Looks like a horseshoe crab. Body leather like, it did not have a hard shell. I did not get a good picture of the underside but, it had reddish legs and gills underneath the main body. Length given was from head to end of tail. Many of these in a seasonal stream along the lava flow. I removed it from the water to take photo then returned it to the water. Not sure if this belongs in BG so, it can be frassed if deemed so.