The Fish and Fish Diversity

As per Fish Base (a global information system about Fishes) November 2019 data there are about 35315 species.

• • HABITAT DIVERSITY: Fishes live in almost every type of aquatic habitat viz., fresh, brackish or marine, from the tropics to the Polar Regions. Many freshwater and marine species are also common in brackish water estuaries. About one-third of families have at least one species with individuals that spend at least part of their life in freshwater. About 12,000 species (43%) of all species, live exclusively in freshwater lakes and rivers that cover only 1 % of the earth’s surface and account for a little less than 0.01 % of its water (the mean depth of lakes is only a few meters). About 15,800 species live in oceans, which cover 70% of the earth’s surface and account for 97% of its water with a mean depth of about 3,700 m.

Fishes are found to live up to an elevation of 5,200 m in Tibet, where some Nemacheilines (Cobitidae) live in hot springs. In the world’s highest (3,812 m) Lake Titicaca (South America), a group of Cyprinodontids has undergone much radiation. Fishes also live in the world’s deepest Baikal Lake (at least 1,000 m) and up to 7,000 m below the surface of the ocean. A few species make short excursions onto land. Subterranean or hypogean fishes are found confined to total darkness of caves or other underground areas or in the fast torrential streams (= Hill Stream Fishes) of Tibet, China and India.

Oreochromis alcalicus or Alcolapia alcalica (the soda Tilapia; Order Perciformes or Cichliformes, Family Cichlidae) is endemic to hot soda Lake (hypersaline lake), Magadi (Kenya) at temperatures as high as 42.5°C. At the other extreme, Cod ice fishes, Trematomus (Order Perciformes, Fam. Nototheniidae) live at about -1°- 4°C under the Antarctic ice sheet, on account of producing antifreeze glycoproteins, preventing formation of ice crystals in blood (hence often called cryopelagic). Many species have acquired air-breathing organs, being essentially independent of water for respiration and live in stagnant, tropical swamps (especially Lungfishes).

Lake-dwelling species may show a preference for deep, cold, oligotrophic lakes or for shallower, warmer and more productive mesotrophic and eutrophic lakes. In lake waters they may show a preference for the open-water limnetic zone, the benthic zone or shallow littoral zones. Stream fishes may prefer riffle or quieter zones and a zonation of species is found from the headwaters to the mouth of a river.

Some fishes also inhabit deep-sea thermal vents in the eastern Pacific Ocean. In oceans, majority of fishes are coastal or littoral. The animals living in the deep sea live under extreme conditions of darkness, high pressure and temperature changes. The fish life of the deep-sea habitat is among the most specialized of any habitats in the world. The continental shelf at about 200 m depth has been considered as a boundary between ‘shallow’ and ‘deep sea’. Most diverse deep-sea assemblages occur between 40°N and 40°S latitudes. In general, any species that is found at extreme ocean depths, more than 500 m and even beyond 8,000 m, is defined as a ‘deep-sea fish’, characterized by elongated to dorso-ventrally flattened or eel-like body, large mouth, pointed fang-like teeth, enlarged or minute eyes, bioluminescence etc. The fish in the deep tend to be much smaller than on the surface, with minimal bone structure and more jelly-like flesh. They are, therefore, slower and less agile than fish living near the surface. They also tend to grow more slowly than surface fish. Some take many years to reach sexual maturity. These adaptive features are, obviously, the outcome of evolutionary adaptations to extreme pressure, cold and darkness and such similarities among unrelated fishes are due to convergent adaptations.

The upper 200 m of the open sea is known as Epipelagic or Euphotic zone. There are three major regions of ‘open water’ viz., Mesopelagic (200 – 1,000 m), Bathypelagic (1,000 – 4,000 m) and Benthopelagic or Abyssopelagic (4,000 – 6,000 m). The deep sea regions below 6,000 m are often referred as hadal depths. The bottom-associated or benthic species swim just above the bottom (= Benthopelagic) or live in contact with the bottom (= Benthic). More than 1,000 species inhabit open waters of the deep-sea and another 1,000 species are benthic, with fairly good representation of Cartilaginous and bony fishes.

