Long live polar invertebrates: part 2

I dipped the edge of the petri dish into the beaker of water, then pulled it back to find I had successfully caught two amphipods. They were small, red, and bug-like, zooming around the cold water in the dish. I had a hard time focusing on them because they were moving so fast, but thankfully, the speedy swimmers slowed down once they were under the lens of the dissecting microscope. I pulled my chair up to the scope and gazed down through the eyepieces. I had to find out what the little bugs were.

I always take for granted that other scientists know invertebrates, just because I spend so much time around those who do. The group of trainees in my Antarctic program encompasses a diverse array of specialties, from physiology to microbiology to planetary science, so obviously not everyone is as excited about invertebrate zoology as I am. I have been able to help others identify the organisms from McMurdo Sound, and if you don't mind, I'd like to introduce you to a few of them.

Orchomenella pinguides, an amphipod in McMurdo Sound

Amphipods are common in a lot of marine habitats, from the poles to the tropics and from the coast to the deep sea. They're crustaceans, related to crabs and shrimps, but they look more like insects. I vividly remember catching amphipods in Svalbard back in 2015, and they were the most common organism we caught on a cruise to hadal trenches (the deepest part of the ocean) in 2013. Many amphipods are scavengers, so they're easy to catch with baited traps.

At the beginning of our training program, we actually had a very hard time catching amphipods. We put out plastic traps baited with raw fish on the seafloor under the ice in McMurdo Sound, but these tried-and-true methods weren't working. Eventually, someone suggested we put the traps at a range of depths, from the sea ice down to the seafloor, and when we checked back later, only the traps right up under the sea ice had caught amphipods - not what we had expected. The bottom of the sea ice can in many ways be considered an "inverted benthos" - like an upside-down seafloor, with lots of algae growing and animals feeding. There are amphipods in the Arctic that live right up under the sea ice for all or most of their life, but nobody in our group knew that Antarctic amphipods lived there too. It was an exciting discovery!

Another animal we caught in the baited traps were nemerteans, also known as ribbon worms. When my fellow trainees came back from the ice with the worms, they asked me to identify them, so I picked up the long, flexible, tube-like organisms in my hands. It only took a few minutes before my skin was covered in mucus, and I found at later that the nemertean's mucus is acidic, with a pH of 3.5. The mucus, plus the worms' size, flexibility, and the fact that they had been caught in a baited trap made me think at first that they were hagfish, but they didn't look quite right. As I turned over the brown worm, I could see a small opening on the underside near its head. I remembered that ribbon worms have a proboscis that shoots out to catch food, and the opening for the proboscis was on the underside of the head. The worm must be a nemertean!

Parborlasia corrugatus in the respirometer

Its scientific name is Parborlasia corrugatus, and it is a voracious predator. Parborlasia eats almost anything it can lay its proboscis on, including sponges, sea stars, and scallops. Photos from the seafloor near McMurdo show massive piles of the brown worms in feeding frenzies.

The thing that gets me about Parborlasia is its size. Most nemerteans are small, only 10s of centimeters long, usually 1 cm wide or less, detectable only be a trained eye. Parborlasia can grow up to 2 m - a clear example of polar gigantism. But like all nemerteans, it has no circulatory or respiratory system, relying on diffusion across its skin to supply its cells with oxygen. Usually only small animals can get away with diffusion, because if they're too big, there's not enough surface area of skin to supply the whole volume of the body with fresh oxygen. Some of us in the training program have measured the metabolic rate of Parborlasia, and so far, it looks like the metabolic rate is exceptionally low. With a low metabolic rate, the worm is less likely to run out of oxygen. What a fascinating creature!