Coloration in 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 icons.

Function of colours

  Functions of colours and colour patterns in reef animals fall into 2 broad categories of SOCIAL and defense, considered here, the latter including the possibility of UV protection. 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 as a broad summary. CLICK ON a topic to learn about it.

Function of colours: defense: warning of toxicity


We know from other parts of the VIRTUAL DIVE that camouflaging in reef organisms can involve combinations of colour, form, and behaviour. What follows in this section are some examples where colour and colour-patterns predominate. This large topic is divided into a section on sea anemones, jellyfishes, & hydroids, considered here, and sections on ZOOANTHIDS, SNAILS, and FISHES presented elsewhere.



Function of colours: defense: warning of toxicity: corals, sea anemones, corals, jellyfishes, & hydroids

seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of giant anemone Condylactis gigantea

"Oh, a nice colorful giant anemone. I've always thought that an anemone's colours are for warning...but that doesn't make much sense if the anemone is trying to catch and eat the same fishes and crustaceans that are seeing the, what's going on?!" - Turneffe Island, Belize.

NOTE Condylactis gigantea


Many cnidarians appear to advertise the toxicity of their nematocysts with spots of colour or white demarcations. Since it would make no sense that photograph of a Pederson shrimptheir small shrimp or fish prey be warned away, it must be a message to non-prey organisms to photograph of a giant anemone Condylactis gigantea with red-tipped tentaclesstay away, rather than to venture too close and get stung. Such encounters would not benefit the sea anemone. In fact, it would just be a waste of its supply of metabolically costly stinging cells (nematocysts).

Are the red-tipped tentacles of this
giant anemone Condylactis gigantea
a warning to stay away? 0.3X

Distinctive white markings advertise the
toxic nematocyst batteries of corkscrew
anemones Bartholomea annulata 1X


Corals are not considered to be particularly toxic, and many fishes and other organisms consume them readily. An exception, however, is the mound coral Montastraea cavernosa which, based on reports that it is avoided by certain corallivorous fishes such as parrotfishes, is thought possibly to contain some type of chemical deterrant. If so, and given that it grows in different colour morphs of cyan, green, and orange, could the colours be warning of toxicity (i.e., aposematic)? This idea is tested by a consortium of U.S. university researchers by exposing green and orange morphs from the Cayman and Bahamanian islands to pufferfishes Canthigaster rostrata in flow-through tanks, and also by testing pelleted squid tissue containing extracts from each colour morph for palatability. Results show, indeed, that M. cavernosa contains both organic and aqueous chemicals that deter feeding by these fishes, but that there is no relation to colony colour. The authors conclude that colour morphs in this coral species may serve other ecological functions. Jarett et al. 2016 Coral Reefs 35: 883. Photographs courtesy the authors.

NOTE while pufferfishes C. rostrata are known to eat corals as part of their diet, it is puzzling why the researchers did not include in their study any of several corallivorous parrotfish species and other mound-coral species present in the study area. This would have fleshed out the study considerably


Upside-down jellyfishes have normal feeding tentacles with potent nematocysts, but they also have what have been described as specially toxic, blue-coloured prey-catching "appendages" that are quite noticeable on some individuals when in their usual upside-down postures. As do many other types of cnidarians (sea anemones, jellyfishes, corals, gorgonians), these jellyfish have abundant photosynthesising symbionts in their tissues. Measurements of amounts of carbon photosynthetically fixed by the zooxanthellae and subsequently translocated to the host jellyfish suggest that up to 1.7 times the host's metabolic demands can be met by activities of their symbionts on a warm, sunny day. The symbionts appear to be Symbiodinium microadriaticum, the same type as found in sea anemones, giant clams Tridacna, and other marine organisms. These observations suggest that prey-catching may constitute a smaller component of the nutrition of the jellyfish than originally thought, although some prey are probably necessary to satisfy specific needs, such photograph of several upside-down jellyfishes Cassiopea xamachanaas nitrogen. The upside-down posture in shallow-water habitats is presumbly to benefit the symbionts by exposing them to greater sunlight, but where does that leave us with the blue-coloured "appendages" and their supposed function in feeding or defense? Is it possible that their bright colours may be a warning to potential predators of their toxicity, a message for the predators to stay away?photograph close view of the special feeding or defensive appendages of jellyfishes Cassiopea xamachana


Upside-down jellyfishes Cassiopea xamachana 0.3X


Close view of the blue-coloured "appendages", thought to contain
especially toxic
nematocysts and thus
possibly to function in
feeding and/or defense 0.6X

  photograph of Christmas-tree hydroids Halocordyle disticha

The white polyps and tentacles of certain hydroids are visually obvious, at least to SCUBA-divers, and give a clear warning of impending pain if touched.




Christmas-tree hydroid
Halocordyle disticha

photograph of a giant anemone Condylactis gigantea with red-tipped tentacles
Well, all of this makes for good reading, but do we accept that what we have seen is truly warning coloration? For example, what is wrong with this picture, most notably in the context of colour perception?

Well, if you are thinking that the photograph was likely taken with a flash and that the red-tipped tentacles of this giant anemone Condylactis gigantea would actually appear black at depth owing to the fast diminution of red wavelengths with depth, so how could the colours be warning, or something along those lines, then you have a point.




photograph of giant anemone Condylactis gigantea
In this regard, check out the accompanying photo of the same species of giant anemone taken in ambient sunlight (i.e., no flash) at a shallow depth. The point is, if coloration is good for warning of toxicity, how do we explain its lack of colourl? Is this just a white variation, or perhaps a bleached individual?

NOTE just as corals may become bleached with loss of their symbiontic zooxanthellae, so do anemones. It is a simple job to determine presence or absence of the symbionts, requiring only a compound microscope, a glass microscope slide, and a scalpel




Giant anemone
Condylactis gigantea 0.7X


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