Joseph Katz is really interested in bubbles, tiny air bubbles in the ocean which are a major cause of noise. But when Katz and his collaborators took holographic equipment to the ocean to view the bubbles' distributions, they were surprised to discover enormous numbers of microscopic plankton instead. Katz had stumbled across the world of copepods. Switching his attention to the microscopic crustaceans, he decided to bring the copepods on land to record the first holographic movies of an animal's movements and the fluid flows around them (p. 3657).
Katz teamed up with a copepod expert, Rudi Strickler, to get the 3-D perspective on the crustacean's activities. Strickler had already tracked the creatures' movements by a variety of methods, so he realised that the digital holographic approach would give him the freedom to follow the tiny animals'trails at high resolution, no matter which way they bobbed. Strickler went fishing for copepods in Lake Michigan and took them to Katz's Johns Hopkins laboratory, ready for the crustaceans to perform in the holographic system's tank.
Using mirrors on the walls of the tank to reflect light beams in orthogonal directions, Edwin Malkiel and Jiang Sheng digitally recorded a crustacean from two perspectives and computationally combined the movies to recover the first fully 3-D views of an animal swimming. By adding microscopic polystyrene particles to the water, the team also followed the swirling fluids as they flowed around the animal's bobbing body.
Watching the 3-D footage, it became apparent that the tiny animal was engulfed in a recirculating jet of water, but how did the copepod produce this unexpected flow? Katz already knew that most copepods are heavier than water,so they sink continually unless they swim by vigorously flicking their mouthparts as they feed. By watching the polystyrene bead's swirling trails,the team realised that the downward feeding flow combined with the upward moving water around the sinking copepod, encircling the tiny crustacean in a bubble.
Which is a problem for the copepod; once it has exhausted the snacks trapped with it inside the circling jet, it must find fresh food. But the team noticed that after 9 seconds of descent the tiny animal suddenly lurched upwards and broke free. They realised that this was the length of time that it took for a parcel of water to flow from the crustacean's mouthparts to the chemoreceptors on its antennae. The team suspects that when a copepod rejects a foul mouthful, the morsel is carried in the recirculating flow until it passes the antennae, and the copepod recognises the bad taste. This is its queue to acrobatically flip and break free of its bubble to sample waters new.
The 3-D fluid flows also revealed new details of the copepod's lifestyle. The team were able to estimate the minute crustacean's apparent weight, and also noticed that although the crustacean's moving mouth parts should have set it tumbling as it sank, the copepod stabilised its descent by bending its tail upward like a rudder. Which could be very handy information if we are ever to predict how this pillar of the aquatic food chain swarms through the seas.