Physical characteristics

A typical mantis shrimp looks, at first glance, like an elaborated, elongated shrimp, with stalked eyes and, according to species, may sport brilliant splashes or coats of garish colors, or may be more cryptically colored. Colors vary considerably among and within species, and between sexes. The body may be cylindrical or somewhat flattened dorsoventrally. The thorax is shield-shaped and the abdomen is conspicuously segmented. At the front of the head, moveable somites bear the eyes and double antennae. The prominent, stalked eyes are often vividly colored iridescent emerald, ruby, or sapphire.

Eight pairs of limbs arise from the thorax. The front-most five limb pairs, the maxillipeds, are manipulatory, used in hunting and feeding; the second pair comprises the deadly raptorial appendages, the stomatopods' most conspicuous claim to fame. Behind the raptorials are three more pairs of maxillipeds, then three pairs of walking limbs, or pereaopods. The abdomen bears five pairs of swimming limbs, the pleopods, which also bear the gills, and ends in an enlarged, fanlike tail, made up of the shieldlike telson, flanked by a pair of uropods, or tail appendages. Stomatopoda ("stomach-mouth") alludes to the first five pairs of thoracic limbs, since these are used in feeding.

The formidably armed second pair of maxillipeds, the raptorial appendages, are kept folded and tucked away underneath the animal's head and body when not in use. The folded appendages, still partly visible, account for the "mantis" part of the common name, since they recall the praying mantis, the familiar insect, which rests its raptorial appendages similarly.

All mantis shrimp species, and both sexes, are predatory, but employ only two basic attack modes and appropriate equipment, being either spearers or smashers. The most distal joint, or dactyl, on the second pair of appendages bears, in spearers, an array of dactylar teeth, or spines, and, in smashers, a heavily calcified "elbow" used as a very potent club. The raptorial appendages look and act like jacknives, folded sim ilarly into grooves when at rest, and whipping open and into action in a fraction of a second.

The business edge of a spearer's raptorial appendage may bear from two to 20 spines, or dactylar teeth, barbed at their tips, efficient and effective arrays of spears for impaling prey, then hauling it in for feeding.

If a stomatopod loses one or both raptorial appendages through injury, it will regrow them gradually over four successive molts. An individual will forcibly remove its own damaged raptorial appendage by use of its other maxillipeds.

Among the five superfamilies, both smasher and spearer species are found in the Gonodactyloidea and Lysiosquilloidea. Superfamilies Squilloidea, Bathysquilloidae, and Erythrosquil-loidea contain only spearers.

The earliest stomatopods, before the evolution of the raptorial appendages, likely fed by grubbing in seabottom mud for resident small creatures, some of which the stomatopods grabbed in the mud while flushing out more mobile types. The fleeing prey prompted the evolution, within the early stomatopods, of elongated, barbed maxillipeds able to flash out and skewer prey on the run. The smasher type of raptorial appendages developed later, probably several different times, from the original barbed appendages.

Stomatopod vision is extraordinarily keen and the eyes are elaborately constructed, being the most highly developed visual organs among crustaceans. Stomatopod eyes are assemblies of ommatidia, or facets, like those in the eyes of other arthropods. However, those assemblies in a central midband region of specialized ommatidia are unique in stomatopods.

The eyes, in a close-up view, have the appearance of highly burnished ovoids with two dark spots and the distinctive mid-band region. The midband region divides the eye into separate hemispheres, each with a pupil and focal point, allowing binocular vision and depth perception in a single eye. The midbrand region also carries its own pupil and focal point, allowing trinocular vision in each eye. Since both eyes each carry three focal points, stomatopod vision is hexnocular, coordinating imagery from six focal points. The eyes are thereby capable of fine-tuned depth perception and range, enabling their owners to be aware of subtleties of speed and distance of prey to be able to attack with fiendish accuracy.

The midband region of the eye, made up of six rows of specialized ommatidia, can perceive color and polarized light. Four rows carry 16 differing sorts of photoreceptor pigments, 12 for color sensitivity, others for color filtering. The pig-mented filters, arrayed within the ommatidia like sunglasses, allow fine-tuning of sensitivity to certain wavelengths of light. The eyes are capable of distinguishing up to 100,000 colors, ranging from infrared through the visible color spectrum and into the ultraviolet. The remaining two rows within the mid-band region are sensitive to polarized light.

The hemispheres are not equipped for color vision and are primarily sensitive to forms. Although the midband region color-scans only a small segment of the stomatopod's visual field, the animal can move the eyes with delicate precision in a wide range, scanning its environment with the sensitive midland region.

A resting but wary stomatopod will often keep its stalked eyes aloft, like periscopes, achieving 360° vision.

Stomatopods can change the spectral sensitivity of their eyes if descending into deeper waters, where the remaining light is mostly blue and highly polarized. The eyes will adjust to the low, blue-shifted light by becoming more sensitive to the blue end of the visual spectrum.

Supplementing the eyes are the antennae, or antennules, sensitive to water-borne odors and turbulence. A special feature of stomatopod antennae is a leaflike flap, or scale, fringed with hairlike setae, to amplify the sensitivity of the anten-nules. Studies by Caldwell have found that, at least in some species, the antennal scales and the uropods, the paddle-shaped appendages of the tail, reflect polarized light. Cald-well's findings suggest that by moving the antennae and uropods, and thereby communicating flashes of polarized reflected light, which stomatopod eyes can see, individual animals may communicate with one another. Experiments lend support to the probability that an individual mantis shrimp can detect and identify the odor of another individual of the same species.

Stomatopod sexes can be easily distinguished. Males bear a pair of long, slender penes, or sperm transferal organs, articulating at the bases of the last paraeopods, or walking limbs. The female gonopores, or sperm receptacles, are visible as a narrow slit on the sternum between the first peraeopod pair. Many species go further with sexual dimorphism, the males being bigger, with larger raptorial appendages and telsons than females.

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