The Plankton: The Soup of Life

Patiently plucking plankton: Blue chromis facing into the current (well, most of them) above a coral head to capture passing plankton. For that matter, the gorgonians and hard corals around them are doing the same thing.

NOBODY GOES UNDERWATER TO LOOK AT THE PLANKTON.  Well, actually, corals, gorgonians, sea anemones, baleen whales, whale sharks, sponges, brittlestars, a lot of kinds of fish and many other denizens of the deep do.

But divers fixated on large, celebrity species like sharks and mantas may find omni-present microscopic organisms they can’t particularly see less than engrossing. They’re missing an enormous point. To the aforementioned denizens, the plankton are the staff of life, the dinner pail, the feed bucket, their nutritional sustenance.


And if you want to understand the activity you’re seeing on the reef, you need to understand the planktonic soup of bacteria, algae, eggs, larvae, radiolarians, diatoms – organisms, barely visible to the human eye and pretty much everywhere.

  • The blue and brown chromis fishes that station themselves above coral heads facing into the current? As Planktivores, they’re in position to intercept plankton sweeping down the currents.
  • The corals they’re stationed above? They extend their tentacles to capture plankton passing by.
  • Sea fans and other gorgonians? Their entire architecture is designed to maximize the ability of their polyps to capture passing plankton.
  • Those creole wrasses that surge up and down the reef in great columns, looking as if they’re gulping water? Actually, they’re snatching plankton from the waters.
  • Those giant whale sharks you yearn to swim with as they sweep through the seas? Planktivores. Same with humpback whales and other baleen whales.
  • The tiny roughhead blennies that poke their heads out of holes in the corals, as if they’re just watching the world go by? Along with bottom-dwelling jawfish and some gobies, they’re on the alert for passing plankton to grab.
  • Anemones, sponges, featherstars, brittlestars, you-name-its, they all live on plankton they pluck, filter or otherwise take from the waters.
We generally focus on sea anemones for the cleaning shrimps and fishes that hide within their tentacles. But anemones themselves are plankton feeders and their tentacles are actually plankton pickers.


The quick definition of plankton is any organism that lives in the water and drifts with the currents due to total lack of or limited capability for propulsion.

Plankton can be really, really small – smaller than the width of a human hair. In fact nanoplankton  can be as much as 10 times smaller than that human hair and picoplankton another 10 times reduced.

We swim through this stuff, very unlikely to be able to identify them as individual organisms. We’re most likely to see them when they aggregate for mass events, like red tide blooms.

That said, bigger organisms – like seaweeds, jellyfish and even some small species of squid – also qualify as plankton. Seaweed just drifts. Jellies mostly drift, with limited propulsion capabilities. And krill, the little crustaceans that whales and whale sharks scoop up do swim but are regarded as part of the plankton.


Plankton can be categorized many ways – nature, size, life patterns. The most basic distinction is between phytoplankton (literally, “plant plankton”) and zooplankton (“animal plankton”).

  • Phytoplakton include bacteria and protists like diatoms and dinoflagellates, many of them autotropic – that is, transforming solar energy into nutrients. And seaweeds.
  • Zooplankton include a broad range of microscopic animals like foraminiferans and radiolarians, often beautiful in their construction. And carnivorous.

Zooplankton would include eggs, larvae and juvenile forms of every creature in the sea – fish, coral polyps, other cnidarians, arrow worms, crabs, snails and sea stars – that tend to come with the own egg yolks and may not be carnivorous. Copepods and the aforementioned krill, jellyfishes and squids would be zooplankton.


Much of this planktonic soup originates in broadcast spawning in which fishes, corals, worms, echinoderms and whatever else throw sperm and eggs into the water column to create larvae that are swept wherever the waters take them, hopefully finding a friendly habitat to land in.

By and large, when they’ve developed to a point where they’re ready to settle, they’ll be guided by chemical cues toward a suitable landing spot. Coral larvae for example, will settle on hard substrates, immediately transform into polyps and begin reproducing themselves into coral colonies.


Juvenile fish may be drawn to mangrove forests or seagrasses – famous as nurseries for reef life – before heading out for life on the reefs. Many species of fish focus on plankton during their juvenile stages s but shift to other foods as adults. No matter what, at all times most bony fish capture food in the same way – drawing it in with suction power as they rapidly open their mouths and flare their gill covers.


Many, many more begin the planktonic journey than finish it successfully. Whether any individual succeeds or not is chancy, if not random – whether sperm and eggs quickly meet in the water column to achieve fertilization, whether those viable eggs survive long enough to become larvae, whether those larvae are swept to suitable places to settle and become the animals their genes dictate, whether they’re eaten by their fellow plankton before any of those stages or whether they just never find a suitable settling spot and perish.  Unlucky ones will drift without settling, dying in the open waters.

Or whether they’re captured on the reef by planktivorous fishes, corals, sea fans and plumes, anemones, brittlestars, featherstars, shrimps or sponges.

PRINCIPAL SOURCES: Marine Biology, Fourth Edition, Peter Castro, Michael Huber; Marine Biology, An Ecological Approach, James Nybakken; Life and Death of Coral Reefs,  Charles Birkeland (Editor); The Coral Reef at Night, Joseph Levine, Jeffrey Rotman;  Reef Fish Behavior, Ned DeLoach.