LECTURES FOR ZOO 1010—CHAPTER 16
BASED ON HICKMAN ET AL. ANIMAL DIVERSITY
Chapter Prologue—What is a Fish?: A fish is a gill-breathing, ectothermic, aquatic vertebrate that possesses fins, and skin that is usually covered with scales. However, fishes do not comprise a monophyletic group. The common ancestor of fishes is also an ancestor to land vertebrates. The fishes constitute the most successful group of vertebrates, with some 24, 600 living fishes having been described, more than all other vertebrates combined.
Fishes are supremely adapted to life in an aquatic medium. In this medium, 800 times more dense than air, fishes can remain motionless, varying neutral buoyancy by adding or removing air from the swim bladder. Fins are used for propulsion, braking, and as tilting rudders. Body fluid composition in marine and freshwater environments is regulated by organs for salt and water exchange. Gills are the most efficient respiratory organs know, able to extract oxygen through a countercurrent system from a medium containing less than 1/20 as much oxygen as air. Fishes have excellent visual and olfactory systems, as well as a lateral line system that is sensitive to water currents and vibrations. Thus, early fishes evolved a basic body plan and set of physiological strategies that both shaped and constrained the evolution of their descendents.
Ancestry and Relationships of Major Groups of Fishes:
Fishes are of ancient ancestry. The earliest fishlike vertebrates were a paraphyletic group of jawless or agnathan fishes, the ostracoderms. One lineage of ostracoderms gave rise to the gnathostomes. The agnathans include the extinct ostracoderms and the living lampreys and hagfishes. Lampreys and hagfishes have an uncertain ancestry and are placed in separate classes.
The remaining fishes have paired appendages and jaws and with the tetrapods comprise the monophyletic group, the gnathostomes. An early group of gnathostomes, the extinct placoderms left no descendents. The cartilaginous fishes (class Chondrichthyes) lost the heavy dermal armor of the early jawed fishes and evolved a cartilaginous skeleton. The bony fishes (class Osteichthyes) have the most extensive radiation and are the dominant fishes today. There are two distinct osteichthyan lineages, the lobe-finned fishes (sister group of tetrapods) and the ray-finned fishes (modern bony fishes). The lobe-finned fishes include the living lung fishes and the coelacanth.
Superclass Agnatha: Jawless Fishes:
The jawless hagfishes (class Myxini) and lampreys (class Cephalaspidomorphi) are remnant groups having an eel-like body form without paired appendages, a cartilaginous skeleton (with no vertebrae in hagfishes and rudimentary ones in lampreys), a persistent notochord, and a disclike mouth adapted for sucking or biting.
Cartilaginous Fishes: Class Chondrichthyes:
Members of the class Chondrichthyes are a compact group having a cartilaginous skeleton (an apomorphy), paired fins, excellent sensory equipment, and an active, characteristically predaceous habit. The group includes the sharks, rays, and chimaeras).
Bony Fishes: The Osteichthyes—Origin, Evolution, and Diversity:
Bony fishes of the class Osteichthyes may be divided into two groups. One is the class Sarcopterygii or lobe-finned fishes, represented today by lungfishes and the coelocanth. Tetrapods arose from within one lineage of this group. The other group is the class Actinopterygii or the ray-finned fishes, which is a very large and diverse group of modern fishes, containing most freshwater and marine fishes.
Structural and Functional Adaptations of Fishes:
Modern bony fishes or teleosts have radiated into approximately 23,600 species revealing an enormous array of adaptations, body forms, behaviors, and habitat preferences. Most fishes swim by undulatory contractions of their body muscles, which generate thrust and lateral force. Flexible fishes oscillate the whole body, but in more rapid swimmers the undulations are limited to the caudal region or tail fin alone.
Most pelagic bony fishes achieve neutral buoyancy in water using a gas-filled swim bladder, the most effective gas-secreting device known in the animal kingdom. Gills of fishes, having efficient countercurrent flow between water and blood, facilitate high rates of oxygen exchange.
Many fishes are migratory to some extent, and some, such as freshwater eels and anadromous salmon, make remarkable migrations of great length and precision. Fishes reveal an extraordinary range of sexual reproductive strategies. Most fishes are oviparous, but ovoviviparous and viviparous fishes are not uncommon. Reproductive investment may be in large numbers of germ cells with low survival (many marine fishes) or in fewer germ cells with greater parental care for better survival (many freshwater fishes).