CHORDATES: CHARACTERISTICS, CLASSIFICATION, HABITAT, REPRODUCTION - BIOLOGY - 2024

The chordates (Chordata) are a broad and diverse phylum of animals with bilateral symmetry that share five essential diagnostic features: notochord, endostilo, pharyngeal grooves, dorsal nerve cord and postanal hollow tail.

In some species, the permanence of these traits is not maintained throughout the individual's life; in some chordates the characteristic is lost even before the organism is born.

Branchiostoma lanceolatum. Source: © Hans Hillewaert /

The structural plan of the members of this group may be shared by some invertebrates, such as bilateral symmetry, the entero-posterior axis, the coelom, the presence of metamers and cephalization.

Chordates, in terms of diversity and number of species, are in fourth place - after arthropods, nematodes and mollusks. They have managed to colonize a very wide series of ecological niches and present innumerable adaptive traits for various forms of life: aquatic, terrestrial and flying.

The origin of chordates has sparked an interesting debate among evolutionary biologists. Molecular biology and embryonic characteristics make clear the relationship of this group with echinoderms in deuterostomes.

Different hypotheses have been proposed to explain the origin of chordates and vertebrates. One of the most famous is the Garstang hypothesis, which proposes that an ascidian larva underwent a process of pedomorphosis and gave rise to a sexually mature individual with juvenile characteristics.

The current representatives of the group are grouped into three heterogeneous lineages: the cephalochordates, known as amphoxes; the urochordates, called asidias, and the vertebrates, the largest group, made up of fish, amphibians, reptiles and mammals.

Within this last group, in a small family, we find ourselves, humans.

characteristics

The first impression when evaluating the three groups of chordates is that the differences are more noticeable than the shared characteristics.

In general terms, vertebrates have as their main characteristic the rigid endoskeleton that is found under the skin. Although the fish are aquatic, the rest of the group is terrestrial, and both feed on jaws.

In contrast, the remaining groups - urochordates and cephalochordates - are sea-dwelling animals, and none of them possess a bone or cartilaginous supporting structure.

In order to remain stable, they have a series of rod-like structures composed of collagen.

Regarding the feeding modality, they are filter feeders and their food consists of the particles suspended in the water. It has devices that produce substances similar to mucus, which allow the capture of the particles by adhesion. However, these differences are purely superficial.

In addition to the fact that chordates have an internal cavity filled with fluid, called coelom, they all have five diagnostic characteristics: notochord, pharyngeal clefts, endostyle or thyroid gland, nerve cord and post-anal tail. We will describe each one in detail below:

Notochord

The notochord or notochord is a rod-shaped structure of mesodermal origin. The name of the Phylum is inspired by this characteristic.

It is flexible up to a point, and extends throughout the entire body length of the organism. Embryologically, it is the first structure of the endoskeleton to appear. It serves as an anchor point for the muscles.

One of its most important characteristics is the ability to bend without undergoing shortening, which allows a series of wave movements to be made. These movements do not cause the structure to collapse - analogous to how a telescope would.

This property arises thanks to the fluid inside the cavity, and it works as a hydrostatic organ.

In the basal groups, the notochord persists throughout the life of the organism. In most vertebrates it is replaced by the spinal column, which performs a similar function.

Pharyngeal clefts

It is also known in the literature as "pharyngotremia." The pharynx corresponds to a portion of the digestive tract that is located just after the mouth. In the chordates, the walls of this structure acquired openings or small holes. In primitive groups it is used for food.

It is important not to confuse this feature with the gills, because the latter are a series of derived structures. They can appear in very early stages of development, before the organism is born or hatches from the egg.

Endostyle or thyroid gland

The endostilium, or its structure derived from the thyroid gland, is only found in chordates. It is located on the floor of the cavity of the pharynx. The endostyle is found in protochordates and in lamprey larvae.

In these early groups, the endostyle and the clefts work together to promote filter feeding.

Certain cells that make up the endostyle have the ability to secrete proteins with iodine - homologous to that of the thyroid gland in adult lampreys and in other vertebrates.

Nervous dorsal cord

Chordates have a nerve cord located in the dorsal portion (with respect to the digestive tube) of the body and the interior of it is hollow. The origin of the brain can be traced to the thickening in the anterior portion of this cord. Embryologically, the formation occurs through the ectoderm, above the notochord.

In vertebrates, the neural arches of the vertebrae function as protective structures for the cord. Similarly, the skull protects the brain.

Post anal tail

The post-anal tail is made up of musculature and provides the necessary motility for the displacement in the water of the larvae of the tunicates and the amphioxus. As the tail is located posterior to the digestive system, its only function is related to the improvement of the aquatic movement.

