Learning in Stomatopods
A wandering stomatopod once came across a large tubular structure. Long, delicate-looking tentacles sprouted from one end to wave to and fro in the gentle current, and the arthropod moved closer in order to get a better look at the strange creature. In doing so, it brushed carelessly against the anemone, then jerked suddenly back in pain as the latter's tentacular appendages started discharging nematocysts. Stung by the unwelcome reception, the mantis backed off and hastily departed in search of its normal prey.
A few days later, it came upon the same anemone. Warily eyeing the cnidarian, it circled quickly around the large obstruction, and spared not one backward glance as it hurried along its way.
|A studious Odontodactylus glances from its assigned reading as a Gonodactylus student hurries out of the main library with an intimidating pile of books (Odontodactylus pic modified from Jeff Jeffords, Gonodactylus image from Dr. Roy Caldwell).|
Learning can be described as a relatively permanent change in the behavior of an organism because of past experience or practice. In the simplified case above, the stomatopod's previous experience with the sea anemone caused it to modify its behavior when it encountered the creature again. Instead of needlessly brushing against the well-protected anemone, it simply avoided the thing altogether ---- it had "learned" to associate the cnidarian with pain, and behaved accordingly.
It should be noted that in order for the stomatopod to make use of its experience with the anemone, it had to be able to recall and remember the results of the past meeting. The term "memory" refers to the retaining and recalling of what has been learned, and thus the process of learning depends on memory. The two concepts are so closely related that people frequently use them as synonyms in everyday conversations, and both will be the subjects of this short note on learning in stomatopods.
Gonodactylid mantis shrimps usually live in holes in corals and other "hard" cavities. One of the earlier papers on stomatopod learning focused on their ability to learn and remember the characteristics of their burrows. In a series of experiments, Reaka (1980) used black-painted flasks as replacements for the natural burrows of mantis shrimps. She placed a flask in an environment that had no other available cover, introduced a stomatopod into the aquaria, then measured the time it took for the mantis to enter the flask, using this as an estimate of the learning and memory abilities of each of the animals (Mantis shrimps are notoriously retiring creatures, and will tend to seek cover in order to protect their rear ends whenever possible).
A test animal, when exposed to the new flask, usually took quite awhile to find the entrance, and contented itself with exploring the surface of the structure, as well as occasionally giving the thing a whack with its raptorial appendages for good measure (I have seen this as well whenever I place a stomatopod into a new environment). After discovering the entrance, and entering the flask, the mantis shrimp was allowed an hour to luxuriate within the confines of the artificial burrow, before being booted out and returned to its permanent aquarium. It was then exposed to the same flask at a later date, and the time it took to again find the entrance was measured and compared to other results.
Reaka found that a stomatopod was able to find the entrance of the flask faster with each passing day of exposure to the same flask. For example, the median of the individual results went from 52 hrs during the first day of exposure all the way down to 1 hr after five days of repeated daily exposures to the flasks. In addition, different individuals showed different characteristic "learning curves", which suggested that it may be possible for natural selection to work on these differences in the wild.
Reaka also found that the mantis shrimps "forget" the characteristics of the flasks as time passes. Mantis shrimps which had been repeatedly exposed to the flasks for 5 consecutive days were exposed to the same flasks from 1 to 14 days later. Stomatopods exposed to the same flask a day later still had a median entrance time of 1 hr, but those exposed to the flask 14 days later took 15 hrs (median, not mean) to find the entrance and dive in.
|"Identity Pains: Individual Recognition in Stomatopods." by ASJ. Stomatopod images modified from Oscar Braun.|
Human beings take the process of identifying other individual persons for granted. Although many other vertebrates have been shown to discriminate among conspecific individuals, such has not been the case with invertebrates. Stomatopods have been the only invertebrate group in which the ability to tell other individuals apart has been strongly documented.*
Stomatopod smashers who live in coral habitats vigorously compete for the scarce number of preformed hollow cavities available. This competition for living spaces has resulted in the evolution in this group of a behavioral complexity found nowhere else in the Stomatopoda, and has seemingly given rise in the ability of these mantis shrimps to identify other potential competitors as specific individuals.
