Chordata/Fish

Characteristics

1. Notochord: One of the most important characteristics of the Phylum Chordata is the presence of a notochord. This is a flexible rod that runs parallel to the nerve cord and provides support and stiffness to the body. In vertebrates (animals with a backbone), the notochord is replaced by the spine during development.
2. Dorsal nerve cord: Another defining characteristic of the Phylum Chordata is the presence of a dorsal nerve cord. This is a hollow tube that runs along the back of the animal and contains the spinal cord and brain. In most other animals, such as insects or mollusks, the nerve cord is located on the underside of the body.
3. Pharyngeal slits: The pharyngeal slits are a series of openings that are located in the pharynx (the part of the digestive tract between the mouth and esophagus). These slits are used in feeding and respiration, and are modified in different ways depending on the species. For example, in fish, they are used for gill respiration, while in humans, they develop into the ear canals and tonsils.
4. Endostyle or Thyroid gland: The endostyle is a gland that runs along the bottom of the pharynx and is thought to play a role in the production of mucus or hormones. In vertebrates, this gland develops into the thyroid gland, which plays a vital role in regulating metabolism.
5. Post-anal tail: Finally, all chordates have a post-anal tail, which is an extension of the body that lies beyond the anus. This structure is used for swimming or balance in some species, while in others it may play a role in courtship or defense.

Protochordate

Protochordate is a subphylum within the phylum Chordata. They are small, marine, and invertebrate animals that have a few chordate features. The major representatives of protochordates are Tunicates or sea squirts, and Amphioxus, which is also known as Lancelets. The Tunicates exhibit a complex life cycle that includes a sessile adult phase which may be solitary or colonial, and a free-living tadpole larval phase. Amphioxus, on the other hand, is a small fish-like animal that spends most of its time buried in the sand at the bottom of the sea. It feeds on planktonic organisms, and it is a filter feeder.

Craniata

Craniata is a subphylum of chordates that includes all animals that have a cranium or skull. This group of animals first appeared about 530 million years ago during the Cambrian explosion. The Craniata includes fish, birds, mammals, amphibians, and reptiles. The skull or cranium protects the brain, which is the central processing center in these animals. The Craniata also have a well-defined and complex nervous system that allows them to respond to environmental stimuli in a much more sophisticated manner than the invertebrate counterparts.

Agnatha

Agnatha is a superclass of animals within the phylum Chordata that are jawless fish. These animals are considered to be the most primitive of all the vertebrates. The name Agnatha comes from the absence of jaws in these fish. The Agnatha includes lampreys and hagfish, which are the only two extant groups of jawless fish. The lampreys have a protrusible, toothed tongue that they use to bore into the flesh of other fish, whereas hagfish feed on decaying organisms. They have a cylindrical body with no scales and a cartilaginous skeleton.

Gnathostomata

Gnathostomata are vertebrates with jaws, and they are the most advanced animals among the chordates. They evolved from Agnatha, which were jawless fish. This superclass includes fish, amphibians, reptiles, birds, and mammals. The presence of jaws sets them apart from the Agnatha and allows for much more effective feeding strategies. The Gnathostomata also have paired appendages, which include fins in fish and limbs in tetrapods. The limbs in tetrapods have played a significant role in allowing these animals to move onto land. The evolution of jaws and the paired limbs have contributed to the success of the Gnathostomata in dominating the planet.

Phylum Urochordata, commonly known as tunicates or sea squirts, is a group of marine invertebrates that belong to the subphylum Tunicata. They are characterized by a unique body structure consisting of a tunic, or outer covering, made of tough cellulose material. Urochordates are filter feeders that live in the ocean, where they take in seawater through an oral siphon, filter out tiny food particles using a mucus net, and then expel the filtered water through a second, exhalant siphon.

The body of a urochordate is generally cylindrical or sac-like in shape, and can range in size from a few millimeters to several centimeters. There are two distinct body types within this phylum: solitary and colonial. Solitary urochordates are free-living and can move about independently, while colonial urochordates are connected to one another and live together in a cluster or chain.

