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Stomatopod Biology

The Muscular Physiology of the Stomatopod Raptorial Appendage

The prey strike of the stomatopods is something that has slowly become enshrined in superlatives, with commentators and writers waxing lyrical about the speed and power of this beautifully simple, yet undeniably deadly, act. In many ways it has become the stuff of which myths have been made, and stories about mantis shrimps breaking aquarium glass and slashing and severely wounding human handlers abound. The awe and respect accorded to it has filtered down even to those who dabble in marine aquaria, a fiercely anti-stomatopod lot who regularly loathe the beast with a vehemence reserved only for those things which would deny them their elaborate saltwater and reef glass communities.

The superbly constructed mechanism that drives this strike lies in a stomatopod’s second pair of maxillipeds, highly specialized raptorial appendages that evolved as a formidable means of defense and offense for the mantis shrimp. In some of the larger stomatopods, these structures have become capable of instantly exerting forces of up to 5 kg in the space of a few milliseconds, formidable numbers indeed considering that the source is a creature that rarely exceeds 20 cm in length.

 
Image used with permission of Dr. Malcolm Burrows.
Modified by addition of copyright info and new labels.
 

The muscles that power the stomatopod strike has been studied extensively by Burrows, and consists of four muscles in the merus of the appendage, including two large extensors and two relatively smaller flexors. Extensor muscles are those which extend or straighten a limb or some other extremity, whereas flexors bend or "flex" a limb or extremity. Since muscles can only work by contracting, in order for the organism to accomplish effective movements, each muscle must be partnered with another muscle or set of muscles that have opposite actions. Thus, flexors and extensors would work against each other as a so-called "antagonistic pair" to bend and straighten appendages. In humans, for example, the relatively well-developed biceps brachii muscles in our upper arm serves to raise the forearm, and thus functions as a flexor muscle, while the contraction of one of its antagonist , the triceps brachii, lowers the same forearm and functions as an extensor.

Like other arthropod appendages, the raptorial limb of a stomatopod is divided into distinct segments. The two extensors originate from the merus and insert at the dorsal inner surface of the carpus. Contraction of these muscles would therefore straighten the limb and cause the dactyl and propus to shoot forward, impaling, cutting, or clubbing the prey or predator into submission. The two flexors also originate from the large merus, but do not insert directly into the carpus. Instead, the muscles attach to and control two carpal sclerites, which together form a "click-joint" that is responsible for "locking" the whole limb in position just before a strike.

The speed of the prey strike and the rapid unfurling of the limb might at first glance be attributed mainly to the similarly rapid contraction of the extensor muscles, but studies showed that all four muscles are specialized for slow, as opposed to fast, contractions. Slow muscles have fibers whose "twitches" (i.e. contractions) lasts significantly longer than those of fast fibers, which are usually found in muscles that require fast, powerful contractions (e.g. flight muscles of birds). Both the lateral and medial extensors have relatively slow rates of contraction, and thus the actual contractions of these muscles could not be the sole and main cause of the extremely fast prey strike.

The origins of the strike seem to lie instead with the use of a mechanical "click joint" that occurs between the carpus and the merus. This mechanical structure consists of two sclerites whose actions are controlled by the insertions of the two flexor muscles. During "cocking" of the limb in preparation for a strike, the lateral flexor contracts and its attached sclerite (termed sclerite #2) locks into position behind a stop located on the ventral surface of the merus. The medial flexor and its associated sclerite (#1) is presumed to help the main sclerite lock into position. If the external environmental cause of the initial cocking continues (e.g. if the prey object continues its approach), the extensor muscles start to slowly contract, but the "click joint" allows the weaker flexors to restrain the contraction of the much larger and stronger lateral and medial extensors. The result is the development of a very large amount of potential energy stored in the structure of the folded raptorial limbs. This bundled energy is suddenly liberated when the click joint is released and the flexors undergo relaxation, causing the extensors to rapidly contract and the forelimb to quickly extend and strike.

The power behind the strike is astonishing. A small 3 cm long smasher that I care for, whose raptorial limbs barely reach a centimeter in total length, was able to muster a force sufficient to jerk a wooden chopstick that I once held in front of it. It shares a large abandoned shell with two sea anemones, and it occasionally seems to want to demonstrate its ownership by striking at the sides of the structure, thus throwing the whole thing backwards slightly, chipping pieces from the sides, and causing the two Cnidarians to cower and retract their stinging tentacles.

References:

Burrows M and Hoyle G. Neuromuscular physiology of the strike mechanism of the mantis shrimp, Hemisquilla. J Exp Zool 179, 379-394.

Explanation of Terms:

Web Site Author: A. San Juan
Site Created February 3, 1998
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