All aspects of stomatopod behavior are defined or influenced by the animal's unique raptorial appendages, and that behavior is bewilderingly complex. Mating rituals and competitive interactions between rivals of a species involve elaborate visual communication cues and actions.
Mantis shrimp activity, depending on species, may be diurnal, nocturnal, or crepuscular (dawn and dusk). Some become active during moonlit nights. Males guard pregnant females. Individuals of some species probably recognize other individuals of the same species by sight and odor. So far, stom-atopods are the only invertebrate type in which the ability to tell apart non-mated individuals has been supported by studies. Individual recognition among stomatopoda would be especially important in coral reef habitats, with their limited available cavities. In this high-competition, high-risk environment, it is best to be able to recognize the bullies on the block.
All mantis shrimp species live in soft-substrate burrows or hard rock crevices. Burrow-dwellers either dig out their burrows or move into one abandoned by some other creature. A burrow may be up to 33 ft (10 m) long, and may have several entrances.
When resting, mantis shrimp, depending on species, may plug the burrow entrance with a rock, or position the claws or telson at the entrance. Individuals of the species, Echi-nosquilla guerini, have telsons equipped with spines, suitable to their function as shields for the burrow entrance.
Newly mature individuals of either sex compete for living space. In substrates suitable for burrowing, most commonly sand, the competition can be fierce, but less so than in coral reefs, since malleable sand permits flexibility in where an individual establishes its burrow. Coral reefs, on the other hand, are built of hard material, always limiting the number of crevices available for colonization by mantis shrimps, so that the competition among coral reef stomatopods for living space is intense. It is among the rock-dwelling species that senses are most keen and communication between individuals is most complex.
Size difference is the usual criterion in individual competition, with the larger opponent chasing off the smaller. More similarly sized opponents, when threat displays fail, may go on to a fight. A spat between rivals is a dangerous affair, since the sharp tines of spearers' raptorials can cut and shred, while the clubbed appendages of smashers can deal a single, fatal blow. In fights, opponents use their flexible telsons as shields and fenders. The battle ends in retreat by one individual or in injury.
A burrowing stomatopod, once ensconced, will defend its home vigorously against other mantis shrimps of the same species, and drive off other marine animals that come too close to the burrow. Depending on size and strength, newcomer stomatopods may force weaker residents out of their burrows or crevices, or the established stomatopod may drive off the intruder.
Burrowing stomatopods show less in-species aggressiveness than do species that live in rock or coral crevices. Competing burrowers generally go through elaborate threat rituals until one admits defeat and leaves, without any actual fighting, since the loser can simply go elsewhere and dig out a new burrow. This sort of flexibility does not exist for stomatopods dependent on rocky crevices for shelter, since the animals cannot carve out new crevices in the tough material; the number of available crevices is always limited. Contenders for crevices have no choice but to fight in earnest. On the other hand, since these are creatures that can kill opponents with a single blow of the raptorial appendage, the rock-dwelling stomato-pod species, for defense, sport the brightest and most varied colors and stage the most elaborate threat behaviors among stomatopod species.
A common but dramatic threat display among stomatopods is the meral spread, in which an individual elevates its thorax and spreads wide its antennules, antennal scales, raptorial appendages, and peraeopods. The display creates an impression of inflated size and exposes a brightly colored depression, the meral spot, on the merus, or proximal segment, of the raptorial appendage. Throughout subclass Hoplocarida, flashing the meral spot is the prelude to attacking the rival, so that displaying the meral spot has an unusually intimidating effect on a rival, even one of another stomatopod species. Gon-odactylus smithii is quick to resort to the meral spread and display, sometimes able to chase off other, more aggressive species with its vivid purple and white meral spots.
The efficacy of the flashed meral spot in banishing rivals seems to depend on how early and how aggressively an individual displays the meral spread in an in-species confrontation. Victory in most cases goes to the individual who displays earlier and more aggressively. Flashing the meral spot early in an encounter between individuals usually ends in victory for the displayer, without a fight ensuing. Females guarding egg clutches are quick to display the meral spread to other stomatopods.
Mantis shrimp molt, or shed and replace, their exoskele-tons, like other arthropods. Molting times vary with species, size, age and reproductive state. Adults of superfamily Gon-odactyloidea molt every three or four months.
Learning experiments on gonodactylid mantis shrimps were carried out by M. J. Reaka, who set up an aquarium furnished with a black-painted flask and no other available cover, introduced a single stomatopod individual from a permanent aquarium, and then noted its behavior and how long it took to locate the entrance and move into the flask. The stomato-pod was allowed to rest within the flask for an hour, then withdrawn and returned to its original aquarium. A few days later, the same individual was introduced to the testing aquarium and its behavior noted. Each tested individual was exposed to five such trials. The more subsequent the trial, the less time the stomatopod took to find the flask, on the average cutting its time from 52 hours on the first trial to only one hour on the fifth.
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