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Chameleon shrimp can adapt camouflage to invasive algae species

Small shrimps of the genus Hippolyte have the ability to change color to camouflage themselves in the algae where they live and escape predators. Depending on the algae they are in, they can turn red, green, brown or transparent.

A study in the Journal of Animal Ecology shows for the first time how these crustaceans interact with invasive algae. The results also show that they can camouflage themselves in algae from other oceans with which they did not evolve together.

The authors, from the Federal University of ABC (state of São Paulo, Brazil) and the University of Exeter (United Kingdom), tested how the species found at European beaches, the chameleon shrimp (Hippolyte varians), interacts with two exotic algae, one from Asia and the other from Australia.

The European species, which is about 3 centimeters long, is evolutionarily related to the (Hippolyte obliquimanus), which is common along the north coast of São Paulo.

"When the chameleon shrimp encounters two different algae, it doesn't matter which is native and which is exotic, it chooses the one that offers the best color match so it can hide," reports Rafael Duarte, first author of the study, which was conducted during his postdoctoral studies at the Center for Natural and Human Sciences (CCNH) at UFABC.

The collections and experiments were conducted during Duarte's internship at the University of Exeter under the supervision of Martin Stevens, co-author of the article.

The researchers collected the shrimp and the algae they lived on from tide pools formed on rocks at two beaches in Falmouth, southwest Britain.

In the laboratory, the animals were placed in a box with two algae to hide in, one native and one exotic. Combinations of two of the four species studied were tested, always one native (green or red) and one exotic (brown or pink).

"The brown one is an Asian sargassum species that has increased its presence in the last decade and is causing problems in several places in Europe. It's taking over the habitat of native species, which are being restricted to deeper areas. However, it has proven to be an excellent shelter for shrimp," says Duarte, currently a researcher at the University of Aveiro in Portugal.

Complexity rather than color

In the Y-shaped boxes, the shrimp were observed for ten minutes. In most cases, they chose one of the two algae offered and remained there until the end of the experiment.

In general, there was no preference for choosing the native algae, but rather the algae where they could best hide. The green shrimp showed no preference between the green native algae and the brown exotic sargassum.

But when they encountered the native red algae and the exotic brown algae, the shrimp preferred the sargassum. The researchers believe that when there is no color match with the native plant, the structure of the invasive plant serves as better protection, favoring its choice.

"While the native green algae are quite leafy and flat, the sargassum has a more complex three-dimensional structure, like a fern. In nature, this probably makes it easier for them to protect themselves better from predators, even if the colors don't quite match," Duarte points out.

Moreover, he adds, the native green algae are quite seasonal. In the periods when they're not as available, the shrimp can use sargassum, which is more stable throughout the year.

It takes up to 30 days for the chameleon shrimp to turn the same color as the algae. The researchers observed that the color change from red to green is faster than the other way around. The most likely hypothesis lies in the pigments inside the cells that give the animals their color, called chromatophores.

Work on pigmentation carried out by other groups shows that red animals have all the necessary pigments in their chromatophores and only need to get rid of the other colors to change their color.

In the case of green animals, however, the red pigment must be acquired in order to change color, which requires more time and energy from the organism. In the case of chameleon shrimp, the researchers believe that it is necessary to eat red or pink algae in order to acquire the pigments of these colors.

However, a number of questions remain about this complex interaction. One of them is how the shrimp recognize the algae. Previous studies have shown that the shape of the algae is important, but it is not certain that the choice is visual. The group is now investigating whether algae have chemical signals that can be detected by crustaceans, and whether the presence of pollutants in the water can alter this perception.

"Although the interaction with exotic algae doesn't seem to affect the species' ability to camouflage itself, we don't know the long-term effects of the presence of these invaders. Throughout the world, invasive species are wreaking havoc on ecosystems. We need to monitor to better understand these relationships," he concludes.