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Stomatopod Log Entry: Trench construction in a stomatopod (March 16,1998)

Contributor: Alan San Juan (Airlan)

Caveat before reading the article: Some people will doubtless feel that I have been too generous in seemingly crediting stomatopods with some degree of "reasoning" in their dealings with external events and their environment. Note, however, that if they are similar to other crustaceans, then stomatopods actually have quite tiny "brains" when compared to their total body mass, and in fact I read in some textbook somewhere (and I will check again) that they actually do not even have a centralized, single brain mass. There have been studies of short and long term memories and learning in these creatures. Caldwell (1985,1992), for example, determined that stomatopods seem to be capable of distinguishing former mates for at least 2 weeks after they have parted, and stomatopods are also seemingly able to distinguish other conspecific stomatopods as individuals based on past aggressive interactions. Reaka (1980) also found that stomatopods seem to be able to learn the characteristics of artificial cavities with repeated exposure to the objects (and intertestingly enough, she also found individual differences in the learning curves of the lab subjects). Finally, Caldwell and Dingle noted in a review paper (1975) that individual stomatopods seem to become more efficient when handling (ie breaking open) unknown hard-shelled prey material as time passes, suggesting that they "learn" from past experience.

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I have always felt that one possible driving force for the development of intelligence in humans has been our ability to extensively manipulate objects in the environment, and this has caused me to become fascinated with those creatures who are also able to alter their surroundings to some degree. For example, I used to watch weaver ants (Oecophylla smaragdina) construct their aerial dwellings out of tree leaves and the silk spun from their larval charges, and I wondered how the termites in our yard knew enough to construct covered runways that started from the ground and extended up trees for several meters into the tree canopy.

Stomatopods are fascinating because they seem to be capable of altering their (admittedly very local) surroundings to a significant extent. Unlike fish, these arthropods feature an array of specialized appendages that are quite capable of both fine and gross manipulations of objects. One of the things that aquarium books sometimes point out is that mantis shrimps are interesting because they are prone to moving structures and objects around the tank.

When I bought my first mantis shrimp, I placed the shell and its occupants (mantis and anemones) into a ten gallon tank with a ground cover consisting of small pebbles (around 0.5 cm diameter on average, I guess), which extended to a depth of 1-2 cm. I fed the thing by placing bits of shrimp away from the shell (10-15 cm). In order to allow the critter to feel comfortable during those first few days, I usually left the area, but filmed the tank using a vhs camcorder.

The first time that I did this, I fully expected the animal to simply rush forward and pick the piece up, then dash back to the shell shelter. Instead, what the mantis shrimp did was use its forward appendages, including the raptorial ones, to create a "trench" extending to the food, by carrying pebbles and depositing them to the side of this irregular straight-line path. The mantis used this trench to get to the food and back to the shell.

I did not think about it that much the first time it happened (since I had placed the food relatively close to the shell). However, sometime later, I placed another shrimp piece quite a bit aways from the shell (>10 cm?), and again taped the whole thing and left. Over the next 30-40 min, the mantis created a shallow trench all the way from the base of the shell to the food. It did this intermittently, going out to push or carry pebbles to the sides of the trench, then rushing back into the shell, then going back out again after awhile to clear the path. It sometimes gave threat displays towards the camera (or glass walls?) while constructing this trench. When the trench reached the food, the mantis simply took the shrimp piece, rolled around once in the trench like some sort of demented miniature puppy, then rushed back into its shell.

One fault that is oftentimes repeated in our relationship with other animals (either as pets or as lab studies) is our tendency to ascribe human feelings and/or abilities and motivations to these creatures. I must admit that at first I longed to believe that the mantis shrimp in this case, being of a cautious nature and wary of its new surroundings, had deliberately and consciously reduced its exposure to the unknown, by constructing the trench all the way to the food. The mantis shrimp has not repeated such actions, either because I have started to give it the food directly, or because it has become more familiar with its surroundings. Could blind instinct, and the evolution of this animal's behavior patterns, be the sole reason for this particular activity? Or was there a deliberate chain of events in the animal's tiny neuronal masses, that went thus: the food is too far away---I am new to this place, and there may be dangers around ----- if I construct a trench to the food, I will be more protected --- therefore, I will construct the trench.

Unfortunately, until someone can prove otherwise through careful experiments, it would be wise to proceed with the first assumption.

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Web Site Author: A. San Juan
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