological changes, including increased muscle tension, that use energy and increase the metabolic rate.

Due to these issues, the majority of the corallivorous butterflyfish species are not suitable for aquarium maintenance, which is a great pity—but keeping them under unnatural conditions, and relying on regular doses of medication to keep pathogens at bay, is not the solution. As marine aquarists, we should strive to replicate as closely as possible their wild environment so that our butterflyfishes can enjoy stress-free lives in our aquariums.
 

References
Thaler, E. 2003. Scheinauge, sei wachsam—Augenflecken bei Fischen. KORALLE 23, 4 (5): 26–33.
—. 2003. Auge oder Scheinauge, das ist hier die Frage. KORALLE 23, 4 (5): 34–37.
—. 2007. Schwimmbewegungen und Körpermusterung entwicklungsgeschichtlich gesehen—oder: Wozu hat der Fisch seine Streifen? KORALLE 46, 8 (4): 64–69.