Scarcely resembling a crustacean so much as some bloated, otherworldly maggot, Sarcotaces arcticus is virtually invisible when it swims up the backdoor of its preferred host, the rockfish. Attaching like a tick to the delicate membranes of the rectum, it begins to gorge on blood and releases an enzyme that causes the host's own body to grow a protective bag or "gall" around the intruder. A bag made of anus. Now impossible to dislodge, the parasite freely grows to around the size of a golf ball, which would be bad enough if the host were our size, but we're talking about a somewhat smaller fish here. Not only does the parasite make a sleeping bag out of rectum skin, it grows too large to ever fit back out either way. All the while, the parasite reproduces and releases its microscopic young, making it not only rude for the host to break wind at social gatherings, but positively terrifying.
Post by Jonathan Wojcik (of BogLeech fame - check out his cool parasite gear) and photo by Jonathan Martin (and if you're brave, check out the video that he took as well!).