So my new fortress started up in a wilderness biome next to a river. I started getting a few messages that dwarves with fishing or hygiene on their mind were being interrupted by a giant sponge. On investigation I found it anchored 2 tiles into the river. What to do?
Plan A. Ignore it. It's a sessile creature without nervous, circulatory or digestive systems and no limbs. Also made of sponge. Apart from surprising the more nervous dwarves what harm can it do? This plan lasted a few seasons until, in a sudden fit of homicidal rage, the sponge slaughtered 3 dwarves in rapid succession with mighty blows of its <not exactly sure>·
Plan B. Send the militia. I've got some competent axe and hammer dwarves in a squad. They won't accept a kill order as they can't reach the target but stationing them on the river bank soon gets a fight going. The axe dwarf manages to hack into the sponge a few times before joining his squadmates at the bottom of the river. The sponge appears unharmed. This makes sense as there are some species of sponge that you can force through a mesh until they're a puddle of disconnected cells which, if left to their own devices, will re-assemble themselves back into sponge.
Plan C. Wall off that section of the river until I strike magma at which point Spongebob is going to get a surprise. Unless I can institute a sponge breeding program and create a moat filled with sponges around the fortress. That should give the gobbos something to think about.
So props to whoever sponsored the sponge's inclusion in DF. A pretty boring creature IRL becomes a source of endless fun.