Mantis shrimp: Eyes
Translations of this material:
- into Russian: Раки-богомолы: Глаза. Translation complete.
Submitted for translation by EvilCat 16.12.2016
Published 3 months ago.
The mantis shrimp has one of the most elaborate visual systems ever discovered. Compared to the three types of colour receptive cones that humans possess in their eyes, the eyes of a mantis shrimp carry 16 types of colour receptive cones. Furthermore, the shrimp is capable of tuning the sensitivity of its long-wavelength vision to adapt to its environment. This phenomenon, known as “spectral tuning” is species specific. Cheroske et al. did not observe spectral tuning in N. oerstedii, the species with the most monotonous natural photic environment. In N. bredini, a species with a variety of habitats ranging from 5-10m (although it can be found up to 20m), spectral tuning was observed, but the ability to alter wavelengths of maximum absorbance was not as pronounced as in N. wennerae: a species with much higher ecological/photic habitat diversity.
The midband region of its eye is made up of six rows of specialised ommatidia - a cluster of photoreceptor cells. Four rows carry up to 16 different photoreceptor pigments, 12 for colour sensitivity, others for colour filtering. The vision of the mantis shrimp can perceive both polarised light and multispectral images. Their eyes (mounted on mobile stalks and capable of moving independently of each other) are similarly variably coloured and are considered to be the most complex eyes in the animal kingdom.
Each compound eye is made up of up to ten thousand side-by-side ommatidia. Each eye consists of two flattened hemispheres separated by six parallel rows of specialised ommatidia, collectively called the midband. This divides the eye into three regions. This configuration enables mantis shrimp to see objects with three parts of the same eye. In other words, each eye possesses trinocular vision and therefore depth perception. The upper and lower hemispheres are used primarily for recognition of form and motion, like the eyes of many other crustaceans.
Rows 1–4 of the midband are specialised for colour vision, from ultra-violet to longer wavelengths. Their UV-vision can detect five different frequency bands in the deep ultraviolet. To do this they use two photoreceptors in combination with four different colour filters. They are not currently believed to be sensitive to infrared light. The optical elements in these rows have eight different classes of visual pigments and the rhabdom (area of eye that absorbs light from a single direction) is divided into three different pigmented layers (tiers), each 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. It is these colour filters that allow the mantis shrimp to see with diverse color vision. Without the filters, the pigments themselves range only a small segment of the visual spectrum: about 490-550nm. Rows 5–6 are also segregated into different tiers, 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 upper and lower hemispheres of the eye.
The midband only covers about 5°–10° of the visual field at any given instant, but like most crustaceans, mantis shrimps have their eyes mounted on stalks. In mantis shrimps the movement of the stalked eye is unusually free, and can be driven in all possible axes of movement – up to at least 70° – 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 movements in the same direction, 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 six which are sensitive to ultraviolet light) and four of them for analysing polarised light. By comparison, most humans have only four visual pigments, of which three are dedicated to see colour, and the human 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 circularly polarised light. Some of their biological quarter-wave plates perform more uniformly over the visual spectrum than any current man-made polarizing optics, and it has been speculated that this could inspire a new type of optical media that would outperform 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 to measure all four Stokes parameters, which yield a full description of polarization. It is thus believed to have optimal polarization vision.
The huge diversity seen in mantis shrimp photoreceptors likely comes from ancient gene duplication events. One interesting consequence of this duplication is the lack of correlation between opsin transcript number and physiologically expressed photoreceptors. One species may have 6 different opsin genes, but only express one spectrally distinct photoreceptor. Over the years certain mantis shrimp species have lost the ancestral phenotype, although some still maintain 16 distinct photoreceptors and 4 light filters. Species that live in a variety of photic environments have high selective pressure for photoreceptor diversity, and maintain ancestral phenotypes better than species that live in murky waters or are primarily nocturnal.
Suggested advantages of visual system
What advantage sensitivity to polarization confers is unclear; however, polarization vision is used by other animals for sexual signaling and secret communication that avoids the attention of predators. This mechanism could provide an evolutionary advantage; it only requires small changes to the cell in the eye and could be easily selected for.
The eyes of mantis shrimp may enable them to recognize different types of coral, prey species (which are often transparent or semi-transparent), or predators, such as barracuda, which have shimmering scales. Alternatively, the manner in which mantis shrimp hunt (very rapid movements of the claws) may require very accurate ranging information, which would require accurate depth perception.
During mating rituals, mantis shrimp actively fluoresce, and the wavelength of this fluorescence matches the wavelengths detected by their eye pigments. Females are only fertile during certain phases of the tidal cycle; the ability to perceive the phase of the moon may therefore help prevent wasted mating efforts. It may also give mantis shrimp information about the size of the tide, which is important to species living in shallow water near the shore.
It has been suggested that the capacity to see UV light enables observation of otherwise hard-to-detect prey on coral reefs.
Research also shows their visual experience of colours is not that different from humans'. The eyes are actually a mechanism that operates at the level of individual cones and makes the brain more efficient. This system allows the visual information to be preprocessed by the eyes instead of the brain, which would otherwise have to be larger to deal with the stream of raw data and thus require more time and energy. While the eyes themselves are complex and not yet fully understood, the principle of the system appears to be simple. It is similar in function to the human eye but works in the opposite manner. In the human brain, the inferior temporal cortex has a huge amount of colour-specific neurons which process visual impulses from the eyes to create colourful experiences. The mantis shrimp instead uses the different types of photoreceptors in its eyes to perform the same function as the human brain neurons, resulting in a hardwired and more efficient system for an animal that requires rapid colour identification. Humans have fewer types of photoreceptors, but more colour-tuned neurons, while mantis shrimps appears to have fewer colour neurons and more classes of photoreceptors.
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