I think the side depends on the species of flounder. Lying flat makes it less vulnerable to predators. One bizarre feature, the eye on the side it lies on migrates to the top side, and its skill looks bizarre. read more
In its life cycle, an adult flounder has two eyes situated on one side of its head, while at hatching one eye is located on each side of its head. One eye migrates to the other side of the body through a process of metamorphosis as it grows from larval to juvenile stage. read more
Viewed from above, plaice and sole have an essentially oval profile while halibut and flounder look more diamond shaped. Members of this group are almost exclusively found in the northern hemisphere. *The California Halibut is apparently classed as a “Left-eye Flounder” aka “Sand Flounder” (family Paralichthyidae). read more