Chordates: Videos & Practice Problems

Chordates are a diverse group of bilateral deuterostome animals characterized by four key features that must be present at some stage of their life cycle: the dorsal hollow nerve cord, the notochord, a postanal tail, and pharyngeal gill slits. The dorsal hollow nerve cord, which develops from the ectoderm, serves as a precursor to the central nervous system, eventually forming the brain and spinal cord in some species. The notochord, derived from the mesoderm, provides structural support and serves as an attachment point for muscles, facilitating movement.

The postanal tail extends beyond the anus and is a defining characteristic of chordates, while pharyngeal gill slits are openings that connect the pharynx to the external environment. Interestingly, humans exhibit these gill slits during fetal development, although they are lost before birth. This highlights that the defining features of chordates may only be present temporarily during their life cycle.

Among the non-vertebrate chordates are cephalochordates, such as lancelets, which resemble fish and utilize their pharyngeal gill slits for suspension feeding. They possess muscle attachments to the notochord, allowing them to swim effectively. Another group, the urochordates, includes tunicates, which exhibit chordate features only in their larval stage. As adults, tunicates lose these characteristics, leading to their unusual appearance. Despite their oddity, some tunicates are considered delicacies in certain cultures, such as sea pineapples in Japan, which are enjoyed despite their ammonia-like flavor when paired with sake.

Vertebrates are distinguished by their vertebrae and cranium, which provide structural support and protect the brain. The vertebrae form a segmented column of bone that encases the spinal cord, allowing for flexibility and movement. In many vertebrates, these structures develop from the notochord, a flexible rod that serves as a precursor to the spine. For instance, in snakes, numerous vertebrae are visible, each contributing to the overall structure and function of the spine.

The cranium, another defining feature of vertebrates, encases the brain and can be composed of bone or cartilage. The vertebrate brain is typically divided into three main regions: the forebrain, midbrain, and hindbrain. The forebrain is the most prominent part, while the midbrain and hindbrain consist of deeper structures, including the brainstem. Interestingly, the jaws of vertebrates originate from neural crest cells, which are embryonic cells that contribute to various structures in the body. The evolution of jaws marks a significant milestone in vertebrate history, leading to the emergence of jawed vertebrates known as gnathostomes, which include modern sharks and other chondrichthyans.

Another critical evolutionary development is the transition from a bony exoskeleton to a bony endoskeleton, which is seen in ray-finned and lobe-finned fish. This adaptation allowed for greater mobility and support. The next major evolutionary step involved the development of limbs for terrestrial movement, leading to the emergence of tetrapods, or four-limbed vertebrates, which include amphibians as their living relatives.

Finally, the evolution of the amniotic egg represents a significant adaptation for life on land, allowing for reproduction away from water. This trait is found in reptiles and is characteristic of a group known as amniotes. Understanding these evolutionary milestones provides insight into the complexity and diversity of vertebrate life.

Fish represent the most diverse group of vertebrates, boasting more species than all other vertebrate groups combined. A significant evolutionary milestone was the development of jaws, but prior to jawed fish, there existed jawless fish. These jawless fish, known as cyclostomes, possess cartilage vertebrae instead of a true backbone. Two notable examples of cyclostomes are hagfish and lampreys, both of which exhibit unique adaptations.

The hagfish, often considered one of nature's less appealing creatures, are scavengers that swim in a snake-like manner. They are equipped with glands along their bodies that secrete a pre-slimy substance. This material absorbs water, transforming into a thick mucus-like slime. This slime serves a dual purpose: it helps the hagfish evade predators and contributes to its reputation as an unappealing organism.

On the other hand, lampreys are parasitic fish characterized by their circular mouths lined with sharp teeth, which they use to latch onto and feed on their hosts. Like hagfish, lampreys have a cartilage skeleton; however, their cartilage differs in that it is not composed of collagen, a common component found in the cartilage of many other animals. This distinction highlights the unique evolutionary adaptations of these jawless fish.

Understanding these organisms provides insight into the early stages of vertebrate evolution and the diverse adaptations that have emerged within the aquatic environment.

