During a 2008 research cruise, scientists imaged a massive gathering of a type of salp—a barrel-shaped, jellyfish-like organism. Such aggregations occur periodically off southeastern Australia in response to annual blooms of tiny plants called phytoplankton, the salps' prey.

Study leader Jason Everett knew that springtime was the right time to be on the lookout for salp explosions, he said by email.

Even so, "we were extremely lucky—from the first net we put down right through to the last, we were getting thousands of salps," said Everett, of the Fisheries & Marine Environmental Research Facility at the University of South Wales in Australia.

Transparent except for their guts, salps are difficult to see—especially the species studied, the one- to ten-millimeter-long Thalia