Ocean Watch
By Susan Scott
Sharks and rays get goiters. I learned this surprising fact in a recently published article by Waikiki Aquarium biologist Jerry Crow. This shark expert and his colleagues studied the thyroid glands of 12 sharks and rays, three free living and nine captive. Sharks get goiters, too,
but no one is really sure why
Of the three free-living, one had a goiter. Of the nine captives, seven had goiters and the eighth had an inflamed thyroid gland. That means that of the 12 sharks and rays studied, only three had normal thyroid glands. A goiter is an enlargement of the thyroid gland, often visible as a swelling.
This surprised me because in the 1950s, when goiters were common in my Midwest community, we kids took school-issued iodine tablets to prevent the malady. We had to do this, we were told, because we didn't eat enough seafood, which was rich in iodine, which prevented goiters.
Well, sharks and rays eat seafood. How is it then that they get goiters?
The answer starts with iodine.
Iodine is a natural element occurring in both soil and sea water. But there isn't much of it. The iodine concentration in sea water is less than 1 part per million.
To make matters worse, the little bit of iodine we do have here on Earth is constantly leaving us. That's because when sunlight hits iodine, the substance vaporizes into the air. Rain returns some of this escaping iodine to the ocean and soil, but much of it keeps going and is eventually lost in the stratosphere.
Fortunately, nature provides several good iodine storage systems. One excellent place to find this element is in seaweed, which accumulate iodine from sea water. About 900 pounds of seaweed produce one pound of iodine. Fish that eat seaweed, then, are also a good source of iodine.
Another high-quality iodine source comes from the salt water found in oil wells. Iodine is abundant there because oil comes from the decay of marine plants and animals, many of which stored iodine when they were alive.
So what does iodine have to do with goiters?
All animals with backbones, including humans, use iodine in the structure of thyroid hormones, crucial to metabolism. We don't need much iodine to make these hormones -- one molecule of thyroxin contains only four atoms of iodine. But we do need some, because when iodine gets scarce, the thyroid gland starts working overtime trying to produce its hormones.
This overwork causes the gland to get bigger and bigger, swelling the victim's neck and unbalancing the person's metabolism. Such enlarged thyroid glands, called goiters, are a major health problem throughout the world, affecting about 300 million people worldwide.
It's easy to see why goiters occur in some people: They don't eat enough plants and animals containing iodine. And in the case of captive sharks and rays, the element in the water gets used up by the aquarium's inhabitants.
It seems obvious then that adding a little iodine to a goiter victim's diet, or an afflicted shark's water, would make the goiters disappear. This does work sometimes, but not always. Surprisingly, some people -- and sharks -- getting plenty of iodine still get goiters.
Something else, researchers believe, is causing some types of goiters in both fish and humans. No one currently knows what that something is, but studying sharks may help researchers find out.
Jerry's study is a good example of why we need to respect and protect all the species that share our planet, and not just the cute, cuddly ones. Sharks may not be most people's favorite kind of fish, but in this case, they could help save millions of lives.
Marine science writer Susan Scott's Ocean Watch column
appears weekly in the Star-Bulletin. Contact her at http://www.susanscott.net.