Gnathostomes, or jawed vertebrates, mark a significant evolutionary advancement with the development of jaws. This diverse group encompasses fish, amphibians, reptiles, and mammals, with early gnathostomes primarily being jawed fish. Key characteristics of gnathostomes include the presence of jaws, larger forebrains, and paired appendages. For instance, humans possess two paired arms, while many fish, such as sharks, have paired fins on either side of their bodies.

One notable feature in some gnathostomes is the lateral line system, a sensory mechanism that detects movements and vibrations in the water. This system is particularly advantageous for aquatic predators, enhancing their ability to sense their environment. In sharks, the lateral line can be observed as a series of white dots along their bodies, serving as an effective adaptation for survival.

The living relatives of early gnathostomes are the cartilaginous fish, known scientifically as chondrichthyans, which include sharks, rays, skates, and chimeras. These organisms are characterized by their cartilaginous skeletons, which are made of cartilage rather than bone, and their paired fins. Although the paired fins may not always be visible in profile images, they are a defining feature of these species.

Sharks utilize internal fertilization and exhibit various reproductive strategies, including oviparous (egg-laying) and viviparous (live-bearing) methods. For example, skates are oviparous, laying eggs, while rays are viviparous, giving birth to live young. Chondrichthian eggs, often found washed up on beaches, can be identified by their unique appearance, with developing embryos visible within.

Understanding the evolutionary significance and biological characteristics of gnathostomes provides insight into the complexity and diversity of vertebrate life, highlighting the adaptations that have allowed these organisms to thrive in various environments.

The evolution of bony fish, scientifically known as Asteichthyes, marks a significant advancement in vertebrate development, characterized by the presence of a bony endoskeleton. This internal skeleton, primarily composed of bone, contributes to the fish's increased density compared to their cartilaginous relatives, the chondrichthyans, such as sharks. To counteract the tendency to sink due to their heavier structure, bony fish have evolved a specialized organ called the swim bladder. This gas-filled sac, which is a modified version of lungs, enables these fish to maintain buoyancy at various depths without expending excessive energy.

In contrast, sharks, which are denser than water, must continuously swim to avoid sinking, highlighting a key difference in buoyancy strategies between these two groups. Another notable feature of bony fish is the operculum, a bony flap that protects the gills. While the operculum is visible externally, the actual gills are fleshy structures located internally.

Bony fish can be categorized into two primary groups: ray-finned fish and lobe-finned fish. Ray-finned fish are distinguished by their fin structure, which consists of parallel bones supported by webs of skin. This group is the most diverse among vertebrates, thriving in a variety of marine and freshwater environments. In contrast, lobe-finned fish, exemplified by the coelacanth, possess muscular lobe fins that are significant in the evolutionary transition from aquatic to terrestrial life. The discovery of a living coelacanth, previously thought to be extinct, has provided valuable insights into the evolutionary lineage leading to land vertebrates.

These muscular fins are considered precursors to limbs, illustrating the evolutionary adaptations that facilitated the transition of life from water to land. Understanding these evolutionary milestones is crucial for comprehending the development of vertebrate life and the eventual colonization of terrestrial habitats.

Tetrapods mark a significant evolutionary transition as life adapted to terrestrial environments. These four-limbed vertebrates, which include amphibians, reptiles, and mammals, evolved from muscular fish fins, enabling them to support their weight on land. Initially, early tetrapods were fully aquatic, utilizing their limbs for movement in water rather than for walking on land. Over time, these organisms developed a heart and a closed circulatory system to efficiently deliver oxygen to their limb muscles, a crucial adaptation for life on land.

The term "amphibian," derived from Greek meaning "double life," reflects their ability to thrive in both aquatic and terrestrial habitats. Amphibians, such as salamanders, frogs, and the lesser-known Apodens (which resemble worms), are ectothermic, meaning they rely on external sources for body heat. This contrasts with endothermic organisms, like mammals, which generate heat internally through metabolic processes, including cellular respiration that produces ATP.

Amphibians possess lungs for respiration but can also absorb oxygen through their moist skin, necessitating a humid environment. A defining characteristic of amphibians is their life cycle, which typically includes a metamorphosis from a larval stage in water to a terrestrial adult form. For instance, frogs begin as tadpoles, aquatic creatures that eventually undergo metamorphosis, losing their tails and developing limbs to become adult frogs. This remarkable transformation highlights the diversity and adaptability of amphibians in various ecosystems.