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What is a shrimp? Well, it is pretty much whatever you want to make it. In the aquarium hobby, we have mysid shrimp, and brine shrimp and seed shrimp, and mantis shrimps and gammarus shrimps and side shrimps and then we have “the true shrimps.” In both the common vernacular and the aquarium hobby, a “shrimp” is generally considered to be any more-or-less cylindrical or tubular swimming or walking crustacean that isn’t a crab or lobster. I suppose that definition will work, but the line between lobsters and shrimps is very blurry. Ignoring all the pretenders to the shrimpy throne, I will concentrate this discussion on the so-called “true shrimps.”

Normally, the taxonomic classification of an animal group can be used to get to a nice defined set of characteristics that determine, rather precisely, into what group the animal is placed. That is not so with the shrimps. It appears that although all the shrimps share a common ancestor, probably a shrimpy-crab that lived about the time of the brontosaurs, the living animals are distributed across many, only distantly related, lineages, each with its own character sets. This means that there are relatively few good characters that describe shrimp while excluding other sea-going bugs.

Shrimps are swimming or walking crustaceans with tubular walking legs as opposed to the paddle-like legs of brine shrimp and mysids. They have a shell, or carapace, that covers the segmented regions of the head and thorax. Five pairs of appendages arise from the middle body region, also called the thorax. Crabs also share these characters; because of this, both crabs and shrimps are termed “decapods,” a term meaning “ten feet.’ In the crabs the front-most of these five pairs of appendages are the “pinchers” or “claws” of the crab, so it really only walks on the last four sets of legs.

The situation is similar in the shrimps except that they have more claws than do the crabs. Depending upon the group, either the first two or three pairs of appendages, counting from the front end, may have pincher-type claws. For example, while the banded coral shrimps of the genus Stenopus possess pincher-type claws on all of the first three pairs of thoracic appendages, most of the other shrimps we keep in aquaria have only the first two sets of thoracic appendages with pinchers. All of the thoracic appendages are often called “walking legs” in shrimp; however, the animals generally use only on the last two or three pairs of legs for walking.

These small pinchers are used to continually pluck and snip at small things on the substrate. There are “taste” receptors in and on the claws, and one of the most interesting things an aquarist can do is to closely watch these small claws “working” across a rock or sand bed. Shrimp are all basically omnivorous although some species tend more toward carnivory and some others lean toward herbivory. Because of their integument, or exoskeleton, they cannot expand the mouth, and because their jaws cannot chew, they are not able to eat large pieces of food. When the probing appendages find acceptable minute food items, these are manipulated, tasted further and passed to other appendages. Watch a shrimp feed and you will see that the subsidiary appendages covering the mouth often seem to pull the food into progressively tinier pieces so that what is actually eaten is a very tiny morsel.

Relatively few shrimps are dietary specialists. This is both to our advantage and disadvantage when we try to keep them in our systems. It is an advantage in that most crustaceans will eat pretty much any food we give them. It is a disadvantage in that many crustaceans will eat many of their neighbors in the aquarium. While shrimps lack the robust claws of their crab cousins, they are often predatory, nonetheless. Shrimps in the genera Saron and Lysmata often eat cnidarians. I have watched peppermint shrimps, Lysmata wurdemanni, which are often purchased as predators on the plague anemones, Aiptasia, eat zoanthids and pluck tentacles off elegance corals.

Many shrimp species have symbiotic relationships with other animals. Cleaner shrimps have a relationship with many reef fishes. The shrimp will crawl over a fish’s body eating small parasites and pieces of tissue debris. These shrimps, mostly in the genera Lysmata, Periclimenes and Stenopus, are recognized by their long white antennae and their habit of standing out in the open waiting for fish at a “cleaning station.” In addition, to cleaning, however, these shrimps are also opportunistic predators and scavengers. Many shrimp species that are kept in aquaria have been called commensals of sea anemones. Recent publications (Fautin, et al., 1995; Guo, et al., 1996) have indicated the relationship may be more ectoparasitic than commensal, as at least some – and perhaps all – of these shrimps eat tentacles and tissues from their host. Nevertheless, all of these shrimps have interesting color patterns and behaviors, and many aquarists consider them to be desirable additions to marine reef aquaria.

While most advanced crustaceans tend to have separate sexes, the situation is truly “ambiguous” within the groups of animals we call shrimps. Many of them appear to have separate sexes and are either male or female. At least at any given time; however, many species change from one gender to the other. If this is the case, they most often are males early in their lives and change into females as they age. Other species, such as those of Lysmata, commonly found in aquaria, are simultaneous hermaphrodites. Still others appear to be either male or female all of their lives. Most shrimps have larvae that pass through a prolonged planktonic existence. This means they don’t reproduce successfully in most hobbyist tanks. It is possible to raise most of them, though. It just is very difficult, due primarily to the fragility of the larvae. Like all crustaceans, they must molt to grow or repair injury. If they are rapidly growing, they may molt frequently. Molting is a hazardous process, however, and most of the mortality seen in aquaria seems to occur during a molt.

Cited References:

Fautin, D. G., C.-C. Guo and J.-S. Hwang. 1995. Costs and benefits of the symbiosis between the anemone shrimp Periclimenes brevicarpalis and its host Entacmaea quadricolor. Marine Ecology Progress Series. 129:77-84.

Guo, C. C., J. S. Hwang and D. G. Fautin. 1996. Host selection by shrimps symbiotic with sea anemones: A field survey and experimental laboratory analysis. Journal of Experimental Marine Biology and Ecology. 202:165-176.

Text by Ronald L. Shimek, Ph. D.
Photos by Reef Central members.
A special thanks goes out to Dave Bayne (Nanook)
for his assistance on this project.

Reefkeeping Magazine™ Reef Central, LLC-Copyright © 2008