In the vast areas of the planet covered by water, human activity threatens the survival of countless species.

There was a time, for example, when manta rays were tossed back, dead or alive, when they were accidentally trapped in fishermen’s nets in places like Sri Lanka. Now their dried gills are prized in China for treating everything from cancer to measles — without any proof that they are effective — and one of the sea’s most majestic creatures is being fished nearly out of existence.

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