How do I use dyntopo and mirror together? Blender newbie here!

I made an animal following the method used in this youtube tutorial: https://www.youtube.com/watch?v=JjW6r10Mlqs

I now want to make it more detailed, so I tried sculpting on it. To get detail, I turned dyntopo on, but that turned the mirror modifier off. When I turned dyntopo off after I did some sculpting, and turned mirror back on, I noticed some holes formed around the 'seams'/ the edge where the mirrored sides meet.

Am I going about adding detail to the low poly creature in the right way? How can I fix these holes? Is there a way to keep mirror and dyntopo both on at the same time?

Thanks in advance!
You cannot effectively use the mirror modifier with dynamic typology (or sculpting in general, for that matter). 

Remove the modifier and then under the Dyntopo settings in a sculpt mode, ensure the "Direction" setting is correct and click "Symmetrize". You may get some pinching down the mirror axis, but this can be smoothed out manually.

After that, go to "Symmetry/Lock" in the same panel and select the correct mirror axis.