Fish are aquatic craniate animals that lack limbs and can survive only in water. The exact number of fish species worldwide is unknown, as it is believed that some species have not yet been discovered. However, scientists estimate that the total number of fish species in the world is approximately 33,600. Different species of fish exist around the globe and have adapted to living in diverse aquatic environments, such as rocky shores, kelp forests, deep-sea waters, rivers, streams, and ponds.
The first species to be classified as fish were soft-bodied chordates during the Cambrian Period. These species were not full vertebrates, as they lacked a true spine, but had notochords that enabled them to be more agile than other invertebrates. A majority of the features that characterize fish are believed to have developed during the Paleozoic Era, which was one of the earliest of the three geological eras of the Phanerozoic Eon, as well as the longest era in the eon. During this time, fish developed into different forms, and the majority of fish species developed an external armor that provided protection from predators. The first species of fish that had jaws that appeared later, during the Silurian period. Over time, various species evolved to become the different types of fish known today.
Species of Fish
The majority of the world's fish species (approximately 70%) live in the Pacific Ocean, which is the largest ocean and has the highest number of coral reefs. In fact, the Pacific Ocean has about twice as many coral species as the Atlantic Ocean, which is the world's second largest ocean as well as the saltiest. Approximately 20% of the world's fish come from the Atlantic Ocean, which ranks second, while 8% comes from the Indian Ocean, which is the warmest ocean and therefore has low concentrations of oxygen. China is the world's leader in terms of fish production, harvesting approximately 58.68 metric tons of fish each year. In addition to being the largest producer, China is also the leading consumer of fish.
Characteristics of Fish
The majority of fish species are ectothermic, meaning they cannot regulate their internal body temperature without the aid of an external source. Fish are also able to communicate through acoustic communication, which involves the transfer of an acoustic signal from one species to another in underwater environments. Most fish species breathe through gills and their bodies are also covered by scales.