Something piled a bunch of clam shells on the swim ladder at the stern of my boat. The water is brackish, there are otter, nutria, beaver and muskrat. Do river otters pile up clam shell in a spot? I have seen other piles of clam shell under overhangs, the clams are about 2-3"

Otters and muskrats will both make shell middens. They also typically have a set locations where they make them as you can take all of the shells away, come back in a few weeks, and another pile will be there. Generally if the shells are laying open, muskrats are the culprit. If the back end of the shell is broken off or crushed, otters were likely responsible.

Coons will also crunch open the back end but usually only do that when otters or muskrats pile live animals up into a midden before eating them.

Around here It would be done by rats.

Muskrats do it here too but when I first read the title I thought you were asking about fish houses.

It's muskrats.

The clam half shells are whole like they were opened with a clam knife. I thought at first somebody was playing a prank except there are no people here! Thanks for the replies ..... added new words to my trapping vocabulary - "shell midden"

Muskrats definitely.

I have seen piles of shells like that which wouldn't fit in a 55 gallon drum. It was muskrats without question.