One of the fascinating groups of marine invertebrates that we study here in Antarctica are the marine sponges (yes, there is also a small group of freshwater sponges). Sponges are ancestrally primitive animals (first considered plants, Aristotle identified their animal nature) that while simple in their construction, have a number of tricks up their sleeves (well actually, up their ostia, the little pores that lead to channels that radiate through their bodies bringing food and fresh sea water for respiration). Sponges are so basic in their design that there is some argument over whether their different cells can even be described as being organized in to discrete tissues. They come in a remarkable variety of sizes, colors and shapes, but all are immobile and live on the sea floor.
They do not have protective shells to live inside, but rather depend on tiny little skeletal elements called spicules to give their bodies rigidity. Filter feeding is accomplished using specialized feeding cells called choanocytes (collared cells) that have a central beating flagella (a little whip-like tail) ringed by small filaments. The flagella beats, creating a water current, and as the water flows past the ring of filaments, little particles of food such as phyto (plant) and zoo (animal) plankton are captured. The nutrition garnered by the choanocytes is then shared with the other cells in the sponge body.
Because sponges cannot move and lack a hard outer protective shell, they are highly susceptible to marine predators such as fish, turtles, and invertebrates. A number of these predators are “spongivorous” making their living eating only sponges. Thus, it is not surprising that sponges have evolved a wide suite of defensive chemicals to deter predators from eating them. They also use their defensive chemicals to keep the offspring of small plants and animals (fouling organisms) from settling on to their outer surfaces and smothering them.
Finally, in a sort of chemical warfare, sponges engage in chemical defenses that prevent other species of sponges and marine invertebrates from growing over their bodies and taking away one of their most precious resources, space to grow! The toxic defensive chemicals that sponges produce have the potential to fight a host of human diseases including cancer, AIDS, tuberculosis, bacterial infections and cystic fibrosis. Today there are no less that five drugs, including several that fight cancer, under development by the National Institute of Health that originate from sponges.
Antarctic sponges have some very unique attributes. First off, they are remarkably diverse. Indeed, while not as diverse as tropical sponges, there are more species of Antarctic sponges than there are in many temperate seas. This high diversity is probably linked to the very old age of the Antarctic marine environment. It was some 25 million years ago that Antarctica finally separated from South America. It then became further isolated by the development of the Antarctic Circumpolar Current, the world’s largest current that sweeps around the entire continent in a clockwise direction. This isolation allowed Antarctic sponges plenty of time to evolve into many different species.
There are also some Antarctic sponges that are related to deep sea sponge species or those from southern South America. Antarctica is also unique in its very high abundance of a small group of sponges known as “glass sponges” (sponges whose skeletons are made of siliceous skeletal elements). Some of these glass sponges are huge, vase shaped, animals, so large that even a Scuba Diver can fit inside their cup shaped body! These sponges are probably extremely old as studies have shown that even over a ten year period there is no detectable growth. They are likely the “redwoods” of the ocean!
Our studies and others have shown that Antarctic sponges are also vulnerable to predators. While there are no fish or turtle predators of sponges in Antarctica, there are plenty of sea stars that enjoy making a meal of sponges. Early investigators predicted that polar sponges might lack chemical defenses because of the lack of conspicuous fish predators. Nonetheless, our studies have shown that you don’t need fish predators to need chemical defenses. Just like their temperate and especially their tropical relatives, Antarctic sponges employ a variety of chemicals to defend themselves from sea stars, fouling plants and animals, and probably overgrowth by other marine invertebrates.
Importantly, some of the chemicals made by sponges in Antarctica have been shown to have the potential for drug development. For example, the Antarctic red sponge Kirkpatrickia variolosa harbors chemical compounds that have bioactivity against cancer, and is undergoing further study to explore its drug potential.
Another interesting thing about Antarctic sponges we have noticed revolves around those species that are pigmented. Sponges in Antarctica have no business being brightly colored. The possible reasons for being colored include “warning coloration” (a means of scaring away potential predators who associate the color with toxic chemicals) and protection against ultraviolet radiation (pigment UV screens). In Antarctica there are no visual predators of sponges (sea stars have no eyes), nor at the depths that sponges occur are there harmful levels of UV radiation.
Why then, are there some common Antarctic sponges that remain colored? We hypothesized that the pigments of these sponges were in and of themselves defensive compounds that helped protect the sponges, and therefore even after 25 million years they continue to play a useful role. We tested our hypothesis by examining the role of pigments in four colorful Antarctic sponges. Just as we suspected, in all cases the pigments played a role in defending the sponges from predators or fouling organisms.
In one interesting twist, we found that a sponge pigment mimicked a chemical involved in regulating molting (exoskeleton shedding) of a small crab-like animal called an amphipod known to prey on sponges. When the amphipod eats the sponge, this chemical pigment prevents molting and the amphipod eventually dies. This is the first example of such a relationship in the world’s seas, although many land plants make molt-interfering compounds to keep herbivorous insects from eating them.
In summary, sponges, while simple in design can be remarkably complex. Our research team continues to be amazed at the many secrets they harbor, and are confident that our studies of Antarctic sponges will continue to provide important information that paints a new view about very old animals.