Monday, 16 September 2019

Mantis shrimp Facts

Mantis shrimp Facts

Mantis Shrimp Facts

The reputation of a mantis shrimp eyes Facts often precedes it. Thumb-splitter, tank-smasher and unwanted hitchhiker are all accusations levelled at them, but there is more to these sophisticated arthropods than just a pair of brutal raptorial appendages.

Life Through the Eyes of a Mantis Shrimp

With over 450 species currently known to science, stomatopods are found throughout the world’s temperate and tropical oceans and vary considerably in size, with the largest specimens reaching in excess of 12 inches.

Their predefined reputation is garnered from the weaponry adorning their raptorial appendages. Formed of three segments, the final section boasts either an enlarged club, giving rise to the ‘smashers’, or a fiendish collection of between 3 and 17 outward facing spines, the ‘spearers’. Spearer mantis species rely on muscle power to lick their limbs out and impale their prey, while smashers use a punching technique to overcome their prey, utilising a lock-and- spring-type mechanism. 

When retracted, a tiny saddle-shaped structure in their arm is compressed, which works to store energy, much like a coiled spring. When released, the limb lies forward at speeds of over 50mph, causing a blow similar in impact to a small calibre bullet. Such is the speed of the delivery, a secondary release of energy occurs through the process of cavitation, creating a second blow to the prey. Even if the initial strike by the mantis shrimp misses, the following shock wave can be enough to stun their quarry.

The secret to their punch is in the composition of the chitin in the mantis’ club, which is stacked in horizontal layers with each set at a slightly rotated angle to the one above, acting as a shock absorber to prevent the club from fracturing. The design is so effective that the aviation industry has taken notice, replicating the layout with carbon fibre to develop a more impact-resistant material from which to construct aircraft panels.

While their impressive weaponry arsenal is hard to overlook, their visual capabilities often ly under the radar and it’s these that really deserve a closer inspection. As with many other arthropods, mantis shrimp have compound eyes. That is, the eye consists of a collection of thousands of individual photoreception units – known as ommatidia – situated on an outwardly curving hemisphere. As a result of this curvature, the ommatidia do not face a uniform direction, rather they are set at differing angles with each covering one small area within a much wider field of view. These individual ommatidia work to collect tiny snippets of information, which then feed in to produce a composite image, enabling the mantis shrimp to view the world around them.

Mantis Shrimp FactsMantis Shrimp Facts

While they perform the same function, not all mantis shrimp eyes are the same, and range from the tall and elongated eyes of the zebra mantis, Lysiosquillina sulcate, to the almost spherical eyes of the peacock mantis, Odontodactylus scyllarus. The eyes are then split into three distinct sections, consisting of two flattened hemispheres with a mid-band running through the centre. These three segments allow the mantis shrimp to see with three parts of the same eye, meaning they have a trinocular vision. While we as humans require both eyes to perceive the depth of field, mantis shrimps can do this with just one. The specialised ommatidia that comprise the mid-band contain the photoreceptors – the cells that convert light into signals, which are then forwarded on to the brain. While dogs possess two different photoreceptors, enabling them to detect green and blue, and humans have three, adding red light to the mix, mantis shrimps have between 12 and 16, meaning that they not only detect visible light, but their photoreceptors are responsible for colour, ultraviolet (UV) and polarised vision too. 

Further to just having increased numbers of photo-receptors, mantis shrimps are capable of spectral tuning – that is, tuning the sensitivity of their long wavelength colour vision to adapt to their environment. Filters will block the passage of certain light to the photoreceptor whilst allowing other colours through. As a result, colours such as red and orange, previously unattainable beyond a certain depth, now become visible. Naturally, these filters vary between individuals found at different depths. Mantis shrimps calling a shallow reef home have red filters, whereas those living at depths of 15m don’t, as red light does not penetrate down that far. This encouraged a study by Tom Cronin at the University of Maryland, who showed that by replicating the lighting of shallow and deep-water environments, juvenile mantis shrimps raised within them could tune their eyes into their individual environments.

But before we start thinking that stomatopods can see a range of colours that humans only dream about, it appears they can’t. With that many photoreceptors, the assumption would be that they could detect and discriminate between colours better than any other animals, but when tested, this turned out not to be the case. Using Haptosquilla trispinosa as a test subject, researcher Justin Marshall from the University of Queensland set a challenge using coloured optical fibres, with the mantis shrimp rewarded with food if they attacked a particular colour. The colours of the non-reward fibre were then repeatedly changed to a point where the mantis could not distinguish between them.

While a mantis shrimp reached this point at colours with a 15- 25nm difference, humans can go 3-5 times better, down to just a 5nm difference. So why a large number of photoreceptors? The reasoning behind this is still unproven, but it is proposed that they use the raw output from all of their photoreceptors to send a complete picture to the brain which, rather than try and distinguish between colours, seeks to recognise them instead. Marshall linked mantis vision of the algorithms used by satellites when mapping areas, and because they are ambush hunters relying on stealth and speed, it’s easy to see how a reduced processing time would benefit them. A quick scan of their surroundings could be all it takes to determine prey from a predator or competitor.

