Thanks guys!
I did try again after posting these and found a way to get the better edge definition in the photos of the mantles that I was hoping for (the posted pics above have a certain "fuzzyness" to them and some of the finer details of the mantle, such as the little dots, don't show up as well as I'd have hoped). However in the next attempt, what I did was go with the smallest f-stop possible (to gain the largest depth of field) and that seemed to extend my focal range. Whereas before it's like a paper thin slice of the picture that's in correct focus. Of course at the smallest aperture, the shutter speed is open longer, so I had to step up the f-stop until the camera selected a shutter speed that I was able to hold the camera steady at (about 1/30s).
I was already shutting off the pumps and powerheads for the shot so that there is no current to move the mantle around which I think also helped.
This gave me results like this:
The image is possibly slightly underexposed, but that seems to allow better detail to come through. So I think I like it better slightly underexposed.
Christy, the crocea's are about 6" at full mantle extension. Shells are probably more like 4" end-to-end. The Squamosa is easily 12" at full expansion, and the shell is more like 9". The maxima (which you can only see peeking out between the two crocea's) is about 3" and maybe about 2" at the shell.