Swimming butterflies

The common Antarctic pteropod, Limacina
antarctica, photographed under a dissecting
microscope by Brandon Hassett.

The jar was full of cold, clear Antarctic water. Inside, I could see three or four black specks that seemed to be bobbing up and down in the water. The jar had been sitting still for a while, so I didn't expect the water to be moving that much. Leaning closer, I noticed the black specks were surrounded by clear, spiral-shaped shells. Two lobes of tissue emerged from the shell like little wings. All of a sudden, the animals came into focus. My brain found the word for them: pteropods.

Pteropods are commonly called "sea butterflies" because of those two wing-like lobes. They're related to snails and sea slugs, but they spend their entire life-cycle up in the water column, not on the seafloor. Pteropods use their lobes, which are modified extensions of the foot, to swim up and down in the water column. The lobes ripple and beat like butterfly wings, carrying the animal up to the surface of the water. At the surface, the pteropod constructs a mucus net, which it spreads over itself like a parachute to catch particles to eat as it sinks back down. They're beautiful and fascinating creatures.

Swimming pteropods in one of the respiration
experiments

My small group has gotten interested in pteropod swimming activity and started pursuing an experiment with the most common Antarctic pteropod, Limacina antarctica. We're trying to figure out how energetically expensive it is for the animals to swim in their characteristic up-and-down pattern. How much food does a pteropod have to catch in its mucus net to make swimming back upward worth it? To answer this question, we're measuring the respiration rates of swimming pteropods and resting pteropods at different water temperatures. We're putting pteropods in closed flasks where they can swim, then measuring the concentration of oxygen dissolved in the water over time. As a comparison, we've put resting pteropods in closed flasks that are not large enough to swim in, and we'll measure their oxygen consumption too.

I haven't done any experiments with respiration or animal physiology since I was an undergraduate, so it's fun for me to learn new techniques. The pteropods are also goregous animals that are relatively easy to work with. It will be fun to see what the data show!