The hogfish (Lachnolaimus maximus) is a reef-dwelling fish that can adapt its colouring to its environment in milliseconds. They could start out white, then change as they move around on the ocean floor to mottled brown, then red.
Chameleons, cuttlefish, octopuses, flounders, seahorses, crab spiders, anole lizards, ptarmigan, and arctic foxes and hares can all change their colour to adapt to their surroundings to a greater or lesser degree. It can be a more gradual seasonal camouflage or rapid active camouflage like hogfish can do.
Scientists speculate that animals with active camouflage don’t depend on their eyes to alter their appearance — they also sense light with their skin. But exactly how “skin vision” works remains an enigma.
A 2018 study shows that hogfish skin senses light differently from eyes. The results suggest that light-sensing evolved separately in the two tissues, according to Lori Schweikert, a postdoctoral scholar with Duke University biology professor Sönke Johnsen.
Do Fish Have Chromatophores?
Chromatophores are cells that produce colour. Many types are pigment-containing cells, or groups of cells, located not just in the skin but also in the eyes and internally. Cephalopods like squid, cuttlefish, and octopuses have chromatophores, as do chameleons and anoles. But do fish have chromatophore cells?
Fish that can change color do have these cells, and the hogfish is one of them. Other colour-changing fish with chromatophores include the purple-striped dottyback (Pseudochromis diadema), the peacock flounder, scrawled filefish, and the tasselled anglerfish, and many species of Rainbowfish.
With dermal photoreception, as it is called, the skin doesn’t let animals discern details as they do with their eyes, according to professor Schweikert. But the skin might be able to sense shifts in brightness or wavelength, like moving shadows cast by approaching predators or light fluctuations associated with different times of the day.
Why Do Fish Change Color
Many species of fish can change their colour. The colour change can be fast or slow, gradual or sudden.
Some fish change their skin colour naturally as they slowly grow from larval to adulthood. Fish also change colouring gradually if their diet changes.
A nutritious diet will make their colouring more vibrant, and a sick or poorly nourished fish will fade in colour as it devotes its energy to core functions. Some fish will also change colour during their mating season. It is generally the males that put on their flashy breeding colorations.
Then there are fish that can change colour seemingly on a whim, like a hogfish.
The hogfish lives out its days in shallow waters and coral reefs in the western Atlantic Ocean, from Nova Scotia to northern South America. Why did Schweikert, Johnsen, and Duke postdoctoral associate Bob Fitak focus on it for their 2018 research?
Because it can make its skin whitish to melt in with the ocean bottom’s sandy floor to cloak itself against predators or sneak up on prey. Alternately, it can assume a bright, contrasting pattern to look threatening or attract a mate.
And chromatophores are the crucial ingredient of this strategy. When activated by light, they spread their pigments out or bunch them up to change the skin’s overall colour or pattern.
Seeing With Fish Skin
The researchers removed fragments of skin and retina from a single female hogfish collected near the Florida Keys. Then they analyzed all its RNA transcripts to see which genes were expressed in the two tissue types.
In previous studies of colour-changing animals such as cuttlefish and octopuses, similar analysis suggests the same molecular pathway that detects light in the eyes may have been evolutionarily co-opted to sense light in the skin.
Schweikert and colleagues, however, discovered that hogfish skin works differently.
Almost none of the genes dedicated to light detection in the hogfish’s eyes were activated in the skin. Instead, the data suggest that hogfish skin relies on an alternative molecular pathway to sense light, a chain reaction involving cyclic AMP molecules.
How exactly does the hogfish’s “skin vision” supplement input from the eyes to monitor light in their surroundings and then bring about a colour change? That, Schweikert says, remains unclear.
Light-sensing skin may provide information about conditions beyond the animal’s field of view or outside the range of wavelengths that the eye can pick up. But the results of this study, along with previous work, are evidence that fish have found a new way to ‘see’ with their skin and change colour rapidly.
- Ramirez, M. D.; Oakley, T. H (2015). Eye-independent, light-activated chromatophore expansion (LACE) and expression of phototransduction genes in the skin of Octopus bimaculoides. Journal of Experimental Biology. 218 (10): 1513–1520.
- Schweikert, L.E., Fitak, R.R. & Johnsen, S. De novo transcriptomics reveal distinct phototransduction signaling components in the retina and skin of a color-changing vertebrate, the hogfish (Lachnolaimus maximus). J Comp Physiol A 204, 475–485 (2018).
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