Eye Camouflage and False Eyespots Among Butterflyfish (Final Paper)

This discussion topic submitted by Stacee Smith ( smithsd4@miamioh.edu) at 9:09 pm on 6/9/00. Additions were last made on Wednesday, May 7, 2014.


Eye Camouflage and False Eyespots
Among Butterflyfish
Butterflyfish are perhaps one of the most common and delightful coral reef fish observed by researchers and divers and are easily recognized by their bright coloration. Currently, more than 125 species of the butterflyfish have been identified (1). Most butterflyfish are found in tropical, shallow waters around coral reefs at depths less than 60 feet. However, some newly discovered species are restricted to deeper reef habitats, including depths over 600 feet (2).
The classification for the butterflyfish are as follows:
Kingdom Animalia
Phylum Chordata, Subphylum Vertebrata
Class Osteichthyes
Order Perciformes
Family Chaetodontidae

Ichthyologists classify butterflyfish in the Family Chaetodontidae, named for their tooth design. Even though the diet of butterflyfish varies from species to species, all have comb-like teeth and are primarily carnivores. These fish prey on different kinds of soft-bodied invertebrates including crabs and barnacles (3). Other butterflyfish, called corallivores have specialized diets of coral polyps or mucus, and these species are usually territorial, or they maintain their territory. The last type of butterflyfish based on diet is the planktivores. As their name suggests, these fish feed above the reef and pluck individual planktonic animals from the water column (2).

Similar to their namesake, butterflyfish spend their days fluttering about in shallow to deep reefs, dock pilings, and rubble fields (4). However, butterflyfish can be fast and aggressive if necessary to escape predators. The common adult butterflyfish usually ranges from 5 to 8 inches, or 12.5 to 20 centimeters long (3). The large body size exhibited by the butterflyfish presents greater metabolic needs. Therefore, the individuals may require large foraging areas. Even though some species of butterflyfish may defend interspecific feeding territories as small as 1 m2 to 2 m2, most adult butterflyfish inhabit large home ranges of 50 m2 to over 10,000 m2 in size (5).

The coloration of butterflyfish is undeniably their most unique characteristic. Butterflyfish exhibit very dramatic colors and patterns consisting of primarily black, white, and yellow (2). Field observations of butterflyfish reveal that a large amount of information is communicated through presentation of the fish's coloration to other fish, including potential predators (6). Researchers have concluded that Chaetodontids do not flee large potential predators, instead they turn sideways and display their vibrant coloring (6). Within the coloring patterns, butterflyfish possess two very distinct morphological characteristics which researchers believe serve a variety of functions, including antipredatory devices. The two characteristics are eye bands and "false eyespots".

Eye Bands or Eye Camouflage

Many theories concerning the purpose of the eye bands have been proposed, but most scientists agree that the dark eyebars serve to camouflage the eye from predators. The protective function of the eyebars is through visual confusion of potential predators, especially against the complex backgrounds found in reef ecosystems. When the eyes are disguised or vanish in the background, a primary target for predators is concealed.

Eyebar configuration is tailored to the morphology of the fish so that the camouflage effect is maximized. The direction of the eyeband is correlated with relative body depth. Deep-bodied fish (such as the butterflyfish) have predominantly vertical eyelines, whereas fusiform, or shallow-bodied, fish display longitudinal ones (6).

In relationship to butterflyfish, eye camouflage is practical because diurnal (or daily) aggression is rare and the eyes are not needed for aggressive displays. The fact that 95% of the observed butterflyfish have eyebars or black heads suggests that there has been greater selective pressure for eye camouflage than for brilliance (6). Of the observed butterflyfish in which part of the iris was not covered by the eyebar, the uncovered portion of the eye usually matched the body color surrounding it. Therefore, when a portion of the iris is not hidden, it matches and blends into the surrounding body color of the fish (6).

Butterflyfish that are deep-bodied with bands are usually mobile and active just above a complex substratum. While these fish can turn sharply, they cannot sustain the high swimming speeds of the elongated species. Therefore, these behavioral characteristics of butterflyfish partly explain the need to camouflage eyes from speedy predators (6).

False Eyespots

Butterflyfish also utilize another morphological survival adaptation known as a "false eyespot", or ocellus (6, 7). The occurrence and adaptive significance of realistic, vertebrate-like eyespots on many animals have long intrigued biologists (7). In fact, when including eyespots surrounded by a contrasting color ring, they can be detected in many groups including insects, cephalopods, frogs, pheasants, and fish. In marine fish, they are found in wrasses, damselfish, basses, flatfishes, and pufferfish, in addition to butterflyfish (7).