Back to the mid-band, and this area of the mantis shrimp’s eye consists of multiple ommatidial rows, ranging from 2 to 6 depending on the species, with the greater the number of rows, the more elaborate the visual capacity. Species from the superfamilies Gonodactyloidea, Hemisquilloidea and Lysiosquilloidea have 6 rows, with the first four containing photoreceptors that react to the different wavelengths of light.

This enables the mantis shrimp to see well beyond just the visible spectrum of light that we can see, and into the infrared and ultraviolet spectrum too. Compound eyes work by creating a single picture from the numerous individual ommatidia outputs, and this means that each ommatidium reacts only to whatever is directly in front of them. Within each ommatidium are groups of photoreceptor and pigment cells, overlaid with the cornea, and a bundle of axons which transmit the image from the individual ommatidia to the brain, where individual outputs are compiled to form a single, overall image. Underneath the corner sits a crystalline cone structure, which serves to direct light down onto the top of a receptive structure – the rhabdom. The rhabdom is constructed from 8 receptor cells, which form a rod-like tube that guides and focuses the light that passes down through the cone.

The small receptor cell number 8 (R8) sits as an upper layer, while the 7 longer receptor cells (R1-7) form an underlying tier, and each of these contains thousands of tiny microvilli. Any light that enters from an angle has to pass through a pigment cell first, with\ these situated around the side of each ommatidium, separating them from the neighbouring ones. These pigment cells work to absorb light before it can pass into neighbouring ommatidia and distort the final image.

Mantis Shrimp FactsMantis Shrimp Facts

The differing photoreceptors are not limited to the light of the same wavelength though. Of those present, up to 8 cover light within the visible spectrum - the section of the electromagnetic spectrum that humans can see – with wavelengths typically between 400-700nm. 

In comparison, UV light has a wavelength between 10-380nm, so out of our visual range. But most mantis shrimps have another 4 photoreceptors which cover light in the UV spectrum too, and one impressive species, the outward rock mantis, Neogonodactylus oerstedii, have 6 photoreceptors tuned in. A study by Michael Bok, a renowned researcher in the field of marine invertebrate vision, found that the crystalline cones that cover the photoreceptors in the eye contain a number of different mycosporine-like amino acids (MAA’s), and each type of MAA acts as a filter, blocking a different wavelength of UV light to the others. Bok pointed out that lots of marine animals simply have one or a couple of different MAA’s for the purposes of blocking damaging UV from reaching their eyes, but the presence of 6 suggested another purpose, in this case shifting their sensitivity and turning their eyes into UV detectors.

But why the need for UV vision? Well, at present, there is no definitive answer, but given that we know numerous animals glow via fluorescence under the waves, it could be that this enables them to spot and predate on even the best- camouflaged of prey items. That then leaves the other two layers in the mid-band, and these deal with polarised light, so waves of light all vibrating in the same plane. Sunlight travels in all different directions in an un-coordinated fashion. But should the light be redirected of a certain surface – say the scales of an ish – then this changes the way the light is reflected, meaning it becomes organised in a way in which it vibrates in just a single direction, known as linear polarisation. Humans cannot see or distinguish this, but many animals, including mantis shrimps, can. What makes the mantis shrimps detection of polarised light all the more impressive is that they can perceive circular polarisation, whereby the light travels in a helix shape, in either a clockwise (right-handed) or anti-clockwise (left-handed) direction as well. In order to make use of the circularly polarised light, mantis shrimps need to convert it to linear polarised light. Typically,

the R8 receptors in the rhabdoms are polarisation insensitive, as the microvilli arrangements are arranged in a random fashion. But this is not the case within this section of the mid-band, as the microvilli are in parallel alignment within an enlarged R8 receptor, making them polarisation sensitive. In order for the circular to linear conversion to take place, you need to slow down or retard, the light by ¼ of its wavelength in one orientation and this is exactly what happens when light passes through these R8 receptor cells. Once the polarised light is in a linear form, it can then be processed.

So, what are the benefits of being able to detect circularly polarised light? A paper by Tsyr-Huei Chiou looking at circular polarisation light (CPL) vision in stomatopods sets out a couple of theories. Firstly, when light is scattered in particularly turbid conditions, it can become circularly polarised, so in this instance, it may allow mantis shrimps greater accuracy when distinguishing between objects and their environment. As well as aiding prey discrimination, it could also act as a secret communication channel. Mantis shrimp are known to use polarised light signals during social interactions, and the CPL is reflected from the parts of the telson that are often used during courtship displays.

