There are something like 25,000 different species of fish in the sea, so the variety of life encountered by ichthyologists, or scientists who study fish, is enormous.There are certain characteristics that are common to every fish, like that they are vertebrates, they breathe oxygen from the water through their gills, and they swim. But even in such basic qualities, the diversity is evident. While most fish swim horizontally, a few, like the seahorse, swim while vertical; and while a majority of fish can only swim forward, there are some species of eel that can swim backward as well.
Beyond that, the diversity only increases. The greater percentage of fish reproduce by laying eggs, but sharks eggs hatch inside the mother, so sharks give birth to live young. When it comes to food, fish can be carnivores, omnivores, herbivores, or live off of plankton alone. The feeding cycles of these different fish do tend to depend on each other, so that the ecosystem they share is beneficial for all. An occasional difficulty with researching this is that the presence of the researcher can affect the behavior of the fish.
Many fish live in the thriving ecosystem of the coral reef, which has been compared to the tropical rain forests on land, since both brim with a vast number of species. Some fish swim back and forth between ocean water and fresh water, depending on whether they are spawning. Still others travel in schools and live in certain parts of the ocean, based on the temperature and salinity of the water, and the available food.
One current subject of interest is vocalizations of fish. Although the sounds that fish make can often be audible to the naked human ear, not much is known about the underwater acoustics of most fish species (whereas mammalian species like whale and dolphin have been studied for some time). This phenomenon may have often been ignored due to sound-proof aquariums. However, research is developing, and the various grunts, hums, or thumps that can come from the sonic muscle of a fish are now being studied.