Speaking of penguins.. they are awesome. So are blue crabs.
When a mature male encounters a female that is about to molt to sexual maturity, the male will perform a rather elaborate courtship ritual, or dance, to get the female's attention. Upon initial contact, the male will stand up high on the tips of his walking legs. He will then wave his claws, stretching them out wide, extending them fully outwards, and will begin to fan pheromones (a chemical scent which attracts females) contained in his urine towards the female with his swimming paddles. Finally, he will snap his body backwards and kick up sand with both his swimming and walking legs. Should the female fail to respond, he will repeat the process again.
The female shows her interest by rocking and waving her claws in and out. She may or may not approach the male, turn around backwards, and attempt to wedge herself under him. Still waving her claws, the male responds by tapping and rubbing her claws with his. Soon she quiets down, tucking her claws into a submissive posture, and allows the male to clasp and carry her with his walking legs. In this position, the mated pair is called a "doubler" or a "buck and rider." She is right side up and facing forward. This is also known as a precopulatory embrace.
The male will "cradle carry" the female for 2 to 7 days until ecdysis (molting) is imminent. This serves two purposes. First, the male is able to protect the female from predators. Second, it assures that the male will be present during the brief time that the female's shell is soft and she's able to mate.
Once the female begins to molt, the male releases her and stands guard over her by making a cage with his walking legs. The molting process may take several hours to complete. This terminal molt marks the female's transition into sexual maturity, and is usually the last time she will shed.
During this transition, her abdomen changes from a "V" shape to a more rounded "U" shape. She is now called a "sook" and will only mate this one time in her life while her shell is still soft. The female must rest briefly to fill the voids of her new shell with water. Next, the male turns the her upside down so that their abdomens are touching, clasping her with his walking legs. This is known as a copulatory embrace.