Beneath the placid antarctic pack ice of McMurdo Sound lives a highly competitive marine community. The sea floor is home to sponges and tunicates; above them in the water column swim fishes and a variety of plankton - including small molluscs and crustaceans and the larvae of a number of species. It was long thought that the sessil sponges and tiny plankton had few predators, but that is not the case. Sponges are routinely preyed on by sea stars, and plankton make a tasty meal for fish. Still, with the right tactics, even the sluggish and sessile, the small and toothless can defend themselves. Many of these seemingly helpless species have evolved potent chemical defenses that give tham a flighting chance against the Goliaths of the antarctic seas. Some species that have not developed chemical weaponds have learned to take advantage of other creatures that have. (Drawings are not done to scale). Names species are mentioned in this article; a key to the numbered species is on page 263.