In his book The Crystal Desert: Summers in Antarctica, biologist David G. Campbell describes how the chemical fluorine is
magnified up the Antarctic marine food chain, from krill to penguin:
Ecologists describe krill as the “keystone” species of the Southern Ocean. They
transform diatoms into food eaten by just about every other large predator in the
Southern Ocean. But along with being universally appetizing, krill are toxic because they
contain high concentrations of the element fluorine, a highly reactive chemical relative
of bromine and chlorine (both of which are used to disinfect drinking water and
swimming pools). Fluorine is harmless in small quantities; indeed, for humans, ingesting
a milligram per day helps prevent tooth cavities. But in quantities greater than ten
milligrams per day, fluorine is poisonous, inhibiting enzymes, diminishing growth and
fertility, and, because it concentrates in bones, deforming the skeleton. Krill scavenge
fluorine from seawater (which contains about one milligram of fluorine per kilogram)
and concentrate it in their chitin shells, where levels exceed 3,000 milligrams per
kilogram.
Everything that eats krill ingests potentially harmful levels of fluorine. An Adélie
penguin, which is about one-tenth the weight of a human, ingests about 240 milligrams
of fluorine per day from the krill that it eats. How does it deal with this toxic load? One
of the best strategies is simply to rapidly warm the ingested krill with body heat. When
the krill die, decomposition causes the fluorine to migrate rapidly from the chitin into
the digestible soft tissues; however, the enzymes that release fluorine from the cuticle
are denatured at temperatures above 30º C. A penguin’s internal body temperature is
38–40º C, so much of the fluorine remains in the indigestible chitin and is excreted in
the feces. Most birds, including ducks and chickens, have gastric ceca that enable them
to digest cellulose (and its chemical relative chitin). But penguins lack ceca and pass the
chitin undigested through their gut. Also, it takes only three to four hours for a krill shell
to pass through an Adélie’s gut, minimizing the potential for absorption of fluorine. Yet
even these adaptations aren’t enough, and penguins do absorb high levels of fluorine,
which is sequestered in the bones until it can be secreted by the kidneys.
The Crystal Desert: Summers in Antarctica by David G. Campbell.
© 1992 by David G. Campbell. Reprinted by permission of Houghton Mifflin Company. All rights reserved.
Campbell goes on to describe how humans are considering how to improve existing krill fisheries and develop
new ones to help feed the growing human population, but the high level of fluorine in krill poses a problem. Even when
frozen, the fluorine in the chitinous shells of krill can migrate to the meat. This means that even though humans do not
eat the shells of krill, they could end up ingesting much of the fluorine that was at one time sequestered in the inedible
shell.
Answer in complete sentences:
1. How many times greater is the concentration of fluorine in the chitin of krill than in seawater?
2. If a typical Adélie penguin ingests 240 milligrams of fluorine every day from the krill that it eats, how many
kilograms of krill must it be eating each day?
3. What are three adaptations that allow the Adélie penguin to minimize the absorption of fluorine from the krill
that they eat?
4. In order to prevent fluorine from migrating into the krill meat, what type of processing might need to occur on
fishing boats immediately after krill have been caught?
5. The krill that some humans target through commercial fishing are an essential part of the diet of whales,
including the endangered blue whale. What adaptation that minimizes the absorption of fluorine might the blue
whale have in common with the penguin?
6. In terms of biomagnification of toxins up a food chain, why might the warm body temperature of the blue
whale, an animal that can grow to 100 feet, be an inadequate defense against fluorine absorption?
7. Leopard seals are top level predators in the Antarctic marine food chain. They eat penguins, squid, fish, sea
birds, and other seals, many of which feed on krill. Young leopard seals are themselves dependent on krill for
food. Who do you think would have more fluorine built up in its tissues: a young leopard seal pup or an adult
seal? Justify your answer with three reasons