Urochordates are hermaphrodites, meaning they possess both male and female reproductive organs. They typically reproduce sexually, with fertilization occurring externally in the water column during spawning events. Larvae of urochordates are free-swimming and possess a notochord, which is a long, rod-like structure that acts as a primitive backbone. As the larvae mature, they undergo a process called metamorphosis, during which the notochord is reabsorbed and the adult form develops.

There are approximately 3,000 species of urochordates, with the majority of them living in shallow tropical waters. They play an important ecological role in marine ecosystems by filtering seawater and exchanging nutrients and gases with the surrounding environment. Urochordates also have potential applications in the biomedical field, where their unique cellulose tunics have been used in developing tissue-engineering scaffolds and drug-delivery systems.

Tunicates have an endostyle, which is a glandular tissue that runs along the length of the pharynx. This structure helps with feeding and is responsible for capturing plankton and other small particles that are present in the water. The captured food is then transported to the stomach, where digestion takes place.

The body of a tunicate is made up of three main parts: the siphons, the pharynx, and the endostyle. The siphons are a pair of tubular structures that are used for water intake and expulsion. The pharynx is a muscular tube that extends from the mouth and into the body. The endostyle is located at the base of the pharynx and is responsible for producing mucus, which helps to trap food particles.

Tunicates have a simple nervous system that consists of a nerve net, which is comprised of nerve cells that are interconnected. The nerve net is responsible for coordinating the various activities of the tunicate, such as feeding and movement.

Subphylum Cephalochordata is a group of marine invertebrates that represents the most basal lineage of chordates. They are commonly known as lancelets or amphioxus and are small, elongated animals that range in size from a few millimeters to about 8 centimeters.

Cephalochordates are characterized by having a flexible, fish-like body plan that is divided into three distinct regions: the anterior, middle, and posterior regions. The anterior region contains the mouth, while the middle region has many short, frilly structures called branchial bars that filter food from the water. The posterior region contains the anus and the tail, which extends beyond the body and helps the animal to swim.

One of the most important features of Cephalochordates is their notochord, which is a long, flexible rod that runs the length of their body. The notochord provides support and allows for flexibility in movement. The notochord is also found in all other chordates, including humans.

Cephalochordates also possess a hollow, dorsal nerve cord that runs along the length of their body. This nerve cord serves as the main pathway for transmitting information between the animal's brain and the rest of its body. In addition to the nerve cord, they have a number of sensory structures, including tentacles, photoreceptor cells, and a specialized balancing organ called a statocyst.

In terms of reproduction, Cephalochordates possess both male and female gonads, but they usually spawn eggs and sperm directly into the surrounding water. After fertilization, the zygote develops into a larval stage that is similar in appearance to other chordate larvae, including those of fish, amphibians, and mammals.

Superclass Agnatha is a taxonomic group that includes primitive jawless fish. Agnatha is derived from the Greek terms 'a' meaning 'without' and 'gnathos' meaning 'jaw', indicating that these organisms lack a proper jaw. Instead, they possess a circular sucker-like mouth, which they use to suck in prey. Members of this superclass are typically ectothermic and possess a cartilaginous endoskeleton.

Agnathans are divided into two major groups: hagfishes (Myxini) and lampreys (Petromyzontida). Hagfishes are unique in possessing a skull but lacking a vertebral column, while lampreys possess both a skull and a vertebral column.

These organisms are considered 'primitive' in nature, as they are believed to be some of the earliest vertebrates and have changed little over the course of evolution. These ancient fish typically inhabit marine environments and are known for their scavenging and parasitic feeding habits.

In terms of reproduction, Agnathans typically have anamniotic eggs, which means that their eggs lack an amnion and are deposited directly into the environment. They also lack a placenta, resulting in the embryos feeding off of yolk during development.

Hagfish, also known as slime eels, are primitive jawless fish that belong to the class Myxini of the subphylum Vertebrata. These eel-like marine creatures are found in cold, deep waters around the world, and have existed since the Cambrian period, about 530 million years ago.

