Mantis Shrimp - Eyes

Eyes

The midband region of the mantis shrimp's eye is made up of six rows of specialized ommatidia. Four rows carry 16 differing sorts of photoreceptor pigments, 12 for colour sensitivity, others for colour filtering. The mantis shrimp has such good eyes it can perceive both polarized light and hyperspectral colour vision. Their eyes (both mounted on mobile stalks and constantly moving about independently of each other) are similarly variably coloured and are considered to be the most complex eyes in the animal kingdom. They permit both serial and parallel analysis of visual stimuli.

Each compound eye is made up of up to 10,000 separate ommatidia of the apposition type. Each eye consists of two flattened hemispheres separated by six parallel rows of highly specialised ommatidia, collectively called the midband, which divides the eye into three regions. This is a design which makes it possible for mantis shrimp to see objects with three different parts of the same eye. In other words, each individual eye possesses trinocular vision and depth perception. The upper and lower hemispheres are used primarily for recognition of forms and motion, not colour vision, like the eyes of many other crustaceans.

Rows 1–4 of the midband are specialised for colour vision, from ultra-violet to longer wavelengths, but aren't currently believed to be sensitive to infrared light. The optical elements in these rows have eight different classes of visual pigments and the rhabdom is divided into three different pigmented layers (tiers), each adapted for different wavelengths. The three tiers in rows 2 and 3 are separated by colour filters (intrarhabdomal filters) that can be divided into four distinct classes, two classes in each row. It is organised like a sandwich; a tier, a colour filter of one class, a tier again, a colour filter of another class, and then a last tier. Rows 5–6 are segregated into different tiers too, but have only one class of visual pigment (a ninth class) and are specialised for polarisation vision. They can detect different planes of polarised light. A tenth class of visual pigment is found in the dorsal and ventral hemispheres of the eye.

The midband only covers a small area of about 5°–10° of the visual field at any given instant, but like in most crustaceans, the eyes are mounted on stalks. In mantis shrimps the movement of the stalked eye is unusually free, and can be driven in all possible axes, up to at least 70°, of movement by eight individual eyecup muscles divided into six functional groups. By using these muscles to scan the surroundings with the midband, they can add information about forms, shapes and landscape which cannot be detected by the upper and lower hemisphere of the eye. They can also track moving objects using large, rapid eye movements where the two eyes move independently. By combining different techniques, including saccadic movements, the midband can cover a very wide range of the visual field.

Some species have at least 16 different photoreceptor types, which are divided into four classes (their spectral sensitivity is further tuned by colour filters in the retinas), 12 of them for colour analysis in the different wavelengths (including four which are sensitive to ultraviolet light) and four of them for analysing polarised light. By comparison, most humans have only four visual pigments, three dedicated to see colour but the lenses block ultraviolet light. The visual information leaving the retina seems to be processed into numerous parallel data streams leading into the central nervous system, greatly reducing the analytical requirements at higher levels.

At least two species have been reported to be able to detect circular polarized light, and in some cases their biological quarter-wave plates perform more uniformly over the entire visual spectrum than any current man-made polarizing optics, the application of which it is speculated could be applied to a new type of optical media that performs even better than the current generation of Blu-ray disc technology.

The species Gonodactylus smithii is the only organism known to simultaneously detect the four linear and two circular polarization components required for Stokes parameters, which yield a full description of polarization. It is thus believed to have optimal polarization vision.