|Bathynomus giganteus, the largest of the isopods, is quite startlingly large.|
|B. giganteus next to a scale (in cm)|
Here is a video of these strange creatures scavenging a tuna, narrated by the ever-great David Attenborough. Begins at 0:30.
Sources are Biology of the Invertebrates, Seapics.com, and Daily Mail. Images are from Wikimedia Commons and are copyright free: one, two.