• • DISTRIBUTIONAL DIVERSITY: In both fresh and marine waters, the largest number of species occurs in the tropics. The number is reduced towards Polar Regions. For the tropics on one hand, a large number of freshwater fishes are found in tropical Africa, southeastern Asia and the Amazon River, but on the other hand, because of the characteristic physiographic and geological history, relatively few freshwater species are found in Central America. Africa exhibits the greatest diversity of non-ostariophysan (i.e., lacking Weberian Ossicles) freshwater fishes but South America is poor on this basis. Contrary to it, in temperate regions, Eastern North America shows the greatest diversity of non-ostariophysan fishes. Most oceanic islands and continental areas recently exposed from the last ice age (e.g., northern regions of North America, Western Europe and Asia), lack indigenous fishes confined to freshwater.

As far as the marine environment is concerned, representatives of many marine genera and of some species occur in the temperate and polar fauna of both hemispheres. The vast majority of species, however, are tropical, most of the rest occurring only in the Northern or only in the Southern Hemisphere.

About 130 marine species are known to extend around the world in tropical or subtropical waters. The Indo-West Pacific (Red Sea and Indian Ocean to Northern Australia and Polynesia) is the richest, with most species occurring in New Guinea to Queensland. In terms of diversity, Southeastern Africa and Queensland have the largest number of marine shore fish. The West Indian or Caribbean fauna (southern Florida to northern Brazil) is also appreciably rich. The West African marine fauna, however, is relatively poor. Arctic and Antarctic faunas are depauperate i.e., lacking in numbers. In all, the greatest number of fish in the world inhabits the southeastern Asian region.

• • BIOGEOGRAPHICAL DIVERSITY: Biogeography is a new discipline that attempts to document and understand spatial patterns of biodiversity. It is actually a branch of ‘Physical Geography’, concerned with the present and past distribution of animals and plants along with the examination of physical environment affecting the distribution of a particular species across the world. In other words, the science of biogeography attempts to document the geographic distribution of taxa (i.e., descriptive biogeography) and to explain their distributional patterns (i.e., interpretive biogeography). It is an active field of study in Ichthyology, finding solutions to various problems related with the distribution of fishes.

There are two approaches to interpretive biogeography i.e., the ecological biogeography which involves various environmental factors (viz., temperature, pH, dissolved gases, turbidity, salinity, currents, competition etc.), limiting the distribution of a species within a water body; and historical biogeography which focuses on the origin of distributional patterns (i.e., paleontological studies or the study of fossils). Paleoclimatic changes are often invoked in historical biogeography, especially when postulating that discontinuous distributions result from dispersal events.

Dispersal and vicariant events are two methods of distribution of fishes. Dispersal, active or passive, is a movement to areas new to the existing population. Barriers of varying effectiveness may be involved. It is of greatest biogeographic significance if the breeding range of the species is increased. Vicariance, on the other hand, is a process by which the geographical range of an individual taxon or a whole biota is split into discontinuous parts by the formation of a physical or biotic barrier to gene flow or dispersal e.g., through tectonic plates. Both dispersal and vicariant approaches are used to explain disjunct or discontinuous distributions i.e., the occurrence of a taxon in different areas with a marked geographical gap between them e.g., Prosopium coulteri (Fam. Salmonidae) in western North America and in lake Superior; Geotria australis (Fam. Petromyzontidae) in Australia, New Zealand and South America etc. Further, Lungfishes found distantly in Africa, South America and Australia are potential examples of vicariance, suggesting that this group represents an ancient distribution limited to the Mesozoic supercontinent Gondwana and the fossil record suggesting that advanced lungfish had a widespread freshwater distribution and the current distribution of modern lungfish species reflects extinction of many lineages.