How polluted are open waters? Parasites inside sharks could help to tell.AddStyle

Parasitic worms inside the guts of sharks are absorbing high concentrations of toxic heavy metals, researchers have found. The worms could be useful to scientists trying to check up on the health of ocean waters. And they could be saving the sharks from metal poisoning — at least for now.

"What we are seeing with these parasites is a classic canary in a coal mine situation," says Kenneth MacKenzie, a parasitologist at the University of Aberdeen. "If the parasites start dying off as a result of metal poisoning we will know that levels have gone too high and can expect larger species to start suffering too."

Heavy metals are a common toxin in marine environments thanks to industrial waste and mining activities. Close to shore, scientists often look at clams, oysters, and other filter feeders, which accumulate the metals in their tissues, to keep tabs on how polluted an area is. But such 'bioindicators' don't exist for the open ocean, and levels of heavy metal pollution there are unknown — although they're thought to be on the rise, leaving scientists concerned about their effects on marine life.

MacKenzie and his colleagues, led by Masoumeh Malek at the University of Tehran in Iran, looked at coastal sharks to see whether parasites in their guts might be accumulating such toxins in open waters. They collected sixteen whitecheek sharks (Carcharhinus dussumieri) in the Persian Gulf, dissected them, and collected their intestinal tapeworms (Anthobothrium sp. and Paraorigmatobothrium sp.). They then tested both shark tissues and parasites for presence of cadmium and lead.

The tapeworms had 278 to 455 times higher metal concentrations than the sharks themselves, the researchers report in Parasitology¹. "This is very exciting because we have effectively found filter feeders in the open water," says MacKenzie.

Heavy metal

The result mimics those of previous studies, which found parasites in freshwater fish were accumulating heavy metals²,³.

Together, such studies leave researchers thinking that these intestinal creatures are protecting their hosts. "If the heavy-metal concentrations found in the parasites were found in the shark tissues instead, I have little doubt that the sharks' health would be poorer," says Malek, although she doesn't know whether this level of pollution would make the sharks ill.

The theory needs further checking says parasitologist Dave Johnston from the UK's Natural History Museum in London. "Identifying whether a protective effect is being generated is a complex issue," he says. The relative mass of parasite to host is much higher for freshwater fish than for sharks, he notes — a shark might need a lot of parasites to suck up enough metal to protect it.

In the meantime, researchers are also keen to use the parasites to study heavy-metal levels in deeper waters, where hydrothermal vents also dump metals into the water. "We know little about heavy-metal concentrations in the deep oceans. We don't know how much pollution gets there," says MacKenzie. "Collecting and analysing parasites from sharks in the deep sea would be very interesting."

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• References

  1. Malek M., Haseli M., Mobedi I., Ganjali M. R.& MacKenzie K. Parasitology, doi:10.1017/S0031182007002508 (2007).
  2. Thielen F., Zimmermann S., Baska F., Taraschewski H.& Sures B.Env. Poll., 129. 421 - 429 (2004).
  3. Turcekova L., Hanzelova V.& Spakulova M. Helminthologia, 39. 23 - 28 (2002).