Julia Borys ’22
Coloration within organisms, which allows for camouflage, is a common example of natural selection. This mechanism increases an organisms’ ability to avoid being seen by predators. While camouflaging is ubiquitous in natural selection, organisms’ ability to change color at a relatively rapid rate in response to a changing environment is not. This rapid adaptation, however, is seen in chameleon prawns (Hippolyte varians). In the work performed by Green et al., not only is the ability of rapid coloration change in chameleon prawns illustrated, but so is a preference of background habitat to match their possessed coloration.1 Green’s work also provides evidence for the benefits of this cryptic coloration through the detectable visual differences by the chameleon prawn’s main predators.
If the cryptic coloration of chameleon prawns really is beneficial, then their main fish predators should not be able to detect them when they’re against a color-matching substrate. To investigate this, researchers converted prawn and seaweed images to “fish vision,” which predicts the amount of light caught by the cones in the dichromatic eyes of pollacks (Pollachius pollachius) and trichromatic eyes of gobies (Gobiusculus flavescens) , which are two of the chameleon prawn’s predators. The noticeable differences of green-colored prawns against green sea lettuce (Ulva lactuca) were almost undetectable in the eyes of the fish while they were spotted against the seaweed red dulse (Palmaria palmata). The opposite was true of the red-colored prawns, who were barely detectable against red dulse but could be seen against green sea lettuce. This confirms that cryptic coloration could be adaptive for the chameleon prawns in avoiding fish predators since they become less visible.
If the color of the chameleon prawns helps them be less visible against their natural background, then it is not much of a surprise for natural selection to seemingly result in this adaptation. What is interesting about chameleon prawns is their ability to change color based on the background substrates available and choose the habitat most closely resembling their current color. In an experiment performed by Green, chameleon prawns were placed on the contrasting color of seaweed for 30 days.1 Especially within the first ten days, the green prawns became redder and the red prawns became greener during the trial. Whatever color the prawn currently was, when given a choice between choosing a color-matching background or not, the prawns chose the color-matching background 75-80% of the time. This behavior of choosing habitat is advantageous because, while the chameleon prawns can change color fairly rapidly, hue alteration takes time and does not protect the prawn from an immediate predator threat. Additionally, different backgrounds are only available during different times of the year. Green sea lettuce grows rapidly during warmer months, but it loses its fronds during the colder months. Red dulse also grows rapidly in summer but remains steadfast during the winter months as well. This results in a change in the availability of a specific-color habitat that the chameleon prawns can use to hide (Fig. 1). During the summer, prawns can choose between green sea lettuce or red dulse. During the winter, green sea lettuce becomes much less available, so it would be expected for a shift to red coloration in chameleon prawns to be necessary in matching the remaining red dulse.
Both the behavioral and morphological characteristics of chameleon prawns impact their crypsis ability and avoidance of potential predators. This allows the prawns to use seasonal changes in their environment to their strategic advantage. Since there may be periods of time where seasonal change is slow, the behavioral aspect of the prawns’ crypsis allows them to continue to avoid predation through camouflage. There are currently other studies that are also testing the behavioral and morphological influences of crypsis in different phylogenetic systems, suggesting convergent evolution of avoidance strategies and their importance to camouflage.
Green, S.D., Duarte, R.C., Kellett, E., Alagaratnam, N., Martin Stevens, M. Communications Biology 2, 230 (2019).