Caldwell (1985) sequentially matched each individual subject in his experiments against two different resident stomatopods, one being significantly larger than the subject, while the other was smaller. One or the other of the resident stomatopods had been introduced earlier into the test area, which contained only a single black plastic bottle for cover. Remember again that stomatopods are usually extremely averse to being exposed and without any ready sources of cover, and so the mantis shrimp would scurry inside the artificial cavity as soon as it discovered the flask.
The test animal was then be introduced into the arena, whereupon it soon came upon the flask with the resident stomatopod inside. If the test subject was larger than the resident stomatopod, then it usually expelled the latter and took over the burrow. If the resident was significantly larger than the test animal, then the newly-introduced mantis shrimp was usually not able to expel the resident, and was driven off by a flurry of blows from the bigger animal.
After sequential exposure to these two resident mantis shrimps, the subject stomatopod was returned to its home cavity for a short while, then placed sequentially into two test areas. One contained a flask with water from the home cavity of the smaller resident, and the other with a flask containing water from the home cavity of the larger resident. The water for each flask was drawn from their respective resident cavity 1 hour before the test, and therefore contained the odor of the animals before the actual interactions of the test subject and the resident. The time it took the test stomatopod to enter each cavity was then recorded.
The time it took the test subjects in the experiments to enter the flasks containing water from the "smaller" resident (which they had evicted earlier) was significantly lower than the time it took them to enter the flasks containing water from the "larger" resident (which they had failed to evict). Thus, this series of experiments showed that test subjects were able to distinguish between the odors of the two different individuals that they had encountered earlier, and to modify their present behavior according to the results of their previous interactions and experiences.
|"Mate Recognition in Stomatopods" by ASJ. Stomatopod images modified from Oscar Braun.|
Caldwell (1992) also showed that former mates recognized one another even after two weeks of separation. In stomatopod breeding pairs, the male stands guard over the female before spawning, and only leaves to find another cavity when the female spawns. The female then broods the eggs for around three weeks, and stands guard herself over the larvae for another week after they hatch. The search for a new home by the male means that there is a possibility that he might come across the same female during this time, and by forcing her out and taking over the cavity, ensure the destruction of his own eggs or larvae.
Through a carefully controlled series of experiments, Caldwell revealed that both the male and female refrained from initiating or escalating aggressive behavior against one another even after two weeks of separation. However, both were very aggressive against other male and female stomatopods. The identification seemed to be facilitated by a form of maxilliped behavior in the female, whereby she "fanned" water currents (presumably containing chemical signals about herself) out the cavity and towards the approaching intruder.
A very interesting proposition that can be derived from all these experiments concerns the evolution of certain behavior in invertebrates. Briefly, that the formation and retention of behavioral traits in animals can only happen if the following conditions are met:
In the case of mate recognition above, for example, we can clearly see how these conditions have resulted in the ability of stomatopods to recognize their mates even after extended periods of absence. The scarcity of preformed cavities in the home area of these animals, and the frequent relocation of mantis shrimps from one cavity to another means that they have ample opportunity to aggressively interact with others of their species. Thus, it would be necessary for the male stomatopod to be able to recognize at least his recent mate in order to ensure the continued survival of his own progeny.
Another, contrary, example might be the recent controversy over observational learning in octopuses, wherein naive animals were reported to be able to quickly solve problems by observing the actions of a conspecific. Although octopuses may indeed have the capability to express such complex behavior, many people have argued that the opportunities to evolve the trait are not present, since octopuses are solitary animals who seldom interact with one another except during mating. In this case, researchers have not been able to duplicate the results of the controversial work done in 1992, and a consensus may be growing that such learning behavior is not present in these animals.
Web Site Author: A. Sunjian
Site Created February 3, 1998