Coloration of reef organisms
Coloration of reef organisms

seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of colorful reef with fishes taken from a video "Well, we've seen throughout the Virtual Dive that colour plays a role in many features of coral-reef life...from warning of potential territorial display. Now let's look at colours of reef animals in a bit more detail." - Little Cayman Island 2002

photograph of a clown crab Platypodiella spectabilis courtesy John Lewis, Montreal
Although there is no shortage of ideas regarding the function of colours in reef fishes and invertebrates, proper experimental testing of these ideas is difficult. There are 3 main reasons for this: 1) for many reef organisms we know so little about them that formulation of testable hypotheses is virtually impossible, 2) colours that we see now may have evolved in response to environmental conditions that are no longer present, and 3) the wavelengths photograph of polkadotted hermit crab Phimochirus operculatusof colours that we see are unlikely to be the same as those that are perceived by reef fishes, mammals, and birds, and by image-forming invertebrates.

What do we know about the
habits of the gaudy clown-crab
Platypodiella spectabilis?
Photo courtesy John Lewis, Montreal

And what could be the function(s) of the colours in the
polkadot hermit-crab Phimochirus operculatus? 2X

Functions of colours in clown crabs Platypodiella spectabilis would be difficult to study, owing to their highly variable colour patterns. For example, check out the 3 differently coloured individuals below, all collected from burrows within encrusting zoanthids and zoanthid-covered sponges in St. Eustatius, Dutch Caribbean. While zoanthids are their usual hosts, they also may be found in vase sponges Niphates digitalis, but only in ones with a zoanthid associate. There is not enough in this small study to correlate colour patterns with island geography or even local hosts, but it certainly would be an interesting research project. Garcia-Hernandez et al. 2016 Coral Reefs 35:209. Third photograph below courtesy the authors; others, courtesy Mike Harterink, Scubaqua Dive Center, St. Eustatius.

NOTE as noted, the crabs tend to associate with zoanthids, or with vase sponges Niphates digitalis that host parasitic zoanthids Umimayanthus parasiticus, as evident in the photograph below Right. The authors remark that if zoanthids are absent, then so is the crab, so this adds another level of interest in any future study of the relationship

photograph of clown crab Platypodiella spectabilis from St. Eustatius
photograph of clown crab Platypodiella spectabilis from St. Eustatius
photograph of clown crab Platypodiella spectabilis from St. Eustatius
Clown crab Platypodiella spectabilis in a burrow hollowed
out within the zoanthid Palythoa caribaeorum

This individual is just beginning to fashion its protective
hiding place in P. caribaeorum

This one is in a vase sponge Niphates digitalis, but note the growth of zoanthids Umimayanthus parasiticus on the sponge

Another collection of clown crabs Platypodiella spectabilis is featured below, this time from Guana Island, British Virgin Islands. As you can see, clown crabs are small, and since they are mostly hidden in their burrows or in crevices during daytime they are easily overlooked by the casual observer, especially SCUBA-divers. All individuals shown here are male, and were collected from dead coral (mostly finger coral Porites) in shallow water (<1m depth), within an area of just a few square meters. Although no females are featured in the array below, the authors note that there is no apparent sexual difference in colours in the species. Martin & Zimmerman 2007 Gulf & Caribbean Res 19 (1): 59. Photographs courtesy the authors.

NOTE based upon the variability in colours and patterning in the specimens shown here and in the previous entry, it seems unlikely that there is any habitat-specific patterning to be found, but it never hurts to look. What is needed first is a method of indexing each individual crab's colours so that a categorisation guide can be developed. The crabs appear to be numerous and handy enough for collections to be made in shallow water, so it behooves someone to get working on what should be a fascinating project

photo array of male clown crabs Platypodiella spectabilis collected in the British Virgin Islands

Before we go any further with colours and their functions, let's see how much we know about colour vision in reef animals. Review the list below andmentally sort the animals into those that SEE COLOUR and those that DON'T SEE COLOUR. Then check the correct listings shown in the boxes below.

NOTE coloration in reef plants is considered elsewhere: PRIMARY PRODUCTIVITY: SEAWEEDS & SEAGRASSES



mantid shrimps

photograph of a mantid shrimp Gonodactylus curacaoensis
The results may surprise you, but only in part because some of the animals listed may not be that familiar. Reef animals that see colour are FISHES, REPTILES, and CRUSTACEANS. Those that don't are MARINE MAMMALS and all NON-CRUSTACEAN INVERTEBRATES. Surprisingly, cephalopods (octopuses and squids) do not see colour, yet their colour repertoire and ability to change colour quickly suggest otherwise. Mantid shrimps can see in a broad range of ultraviolet wavelengths, although how they use this ability is not known. Marshall & Oberwinkler 1999 Nature 401: 873.

The eyes of mantid shrimps Gonodactylus curacaoensis are uniquely mobile, and their
swivelling adjustments when they look at objects suggest good depth perception 2X

  list of reef animals capable of colour vision   list of reef animals that don't see colour  

hot buttons for colours section of Biology of Caribbean Coral Reefs website
Topics relating to colours include how colours are created, considered here, and HOW COLOURS ARE PERCEIVED and the FUNCTION OF COLOURS, dealt with in other sections.

How colours are created

Colours in marine animals are created by movement of pigments in special cells called chromatophores, considered here, and by PIGMENT DEPOSITS, by refraction of light from STRUCTURAL ELEMENTS, and by the PRESENCE OF OTHER ORGANISMS.

