29.2 Fishes
3 min read•june 14, 2024
Fish are fascinating aquatic vertebrates with diverse adaptations. From primitive jawless species to advanced jawed fishes, they've evolved unique features for life underwater. Their respiratory, circulatory, and osmoregulatory systems are finely tuned for aquatic environments.
Sharks and rays, with cartilaginous skeletons, have keen senses for hunting. Bony fishes, including ray-finned and lobe-finned species, show remarkable diversity. Their adaptations, like swim bladders and specialized , have allowed them to thrive in various aquatic habitats.
Jawless and Jawed Fishes
Jawless vs jawed fishes
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- Jawless fishes () lack jaws and paired fins, are primitive vertebrates with a cartilaginous skeleton, and the persists throughout life without being replaced by a vertebral column ( and lampreys)
- Jawless fishes lack and represent the earliest vertebrates in evolutionary history
- Jawed fishes () possess jaws and paired fins, are more advanced than jawless fishes, have a bony or cartilaginous skeleton, and the notochord is replaced by a vertebral column (sharks, rays, and bony fishes)
- Most jawed fish species have scales and the evolution of jaws allowed for diversification of feeding strategies and increased success compared to jawless fishes
Sharks, Rays, and Bony Fishes
Characteristics of sharks and rays
- Sharks and rays () have a cartilaginous skeleton and that are tooth-like in structure
- Sensory systems of sharks and rays include:
- that detect electrical fields produced by prey
- system that detects water movement and pressure changes for navigation and hunting
- Keen sense of smell for tracking prey over long distances
- Reproductive strategies of sharks and rays involve internal fertilization and either:
- where eggs are laid and develop externally (skates and some shark species)
- where live birth occurs after internal development (most sharks)
- where eggs hatch internally and live birth occurs (some sharks and rays)
- Sharks and rays have low fecundity, slow maturation, and long gestation periods compared to bony fishes
Adaptations of bony fishes
- Bony fishes () have a skeleton composed of bone rather than cartilage
- Many bony fishes possess a , a gas-filled organ that aids in buoyancy control
- Some bottom-dwelling species lack a swim bladder (flatfishes and some eels)
- In some species, the swim bladder functions as an accessory respiratory organ (lungfishes and gouramis)
- () have thin, bony rays supporting their fins and are a highly diverse group that includes most common fish species (salmon, tuna, and perch)
- () have fleshy, muscular fins supported by bones and include:
- Coelacanths, ancient fish once thought to be extinct until rediscovered in 1938
- Lungfishes, which have and a highly vascularized swim bladder that functions as a lung
- Lobe-finned fishes are evolutionarily significant as they gave rise to , the first four-legged vertebrates to transition onto land
Fish Physiology and Adaptations
Respiratory and circulatory systems
- Fish use for gas exchange, which are highly efficient due to
- Gills are composed of numerous filaments with lamellae, increasing surface area for oxygen absorption
- Blood flows through capillaries in the lamellae in the opposite direction of water flow, maximizing oxygen uptake
Osmoregulation and excretion
- Fish maintain osmotic balance through , which differs between freshwater and marine species
- Freshwater fish actively uptake ions through their gills and excrete dilute urine to counteract water influx
- Marine fish drink seawater and excrete concentrated urine to counteract water loss
Thermoregulation
- Most fish are ectothermic, relying on environmental temperature to regulate their body temperature
- Some species, like tuna and certain sharks, exhibit regional endothermy to maintain higher body temperatures in specific organs
External features and locomotion
- Fish possess fins for propulsion, stability, and maneuvering in water
- Scales cover the body of most fish species, providing protection and reducing drag during swimming
Key Terms to Review (32)
Actinopterygii: Actinopterygii, also known as ray-finned fishes, is a diverse class of bony fishes characterized by the presence of rays in their fins, which are supported by dermal bones. This group includes the majority of fish species, ranging from tiny guppies to large sturgeons, and plays a crucial role in aquatic ecosystems as both predators and prey. Their evolutionary adaptations allow them to thrive in various environments, contributing significantly to marine and freshwater biodiversity.
Agnatha: Agnatha refers to a superclass of jawless fish that includes modern lampreys and hagfish. These ancient aquatic vertebrates are characterized by their lack of jaws and paired fins, which distinguishes them from other fish species. Agnatha plays a vital role in the evolution of vertebrates, showcasing some of the earliest adaptations to a life in water.