Chiou’s study showed that this is sex-specific, with males from 3 species of mantis shrimp from the Odontodactus genus reflecting CPL, whereas the females did not. It could provide a great way to advertise to mates while avoiding predators and to avoid male-male confrontations. One such species, the purple spot mantis shrimp, Gondactylus smithii, has been shown to have dynamic polarisation vision, using rotational eye movements to maximise the contrast between the object the mantis shrimp is focussing on and the background against which it is set. Chiou also noted in his paper that the peacock mantis shrimp is particularly sensitive to circularly polarised light, more so than any other species, so it could be that even other mantis shrimp cannot see their behavioural ‘conversations’. 

It isn’t just the adults with enviable eyesight. Stomatopod larvae ride the ocean's currents, living life in the water column, unlike the adults who live a more benthic lifestyle. While their bodies are transparent to aid with their camouflage, their eyes contain the screening pigments between photoreceptors, which aid the preservation of image resolution but render them opaque. To combat this, the larval mantis shrimp have reflective light structures positioned above their retina, which acts as a mirror, reflecting light back and allowing them to blend in with the light in their immediate surroundings.

If their vision is impressive, then the structure and mobility of their eye are even more so. Mantis shrimp eyes are set on highly mobile stalks, and not only can each eye be moved independently from the other, but they can rotate them through all three degrees of rotational freedom. That is, they can move their eyes up and down (pitch), from side-to-side (yaw) and twist them about their eyes-stalk (roll). While they do make the same gaze stabilising movements that other animals do to keep their view of the world steady as they move, they also continually roll their eyes. Such behaviour in humans would be like trying to visually track a moving target while rolling your head through every possible rotation and angle – one sure to induce severe vertigo and motion sickness. For a mantis shrimp, it appears that it doesn’t matter in which direction their eyes are orientated, it has no bearing on their actual vision or spatial perception, so up still means up whatever angle their eye faces.

These stomatopod discoveries have not gone unnoticed across the rest of the science world. Humans use CPL filters in photography, to eliminate the glare from reflective surfaces such as glass while preventing linear polarised light affecting the cameras internal sensor. As such, it is the camera world where most of the research has entered into. Earlier this year, a new type of camera was designed to detect polarised light much in the same way as a mantis shrimp does, that would then enable autonomous vehicles to better gauge their surroundings. Perhaps most pertinent use of all is in the field of cancer detection. Cancerous cells have been shown to reflect polarised light differently from surrounding healthy tissues, and this occurs early on, prior to the onset of other symptoms. Viktor Gruev, who led a team from the University of Illinois, designed cameras inspired by stomatopod vision that can not only detect polarisation patterns on living tissue but are small enough for use in conjunction with endoscopes too.

Mantis Shrimp Facts

But with all the recent focus and research on the incredible visual capacity of the mantis shrimp, a certain stomatopod wound up in the public eye for a different reason. After their stint in the spotlight on Blue Planet II, the zebra mantis, Lysiosquillina maculata, has found itself burdened with a rather unwanted reputation – that of infidelity. As one of the largest species of ‘spearer’ mantis shrimp, they inhabit long, sandy U-shaped burrows dug around the base of coral bommies or rocks, away from the main reef, or along the outer sections of mangrove forests. While most stomatopods prefer a solitary lifestyle, zebra mantis shrimp form monogamous pairs, with the female spending much of her time in the burrow tending to her eggs, whilst the male spends his time hunting for two. Shacking up away from the reef, and potentially their main source of food might sound counter-productive, but the reef metropolis also harbours much larger predators – ones more than happy to snack on a roving mantis shrimp. Life is much safer out in the suburbs, and mangrove forests have the added bonus of acting as a nursery ground for main reef fish species.

Mantis Shrimp Facts

But being the main provider does prove a risk for both though. Spending his time at the entrance of his burrow, waiting to ambush, or roaming the seafloor for prey leaves the male open to predation. If tragedy strikes, without the steady supply of food, the female is left at risk of starving and the potential offspring have little chance of surviving. In this situation, the female mantis needs a new suitor but cannot leave her eggs unattended. However, she has a trick up her sleeve. Despite their size, prime real estate is worth a reduced territory, so pairs will excavate burrows as little as a metre away from their neighbours. Knowing that other individual males will be roaming the seabed above, the female sends out an S.O.S call, signalling her availability. Using a low-frequency call, females not only advertise their whereabouts, but it is thought that the frequency emitted indicates her size, and in the mantis world, big is most definitely beautiful. Larger females are likely to produce more eggs, so if a male hears the call of a female who sounds larger than his current partner, he trades up, leaving his previous mate to find a new suitor.  

Mantis Shrimp FactsMantis Shrimp Facts

Understanding the social relationship between these zebra mantis pairs also shed some light on those specimens found in the aquarium trade – based on the males being the hunter-gatherers and the females being stay-at-home mums, if you have a zebra mantis scuttling around your tank at home, it’s almost certainly male. Aside from the zebra mantis shrimp’s marital dalliances, the more we probe into the hidden secrets of the mantis shrimp, the more we discover, and at times with stomatopod vision, it feels like we have barely scratched the surface.

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