Part of mesh goes outside Hello Blender community :)

So this is a shark model, and one of the side fins and upper fin gets separated from the main mesh of the shark. Same thing goes for the teeth of the shark. This model was made in 3DS and then exported to Blender. When I look at the contents menu on the right, I see that the mesh model is one object, so why does it get separated? I've uploaded the blender file here:



Thank you all again for all your help! :)
The issue is your topology. In Blender, everything must be one piece, and your fin and teeth are separate from the body even though they appear to be together. To prove this, select one vertex of the body mesh, key Ctrl + L, then G and move your cursor. Watch as it separates.



Normally, W > Remove Doubles would help, but your mesh is too fragmented.

A noticeable seam that is too big for Remove Doubles:



What I have done is deleted several faces around the pieces in question and joined manually using F to create faces. I have also used Alt + M > At Center to join some suspicious vertices. Lastly, I touched up the weight painting. It's not perfect, especially in the mouth area, but it's a lot better.