February 24, 2003
Fish Evolution and Swimming
The "fish triangle" of functional morphology was reviewed, showing morphological differences among specialists for accelerating, manuevering, and cruising.
We continued with a video on evolutionary changes within fish, concentrating on the marine -> fresh water -> (ultimately) terrestrial transition within vertebrates. Featured in the video were three fish genera. The agnathan Pteraspis ventured into brackish water (salinity levels between normal sea water and fresh water), retaining salts with the help of a kidney and protective bony plates. The actinopterygian (ray-finned) osteichthyan Cheirolepis was adapted to fresh water and its unstable calcium supply with its extensive, bony internal skeleton. The sarcopterygian (lobe-finned) osteichthyan Eusthenopteron was even better adapted to rivers and streams, its bony fin axes aiding it in locomotion in shallow water. Eusthenopteron also possessed paired lungs which aided the fish in oxygen-poor freshwater.
Paired lungs (found in lungfish, osteolepiforms [including Eusthenopteron], and tetrapods [tetrapods, the terrestrial vertebrates, are modified sarcopterygian descendants]) are a modification of an air sac that was most likely present in the earliest osteichthyans. An air sac is primitively present in some living actinopterygians, and in unmodified form aids fish in two respects (direct uptake of air and increased buoyancy).
The nonrespiratory swim bladder of teleost fish (the predominant actinopterygian group today) also likely arose through modification of an existing air sac involving dorsal and posterior migration (i.e, it migrated toward the center) and reduction in volume. In open marine waters with good circulation, cool temperatures (warm water holds less dissolved oxygen), and subsequent reliable oxygen content, the improved buoyancy and stability offered by a swim bladder outweighs the benefits of an additional oxygen source.
In warm, tropical, restricted waters (freshwater and some nearshore marine environments), such as those favored by Eusthenopteron, the combination of warm water, restricted circulation, and organic input from the continents (bacterial decay of organic matter depletes oxygen further) likely caused the respiratory function to take priority.