Chordates are bilaterally symmetrical animals, making up the third largest and the most diverse phyla in the animal kingdom, Chordata. They are distinguished by five characteristics: an elastic, rod-like notochord, a dorsal hollow nerve cord, an endostyle or thyroid gland, pharyngeal slits, and a post-anal tail. These features are present at some stage in their life cycle and are integral to their classification.
Presently, around 81,000 extant chordate species are divided into three subphyla: cephalochordates (Cephalochordata), tunicates (Tunicata), and vertebrates or craniates (Vertebrata).
Chordates possess five characteristics that distinguish them from other animal phyla.
Tunicates and vertebrates have a distinct heart that circulates blood throughout the body. In contrast, cephalochordates use specialized blood vessels for circulation. Deoxygenated blood flows into a series of branchial arteries in the gills via the ventral aorta, and after gas exchange, oxygenated blood returns through the dorsal aorta. While vertebrates have a complex network of capillaries for blood distribution, such networks are absent in tunicates and cephalochordates.
While tunicates, cephalochordates, and aquatic vertebrates possess gills for respiration, terrestrial vertebrates have evolved to use lungs as their primary respiratory organ.
Their digestive system is a simple tube consisting of a ciliated pharynx, a sac-like stomach, and specialized gland-like structures found only in vertebrates, such as the liver and pancreas.
Although nitrogenous waste is typically expelled through the kidneys in terrestrial vertebrates, the gills serve as the primary excretory organ in fish and other aquatic vertebrates.
In contrast, cephalochordates and tunicates use different organs for excretion. Cephalochordates possess specialized protonephridia, while tunicates expel waste by diffusing ammonia across their body tissues.
Although the anterior part of the dorsal nerve cord in tunicates and cephalochordates enlarges into a brain-like structure, a well-developed brain is only found in vertebrates. Both cephalochordates and vertebrates are characterized by segmentally repeated nerves arising from the nerve cord, but this segmentation is not observed in tunicates.
Tunicate larvae possess simple light-sensitive organs and other sensory structures that detect the direction of gravity. Similarly, cephalochordates have pigment spots and light receptors within their nerve cord, whereas vertebrates are far more complex and sensitive to external stimuli.
Several DNA sequencing efforts to determine the interrelationships among chordates have produced various hypotheses, but the current consensus supports the monophyly of chordates. This means that all chordates have descended from a single common ancestor. Recent phylogenetic data found two conserved signature indels (CSIs) in cyclophilin-like protein and mitochondrial inner membrane protease ATP23, shared by vertebrates, tunicates, and cephalochordates. This discovery reinforces the monophyletic nature of chordates.
Vertebrates and tunicates share two additional CSIs in the exosome complex protein RRP44, indicating that vertebrates are more closely related to tunicates than cephalochordates. Consequently, the subphylum Cephalochordata is considered a sister group to Olfactores, which includes vertebrates and tunicates.
Chordates share several features with their closest non-chordate relatives, hemichordates and echinoderms. Hemichordates possess branchial openings resembling gill slits and stomochords similar in composition to the notochord. Echinoderms share the tube-within-tube deuterostomic body plan and are believed to share a common ancestor with chordates.
Based on the above studies, over 81,000 living chordate species are classified into three subphyla and ten classes.
| Cephalochordata | 1. Fish-like, filter-feeding animals lacking distinct heads or brains. |
| Tunicata | 1. They exist in two major forms: sea squirts and salps. Both forms retain the characteristics of a chordate only in their larval stages. 2. While sea squirts are sessile filter-feeders, salps float passively and possess a two-generation life cycle (solitary and colonial). 3. The third group, larvaceans, resemble tadpole larvae and swim actively throughout their lives. |
| Vertebrata | 1. As their name suggests, they have modified their notochords into an ossified vertebral column. They also possess a distinct cranium. 2. The vertebral column consists of a series of bony or cartilaginous cylindrical vertebrae. However, hagfish lack vertebrae and possess an incomplete cranium, placing them under the group Craniata, which includes all vertebrates. |
Although fossil records of early chordates are sparse, several biochemical techniques based on RNA sequences have been used to study the evolution of this phylum. It is suggested that deuterostomes evolved as early as 900 million years ago, with the first chordates appearing around 896 million years ago. Nearly all the earliest chordate fossils have been discovered in China’s Chengjiang Fauna from the Early Cambrian Period.
Both tunicates and cephalopods filter-feed on suspended food particles in the water. They draw the water current into the mouth by using their gill slits. The food is then coated using mucus secreted by the endostyle, which is then brushed into the posterior part of the gut using the cilia lining the pharynx.
Chordates move from one place to the other by the movement of muscles.
Most chordates are gonochoric, meaning they exist as either males or females. However, some fish and tunicates are hermaphrodites, possessing both male and female sex organs in a single individual. Typically, males and females produce gametes (sperm and ova) that fertilize internally to form embryos. In contrast, primitive chordates released gametes outside their bodies, with fertilization occurring externally. Additionally, some fish and lizards can reproduce asexually, completely bypassing fertilization.
Tunicates and vertebrates are often viviparous, giving birth to live young. On the other hand, cephalochordates are oviparous, laying eggs that sometimes undergo a larval stage before reaching adulthood.
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