Ampullae of Lorenzini: Ampullae of Lorenzini are specialized electroreceptors found in cartilaginous fishes like sharks and rays. These sensory organs allow these fishes to detect electric fields produced by other organisms, which is crucial for hunting and navigation in their aquatic environment.
Chondrichthyes: Chondrichthyes are a class of fish characterized by their cartilaginous skeletons, which differentiates them from bony fishes. This group includes sharks, rays, and skates, and they are known for their unique adaptations such as acute sensory systems and powerful jaws. These features make Chondrichthyes efficient predators in marine environments.
Coelacanth: The coelacanth is a rare deep-sea fish that belongs to the order Actinistia, and it is known for its unique lobed pectoral fins and distinctive body structure. Once thought to be extinct for about 66 million years, the discovery of a living specimen in 1938 off the coast of South Africa reshaped our understanding of vertebrate evolution and the history of fishes.
Countercurrent gas exchange: Countercurrent gas exchange is a biological mechanism that maximizes the efficiency of oxygen uptake in aquatic organisms, particularly in fishes. This process occurs in the gills where water flows in one direction while blood flows in the opposite direction, creating a gradient that facilitates the transfer of oxygen from water to blood. This system allows fishes to extract a greater amount of oxygen from the water compared to a concurrent flow system.
Ectothermy: Ectothermy refers to the physiological condition where an organism relies on external environmental sources to regulate its body temperature, as opposed to generating heat internally. This adaptation allows ectothermic animals, such as many reptiles and fish, to conserve energy and survive in varied habitats, affecting their behavior, physiology, and ecological interactions.
Fins: Fins are specialized appendages found on fish and other aquatic animals that aid in locomotion, balance, and steering in water. These structures play a critical role in the movement of fish, allowing them to navigate their environment effectively while also helping to maintain stability and control during swimming. The variety and structure of fins contribute significantly to the diversity of fish species and their adaptations to different aquatic habitats.
Gills: Gills are specialized respiratory organs found in many aquatic organisms that facilitate gas exchange by extracting oxygen from water. They consist of thin filaments rich in blood vessels, maximizing surface area for efficient oxygen uptake and carbon dioxide release.
Gills: Gills are specialized respiratory organs found in many aquatic animals that allow for the exchange of gases, primarily oxygen and carbon dioxide, with the surrounding water. They play a crucial role in the survival of fish and some amphibians, as they facilitate breathing underwater by extracting dissolved oxygen from water as it flows over them.
Gnathostomata: Gnathostomata refers to a major clade of animals that possess jaws, which distinguishes them from jawless vertebrates. This group includes a vast diversity of species, including fishes and tetrapods, and is characterized by features such as paired fins and complex sensory systems.
Gnathostomes: Gnathostomes are jawed vertebrates, encompassing a broad group including fishes and tetrapods. They evolved from jawless ancestors and are characterized by the presence of true jaws formed from gill arches.
Hagfish: Hagfish are a unique group of jawless fish known for their eel-like appearance and slimy bodies. These fascinating creatures play an essential role in marine ecosystems as scavengers, feeding on dead or dying fish by burrowing into them. Their distinct characteristics, such as the ability to produce copious amounts of slime, help them escape predators and adapt to their environment.
Lamprey: Lampreys are a group of jawless fish known for their elongated bodies and distinctive sucker-like mouths. They belong to the class Petromyzontiformes and are considered some of the most primitive vertebrates, showcasing unique features that link them to the early evolution of fishes.
Lateral line: The lateral line is a specialized sensory system found in many fish and some amphibians that detects vibrations and movement in the water. This system consists of a series of fluid-filled canals and sensory cells called neuromasts, which allow aquatic animals to perceive changes in water pressure, currents, and nearby objects, helping them navigate their environment and avoid predators.
Lobe-finned fishes: Lobe-finned fishes are a group of bony fish characterized by their fleshy, lobed fins, which are connected to the body by a single bone structure. This unique adaptation allows for more complex movement in shallow water environments and has evolutionary significance, as it is believed that lobe-finned fishes are the ancestors of all tetrapods, including amphibians, reptiles, birds, and mammals.
Lungfish: Lungfish are a group of freshwater fish known for their ability to breathe air using lung-like structures, allowing them to survive in oxygen-poor environments. These fascinating creatures represent an evolutionary link between fish and tetrapods, showcasing adaptations that enabled the colonization of land by vertebrates.
Myxini: Myxini, commonly known as hagfishes, are jawless marine vertebrates. They are characterized by their elongated, eel-like bodies and their ability to produce copious amounts of slime as a defense mechanism.
