ScienceWeek

EVOLUTIONARY BIOLOGY: ON STOMATOPODS

The following points are made by Thomas W. Cronin (Current Biology 2006 16:R235):

1) Stomatopod crustaceans are commonly known as mantis shrimp, because their prey-catching (raptorial) appendages resemble those of praying mantises. They are strictly marine and are quite distinctive from all other crustaceans, having separated from them roughly 400 million years ago and having evolved on their own unique track ever since. About 500 species are recognized today, most living in shallow tropical waters on sand flats or in coral reefs. While unfamiliar to most people, mantis shrimps are actually very common in these habitats.

2) Stomatopods are destructive creatures. They actively hunt down and attack prey, using their mantis-like arms either as spears to snatch small invertebrates and fish from the water, or as clubs to bludgeon snails and small crustaceans to death. Because they tend to wipe out the animals in small seawater aquaria within hours of their arrival, they are widely hated by aquarium hobbyists, who unwittingly import them in coral rock placed in the tank.

3) Most stomatopods are small, typically a few centimeters long, but some get to be the size of a lobster and can actually smash through the glass wall of an aquarium. Despite their pugilistic nature, mantis shrimps are quite attractive animals, colorfully marked and interesting to watch as they wander about, probing into cracks and crevices for potential food. Their compound eyes extend anteriorly on stalks, and the eyes are constantly in motion, giving mantis shrimps an air of inquisitiveness and intelligence. Being so unusual, mantis shrimps have been called "shrimps from Mars", but they have also been likened to crustacean primates because of their behavior and their alert, active eyes.

4) Stomatopods can distinguish colors. Mantis shrimps have the greatest diversity of color receptor types known for any animal. Many species have ten different photoreceptor spectral classes that see in the human "visible" spectrum (400 to 700 nm) plus another five or six types that operate in the ultraviolet. The color receptors differ in the visual pigments they contain, and some are spectrally tuned by colored filter pigments that overly each actual receptor. Mantis shrimps have been trained to select an object of a particular color from an array of objects of different colors, demonstrating that they have true color vision and can therefore make discriminations based upon hue alone and not brightness. The receptors that mediate color vision are aligned in a linear arrangement in the compound eye and thus sample only a thin strip of visual space. The rest of the visual field is apparently seen only in shades of brightness, so to fill in the missing color, the eyes must scan over objects of interest. To give a sense of the complexity of mantis shrimp color vision, humans have only four classes of photoreceptors, compared to the 15 or so of mantis shrimps, and only three of those are cone types used for color vision (the other is the rods, used in dim light).[1-4]

References:

1. R.L. Caldwell and H. Dingle, Stomatopods. Sci. Am. 234 (1976), pp. 80 89

2. T.W. Cronin and J. Marshall, The unique visual world of mantis shrimps. In: F. Prete, Editor, Complex Worlds From Simple Nervous Systems, MIT Press, Cambridge MA (2004), pp. 239 268

3. http://www.ucmp.berkeley.edu/aquarius/

4. N.J. Marshall, J.P. Jones and T.W. Cronin, Behavioural evidence for color vision in stomatopod crustaceans, J. Comp. Physiol. A 179 (1996), pp. 473 481

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