Hagfish have a unique body structure that sets them apart from other fish. They have a long, tubular body that is covered by mucus-secreting glands, which protect them from predators. They have a cartilaginous skull and a simple, unpaired nostril for detecting odors. Their mouth is equipped with two rows of sharp, keratinized teeth that are used to burrow into the flesh of dead or dying marine animals. They have no true jaws, but instead use their tongue to rasp flesh.

The hagfish's circulatory system is simpler than that of other vertebrates. They have only one heart, and lack a respiratory pigment such as hemoglobin. Instead, they use diffusion to extract oxygen from the water. They do not have a swim bladder, but instead rely on their body structure to maintain buoyancy.

Hagfish are known for their ability to produce slime in large amounts. When threatened, they release a thick, sticky slime from their slime glands, which can suffocate or confuse predators. The slime is also hypothesized to have antibacterial and antiviral properties.

Hagfish play a vital role in the marine ecosystem by scavenging dead or dying organisms, which helps to recycle nutrients. They are also an important commercial fishery, primarily in Asia, where their skin is used to make luxury leather goods. However, overfishing has led to declines in hagfish populations in some parts of the world.

The sea lamprey, or Petromyzon marinus, is a parasitic fish species which undergoes a complex life cycle consisting of multiple distinct stages. These stages involve dramatic changes in morphology, behavior, and habitat in response to both internal and external factors. In this explanation, we will explore each stage of the sea lamprey life cycle in detail.

Stage 1: Egg The sea lamprey life cycle begins when a mature female lays her eggs in freshwater streams or rivers. These eggs are typically laid in nests or on the river bottom and are often guarded by males until they hatch. Lamprey eggs are small and semitransparent, with diameters ranging from 2-4 mm. The incubation period for these eggs can last from two weeks to several months, depending on water temperature and other environmental factors.

Stage 2: Larva Once the eggs hatch, the sea lamprey enters the larval stage of its life cycle. At this stage, the larva is only a few millimeters long and has an eel-like body shape. It lacks jaws, teeth, and even eyes, and instead, relies on a specialized sucker mouth to attach to rocks or other aquatic surfaces while it feeds on microorganisms. During this stage, the larva undergoes multiple molts as it grows in size and develops rudimentary eyes and fins.

Stage 3: Juvenile After several years, the sea lamprey larvae undergo metamorphosis, transforming into juvenile sea lampreys. This stage is characterized by the emergence of jaws, teeth, and eyes, marking the transition from filter-feeding to predatory behavior. Juvenile lampreys feed on small fish and invertebrates, and their bodies become more streamlined and eel-like as they prepare for their migration to the ocean. During this stage, sea lampreys will also develop fat reserves that they will rely on during their oceanic phase.

Stage 4: Adult Once the sea lamprey reaches adulthood, it will migrate to the ocean where it will spend most of its life. During this stage, the sea lamprey will grow rapidly, reaching lengths of up to three feet long. These adult lampreys seek out other fish species to parasitize, attaching to their sides with their sucker mouths and using specialized teeth and digestive enzymes to feed on the host's blood and tissues.

Stage 5: Spawning When the adult sea lampreys reach sexual maturity, usually around five to seven years of age, they will return to freshwater streams or rivers to spawn. Once there, they will select a mate, and the female will deposit her eggs in a nest or on the river bottom while the male simultaneously fertilizes them. After spawning, the adult lampreys will die, marking the end of the life cycle.

Superclass Gnathostomata comprises the jawed vertebrates and is one of the two major groupings of the phylum Chordata, alongside superclass Agnatha, which includes the jawless fishes. There are numerous noticeable characteristics that distinguish them from their ancestors, the agnathans. Gnathostomes have jaws formed from paired and modified gill arches, which played a critical role in their evolutionary success by allowing them to obtain and process food more effectively, which was arguably a key adaptation leading to radiation and diversification.

