Most of the Oceans’ Fishes are Part-Time Plankton

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Starfish larva floats deep beneath the ocean's surface.

The tiny larva of Asterias rubens, or common sea star.

DP Wilson FLPA/AGE Fotostock


If you happen to be traveling to a warm beach this winter, take a close look when you go in water. Whether you see them or not, you’re taking a swim with plankton.

These tiny organisms found near the ocean’s surface are more numerous than you imagine. Some are permanent plankton that will spend their entire existence riding the currents. Others are just going with the flow until they develop into full-fledged swimmers.

 

A sunfish larva floats deep below the ocean's surface.

The Mola mola, or ocean sunfish, is just one species of fish that begins life as a planktonic larva

G. David Johnson/CC BY-SA 3.0


“Many species of fishes start out life as planktonic larvae, suspended in the water column, unable to propel themselves, and at the whim of wind and oceanic currents for their dispersal,” says John Sparks, curator in the Department of Ichthyology who’s overseeing the upcoming exhibition Unseen Oceans, which opens on March 12. “These larval fishes are temporary members of the zooplankton that, in turn, feed on smaller plankton.” 

Among the soon-to-be-swimmers: the blue marlin, Makaira nigricans, one of the world’s most iconic game fishes, which can grow to weigh more than 1,000 pounds. Blue marlins start their lives as humble, millimeter-long eggs that, when fertilized, develop into slightly less tiny larvae and spend their early days floating among other zooplankton.

 

Starfish on a beach, next to a mussel shell.

When Asterias rubens reaches maturity it can be found in and around the oceans’ benthic zone, the region along the seafloor.

Clément Philippe/AGE Fotostock


If they survive long enough—and avoid being eaten—another subset of part-time plankton settle down—way, way down. These benthic species, as they’re known, sink out of the water column and down to the seafloor. Sea stars (you may know them as starfish) and sea urchins, for example, get their start as drifting planktonic larvae before moving to deeper haunts.

Find out more about other amazing plankton in Unseen Oceans, which opens on March 12. 

 

A version of this story originally appeared in the Winter 2018 issue of Rotunda, the Member magazine.