Coloration of reef organisms
 
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  hot buttons for colours section of Biology of Caribbean Coral Reefs website
This section deals with the function of colours. Topics of HOW COLOURS ARE CREATED and HOW COLOURS ARE PERCEIVED can be accessed via the icon.
 
 

Function of colours

  Functions of colours and colour patterns in reef animals fall into 2 broad categories of SOCIAL and DEFENSE, one topic of the latter, mimicry, being considered here. Most or all of these topics have been mentioned elsewhere in the BCCR but, by its nature, this section on FUNCTION OF COLOURS pulls them together in broad summary. A third category of UV PROTECTION is also included in its own, short section. CLICK ON a topic to learn about it.
 
 
 

Function of colours: defense: mimicry

 

Mimicry is when an animal's shape and colour resemble something else in order to attract its prey, hide from its prey, hide from its predators, or to benefit in copy-cat fashion by appearing to be another animal entirely. Fishes such as frogfishes that use lures to attract their prey are employing mimicry, and colours as well as form and behaviour are involved.

This section of mimicry deals with Batesian & Mullerian mimicry, while other sections deal with FISHING LURES & CAMOUFLAGE MIMICRY, EYESPOT MIMICRY, and AGGRESSIVE MIMICRY.

 
 

Function of colours: defense: mimicry: Batesian & Mullerian mimicry

 
 

Batesian mimicry occurs when a palatable animal (the mimic) evolves to resemble an unpalatable (toxic) animal (the model). In this strategy, the non-toxic mimic is mistaken by predators as the toxic model, and benefits by being left alone. Batesian mimicry is relatively common in insects, especially moths and butterflies, but occurs rarely in coral-reef organisms. For obvious reasons, much of our understanding of mimicry in coral-reef organisms is based on casual observation and not on rigorous experimental science. A decade ago, in the first study of its kind on coral-reef fishes, a purported mimicry of a toxic pufferfish by a non-toxic filefish was tested in field experiments by evolutionary biologists in Lizard Island, Australia. Although not exactly pertinent to the Caribbean, the design of the experiments is interesting, and the results provocative. Caley & Schluter 2003 Proc Roy Soc Lond B 270: 667. Photographs of model and mimic courtesy Robert Myers 1989 Micronesian Reef Fishes Coral Graphics, Guam

NOTE named after the famous 19th Century English explorer and naturalist Henry W. Bates who described the phenomenon in 1852 after extensive study of butterflies in Amazonian rainforests

 
photograph of pufferfish Canthigaster valentini courtesy Robert Myers 1989 The MODEL: the pufferfish Canthigaster valentini is laden with deadly tetrodotoxins in its internal organs 1X. It is almost universally avoided as food by predatory groupers and snappers photograph of filefish The purported MIMIC: the filefish Paraluteres prionurus is non-toxic, but does have an erectable dorsal spine typical of all filefishes 1X
The design and implementation of the experiment is elegant in its simplicity:
 
painted models of pufferfish used in mimicry experiment painted models of pufferfish used in mimicry experiment painted models of pufferfish used in mimicry experiment
The researchers construct 6 life-size models of the pufferfish and paint them to mimic the pufferfish's overall colour pattern, but graded in stepwise fashion from realistic (top) to impressionistic (bottom) The models are distributed on the reef flat, each with float and anchor. At Lizard Island the commonest piscivores are groupers (49%) and snappers (17%) so, for our simulation here, these types are represented by 4 Caribbean species gathered along the reef edge on the right Visits by the predatory fishes to the replicas are monitored during numerous 5min periods during daytime. The expectation is that the replicas most closely mimicking the real pufferfish will receive LESS visits, and the ones least closely mimicking will receive MORE visits
  graph showing results of mimicry experiment
The monitoring is done during daytime and there are many replicates of the experiment done in different areas of the reef. The results show that mean number of visits by the piscivores increases as the resemblance of the replicas to the pufferfish becomes less, in accordance with the scientists' prediction. The accompanying graph indicates that that the protection is maintained through 3-4 models, suggesting that even a moderate resemblance to the model will offer protection. The authors note that this provides the theoretical basis for evolutionary selection to a closer match.
 
So, the results support the idea that the filefish (the MIMIC)... photograph of filefish used in mimicry experiement
...by resembling the pufferfish (the MODEL)... photograph of pufferfish used in mimicry experiment

...gains protection from the piscivores...

photographs of Caribbean piscivorous fishes used in mimicry experiment
...and thus provide the first experimental evidence of Batesian mimicry amongst coral-reef fishes.  
 
 

Or does it? What is missing from this experimental scenario, something the scientists who did the study are well aware of? Think about the suggestions given below (there may be others), then CLICK HERE for explanations. Caley & Schluter 2003 Proc Roy Soc Lond B 270: 667.

1. the experiments were done in the field and not in controlled conditions in the laboratory.

