Animating jawless mouth on sphere type creature problem

Hey guys.

I am trying to use blender for some UDK experimentation. For my 1st project, I simply want to create a UV sphere, one side horizontally split, the other side extruded to be a tail.

Now that you have a basic idea of what I’m doing here’s my problem.
The upper jaw remains motion less, since I don’t need to move that. However, the lower jaw has to open in a 90degree angle, using one bone only for rotation(Low_Jaw_Cont). Now when I attempt this you get the obvious problem that all effected vertices will move along with rotation.

However, I want them to “collapse” in on each other on the z axis only.

Now I tried adding a bone with a stretch constraint, but that basically stretches the “jaw” horizontally which is disastrous for the model. And I cant get it’s tail to follow the rotation control bone (Low_Jaw_Cont).

Any ideas on how to pull this off?

–EDIT–
Creating some pictures to fully display the problem

This pic below shows the basic look of the body. Topology isn’t all it could be if you aske me. But I am just looking for a solution atm.
http://s22.postimg.org/99gxhjbod/TP1.jpg
The pic below demonstrates the problem at hand. Rotating the what will be the Lower_Jaw_Control_Bone makes the jaw completely move into the body. Whereas I want it to basically shrink.
http://s16.postimg.org/biwosvjy9/TP2.jpg
Pic below I’m using a stretch constraint. Although I feel like I am getting closer, the mouth gets deformed somewhat awful.
http://s9.postimg.org/b5sj1fraz/TP3.jpg

using one bone only for rotation(Low_Jaw_Cont).

At minimum you will need two bones; one to locate and fix the head / body, and other to rotate the jaw. The vertex group associated with jawbone need to be assigned to jaw geometry vertexes only. Head / body bone vertexes is assigned to head / body bone.

I want to use just the one bone to animate the jaw. The model consists out of 4 tail segment bones, a body bone and a lower jaw bone in its current cleaned out state.

So as the stretch modifier isn’t transforming along the curve like I want it. I am now trying dividing the lower"jaw" up into segments per edge loop. Creating a 6 bone system. I am going to attempt to have the 4 segments rotate along with the controller bone, and setting up the 6th bone as a roadblock for the other bones.

—edit—
Can’t seem to find a constraint that will act as a road block, here’s a pic.
http://s23.postimg.org/84xfq762f/TP4.jpg
I want each bone to stop at the displayed location. Using only 1 bone to control the rotation.

For simple Jaw rig, you can get away with one jawbone and control the mouth deformation with Weigh Paint. It is probably a good idea to create loop geometry around the mouth so that mesh will deform nicely under control. The pivot point of jaw controls how mouth forms as it opens. I placed it behind where mouth splits. This will cause mouth to open wider as jaw drops. This is how human look with mouth open; maybe not for a fish though. Play with pivot point and weigh painting.

http://i1135.photobucket.com/albums/m626/cabby24/Blender%20Pics%202013/fishjaw_zpse5f8fffa.png

Ah them pics, well… On your model this seems to work out better. However, I made the mouth to be part of the UV Sphere and the transformation just looks awefull. BUT, I tried using morph targets(shape keys) to manually put the loops in the position I wanted them. This resulted in the outcome I wanted to achieve. But will result in me having a hell of a lot more problems getting the animations to run in UDK.

Heres a link to a vid I created showing of the animation I wanne accomplish using bones rather then shape keys

Problem could also be the aweful topology I made for this litte test subject