This is the northern clingfish (Gobiesox maeandricus). It's a fish common to the Pacific coast of North America. It's called a clingfish because it has a large sucker on the bottom of its body. This sucker can attach firmly to rough, slimy surfaces and withstand a lot of pressure. The image above shows a 0.8 ounce clingfish attached to a 6 pound rock. Under the right conditions, a nothern clingfish can pull 150 times its own bodyweight.
The sucker consists the pectoral and pelvic fins of the clingfish, which are fused together. They're roughly textured on the edges with microscopic hairs which provide friction between the fish and the object it's attached to.
The northern clingfish puts its sucker to good use. It uses it to stick to rocks in rough tidal areas and to attack mollusks.
Dr. Adam Summers, a professor of biomechanics at the University of Washington, says that this type of sucker would not be useful in dry conditions. But wherever it's wet, it's perfect. Someday, humans may be able to manufacture suction cups based on that of the northern clingfish. These would be ideal for climbing in wet walls, such as up a waterfall.