The external anatomy of female and male seahorses differ, a characteristic that is called sexual dimorphism. On the female seahorse, the lower abdomen joins the tail at a sharp angle and her anal fin is often higher and slightly larger. On males, a brood pouch is found beneath the anal fin, and when empty, tapers gradually to the tail.

With a head that looks like a horse, a bumpy tummy and a spiral tail, the common seahorse reaches a maximum length of 12 inches (30 cm). Males are larger than females. They come in a variety of colors, but the males are usually a bit grayer with dark spots. The females often have some yellow and dark spots on their bodies.