Gnathostomes have a more extensive and diverse assortment of cranial sensory structures, including lateral line systems, olfactory pits, and distinct inner ear structures that allow them to gather more information about their surroundings than agnathans. Additionally, gnathostomes have enlarged forebrains, cerebellums, and optic lobes that are more advanced than those of their jawless relatives, providing them with better spatial perception, memory functions, and visual acuity. They also have a more muscular and flexible muscular system and paired pectoral and pelvic fins that allow for more precise control of their movements, overall improved swimming performance and greater mobility leading to them inhabiting both aquatic and terrestrial environments.

Class Chondrichthyes belongs to the phylum Chordata and is comprised of cartilaginous fish. This class includes approximately 1,200 species of fish, with characteristics such as five to seven gill slits, paired fins, a cartilaginous skeleton, and placoid scales. They range in size from the small lantern shark at 8 inches to the whale shark, the largest fish in the world, reaching up to 40 feet.

Chondrichthyes have a unique type of scale known as placoid scales or dermal denticles. These scales have a dentine core covered by enamel, with a sharp tip protruding from the skin's surface, making them feel like sandpaper. These scales are ultimately responsible for giving these fish their distinct feel and are used in commercial sandpaper manufacturing.

The respiratory system of Chondrichthyes is unusual because they have five to seven pairs of gill slits that allow water to pass over their gills. Because they lack the opercula, bony structures that protect fish's gills, these fish must swim continuously to keep water flowing over their gills.

Another unique feature of Chondrichthyes is their reproduction. Unlike other fish, this class has a slow rate of reproduction with internal fertilization in most species. Most of the females lay relatively few, large yolky eggs, meaning that these fish typically have a low level of reproduction compared to other fish.

This class includes many important and well-known species, including sharks, rays, and skates, that play an essential role in the marine ecosystem. They sit at the top of the food chain, keeping other populations of fish and marine organisms in check, and play an essential role in the ocean's health. Additionally, many species of Chondrichthyes have commercial and recreational importance, leading to many conservation efforts to preserve these animals' populations.

The ampullary organs of Lorenzini are specialized sensory structures found in cartilaginous fish that are used for electroreception, or the detection of electrical fields. These organs are small, sack-like structures that are present in the skin of the fish's head and connected to the surface of the skin through a canal or duct.

Each ampullary organ consists of a small pouch that contains a gel and a modified nerve cell. The gel is electrically conductive and allows the stimuli detected by the organ to be transmitted to the nerve cell. The modified nerve cell is known as an electroreceptor, which responds to the electrical charges detected by the organ.

When a fish swims through a magnetic field, it creates an electrical field around its body. The ampullary organs of Lorenzini are able to detect these weak electrical fields through the use of the electrically conducting gel in the organ. The fish uses this electroreception to detect prey, navigate through underwater obstacles, and to communicate with other fish.

The ampullary organs of Lorenzini are an essential adaptation for cartilaginous fish, which have poor vision and rely on other senses to navigate through their environment. The ability to detect electrical fields allows these fish to sense their surroundings in a way that is unique among vertebrates.

Sharks and rays, collectively known as elasmobranchs, reproduce using internal fertilization. Male sharks and rays have paired reproductive organs called claspers that are used to transfer sperm to the female during copulation.

Female sharks and rays have paired ovaries, but only the left ovary is functional, producing and releasing eggs into the oviduct. Fertilization occurs when sperm from the male is deposited into the female's oviduct and fertilizes the egg(s).

There are two main types of reproduction in elasmobranchs - oviparity and viviparity. Oviparous elasmobranchs lay egg cases that are fertilized internally and then deposited in the environment, either attached to substrate or left to float freely. These egg cases have a tough, leathery exterior that protects the developing embryo inside.

Viviparous elasmobranchs retain the developing embryo inside the female's body during gestation. The embryo receives nourishment from a yolk sac inside the egg, which is then absorbed by the embryo as it grows. Some viviparous elasmobranchs also have a placenta-like structure that provides additional nourishment and oxygen to the growing embryo.

In some species of sharks, there is also a phenomenon known as oophagy, where the developing embryos within the female cannibalize each other, with the strongest embryo ultimately surviving and being born.

