Complex sentence: Coconut crabs have organs known as "branchiostegal lungs", which are used instead of the vestigial gills for breathing and they will drown if immersed in water for long.
Split into simpler sentences: Coconut crabs have organs known as "branchiostegal lungs", which are used instead of the vestigial gills for breathing. They cannot swim, and will drown if immersed in water for long.