Fireflies in the Deep

by Gail Wellenstein, Kitsap Beach Naturalist

Copepod, a type of crustacean zooplankton. Photo courtesy of oceanservice.noaa.gov/facts/plankton.html

Kayaks quietly paddled away from the boat ramp at Mystery Bay, heading toward Indian Island after sunset, seeking the shadows on a moonless night. The water was calm, and only our guide spoke as we approached the steep shoreline, careful to avoid downed trees. Paddles dipped carefully into the water and suddenly there was a spark, then another, and soon every kayak was surrounded by tiny flashes of blue-green light, produced by zooplankton using the chemical luciferin, looking like fireflies in the deep.

There was another event occurring that night throughout all the oceans of the world. Untold numbers of zooplankton rose from darker and deeper waters to feed on the phytoplankton floating near the surface. As dawn approached, they sank several hundred meters to avoid their predators and damaging ultraviolet radiation. When considering the sheer mass of organisms repeating this every 24 hours, it becomes the largest migration on the planet, visible from ships and satellites.

This phenomenon was discovered during World War II, when sonar operators noticed signals that seemed like they came from the ocean bottom, but they existed far above the recorded depth and would creep even closer to the surface at night. Submersibles and nets were sent out to investigate, and what they discovered was a combination of zooplankton and small fish --  trillions of them.  What appeared to be empty, open water at night was actually filled with a staggering variety of tiny animals of different colors, dazzling shapes and bioluminescence.

Zooplankton are small, weak swimmers that usually drift with the current and survive by eating phytoplankton or other zooplankton. They are single celled, animal-like organisms in a group called protozoa. Zooplankton range in size from microscopic to the size of jellyfish and are found throughout the world.

Some zooplankton remain in this drifting stage throughout their life. Others, like the larva of urchins, sea stars, crustaceans and most fish, are only temporary members of this category, leaving this label behind when they become truly mobile.

One of the most common permanent zooplankton is krill, a shrimplike animal just a half inch long, that lives in swarms up to 60,000 krill per cubic meter. These swarms confuse predators into thinking it is one large animal. Krill are particularly important in the southern oceans as they are a main food source for penguins, seals, and baleen whales. They survive the severe winters of Antarctica when phytoplankton is less available by shrinking their bodies to make it through periods of starvation. Despite this harsh environment, the total weight of Antarctic krill is more than the total weight of all the humans on the planet. 

An ongoing research project in the Pacific Northwest takes advantage of vertical migration. The Pacific Northwest Crab Research Group, in collaboration with various organizations including the Port Townsend Marine Science Center, sets up light traps from spring through summer to attract zooplankton at night. The objective is to evaluate how many larval Dungeness crabs are present and whether the number of larvae this year can predict future adult numbers.

These traps collect zooplankton that are drawn by the light. Each morning the trap is emptied, the contents sorted, and Dungeness crabs are counted. The hope is that the data collected will lead to improved,  long-term management and sustainability of the Dungeness fishery.

A side benefit of these traps is the occasional capture of a giant pacific octopuses’ larvae. Kakantu, the most recent octopus at the aquarium in Port Townsend, was captured in a light trap when he was the size of one grain of rice.  He was carefully tended by the staff and grew rapidly for two-and-a-half years. He was released into his home waters last winter upon reaching maturity, hopefully to find a mate and continue the cycle of life.