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Latest Expedition Journal

October 30:  Day 28

Zooplankton Rules

The gyre is an oceanic desert. Chlorophyll levels are low, land is hundreds of miles away, up to five thousand vertical meters of water separate sunken nutrients from the surface layer where sunlight penetrates, and marine animals are rare.  Yet zooplankton dominate. These creatures are drifters; some can swim, others float, glide or bob, but all travel wherever the sea brings them.

We catch zooplankton every day in the nets.  After emptying the cod end jar of the nets into a bucket, then draining the seawater out through a sieve and counting and removing the plastic, we are left with what is the biomass of the zooplankton.  All creatures greater than 2 cm in size are removed and measured.  Then we fill a tiny 1 ml measuring spoon with these critters, empty it into a Petri dish, and identify the first hundred zooplankton we see.  Under the dissecting microscope and magnified up to 50 times, an amazing new world appears.

From the scope, it’s easy to wax sentimental about zooplankton. When asked about his favorite zooplankton, Thom Young said, “Nothing captures my heart like a baby cephalopod!” When the nets bring up one of these juvenile squids, word spreads quickly around the ship. Team Plastic members pop in the lab to admire it up close and watch the squid quickly change to blend in with the different colored backgrounds we place it on.

Madelyn Soldner-Sullivan’s favorite is the nudibranch, Glaucus atlanticus, a 1 cm snail without a shell, “It’s magical,” she says, “like a dragon.” This beautiful, blue and white nudibranch is also a predator.  When it gracefully swims up to Porpita porpita  (a zooplankton that looks like a blue sun hat with dangling tentacles, from the Class Hydrozoa), the nudibranch eats it, stinging tentacles and all, then uses the stinging cells to ward off potential predators.

Videographer Sabrina Schlumberger’s favorite?:  “That would be the Phronemid amphipod that we found in the Atlantic on my student trip,” she says.  “It wears salps like a dress.” This shrimp-like amphipod is particularly fierce-looking, and encases itself in the body of a salp—a see-through, jelly-like, filter-feeding zooplankton.  The salp, disturbingly, is our closest zooplankton relative. It has earned membership to our phylum, Chordata, because its tadpole-shaped larvae has a cartilaginous supporting rod similar to our own backbone. By stealthily bobbing through the ocean surrounded by salp, this amphipod can grab unsuspecting prey before they can escape. Sabrina adds: “The Phronemid amphipod was the inspiration for the scary, tooth-faced invader that tries to eat Sigourney Weaver and her crewmates in the sci-fi thriller, Alien.”

The most common zooplankton in our “hundred counts” are thought to be the most numerous multicellular animals on earth: the copepods. To the naked eye they appear as bright blue, red, or colorless specks. Copepods provide a crucial link in the ocean food web. They eat phytoplankton and are eaten by other zooplankton thus transferring energy from the sun to the herbivores, and up the food web to fish like tuna that are then eaten by people.

Our nets almost always bring in water striders, Halobates, the lone insect of the ocean. These small, black bugs glide over the ocean’s surface with miles of water beneath their legs, but they can’t swim.  When we shine a light into the water on moonless nights, hundreds come scurrying over.
 
Peering into the deep, blue water from the deck of the Seamans, the gyre seems empty of life. But when we look through the microscope, a whole, new universe is unveiled. Above are just a few examples of the many different little animals we find that are able to eke out a living in the inhospitable waters of the gyre. Every one of these creatures has a story waiting to be discovered.