It's easy. Just write down all the sea critters and cross off the ones you don't know.

Math.

I love it when people just make jokes instead of answering a question

number of genera, families, orders, classes, and phyla—a designation above class—in each kingdom. That's a relatively easy task, since the number of new examples in these categories has leveled off in recent decades.

By contrast, the number of newly discovered species continues to rise sharply.

Using complex statistics, Worm and colleagues used the number of genera, families, and so on to predict Earth's number of unknown species, and their calculations gave them a number: 8.7 million.

An Issue of Statistics

Some experts called the research, published August 23 in the journal PLoS Biology, reasonable.

But Dan Bebber, an ecologist at the environmental group Earthwatch Institute, said the study relies on improper statistical methods.

The study team used a method called linear regression to calculate the number of Earth's species. But Bebber thinks this method is the wrong one for the data, and that the team should have used a technique known as ordinal regression.