Under intense predation pressure—birds during low tide, larger fish at high tide—rock gobies have become masters of camouflage. These little saltwater fish can change their color and brightness to conceal themselves in just a minute. The findings were published in PLOS ONE this week.
To remain effectively camouflaged, rock gobies (Gobius paganellus) must respond rapidly since they’re being pushed over many different backgrounds by the tides and waves in rockpools where they live. Previous work on color change for camouflage were limited to a few animals—chameleons, cuttlefish, flatfish, and crabs—and they rarely directly quantified the sorts of changes that occur, how fast they happen, and how they affect camouflage matching.
So, to see if small fish can tune their appearance to match their backgrounds, a trio of University of Exeter researchers led by Martin Stevens collected 80 rock gobies using a dip net in the intertidal zone of Gyllyngvase beach in Falmouth, Cornwall in the U.K. They conducted two experiments right at the beach in under two hours each, and the gobies were returned to their rockpools afterwards. In both experiments, gobies were photographed against different backgrounds over time to see how fast they can change color or brightness and to quantify the extent of the changes.
In the first experiment, 40 gobies were placed in shallow, water-filled trays lined with either black or white backgrounds to test for changes in their luminance (or lightness). The team tested for actual color changes in the second experiment, when the rest of the gobies were photographed against waterproof paper that were either red or blue. These colors sit at different ends of the visual spectrum that gobies are likely able to discriminate.
They found that goby color changes occurred within just one minute. When they were photographed on a bright white or dark background, they changed their brightness accordingly. When photographed on colored backgrounds, they altered their color to become either red or more blue (they couldn't quite get to blue). Some colors are easier for them to adapt to than others, and red may be the most important one to match up with: Red algae and brown stones and seaweed are common in the European and north African pools where they live.
Some examples of brightness changes are pictured here. On the left are three individuals who were placed on a black background, and the same individuals are shown on the right after being on a white background.
“Anyone who’s been rock pooling will probably have encountered rock gobies, and may even have thought they could see them change color in buckets,” Stevens says in a news release. “Our research shows that this is the case and that rock gobies can rapidly tune their appearance to match their background.”
The quick-fire color change is driven by special cells called chromatophores. These work to either condense or spread pigments of varying colors (such as melanin and carotenoids) over the body to change the appearance of the fish. This process is guided by the goby’s visual system as it moves onto new backgrounds.
Images: Alice Lown (top, bottom), 2014 Stevens et al., PLoS One (middle)