Dear Blue Lobster:
I can see the need for iodine in salt water or brackish water animals, but fresh water crays have no natural source of iodine. Has anyone seen any studies that prove it is beneficial?
In the shrimp world we've established that in small doses it dooesn't hurt, but long term keeping in side by side tanks have shown no benifit at all.
Dear Gentle Sir:
In marine environments, the level of dissolved minerals generally satisfies a crustacean's need for iodine. Fresh water, however, lacks the dissolved minerals and crayfish, as well as other crustaceans, may experience iodine deficiency. This can result in a number of disorders.
Goitre, as you mentioned, is one such consequence of hypoiodinism. In crays this manifests itself as a large goiter just the under the head segment, often making the cray look as if its head were about to explode. Native North American tribes would avod eating such specimens, as their flesh was loose and sickly; animals too avoid bulbous-headed crays.
Sluggishness and mental retardation are other symptoms. If your cray is acting slow, weak, or generally lethargic, lack of iodine may be the cause. Likewise, a cray that eats its own appendages or feces may suffer from iodine deficiency.
Some crays have developed numerous methods to obtain it in other ways. One tropical species combs the brackish deltas of the Amazon and its tributaries for kelp detritus with antennas adapted to detect iodine molecules from up to eight kilometers (five miles) away.
If your cray is not lucky enough to have these adaptations, there are ways to introduce iodine to your aquarium. The simplest way is to add iodized table salt. Add one ounce a week until symptoms subside.
If your cray is sensitive to salt, add pure iodine crystals to the water. To obtain these, empty a bottle of iodine tincture onto a flat plastic container and leave in the sun. When the alcohol and water have evaporated, use a straight razor to scrape the remaining crystals into your cray's tank. Stop when the water begins to take a lavender hue.