Crabs...Friend or Foe?
Crabs help to make up the phylum Arthropod and are roughly 4,000 described species of true crabs that make up the infraorder Brachyura characterized by skeletal plates (epistones)fused to a body (carapace). The Arthropod family is made up of about 42,000 shrimp, lobsters, crayfish, crabs and terrestrial insects.
Being that crabs are opportunistic feeders, compatibility problems exist within the confines of our tanks. Many crabs are omnivorous and and predatory in nature thus the need to watch any crab carefully exists.
Many crabs arrive in our tanks as "hitchhikers" with our live rock and coral purchase. Their incredible survival rate through shipping and handling is a credit to their hardiness. Living for days under less than optimal water conditions and even with no water other than wet newspaper covering live rock, then to suffer osmotic shock of being dropped into saltwater without any acclimation time is amazing and a credit to their toughness.
Crabs are either symbiotic, commensal or predatory in nature. Their varied size, behavior and build is rivaled by few other groups in the aquarium trade. The most popular crabs are known to be algae eating species. Many are marked as "clean-up crew" crabs, however many are problematic. Even the so called "reef safe" crabs can become burdensome in large hurds as they are omnivorous and may eat desireable algae and plants including each other with hermit crabs even eating snails to replace its home(Snail shell) with a bigger one if shells are not allocated to them on the sand bed floor. Food which is not limited to includes: worms, snails, mollusks, algae, crustaceans and fish. Many crabs place a burden to the microfauna of live rock and sand in natural marine aquaria.
Not all crabs are true crabs. Hermits, spider, and horseshoe are examples of this.
As for "reef safe" crabs though their are always exceptions to the rule since all reef animals must eat something in ones tank to survive. Knowing this safe crabs will be defined as their likelihood to eat popular fish and inverts as well as corals. You can be told alot about a crab's behavior by looking at key features and appendages. Large pinchers are obviously used for crushing and chipping large mollusk shells and crabs such as boxed crabs with large claws are predatory and must not be trusted in a tank. On the other end of the spectrum are the porcelain crabs with feathery appendages used to filter food out of the water column are quite safe with most fish and invertebrates. Inbetween these two extremes fall most of the other types of crabs.
Care for crabs is basically the same as for other invertebrates. Stable specific gravity between 1.024-1.026, stable ph, zero ammonia and nitrite and <10ppm nitrate. Special care should be practiced when acclimating crabs to your tank to cut down on the chance of osmotic shock.
Iodine supplementation is a good practice as well to aid in the crabs ability to molt(ecdysis). A deficiency of iodine will cause incomplete molts and high mortality rates. Due to the quickness of iodine depletion in tan water(usually a few hours tops) iodine should be added daily. Iodine is consumed quicky due to organic uptake, excess skimming, oxidation, and chemical adsorption. A weekly water change regimen is also advocated as a way to replenish minerals and nitritive elements like iodine, as well as dilluting undesireable contaminents in your water.
Safe types of crabs for your tank are as follows:MOST BUT NOT ALL Hermits, Sally Lightfoot's Emerald's, Porcelain's and Arrow's. Due to their mild temperment, small sizeand their ability to consume nuisance algae, detritus, uneaten food within rockwork and on the sandbed make them desireable and suitable inhabitants...plus, as for the hermits, they are more tolerant of fluctuating and less-than- ideal water parameters. BEAWARE of the hermits that come with large claws ie. the comon Green Hermit as they are predatory in nature.
As for the worst kind of crabs, the decorator(except for the decorator spider crab), sea spiders, box/shame faced, pom-pom, horseshoe, anemone, mangrove fiddler, and galatheid are not to be chanced in an aquarium tank especially for a reef biotope. Predatory and carnivorous in nature feeding on mollusks, fish, beneficial plants and microfauna as well as each other, makes them bad additions to a tank. Plus many of these will not make it due to the rigors of aquarium life cause by repetitive contact with fish and other inverts, duress in shipping, and the specialized niche required by others.
Over time, the omnivorous, even carnivorous tendancies of crabs seem to spill out. What a bad way to go when, you purchase a crab with its supposed "algae eating/control potential, and kills and consumes other algae-grazing inverts!
Now, i'm not suggesting that you don't buy a crab for your tank, as I have some in my tank as well with continued success. I'm just saying to be careful and watch because they could turn at any time.
They do possess a style of their own and a distinctive behavior. Though mainly secretive, they can be a fascinating addition to an aquarium under the right circumstances. 8)
Thanks for all the info!
From what I've been reading the inverts don't "soak up" iodine from the water, but need to actually ingest it. That's why crab pellets and other foods that are designed specifically for crabs, shrimp, and lobsters are high in iodine content.
If you do dose iodine, please be very careful because it's very toxic to a tank when overdosed!
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