If there is a big five, there should be a little five as well. Smaller, maybe lesser known species that still perform a vital role in the Antarctic ecosystem.
The first species that comes to mind is of course Krill. These shrimp-like creatures have a crucial role in the Antarctic food web. Nearly every species is somehow connected to this. One could further think of several species of fish, like silverfish and toothfish, that also form a significant role. But all of these are hard to find, as they live underwater and most people who come to Antarctica don’t dive (for a good reason).
A species I always like, and definitely not for its looks, is the Snowy Sheathbill. Everywhere where you find penguin colonies, you see these white, chicken-like birds. They specialise in eating penguin poo. Fortunately, there is plenty of that available in any penguin colony. Penguins are not extremely good at digesting Krill, so their poo still has quite some nutritional value, which is what the sheathbills use.
And like I said, they are not known for their good looks. The only thing on Antarctica that really beats a Snowy Sheathbill in ugliness? Indeed, a Snowy Sheathbill chick…