2. other non-piscivorous fishes, such as herbivores, were excluded from the study.

3. no experiments were done using live pufferfishes (the model) and filefishes (the purported mimic).

4. the SCUBA-divers may have biased the results by interfering with the normal behaviour of the fishes.

5. because plastic was used to construct the replicas the piscivorous fishes would not have behaved normally.

 
 

In an earlier study in Panama, researchers from the Academy of Natural Sciences of Philadelphia note a similarity in spotting pattern between juvenile bridled burrfishes Chilomycterus antennatus and sea hares Aplysia dactylomela, and infer mimicry of the Batesian type.  In this case, the undefended burrfish is the mimic, while the toxic sea hare is the model.  While it is easy to see common colour patterning in the 2 species in the authors’ photograph (accompanying), it should be noted that these specimens appear to be preserved and the colours appear to be bleached.  When alive the 2 species are less similar, with juvenile burrfishes being yellow with black markings, and Aplysia dactylomela varying from greenish to brown and purple (see photographs below) with black markings.  There are also points of behaviour that suggest that the look-alikes may just represent coincidental colour patterning. Sea hares crawl rather than swim, and tend to rest during the day and be active at night. Burrfishes are active during the day and rest at night. So, for several reasons the authors' inferred mimicry is questionable. Still, based upon the previous study on Batesian mimicry in pufferfishes and filefishes, it should be possible to conduct a similar experiment in sea-grass meadows using plastic replicas of juvenile burrfishes as well as living specimens to test the authors' idea.  Heck & Weinstein 1978 Biotropica 10 (1): 78. Photograph lower Right courtesy Brandi Noble and NOAA, US Government.

NOTE  juvenile burrfishes are able to inflate like adults, but their spines are soft, and even when inflated their small swallowable sizes would not deter even medium-sized piscivores

NOTE requisite for classification as Batesian mimicry is for mimic and model to be exposed at the same time to visual predators. Most of Henry Bates' examples are day-flying butterflies of Family Heliconidae; one, however, is different and underscores the point being made here. It involves a toxic butterfly (model) and a palatable moth (mimic), the latter evolved to fly in daytime

Striped burrfish Chilomycterus schoepfii,
included here just to show the black and yellow
mottled coloration of a juvenile burrfish 5X

photograph of specimens of juvenile burrfish Chilomycterus antennatus and sea hare Aplysia dactylomela
The mimic, a juvenile bridled burrfish Chilomycterus antennatus and the model, a sea hare Aplysia dactylomela
photograph of striped burrfish Chilomycterus schoepfii courtesy Brandi Noble and NOAA
  More photographs relevant to this presentation:
 
photograph of striped burrfish juvenile Chilomycterus schoepfii photograph of seagrass-type sea hare Aplysia dactylomela photograph of sea hare Aplysia dactylomela from a seaweed habitat
Coloration of seagrass-habitat-type sea hares Aplysia dactylomela. Photograph courtesy Anne Dupont, FA. Coloration of seaweed-habitat-type sea hare Aplysia dactylomela 0.6X
Inflated juvenile striped burrfish Chilomycterus schoepfii showing soft spines 2X. Photograph courtesy FISHGUY.  
 
 

A related type of mimicry, known as Mullerian mimicry, occurs when several different species, all toxic or otherwise unpalatable, evolve common colour patterns and often common behaviour, and thus present a common message to potential predators: "eat us at your peril...and remember this colour pattern for next time!". Neudecker 1989 Envir Biol Fish 25: 143.

NOTE described by a German zoologist F. Mueller in 1878. A familiar example is the common pattern of orange, yellow, and black possessed by different toxic/unpalatable insects such as fire ants, wasps, and bees to warn of their unpalatibilty to potential predatory birds and lizards. In addition to presenting a common colour pattern to a predator, making its job of remembering which species are unpalatable easier, this type of mimicry spreads the mortality incurred by the different models (as predators try them out as food) over several species, instead of just one

   
  Butterflyfish species the world over exhibit common colour patterns of yellow, white, and black. Although the function of colours in butterflyfishes is not well understood, they may warn of unpalatability owing to the fishes' sharp spines. Convergent evolution of the type represented by Mullerian mimicry is most convincing when the mimics are not taxonomically close, such as toxic butterflies and moths in Central America and Brazil, but even though butterflyfishs are mostly in a single family Chaetodontidae, the fact that they are widely separated geographically lends greater credibility to the idea.
 
photograph of spotfin butterflyfish Chaetodon ocellatus photograph of pyramid butterflyfish Hemitaurichthys polylepis photograph of racoon butterflyfish Chaetodon lunula
CARIBBEAN: spotfin Chaetodon ocellatus INDO-PACIFIC: raccoon C. lunula INDO-PACIFIC: pyramid Hemitaurichthys polylepis
 
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