Osteichthyes are a taxonomic grouping of bony fishes. This group includes ray-finned fishes (class: Actinopterygii) and lobe-finned fishes (class: Sarcopterygii). This highly diverse group of fishes, which contains almost all fish species, is the most diverse group of vertebrates today. Osteichthyes differ from chondrichthyes by (in most cases) possessing a bony skeleton, a swim bladder, scales (ctenoid, cycloid, or ganoid scales), and external fertilization.
Chondrichthyes are a taxonomic class of cartilaginous fishes that encompass sharks and rays (elasmobranchs) and chimaera. Though there are exceptions, in general, Chondrichthyes have exposed gills, no swim bladder, internal fertilization, and placoid denticles. These characteristics differentiate them from the more evolutionarily derived branch of fishes, bony fish (Osteichthyes).
Placoid denticles are found on sharks, rays, and chimaeras. Not really a true “scale,” like ctenoid or cycloid scales, placoid denticles are actually modified teeth. They have an inner tissue component, which contains both blood vessels and nerves, that is covered by a layer of dentin and an outer enamel. They form a tough protective skin layer for sharks, rays, and chimaeras and also have shown to reduce friction and drag so that these fish can swim more efficiently through water.
This series of small bones links the swim bladder to the inner ear in Ostariophysian fish, including minnows, carp, catfishes, and characins. When a sound wave hits the swim bladder, the vibration is transferred through the Weberian apparatus to the auditory region of the inner ear. This action helps amplify sound and, as a result, these fish have very sensitive hearing and can hear sounds that many other fish are in capable of hearing.
The swim bladder is an air filled organ used by some fish to maintain buoyancy at a desired depth and produce or hear sound.
- Physostomous swim bladders are directly connected to the gastrointestinal tract so that fish with these swim bladders, such as herrings, must “gulp” air to inflate their swim bladder and “burb” or “fart” air to deflate them.
- Physoclistous swim bladders are not connected to the digestive tract so that fish with these swim bladders must diffuse gas from the blood to fill and collapse them.
For many fish, the swim bladder has the additional role of transferring sound waves to the auditory system. And in some fish, such as drums and croakers, the swim bladder is used to make sounds and communicate with one another and other fish.
The lateral line is, literally, an observable line down both sides of a fish. It is also a sensory organ system that helps fish detect motion in the water around them. When water along the lateral line is displaced by movement or a vibration nearby, hair cells (similar in form and function to hair cells in a human ear) translate the displacement into an electrical impulse that is transmitted to the brain. The lateral line helps a fish orient itself upright in the water and relative to other fish (e.g., schooling fish or predators and prey).
Photophores are organs that are used by fish (and invertebrates) to produce light either by chemical reaction or through symbiotic bacteria capable of bioluminescence. Most fish that use photophores live in the deep sea where light from the surface is limited. Like a firefly in the sea, some of these fish use photophores to attract mates; others use photophores as counterillumination and camouflage; others use their photophores like search lights to find prey or avoid predators; and still others use photophores for multiple purposes. Splitfin Flashlight Fish (Anomalops katoptron), for example, use their photophores to communicate with other flashlight fish, attract prey, and confuse predators. They are believed to produce the brightest bioluminescence of any organism – their light can be seen from over 100 feet away!
Thermoregulation is the process by which an organism controls its internal temperature. Fish have many different mechanisms for regulating their temperature. Most fish are ectothermic, using their environmental temperature to manage their body temperature, but some fish are endothermic, having the metabolic ability to internally manage temperature. Poikilothermic fish are ectotherms which have no control over their body temperature; their core body temperature conforms to ambient temperature. Eurythermic fish have evolved to survive in a wide range of environmental temperatures and stenothermic fish have evolved to survive in a narrow range of environmental temperatures.
Thermoregulation is very important for fish because temperature influences the function of many organs and the rate of many metabolic processes. Most fish species have evolved to survive within a specific temperature ranges; outside that range, enzymes can degrade, organs can fail, and the organism can die. Understanding thermoregulation for fish species is particularly important when considering implications for climate change.
In contrast to stenotherms, eurythermic fish can function at a wide range of water temperatures. They are often, but not necessarily, ectotherms. Desert Pupfish (Cyprinodon macularius), for example, can function in ambient temperatures ranging from 4 to 45 degrees Celsius. This thermoregulatory strategy requires that organs, enzymes, and metabolic processes can operate at varying environmental temperatures.