FWIW, there are some organisms that do have a demonstrated need for iodine, and this has been known for a long time. It isn't always clear that they necessarily get their iodine from the water as opposed to in foods, but they do have a need. It's just that I do not think most reefers keep these.
I documented what was known to the point I wrote this article in 2003 :
Chemistry And The Aquarium: Iodine in Marine Aquaria: Part I ? Advanced Aquarist | Aquarist Magazine and Blog
https://web.archive.org/web/20050405045820/https://www.advancedaquarist.com/issues/mar2003/chem.htm
Chemistry And The Aquarium: Iodine In Reef Tanks 2: Effects On Macroalgae Growth ? Advanced Aquarist | Aquarist Magazine and Blog
https://web.archive.org/web/20030419082711/http://www.advancedaquarist.com/issues/april2003/chem.htm
some examples from the first one:
Who uses iodine: Gorgonia and antipatharian corals (black coral)
Another set of creatures of the deep that use iodine are certain
gorgonia, such as
Plexaura flexuosa.42 They have 3,5-diiodotyrosine in their bodies, to the tune of 0.1 to 2.6% of the total dry weight as iodine. This iodoamino acid is presumably incorporated into proteins in the skeleton (stem), but the benefit is unclear. Again, it may be largely an antipredatory effect that is desired. The iodine incorporation in
gorgonia seems to increase with age.43,44 The proteins of many different
gorgonia species contain substantial iodine:
Eunicella otenocalloides 6.5-8.9% by weight%,
Gorgonia verrucosa 4.2-9.0,
G. lamarcki 3.3-6.8,
G. scirpearia 0.4-0.6,
Rhipidigorgia flabellum0.6-1.1,
Euplexora maghrebensis 0.19-0.23, and
Plexaura kukenthali 1.9-2.2.44 It has also been demonstrated that at least one gorgonia (E. verrucosa) takes up iodine in the form of inorganic iodine from the water column.45
One study showed that the organoiodine compound thyroxine, and some related compounds, are localized to certain parts of the gorgonia
L. virgulata.46 Most interestingly, one of the places it is localized to are scleroblasts (spicule-forming cells) and on the spicules themselves. Further, the addition of thyroxine to these cells impacted the uptake of calcium, and it is suggested that the thyroxine functions in spicule formation.
The
antipatharian corals (the black corals) also seem to incorporate a lot of iodine. The basal regions of these corals are especially loaded with iodine,
with more than 23% iodine by dry weight recorded in two species.43, 45 Again, the specific purpose is not known.
Who uses iodine: Tunicates
The known story of iodine in tunicates also appears to be complex. Tunicates apparently use the iodinated amino acids to make proteins, like the gorgonia, but they also make some very complicated organoiodine compounds.47-49 Since some of these molecules are vertebrate hormones (e.g., thyroxine) it has been suggested that tunicates use them in this fashion, while most lower invertebrates do not.48 Others of these iodoorganics are even more complex, such as lukianol B, a cytotoxic compound that might be made as a deterrent to predators.47 Like all of the species discussed above, tunicates have also been shown to take up iodine as dissolved inorganic iodine from the water column.49
Who uses iodine: Sponges
Sponges are also known to produce a wide variety of potentially toxic organoiodine compounds.50 One sponge of the genus
Geodia is known to make cyclic peptides (geodiamolides A and B) that contain iodine, and these toxic compounds are expected to exert a protective effect of some type.51
Overall, the sponges do incorporate large amounts of iodine An analysis of 12 species of sponges from the Andaman Sea showed iodine contents that range from 0.001 to 0.085% iodine.52 I have not, however, seen any studies about where sponges get iodine, whether from inorganic iodine in the water column, or from ingested food.
Who uses iodine: Worms
The marine worm
Glossobalanus minutus has been shown to take up iodine from the water column, but it is not clear what it does with it as no iodinated proteins were detected.53 A different marine worm,
Lineus rubber, accumulates iodide from the water column and stores it mostly as iodide, with a smaller amount of thyronine (an organoiodine hormone).54, 55
Who uses iodine: Shrimp
As one moves up the food chain to more sophisticated organisms, data on their ability to take up iodine from the water column becomes very sparse. Shrimp are known to have a need for iodine to grow, but that is only known in terms of a dietary requirement. The shrimp,
Penaeus chinensis O'sbeck, for example, grows optimally when the diet contains 0.003% iodine.56
Shrimp apparently incorporate substantial amounts of iodoorganics into their bodies. The shrimp
Pandalus borealis, for example, incorporates between 0.04 and 2 ppm iodine as iodoorganic compounds depending on the particular body tissues examined. Their roe were somewhat higher, up to 4 ppm iodine as iodoorganic compounds.57 Shrimp shells and other parts can contain up to 17 ppm by dry weight iodine, the majority of which is iodoorganic compounds58, but the values are still far lower than for other inverts like macroalgae, sponges, or gorgonia.
Still, the amount contained says nothing about whether iodine is an important requirement. I could find no scientific studies that showed that shrimp need iodine from the water column, but neither could I find any that demonstrates that they do not.
Who uses iodine: soft corals such as Xenia
Surely, you say, there must be studies showing that Xenia and other soft corals need iodine from the water column? Well, I could find none. There may be studies that I could not find, and regardless of whether there are studies, iodine in the water column may or may not have a significant impact on these organisms. Nevertheless, there is no published basis (that I could find) for many of the claims about iodine.
There are studies that show that Xenia does contain substantial iodine, and it is likely that it got it from the water column, but what good, if any, that iodine serves is unknown. In a recent publication,
Ron Shimek showed that a wild specimen of Xenia sp. contained 350 ppm iodine on a wet basis and a captive specimen showed 270 ppm on a wet basis and 1350 ppm on a dry basis.29 Those values are as high as some of the macroalgae, and lend some support to the idea that Xenia accumulate iodine (and presumably have a use for it at such high accumulations).
Of course, accumulating iodine from the tank somehow, and showing that supplemental inorganic iodine is beneficial are very different. I am in the planning stages of running experiments on the possible benefits of iodine supplementation to certain soft corals, but the technical challenges are significant (much more so than similar tests on macroalgae), and I’m not certain that they will be successful.