NOTE: lit. "colour carry", referring to cells in which pigments can be moved into and out of extensions of the cell body


seahorse dive leader in Biology of Caribbean Coral Reefs website photograph of a bridled goby taken from a video

"This little fellow sitting so quietly here is a bridled goby. If we get real close, we can see some pigment deposits in the skin and just a hint of chromatophores near the eye. They're just a dusting of small black spots and may be a bit hard to see." - Turneffe Island, Belize 2002. Video courtesy Andy Stockbridge, Belize.

NOTE Coryphopterus glaucofraenum

photograph of juvenile dusky cardinalfish
Although chromatophores are generally hard to see in adult fishes, they are sometimes easily seen in larval fishes because of their large relative size and the transparency of the fish's skin. Photograph courtesy Linda Ianiello, Florida.



Individual chromatophores are quite easy to see in this dusky cardinalfish
Phaeoptyx pigmentria.
The fish seems to be well camouflaged against its
background, but as this photo was taken at night camouflaging is not an issue 1.5X




Chromatophores come in different colours in accordance with the type of pigments that they contain. In fishes the chromatophores are predominantly red, yellow, and black, and are under separate control. Control is mediated by neuronal or hormonal means, or a combination of both. In general, slow colour change, as during breeding, is under hormonal control, while fast colour change, as during territorial confrontation, is under nervous control. Colour change is by differential movement of pigment from a concentrated central area outwards into extensions of the chromatophore, causing the chromatophore to become coloured. The chromatophore goes from a CONTRACTED or blanched state to an EXPANDED or coloured state (see drawings below).
phases of chromatophore expansion in a larval sergeant-major

drawing showing operation of a chromatophore or colour cell
The skin of fishes, as in other animals, is comprised of 2 main layers, an outer epidermis and a deeper dermis. The chromatophores of fishes are located in the dermis layer just beneath the epidermis. Drawing adapted from Torrey 1962 Morphogenesis of the vertebrates John Wiley & Sons, NY.



This drawing shows the skin structure of a fish
that has black and red coloring. Light from the
sun passes through the epidermis into the dermal
layer, is differentially absorbed by the pigments
in the chromatophores, and is reflected back to
the outside minus the wavelengths absorbed by the
pigments. Areas of red chromatophores appear red,
while areas of black chromatophores appear black.

photograph of a blackbar soldierfish Myripristis jacobusBlack colour in reef fishes can be caused by melanin pigment that is either contained within chromatophores (these are called melanophores) as shown in the foregoing example, or is in deposits. Melanin is manufactured by the fishes, as it is in other photograph of squirrelfishes Holocentrus bullisianimals including humans, rather than having a dietary origin. In contrast, yellow and red pigments are always contained in chromatophores, and these are dietarily derived from plant foods, either directly through eating them, or indirectly through consumption of herbivores that are themselves eating the plants.

NOTE lit. "black" G. Melanin is manufactured in animals specifically for darkening the skin or for protection from light. In humans, it creates darkly pigmented skin. Melanin deposits are considered in the subsection "PIGMENT DEPOSITS" that follows.

The red carotenoid pigments in squirrelfishes (Left Photo Holocentrus
0.25X) and blackbar soldierfishes (Right Photo: Myripristis
0.4X) are obtained from the fishes' crustacean prey

photograph of a juvenile squid Loligo with chromatophores easily visible
Several types of reef invertebrates have chromatophores, including even some sea urchins, but they are best developed in cephalopods and crustaceans. Cephalopods change colour during mating, camouflaging, aggression, and fright.




Individual chromatophores, most in the
expanded state, are readily seen in
this juvenile squid Loligo sp. 3X

seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of a dying Humboldt squid Dosidicus gigas

This squid is a Humboldt squid Dosidicus gigas filmed in the Sea of Cortez. It's a male, and from the look of it has finished its mating and is dying. The pulsations of colour in the chromatophores show that even though it is moribund, the squid's nerve cells are still firing

NOTE nerves in the brain operate the chromatophores in one-to-one relationship and are thought to be positioned in the brain in patterns similar to those of the chromatophores that they control. Thus, as the nerves fire off in order in the brain, the chromatophore responses occur in corresponding waves

Because the chromatophores in octopuses Octopus sp. are under direct nervous control, the responses are virtually instantaneous. The nerves connect directly to special muscle bands in each chromatophore. When the nerve fires in the brain, the impulse causes the muscle bands to contract, instantly expanding the chromatophore and stretching out the pigment within. Photograph courtesy Anne Dupont, Florida.

photograph of octopus Octopus sp. courtesy Anne Dupont, Florida drawing showing 1:1 nerve:muscle innrvation of a chromatophore drawing showing chromatophore expansion from nerve discharge

In comparison, chromatophores in crustaceans are under hormonal control and changes may take hours or even days. The hormones are neurosecretions produced in nerve bundles in the eyestalks, and it is thought that each type of chromatophore (yellow, red, black, orange) is controlled by a single neurosecretory bundle or type. When released from their sites of formation in the eyestalks the neurosecretions move into the photograph of lobstermain body compartment and are carried in the circulating hemolymph to the chromatophores in the skin.

NOTE hemolymph is a fluid, coloured greenish-blue in crustaceans because of the presence of a copper-containing respiratory pigment hemocyanin, and has functional properties similar to the blood of other animals. Although partially transported in vessels, especially to and from the heart, hemolymph for the most part percolates through open spaces in the body, known as hemocoelic spaces, to reach all areas

Spiny lobster Justitial longimanus 0.2X

An octopus Octopus sp. changes its colour as it
tries to escape from view on a Belizean reef 0.33X
photograph of octopus changing colour in Belize

hot button for how colours are created part of BCCR hot button for how colours are perceived part of BCCR hot button for functions of colours part of BCCR