Chameleon shrimp

Scientists discover a species of crustacean that uses color shifts as camouflage to hide in seaweed and escape predators.

By Peter Moon  |  Agência FAPESP – They are small; they are numerous; and they change color. With a diameter of no larger than 1.5 cm, shrimp of the species Hippolyte obliquimanus live along the entire coast of Brazil and are associated with algal meadows.

Masses of these shrimp can be found off São Sebastião (São Paulo State) in mats of Sargassum furcatum, brown macroalgae or seaweed up to 2 m long, and Galaxaura marginata, a slightly smaller species of seaweed that is red or pink. There are two morphological types of H. obliquimanus: homogeneous (H) with color variability and translucent (T) with stripes, color bands or dots. Both belong to the same species.

Researchers have discovered that these crustaceans can change color to camouflage themselves in brown or red seaweed as a chameleon-like strategy to elude predators. Scientists refer to these crustaceans as carnival shrimp, no doubt referring to the multicolored costumes of Brazilian Carnival.

The authors of this study are PhD student Rafael Campos Duarte, his supervisor Professor Augusto Alberto Valero Flores, both of whom are affiliated with the University of São Paulo’s Marine Biology Center (CEBIMAR-USP), and Martin Stevens, Associate Professor of Sensory & Evolutionary Ecology at the University of Exeter in the United Kingdom. The study was supported by FAPESP, and the findings were published in the journal BMC Evolutionary Biology.

While the researchers were collecting animals near the seashore, they were struck by the fact that specimens of H. obliquimanus captured in mats of Sargassum were not only brown but also green, yellow, pink, red and black, whereas specimens captured in Galaxaura were predominantly red.

They performed experiments in the lab to investigate the mimicry capacity of the shrimp and the duration of their color changes using aquariums with both species of seaweed, as well as plastic imitations of the plants.

The results were surprising. The scientists found that shrimp with H morphology changed color in minutes to hide in the seaweed, but this behavior was observed only in tanks with live plants.

“No specimen changed color in the presence of artificial plants,” Duarte said. “This suggests that seaweed releases some substance into the water that affects the shrimp and makes them change color.”

Additionally, mimicry was not uniform in Sargassum. Although most of the shrimp specimens displayed shades of brown, other colors were also observed. In the case of Galaxaura, mimicry was more efficient, and all the animals were red or pink. 

“This efficiency may be due to the fact that Galaxaura algae aren’t especially abundant in the sea off São Sebastião, as there is less seaweed, and fewer shrimp live there,” Duarte said.

The fewer animals there are in a seaweed mat, the greater the need for efficient camouflage to avoid capture by fish.

Sargassum algae predominate in this environment where “the shrimp population is very dense indeed,” he explained. “Our dip net brought up hundreds of individuals with each sweep through the seaweed. The mats grow vigorously in summer, and on average, there are 150 individuals per kilo of seaweed.”

Such a high density of animals living in the Sargassum mats suggests a low likelihood of an individual being caught by a predator. “So, the mimicry doesn’t need to be as efficient as that for the shrimp that live among Galaxaura,” he said.

Daytime colors

“Color shifts, such as those highlighted by this study, have to do with the camouflage strategies used by shrimp to mimic the color of the substrate they inhabit,” Flores said.

A change of habitat influences the color of an animal. Thus, brown shrimp in Sargassum become red in Galaxaura and vice versa. “In the UK, there is a shrimp of the same family that is green because green seaweed predominates there,” he noted.

Another observation was that all colors were displayed only during the day. As it grew dark, the crustaceans faded until they became transparent, translucent or blue.

“Their color comes from pigments contained in color cells called chromatophores,” Duarte said. “An individual’s coloring changes according to the arrangement of the chromatophores.”

“Our findings suggest that the T morphotype is more mobile and doesn’t rely on camouflage based on similarity to the background,” said Flores, who is CEBIMAR’s Deputy Head.

“The densities of T-type shrimp in Sargassum and Galaxaura are identical.” The former seaweed is seasonal, occurring mainly in summer, while the latter is non-seasonal. “We therefore concluded that the portion of the T population in Galaxaura is responsible for the stability of the species over time. Additionally, this population is more ‘mobile’ and can make connections between different clusters of algae.”

According to Flores, many other species found along the coast of Brazil use camouflage strategies. “Rapid color shifts taking only a few days are expected for species of crustaceans with a thin exoskeleton, in which a relatively small amount of pigment can have a striking effect,” he said.

In addition to small shrimp such as H. obliquimanus, the researchers expect to find these rapid color shifts in other groups of crustaceans, such as amphipods and tanaids.

The article “Shape, colour plasticity, and habitat use indicate morph-specific camouflage strategies in a marine shrimp” (doi: 10.1186/s12862-016-0796-8) by Rafael C. Duarte, Martin Stevens and Augusto A.V. Flores, published in BMC Evolutionary Biology, can be read at: bmcevolbiol.biomedcentral.com/articles/10.1186/s12862-016-0796-8.