Notochord: The notochord is a flexible, rod-like structure found in the embryos of all chordates, serving as a primary support structure that defines the body's axis. It plays a crucial role in the development and organization of the vertebrate body plan, influencing the formation of the spine and other skeletal structures.
Osmoregulation: Osmoregulation is the process by which organisms maintain the balance of water and salts in their bodies to ensure proper cellular function. It involves various mechanisms to control osmotic pressure, preventing either excessive uptake or loss of water.
Osmoregulation: Osmoregulation is the process by which organisms maintain the balance of water and solutes in their bodies to ensure proper physiological function. This is crucial for survival as it helps organisms adapt to various environments, whether they are aquatic or terrestrial, by regulating internal conditions despite external changes.
Osteichthyes: Osteichthyes, commonly known as bony fish, is a diverse class of fish characterized by their bony skeletons, which differentiate them from cartilaginous fish like sharks and rays. This group includes a wide variety of species, ranging from the familiar goldfish to the enormous ocean sunfish, showcasing significant adaptations that enable them to thrive in diverse aquatic environments.
Oviparity: Oviparity refers to the reproductive strategy where animals lay eggs that develop outside the mother's body. This method of reproduction is commonly seen in various species, allowing for greater offspring production and often involving a higher chance of survival for the young in certain environments. Oviparous animals usually provide some level of care or protection for their eggs, but many species rely on external conditions to ensure the development of their offspring.
Ovoviviparity: Ovoviviparity is a reproductive strategy where embryos develop inside eggs that remain within the mother's body until they are ready to hatch. This form of reproduction combines features of both oviparity (egg-laying) and viviparity (live-bearing), as the young receive nutrients from the yolk of the egg rather than directly from the mother. In the context of certain fish species, ovoviviparity allows for increased survival rates of the offspring in a safer environment until they are ready to face external conditions.
Petromyzontida: Petromyzontida, commonly known as lampreys, are jawless vertebrates characterized by their elongated, eel-like bodies and circular, sucker-like mouths. They are often parasitic, feeding on the blood and bodily fluids of other fish.
Placoid scales: Placoid scales are small, tooth-like structures found on the skin of certain fish, particularly cartilaginous fish like sharks and rays. These scales are unique because they provide protection and reduce drag while swimming, which is crucial for their survival in aquatic environments. The structure of placoid scales is similar to teeth, consisting of an inner pulp cavity and a hard outer layer made of enamel, helping to create a rough texture on the fish's skin.
Ray-finned fishes: Ray-finned fishes are a diverse group of bony fish characterized by their fan-like array of thin, bony rays that support their fins. This group, scientifically known as Actinopterygii, represents the largest and most diverse class of vertebrates, showcasing a wide variety of shapes, sizes, and adaptations to different aquatic environments.
Sarcopterygii: Sarcopterygii, commonly known as lobe-finned fishes, are a clade of bony fish characterized by their fleshy, lobed pectoral and pelvic fins, which are supported by bones. These features connect them closely to tetrapods, as they are believed to be the ancestors of land vertebrates. Sarcopterygii include both extant species like coelacanths and lungfishes, and the extinct ancestors that contributed significantly to vertebrate evolution.
Scales: Scales are protective, often overlapping structures found on the skin of many fish species. They serve multiple functions, including protection from predators and parasites, and play a role in locomotion and hydrodynamics as fish swim through water. Scales can vary in shape, size, and composition depending on the species and their environment, which reflects their evolutionary adaptations.
Swim bladder: The swim bladder is a gas-filled organ found in many bony fish that helps them maintain buoyancy in water. By adjusting the amount of gas in the swim bladder, fish can control their position in the water column without expending energy. This adaptation is crucial for their survival as it allows them to efficiently navigate their aquatic environments.
Tetrapods: Tetrapods are a superclass of animals that have four limbs, which evolved from the fins of ancestral fish. They include amphibians, reptiles, birds, and mammals. This significant evolutionary step allowed tetrapods to adapt to life on land, leading to various adaptations in their anatomy and physiology that enabled them to thrive in diverse terrestrial environments.
Viviparity: Viviparity is a mode of reproduction where embryos develop inside the body of the parent, leading to the birth of live young rather than laying eggs. This reproductive strategy is observed in various animal groups and offers certain advantages, such as increased survival rates for the offspring due to a protected development environment and direct nourishment from the mother.