Mantis shrimp are among the most interesting crustaceans that one might keep in a marine aquarium. Longevities in excess of 20 years are known, and many types form lifelong pair bonds. Their social interactions are incredibly complex – in some species the male hunts for the female while she guards the eggs, while in others two clutches of eggs are laid, each guarded by one parent.
Only distantly related to shrimp, these unique, aggressive predators are actually classified within their own order, Stomatopoda. Over 400 species are known, mostly from the Indian and South Pacific Oceans. Hobbyists are often surprised to learn that one species, the 10 inch long Squilla empusa, ranges along our Atlantic Coast is for north as Cape Cod.
A flurry of new research articles on these fascinating creatures has been published recently, and it turns out that they are even more unusual than we might have suspected. I’d like to summarize some of this new information here — in my next article, I’ll write about caring for mantis shrimp in captivity.
A New and Unique Visual System
Research completed at the University of Queensland, Australia, in March of this year has demonstrated that mantis shrimp have a vision system previously unknown in any other type of animal. Utilizing precisely tilted filters in their eyes, mantis shrimp are able to perceive circular polarized light (CPL) by converted it to a linear form. CPL spirals to the left or right, and appears only as “haze” to us and other creatures (hence the need for polarized sunglasses). The filter within the mantis shrimps’ eyes functions in a similar manner to those used in certain photographic processes – only they beat us to it by about 400 million years!
CPL is reflected by male mantis shrimps’ exoskeletons, leading researchers to believe that it is used for sexual signaling. Furthermore – squid, a major mantis shrimp predator, can detect linear polarized light but not CPL. The use of CPL may, therefore, represent an ingenious strategy by which the mantis shrimp can communicate without drawing the attention of their enemies.
The World’s Most Complex Eyes
Further research in May of this year revealed that mantis shrimp possess the Animal Kingdom’s most complex eyes. Their eyes contain ten pigments sensitive to different light wavelengths, as opposed to our own three pigments. In addition to detecting CPL, mantis shrimp can also see colors ranging from ultraviolet through infrared – far more than any other creature.
Although we have yet to understand all the reasons for the evolution of such a remarkable visual system, we have some hints. Certain of the mantis shrimps’ prey, such as sand shrimp, are transparent and very difficult to see underwater. However, these shrimp are full of sugars that reflect polarized light – making them easy targets for the mantis shrimp. As if all this were not enough, mantis shrimp can also rotate each eye independently of the other, allowing for a very wide circle of vision.
Splitting Thumbs and Shattering Glass
Of more immediate concern to marine aquarists is a recent study demonstrating that a common pet trade species, the peacock mantis shrimp, can extend its hard, club-shaped front legs at speeds of over 75 feet per second. This is the fastest kick known, and explains the why we sometimes find aquariums housing mantis shrimp shattered, and a flood on the floor – the odd creatures actually generate enough force to break glass! In fact, so much pressure is exerted that the exoskeleton at the back of the leg actually wears away over time, but is replaced when the mantis shrimp molts.
This mighty thrust is made possible by a unique hinge in the leg, and was analyzed after being recorded by a camera capable of operating at 100,000 frames per second. The deadly front legs allow mantis shrimp to crack the shells of the snails and crabs upon which they feed, and to defend themselves — indeed, divers long ago christened these colorful terrors “Thumb Splitters”.
Communicating via Florescence
Although many marine creatures fluoresce (absorb one color and emit it as another), mantis shrimp are the only ones known to use fluorescence as a means of communication. This month (May, 2008) researchers at the University of North Carolina demonstrated that the bright yellow spots of the species Lysiosquillina glabriuscula were visible even at depths of over 130 feet, allowing the animals to signal each other despite the dim blue light (which would otherwise render the yellow color indistinct).
Last but not least (“last” for now, I’m sure these oddballs are hiding other secrets!) – certain species of mantis shrimp cover ground by curling into a ball and rolling downhill.
On to captive care next time – until then, please share your own observations and questions. Thanks, Frank.
A video showing just how well a pugnacious mantis shrimp can use its kicking ability is posted at:
You might be interested in an article I wrote for ‘Suite101’ a while back: