Color vision is a vital capability for animals, as it significantly increases the information received from the environment. To discriminate between different colors, the eyes of most animals rely on two to four photoreceptors with different spectral sensitivities, whose electrical responses are compared in a neural process yielding an astonishingly fine color resolution, particularly in vertebrate eyes. In a remarkable study published recently in Science, a team of scientists led by Hanne Thoen from the University of Brisbane, Australia, provide evidence for a completely different mechanism of how animals can see colors. Instead of having only a few visual receptors with different spectral sensitivities, mantis shrimp use 12 different photoreceptors, which enables them to recognize light of different wavelengths without time-consuming neural processing.
Mantis shrimp belong to an ancient group of crustaceans; they look like lobsters that were beautifully painted with vivid colors. They are predominantly found in tropical coral reefs, where they hunt with the help of forceful raptor claws that can explosively unfold to maim and even kill comparably large prey animals. Colors play important roles for these shrimp, because they help the animals to recognize different types of corals, prey species, predators and competitors. To orientate in this colorful environment, the shrimp have stalked ‘compound eyes’ that are capable of moving independently of each other. The eyes resemble those of other arthropods, as they are made of numerous visual units called ommatidia, which produce individual pictures that are finally combined in the brain. The shrimp eyes, however, are special in that they possess a ‘midband’ of six ommatidial rows, two rows of which analyze polarized light and four rows account for color vision.
To examine the mode of color-coding in these shrimp, Thoen's team measured spectral sensitivities using intracellular electrophysiological recordings in the photoreceptor cells. They demonstrated that the 12 different types of photoreceptor cells are contained in the color-detecting ommatidial rows of the midband, and each photoreceptor cell is narrowly tuned to a different wavelength between 300 and 700 nm. As the vertical visual field captured with these ommatidia is very narrow, the scientists concluded that the shrimp need to scan an object of interest with slow eye movements. Indeed, such scanning movements of the shrimp's eye have been noticed in previous studies. This mechanism may generate a temporal signal for each spectral sensitivity, enabling the shrimp to recognize particular colors very rapidly.
Next, the scientists wanted to know how the eyes perform in color discrimination. They trained the shrimp to recognize a certain color by rewarding them with food. Then they used various test colors to determine at which wavelength difference the animals fail to discriminate between the test color and the color they were taught to associate with food. Counter-intuitively, the team found that the shrimp's ability to discriminate between different colors is very poor, despite the fact that they possess these 12 different color receptors.
Thoen's team demonstrated that mantis shrimp see their environment with an array of peripheral color detectors that scan an area of interest. They also discovered that the animal's colour discrimination is not very sensitive, but its temporally efficiency is high because further neural processing steps are not required. Obviously, it is less important for the mantis shrimp to discriminate colors with high accuracy, but advantageous in their environment to instantly recognize the movement of colored patterns that are characteristic of either prey animals or potentially life-threatening predators and competitors.
