"springtime was the right time to be on the lookout for salp explosions"

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 democratica.

Everett has swum in a salp swarm, and he described it as being like "swimming through a pool of jelly balls."

"Every time you take a stroke, you'd touch some little pieces of jelly. It's an odd sensation, given you can't really see the salps in the surf zone due to the waves and whitewater."

Read More | "Huge Swarm of Gelatinous Sea Creatures Imaged in 3-D" | Christine Dell'Amore | National Geographic