Gobies

Although some species are moderately elongated, Gobies are usually very small, compact fish. The smallest vertebrate in the world is, in fact, a goby known as Trimmatom nanus which lives off the Philippine islands. This goby never grows larger than 8-10 mm long. Two other gobies living in the Philippines—the Pandaka pygmaea and the Mistichthys luzonensis—are among the shortest freshwater fishes in the world; the females of these species mature at 10-11 mm long. While most of them get no bigger than 4 in (10 cm) long, the largest range up to 19.5 in (50 cm).

One of the most unusual traits of True Gobies is the "suction cup" located on their undersides near their pelvic areas. Their pelvic bones are fused with each other; thus, their pelvic fins are united, at least at the base. In True Gobies, the fin is connected by a thin membrane which enables the suction cup to create a vacuum; gobies can use this vacuum to gain a firm hold on objects. This suction cup exists in many different variations. In some species, the pelvic fins are completely connected by a membrane; in others, the fins are partially or completely separated.

Gobies are also characterized by the presence of a two-part dorsal fin, a fin located on their backs. The first part of the dorsal fin can have up to eight unbranched rays, although sometimes these rays are completely absent. Gobies usually, but not always, have some scales; these scales are sometimes present only in specific parts of their bodies. Their mouths are usually located at the very tip of their bodies and often protrude from their faces. Their jaws contain powerful teeth which are well suited for eating meat.

In general, gobies have developed in quite diverse ways during the course of their evolution. While they usually live in salt water, they are often found in brackish water, and sometimes even freshwater. In fact, gobies are often the most plentiful fish in freshwater on oceanic islands. A few species even live in rivers in the mountains. They have adapted to live in widely varying habitats, living, for example, inside sponges and on land.