Researchers have proposed many theories concerning the purpose and function of the eyespots in butterflyfish. False eyespots look like eyes, but occur in other regions of the body (6). In fact, the most prevalent theory is based on the fact that most false eyespots are located in the posterior of the butterflyfish. The butterflyfish use the false eyespots to produce an impression of a head at the wrong end. This leads researchers to believe that the eyespot serves to misdirect predator attacks toward less vital body areas, resulting in decreased predation rates or severity (7). In some cases, the predator completely misses the intended butterflyfish by assuming an incorrect (or misguided) plan of attack.

The varying sizes of the false eyespots influence other theories about their function. Given their large size in relationship to real eyes, researchers suggest that the eyespots serve to intimidate a predator into perceiving that the fish is larger or a member of a different species (7). In a study completed in 1993, false eyespots averaged 6.65 (+/- 1.71) times larger than real eyes at all ages and sizes (7). The larger eyespot is thought to discourage rather than misdirect attacks, thus decreasing predation intensity.

Large eyespots are also consistent with a "reaction-distance hypothesis" about the function of the false eyespot (7). A larger eyespot would confuse the predator about its actual distance from the prey and deceive the predator into initiating an attack from further away than normal. Mortality of the butterflyfish would then be reduced if it was able to react quickly enough to escape the attacking predator.

The final theory concerning eyespots is their role in social communication, both inter and intraspecifically (6). The eyespots could serve as warning signals to predators that these fish are deep-bodied, spiny, and may not be worth the effort to attack (6, 7). Another communicative role of the eyespots could occur between mates to identify each other. Most Chaetodontids, butterflyfish, occur in pairs and are thought to be monogamous for up to 10 years (5, 7). Because the butterflyfish are often involved in intra- and interspecific interactions near territory borders, the eyespots may serve as important cues to help them rejoin after being occasionally separated.

Like many other species of the coral reef fish, the butterflyfish (both male and female) are brightly patterned with distinct coloration. As suggested, the designs displayed by the butterflyfish through colors, patterns, and the presence of eyebars and false eyespots may serve many purposes in the life of butterflyfish. The color characteristics may not only serve as communication mechanisms, but also antipredatory devices. The simultaneous occurrence of eyebars and false eyespots in many species of butterflyfish lend to support to using the coloration as protection. The coloration of butterflyfish is not static, but instead changes in varying social situations and sometimes at night (6). Therefore, no single explanation of the role of brilliant coloration of butterflyfish is sufficient or comprehensive enough to account for all circumstances.

References

1. McMillan, W.O., Weight, L.A., and Palumbi, S.R. Color Pattern Evolution, Assortive
Mating, and Genetic Differentiation in Brightly Colored Butterflyfishes
(Chaetodontidae). Evolution Vol 53 Iss 1. pp. 247-260. (1999).

2. Waikiki Aquarium Education Department. Marine Life Profile. Internet:
http://waquarium.mic.hawaii.edu/MLP/search/butterflyfish.html (January 1999).

3. The Columbia Encyclopedia Sixth Edition. Butterflyfish. (2000). Internet:
wysiwyg://88/htp://http://www.bartleby.com/65/bu/butterfl-fi.html

4. New World Publication's Marine Life Learning Center. Butterflyfish. Internet:
http://www.fishid.com/learnctr/bflyfish.htm (1999).

5. Hourigan, T. F. Environmental Determinants of Butterflyfish Social Systems. Environmental Biology of Fishes. Vol 25. No 1-3. pp. 61-78. (1989).

6. Neudecker, S. Eye Camouflage and False Eyespots: Chaetodontid Responses To
Predators. Environmental Biology of Fishes. Vol 25. No 1-3. pp. 143-157. (1989).

7. American Society of the Ichthyologists and Herpetologists. Morphological Variation In Eyespots of the Foureye Butterflyfish (Chaetodon capistratus): Implications for Eyespot Function. Copeia. pp. 235-240. (1993).

Other Resources

McBride, R.S. and Able, K.W. Ecology and Fate Of Butterflyfishes, Chaetodon spp., In
the Temperate, Western North Atlantic. Bulletin of Marine Science. Vol 63. No 2. pp. 401-416. (Sept 1998).

Tricas, T.C. Prey Selection By Coral-Feeding Butterflyfishes: Strategies to Maximize
the Profit. Environmental Biology of Fishes. Vol 25. No 1-3. pp. 171-185. (1989).


Internet Sites:

http://www.flmnh.ufl.edu/fish/Education/Adapt/Adapt.htm

http://www.wmi.org/bassfish/articles/T52.htm


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