We’ve talked a lot in our posts about the macroalgae (seaweeds) here at Palmer Station. But as we’ve also noted, there are many benthic (bottom-dwelling) animals too.
Although there are a couple species of fish here that live near the bottom most of the time, all the truly benthic animals are invertebrates. The bones that make up your backbone are called vertebrae. The prefix “in” means “not” or “without.” So the name invertebrates means that these are animals without backbones. In the literal sense, they are spineless creatures!
At shallower depths where macroalgae dominate, benthic invertebrates other than the amphipods Maggie has posted about or the gastropods (snails and limpets) are relatively uncommon. But at deeper depths where the macroalgae begin to thin out because of a lack of sunlight, sessile invertebrates like sponges, tunicates, and soft corals begin to take over the bottom. “Sessile” means that just like the algae, these are organisms that are attached to one spot on the bottom and do not move. (Or at least, they cannot move much.)
The dominant sessile invertebrates here and, overall, throughout Antarctica are sponges. Sponges are pretty much the most primitive animals there are. In fact, although they have different kinds of cells specialized for different jobs, many people do not even think that they are so complex even to have true tissue-level organization. Or if so, only a couple different kinds. Tissues are groups of similar cells which work together for some function and which have a common origin in a developing, or embryonic, animal. Examples in your or me would be our muscles or nerves.
Sponges in Antarctica can be huge! I’ve seen them as tall as six or seven feet and bigger around than me. I’ve seen photos of them that are so big around that an adult person could actually sit inside one. At depths we can dive to here at Palmer they do not get that big, but we commonly see sponges that are a couple or more feet tall.
Another very common group of sessile invertebrates here are animals called tunicates. Tunicates can live as individuals or very tiny ones can live together in colonies. Like sponges, they eat tiny things living in the water and to do that they bring water inside them, filter the small organisms out, and then expel the water. Sponges usually have multiple water inlets and outlets. Tunicates have only one place where water comes in and another where it goes out per individual. These are called “siphons”. But a colonial form would have one set in each of its thousands of tiny individuals that make it up.
Tunicates in which each individual lives by itself are called “solitary.” Solitary tunicates have the common name “sea squirts” because with many of them, if you pick them up out of the water, they contract. The water that they have taken in to filter then comes squirting out of the animal though its out siphon. Many a new invertebrate zoology student has been asked “do you know why they call them sea squirts?” just as some smart aleck points the out siphon at him or her. It is the biological equivalent of a squirt gun!
Colonial tunicates do not make themselves into squirt guns. But they are very common here and an important component of the benthic communities. We find more of them than of the solitary tunicates.
You have read in our other posts about how we are here studying how the benthic organisms, both invertebrates and macroalgae, use chemical compounds that are called “secondary metabolites” or “natural products” in their ecological interactions with each other. Mostly, though not entirely, these studies look at how the chemicals can help defend the benthic organisms from things that might otherwise eat them. We team up on these studies with our colleagues from the University of South Florida lead by Dr. Bill Baker. Bill is here with us now and you’ve read about and seen photos of his two students, Alan Maschek and Jason Cuce, who have been here with us all season.
Although the invertebrates and algae make these secondary metabolites for their own purposes, they can be helpful to people too. About 60% of all pharmaceutical compounds (medicines) are based on natural products. When we discover new secondary metabolites through our ecological studies, we make them available to medical researchers to test for potential medical uses.
One of the colonial tunicates we study, which has the scientific name Synoicum adareanum, makes a set of secondary metabolites that we have named “Palmerolides” after Palmer Station. As with the other chemical compounds we have discovered, we sent these to the National Cancer Institute for medical screening. And guess what? They came back as excellent candidates for the treatment of melanoma, which is the deadliest form of skin cancer! There is a great deal of work to do to get them from where they are now (testing in mice) to treatments for human cancer, but it is exciting that our ecological studies might someday have such a direct benefit for people stricken with cancer.
There are many other kinds of invertebrates here: soft corals, bryozoans (sometimes called “moss animals”), beautiful snails without shells called nudibranchs, very important sea stars that are a top predator, and the list goes on and on. Perhaps we’ll be able to come back later on with a post about those. For the time being, though, you can see video of many of those as well as sponges and tunicates here at Palmer Station by clicking on the YouTube link below the photos on the right and looking at our video entitled “Invertebrates at Palmer Station.”
Remember… If ever you are by the seashore and someone asks you “do you know why they call them sea squirts?” say “yes!” and prepare to defend yourself from a biological squirt gun!