Myosepta is the segment in between the myotomes (muscle segments) of a fish. It is the site where the myotomes attach, and is composed of collagen, which is the main connective tissue in fishes.
The collagen is arranged in a grid-like formation which gives it the ability to get bent laterally but not longitudinally. This gives the fish the ability to move its body from side to side. Because the collagen is a fairly rigid substance, lateral bended collagen will tend to get unbent. This action is thought to lead to the fish having to spend less energy moving to the other side, because of the drag from the collagen.