Mechanisms for Propelling Plankton Studied With High-Speed Cameras

At the Charleston Marina, scientists recently gathered copepod plankton, which are tiny aquatic crustaceans not more than 2mm in length, and studied them under the high-speed video. They aimed to determine their movement mechanisms. Copepods are the dominant components of plankton.

The interest of Univ. of Oregon's Inst. of Marine Biology researchers Richard Emlet and George von Dassow was piqued by a student's discovery of barnacle larvae's swimming mechanism. They published their paper in the Univ. of Chicago Press Journals' Biological Bulletin documenting the Acartia copepod species' mechanism for aquatic movement. Von Dassow says that they use an elastic material as a paddle to swim and escape the viscosity of the water.

Like barnacles, the copepods form from their cuticle a temporary fan-shaped structure made from extra-cellular filaments that serve as paddles for their legs. They are made of exoskeletal meshwork which has an opening and closing movement lasting less than 10ms per cycle of the swim stroke. They make arc-like and sweeping movements that cause a burst of speed, which helps the copepods escape danger.

Von Dassow explains that unlike us, small creatures such as plankton experience water as a very sticky medium, something akin to a human if they were to swim in thick syrup.

To compensate, copepods have larger limbs that are more powerful and harder than living tissue. Von Dassow compares it to birds having large and vaned feathers.

University of Texas Port Aransas Marine Science Institute researchers have also used high-speed videos to study copepod movement in 2003. Still, they did not delve into the mechanisms by which the animals accomplish it.

Von Dassow shared that their study is curiosity-driven scientific research, which is what marine labs are for. He says that the study was tricky since copepods can be easily physically damaged due to their delicate bodies.

He says that a lot of their recordings were made with the copepods placed in a petri dish having a glass coverslip at its bottom. The movements are in the range of 10 milliseconds per cycle, which requires a camera with an ability to record 8,000 frames in a second. Standard videos have a speed of 30 frames a second or 33 milliseconds in between each frame, which is too slow to capture the copepods' movements.

Furthermore, it is also not possible to aim the microscope; they would just have to wait for the copepods to enter the field where the camera is pointed.

For every 10 to 100 attempts, von Dassow says they would catch only one significant glimpse worth studying. The movements where key sequences were recorded are seen in only one in every one thousand attempts.

The study's findings encompass the movement of adult and pre-adult Acartia stages. As with many insects, these animals live for roughly one month, according to von Dassow.

He adds that the movement mechanism is ecologically vital because small predators like crustaceans eat these animals up to the largest baleen whales. Their rapid escape mechanism may be crucial for survival, and not merely a useless faculty since the mechanism is expensive energetically.