The first ray-finned fishes apeared during the Devonian Period (400-350 MYA) and underwent significant expansion and diversification during the Carboniferous Period (360 MYA). The first actinopterygiians evolved in freshwater but they later expanded into marine habitats. During the Late Triassic a group of ray-finned fishes called 'teleosts' first appeared. Teleosts are today the most numerous group of fish species alive today, with an estimated 23,000 of the 24,000 species (Jonna, 2004).
Ray-finned fishes possess the following characteristics:
- The skeleton of ray-finned fishes is made up of true bone.
- The upper jaw of many actinopterygians is formed by two bones (the maxilla, and the premaxilla).
- The fins of ray-finned fishes consist of a set of bony spines that are covered with a thin layer of skin thus making actinopterygiians distinct from sarcopterygiians, a group of fish whose fins are instead fleshy lobes.
The two-boned structure of the upper jaw in actinopterygians has proven highly adaptable. This structure makes the upper jaw protusible (easily extended) and as a result, a wide range of feeding adaptations have developed in actinopterygiians.
Many actinopterygians face significant threats from humans including habitat destruction, over-exploitation through commercial fishing, and pollution of the waters in which they live.
Classification:
It is worth noting that actinopterygiians do not include the group of fish known as chondrichthyes, whose skeleton is made up of cartilage, not true bone. Chondricthyes or 'cartilaginous fishes' include sharks, skates, and rays. The fossil record indicates that the origin of the chondricthyes dates back further than that of the actinopterygiians to around 450 MYA.
Range and Habitat:
- Temperature: -1.8°C–40°C
- pH: 4–10
- Salinity: 0–90 ppm
- Depth: 0–7,000 m
- Oxygen Saturation: 0–saturation (Jonna: 2004)
Regions with high diversity of freshwater species include the Amazon River Basin and the rivers and wetlands of Southeast Asia. Marine species are most diverse around coral reef systems such as the Great Barrier Reef. Some species of ray-finned fishes are capable of migrating between fresh and salt water, and are therefore considered diadromous. Some species such as salmon make a migration from saltwater upstream to freshwater where they spawn. Other species such as freshwater eels migrate from freshwater to salt water to spawn (Jonna, 2004).
References:
- Burnie D, Wilson DE. 2001. Animal. London: Dorling Kindersley. 624 p.
- Jonna R. 2004. Actinopterygii, Animal Diversity Web. October 12, 2007.
