Fiddler Crab Maintenance
Fiddler crabs are relatively easy to keep in the classroom if you know what you are doing. You can use either a small aquarium or a clear plastic box (much cheaper than an aquarium; so that you can see some of the sand layers and possibly some burrowing activity). Do not overcrowd the container; you should have one or two males along with females. For example, if you use a 10 gallon aquarium, you can have 6-8 crabs. The sand or substrate is critical to keeping the crabs healthy in your captured environment. Pet stores will recommend using aquarium sand or play sand in the tank. This is not ideal. If you can obtain moist sand from a beach, it will have small plants and diatoms on the grains. These are important for crabs to get some of their natural foods. If you do use aquarium sand or play sand, you should obtain some moist sand from a beach to "seed" your substrate with natural food for the crabs. You can simply mix the moist beach sand with the other sand by pouring it on the surface and using a fork or other object to rake through the moist sand until the top few millimeters of the substrates are mixed. You will need at least 10 centimeters of sand to give the crabs enough depth to burrow.
Place your sand in the bottom of the container. Then add seawater so that it is about 2 cm above the surface of the sand. Natural seawater is best, but you can use artificial sea salt as well. Many pet stores will try to sell you chemicals to remove chlorine from tap water. If you place tap water into plastic milk jugs, shake the jugs, then leave the lids off for a few days, most of the chlorine will be removed. I suggest that you use spring water that can be purchased at many stores (DO NOT USE DISTILLED WATER) to mix your artificial seawater. After the water has been added to the container, tilt it so that approximately on third to one half of the sand is covered. This gives the crabs access to seawater and wets the substrate to support the microflora on the sand surfaces. But, it also provides a drier area for the crabs to burrow without drowning. In the dry part of the container, you will need to place a small dish of the spring water. Be sure that the dish is not too deep or if you choose a deeper dish, include a way for the crabs to climb out if they fall in (a rock is an easy object to use). You can also include some egg shell in the water. Crabs need calcium to molt properly. Adding clean egg shell to the water gives the crabs some additional calcium. The crabs do not need other objects in the container, though pet stores will try to sell you plastic plants, "ship wrecks", driftwood, and other materials for your container. Every two or three days, use a spray bottle to moisten the dry area. This keeps the substrate from drying too much, keeps up the humidity in the container, and replaces some of the water that evaporates from the container. If the crabs dry out too much, they cannot breathe properly and will weaken or die.
It is important to provide good food for the crabs to supplement the food that they can get from the sand. Crabs should be supplemented every two to three days; to keep your maintenance time to a minimum, it is best to feed them after you have sprayed the container with spring water. DO NOT OVERFEED THEM. FMR hermit crab food (can be purchased in many pet stores) and dry cat food (crushed into small bits) are good general foods. You can also use slivers of vegetables occasionally. Clean egg shell can also be provided occasionally. The crabs may crunch or scrape the shells as a part of their feeding and may be able to obtain calcium from the shells. You should remove excess food or egg shells before spraying with spring water to maintain a healthy environment with fresh food for the crabs.
If you are going to try to keep the crabs for an extended period (more than a month or two), you should do a complete sand and water change. Waste materials will begin to build up in the container and can cause the crabs to sicken or die. If you are keeping them for a shorter time, replace about half of the water every month. Be sure NOT to put the aquarium in full sunlight for long periods. Place them in an area where there is some sun exposure if possible.
To encourage the crabs to burrow along the sides of the container, you can cover the outside with a skirt of paper to darken it. Keep the paper on the aquarium unless observations are being made.
Students can watch the crabs to observe feeding and interaction behaviors. Be sure to have them look at tide tables to see if the crabs still can keep "sea time" in the classroom.
Background readings for fiddler crab maintenance and observations
Aicher, B., and J. Tautz. 1990. Vibrational communication in the fiddler crab, Uca pugilator. I. Signal transmission through the substratum. Journal of Comparative Physiology 166A(3):345-353.
Chiussi, R., and H. Díaz. 2002. Orientation of the fiddler crab, Uca cumulanta: Responses to chemical and visual cues. Journal of Chemical Ecology 28(9):1787-1796.
Denger, B. C., and R. A. Tankersley. 2005. Rhythmic activity of adult fiddler crabs Uca pugilator from areas with different tidal regimes. Integrative and Comparative Biology 45(6):1124.
Palmer, J. D. 1990. The rhythmic lives of crabs. BioScience 40(5):352-358.
Pratt, A. E., D. K. McLain, and G. R. Lathrop. 2003. The assessment game in sand fiddler crab contests for breeding burrows. Animal Behaviour 65:945-955.
Takeda, S; Poovachiranon, S; Murai, M. 2004. Adaptations for feeding on rock surfaces and sandy sediment by the fiddler crabs (Brachyura: Ocypodidae) Uca tetragonon (Herbst, 1790) and Uca vocans (Linnaeus, 1758). Hydrobiologia 528(1-3):87-97.
Yamaguchi, T; Henmi, Y. 2001. Studies on the differentiation of handedness in the fiddler crab, Uca arcuata. Crustaceana 74(8):735-747.
Yamaguchi, T; Henmi, Y; Ogata, R. 2005. Sexual differences of the feeding claws and mouthparts of the fiddler crab, Uca arcuata (De Haan, 1833) (Brachyura, Ocypodidae). Crustaceana 78:1233-1263.
Yamaguchi, T.; Henmi, Y; Tabata, S. 2005. Hood building and territory usage in the fiddler crab, Uca lactea (De Haan, 1835). Crustaceana 78:1117-1141.
Zeil, J; Hemmi, JM. 2006. The visual ecology of fiddler crabs. Journal of Comparative Physiology, A 192 (1): 1-25.
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