Class Actinopterygii

Class Actinopterygii is a taxonomic class of fish commonly referred to as ray-finned fish. It is the largest class of vertebrates, comprising about 32,000 extant species, and is one of the most diverse groups of fish. They are characterized by their bony, segmented fins, where the fin-rays project from the body and support the fin's structure.

Actinopterygii is divided into two subclasses: Chondrostei and Neopterygii. Chondrostei includes the sturgeons and paddlefish, while Neopterygii includes all other ray-finned fish.

Ray-finned fish includes some of the most well-known and commonly consumed fish such as salmon, trout, tuna, and tilapia. They can be found in nearly all aquatic environments, including freshwater, saltwater, and brackish water.

Their body shapes vary greatly, but most have a streamlined shape that helps them swim efficiently through water. Some species have evolved specialized structures to aid in swimming, such as the sail-like dorsal fin of the sailfish or the elongated lower jaw of the pike.

Actinopterygii includes a wide range of feeding strategies. Some species are filter feeders, others are predators, and still, others feed on plant material. Some species of ray-finned fish are herbivorous, which is rare in fish.

Reproduction in ray-finned fish is diverse. Some species are viviparous, where young are born live, while others are oviparous, laying eggs externally. In some species, parental care is provided to the offspring after hatching or birth, while in others, it is left to the eggs to develop on their own.

Class Actinopterygii

Class Actinopterygii is a taxonomic class of fish commonly referred to as ray-finned fish. It is the largest class of vertebrates, comprising about 32,000 extant species, and is one of the most diverse groups of fish. They are characterized by their bony, segmented fins, where the fin-rays project from the body and support the fin's structure.

Actinopterygii is divided into two subclasses: Chondrostei and Neopterygii. Chondrostei includes the sturgeons and paddlefish, while Neopterygii includes all other ray-finned fish.

Ray-finned fish includes some of the most well-known and commonly consumed fish such as salmon, trout, tuna, and tilapia. They can be found in nearly all aquatic environments, including freshwater, saltwater, and brackish water.

Their body shapes vary greatly, but most have a streamlined shape that helps them swim efficiently through water. Some species have evolved specialized structures to aid in swimming, such as the sail-like dorsal fin of the sailfish or the elongated lower jaw of the pike.

Actinopterygii includes a wide range of feeding strategies. Some species are filter feeders, others are predators, and still, others feed on plant material. Some species of ray-finned fish are herbivorous, which is rare in fish.

Reproduction in ray-finned fish is diverse. Some species are viviparous, where young are born live, while others are oviparous, laying eggs externally. In some species, parental care is provided to the offspring after hatching or birth, while in others, it is left to the eggs to develop on their own.

The gills of class actinopterygii are a complex respiratory system that performs the crucial process of gas exchange, providing oxygen for cellular respiration and releasing carbon dioxide. Actinopterygii or ray-finned fish, which include almost all modern fish species, have a high surface-to-volume ratio and rely on a large surface area for gas exchange.

The gills are made up of gill filaments, which are thin and elongated structures that extend from the gill arch. Each gill filament contains numerous gill lamellae, which are small, flattened, and closely packed structures that provide a large surface area for gas exchange. The gill lamellae are composed of two layers of cells; one layer contains capillaries that allow for oxygen exchange, and the other supports a thin layer of cells that secrete mucus to protect the delicate respiratory surface.

Water enters the fish through the mouth, passes over the gills, and leaves through the gill slits. This process is facilitated by the operculum, a bony plate that covers the gills and pumps water over them. The fish's circulatory system also plays a role in gas exchange, as blood flows in the opposite direction of water on the gill filaments, maximizing oxygen diffusion.

Actinopterygii have evolved several adaptations to optimize their respiratory system, including counter-current exchange, a mechanism that allows for the maximum exchange of oxygen and carbon dioxide from the water. This unique adaptation enables fish to extract nearly all the oxygen available in the water, and allows them to survive in low-oxygen environments.