CITIZENS OF THE SEA — WONDROUS CREATURES FROM THE SENSUS OF MARINE LIFE

It is a curious and distressing fact that, in spite of all we know about life on Earth, so many basic questions still remain, especially when it comes to life in the ocean. What lies underneath the large ocean that covers more than 70 percent of the planet's surface? We know that it's full of salty water and that life can be found in every drop of it—actually, tens of thousands of bacteria in every drop. And, in a place like the western Mediterranean, we can discover as many as 100 different species of plants in an area the size of a dinner plate.

But, when it comes to guessing how much life there is in the entire ocean, we are unsure by a factor of 20. Experts estimate that there may be between 500,000 and 10 million different marine species—some difference! This is why the Census of Marine Life was launched. The Census is an international effort with the goal of answering three basic questions: What lived, lives, and will live in the world's ocean? Over the past decade, 2,000 scientists searched the ocean from Pole to Pole, from top to bottom, for everything from microbes to whales. The goal was not just to estimate the number of species but also to figure out how much their numbers have changed over time and what the future of marine life might be. The Census has been a journey of discovery.

There is no agreed-upon method for making a census of marine life. So the Census used an eclectic array of tools—from scuba diving to satellites—and human ingenuity to construct an impressive collage of information, from the discovery of new species of deep-sea creatures to the tracking of the migration of large ocean predators such as tuna. It's detective work—fascinating, challenging, and often exhilarating. Take reef fish. How can we know how many there are on a particular coral reef? You swim along a 50-meter line, identifying, counting, and estimating the size of every fish within two meters on each side. This is harder than it sounds. Imagine yourself swimming against a current, with schools of hundreds of small fishes moving frantically from one side of the line to the other while other fish dive down into the reef to hide.

To complicate things, males and females are colored differently in some species, just like some birds. In addition, large fishes tend to swim away as fast as fishly possible, which makes it very difficult for one to estimate how big they are. Now multiply this times 300 species or more. For the rest of the ocean, mostly beyond what our human bodies can tolerate, the quest is even more daunting. Exploring the deep sea is similar to space exploration: We need sophisticated submersibles to isolate us from the hostile environment—dark, cold, and under intense pressure. We can catch only glimpses of a world where we are welcome only as temporary guests...