Octopuses are a popular entrée for plenty of predators—including us humans. And for good reason. Octopuses are nutritious, with loads of lean muscle in those amazing arms, and plenty of good minerals.

But are they also harboring hazardous heavy metals?

Surprisingly, "there is no information on the levels and magnitude of octopus contamination by heavy metals or their safety for human consumption" in the western Indian Ocean, where many octopuses are caught, wrote the researchers behind a new paper, published this month in the Journal of Health and Pollution. Previous research has found trace amounts of heavy metals in octopuses caught in European and UK waters.

"Marine resources provide a major source of protein for the costal people of the region," the scientists noted. And octopus is especially popular "because of local beliefs of the species' medicinal effects as an aphrodisiac." So the Tanzania-based team went looking for lead in the local catch, the day octopus (Octopus cyanea), which is consumed locally as well as exported to Europe and Asia.

The researchers sampled 60 octopuses during expeditions in 2013. The team found up to 7.22 micrograms of lead per gram of muscle in the local octopuses. "These values were above the maximum allowable criteria of 0.2 micrograms/gram" set forth by the World Health Organization as well as "above the tolerance limits set forth by the United Nation Environmental Program, the International Atomic Energy Agency and the EPA," the researchers noted. "That is, the octopus collected…are unsafe for human consumption."

If, however, an adult weighing roughly 70 kilograms were eating only the Environmental Protection Agency's recommendation of just 340 grams (three quarters of a pound) of fish per week—all replaced with grams of this octopus, the amount of lead eaten per week (roughly 12 to 18 micrograms) would still be within limits considered safe. But, with higher weekly consumption of octopus, as is likely for locals at least, those eating this animal caught in these waters could be at risk of consuming too much of this heavy metal.

And researchers cautioned that the livers of these octopuses contained even higher levels of lead. "Current harvesting and processing methods may lead to contamination of muscles through contact with contents of ruptured livers," they noted.

Much of the Tanzanian octopus catch is consumed locally. But as global demand has increased, so has the country's exports. By 2002, Tanzania was already exporting more than 870 metric tons of octopus, according to a 2009 United Nations report (pdf). And informal data, the report notes, suggest this number has continued to grow.

And in addition to us humans, other octopus predators might also be suffering from exposure to these toxins. "Lead is know for its neurotoxic characteristics that cause behavioral deficits in most vertebrates, which may result in decreasing growth rates, survival and metabolism," the scientists noted. And likewise, lead might well be affecting the octopuses themselves.

"The presence of lead in the muscles of O. cyanea indicates a contaminated environment," the authors noted. In particular, "sediment acts as a reservoir for many pollutants, including heavy metals." And this could be a distinct issue for octopuses (and the people who eat them). "The feeding habits of the octopus may also contributed to the elevated concentration of lead in the muscles," the researchers wrote. "Being at the top of the local food chain, they feed on various food items such as mollusks, crabs and other crustaceans that spend much of their life cycle in contaminated sediment."

The sampled areas are polluted by runoff from rivers that wash unprocessed industrial and urban waste from the coastal cities of Dar es Salaam and Tanga—especially during monsoon season, the researchers noted. So to clean up the octopus catch, here and elsewhere, we will likely have to clean up our global waterways. For our sake as well as that of other predators. And the octopuses themselves.

Learn more about global octopus fisheries in Octopus! The Most Mysterious Creature In the Sea.

Illustration courtesy of Ivan Phillipsen