The efficiency of the tail increases significantly in later groups, where the fins are added to the body of the organism. In humans, the tail is found only as a small remnant: the coccyx and a series of very small vertebrae. However, many animals have tails that can wag.

Subphylum Urochordata

Tunicates are a subphylum commonly known as sea squirts. They include approximately 1600 species. These organisms are widely distributed inhabitants in the oceans, from the depths to the coasts.

The name “tunicate” derives from a kind of tunic that surrounds the animal, it is composed of cellulose and is not an organ or a living structure.

The vast majority of adult representatives have a completely sessile lifestyle, anchored on some rock or other substrate. They can be solitary or grouped in colonies. The larva, for its part, has the ability to swim and move freely through the ocean until it finds a suitable surface.

The adult forms are extremely modified and have degenerated most of the five diagnostic characteristics of chordates. In contrast, the larvae - reminiscent of a small tadpole - possess all five characteristics of chordates.

There are three classes of tunicates: Ascidiacea, Appendicularia and Thaliacea. The first class has the most common, diverse, and most studied members. Some have the ability to shoot jets of water through siphons when disturbed.

Subphylum Cephalochordata

Cephalochords are small animals, between 3 and 7 centimeters in length. The appearance translucent and compressed laterally. The common name is amphiox (before it was used as the genus, but now they are called Branchiostoma).

There are 29 species, being an incredibly tiny subphylum, in terms of the number of species. In the small body of the animal, the five characteristics of chordates become evident.

The body works in the following way: the water enters through the mouth, thanks to a current produced by the cilia it has, it continues its way through the clefts of the pharynx.

In this step, the particles that serve as food are held together by the secretion of mucus from the endostyle. The cilia carry food to the intestine and are engulfed.

Although at first glance it seems a very simple organism, its circulatory system is quite complex. Although there is no heart, it is a system similar to that found in fish, orchestrating the passage of blood in the same way as in this group.

The nervous system centers around the nerve cord. The pairs of nerves emerge in each region of the muscle segments.

Subphylum Vertebrata

Vertebrates are the most diverse set of animals, in terms of morphology and habitats, of the chordates. All members of the lineage possess the diagnostic characteristics of chordates in at least some stages of their life cycle. In addition, we can distinguish the following features:

Characteristics of vertebrate systems

The skeleton, made of cartilage or bone, is made up of a vertebral column (with the exception of the mixins) and a skull. As for the muscular system, there are segments or myomers in zigzags, which allow movement. The digestive system is of the muscular type, and there is now a liver and a pancreas.

The circulatory system is responsible for orchestrating the passage of blood through all body structures. This objective is accomplished thanks to the presence of a ventral heart with multiple chambers and a closed system composed of arteries, veins and capillaries.

Erythrocytes or red blood cells are characterized by having hemoglobin as a pigment to transport oxygen - in invertebrates there are a variety of pigments of green and blue tones.

The integument has two divisions: an epidermis located in the outer portion or a stratified epithelium derived from the ectoderm and an inner dermis formed from connective tissue derived from the mesoderm. Vertebrates present a series of variations in this sense, finding horns, glands, scales, feathers, hair, among others.

Almost all sexes are separated, with their respective gonads that discharge the contents into a cloaca or specialized openings.

Classification and phylogeny

Where are chordates found?

Source: Arthur Tributino Menezes, from Wikimedia Commons

Before describing the phylogeny of chordates, it is necessary to know the location of this group on the tree of life. Within animals with bilateral symmetry, there are two evolutionary lineages. On the one hand there are the prostostomates and on the other the deuterostomates.

Historically, the distinction between the two groups is based basically on embryonic characteristics. In the protostomates, the blastopore gives rise to the mouth, the segmentation is spiral and the coelom is schizocelic, while in the deuterostomates it gives rise to the anus, the segmentation is radial and the coelom is enterocelic.

In the same way, the application of current molecular techniques has confirmed the separation between the two, in addition to clarifying the relationships between the individuals that make them up.

Protostomes include mollusks, annelids, arthropods, and other smaller groups. This lineage is divided into two groups: Lophotrochozoa and Ecdysozoa. The second group, the deuterostomes, includes echinoderms, hemicordates, and chordates.

Cladist and traditional classification

The Linnaean classification provides a traditional way that allows the classification of each taxa. However, from the cladist perspective, there are certain groups that are not currently recognized, because they do not meet the requirements imposed by this traditional classification school.

The most recognized examples in literature are Agnatha and Reptilia. Since these groups are not monophyletic, they are not accepted by cladists. For example, reptiles are paraphyletic because they do not contain all the descendants of the most recent common ancestor, leaving birds on the outside.

However, most texts and scientific literature maintain the traditional Linnaean classification to refer to the different groups of chordates that exist. Changing subfields in zoology represents an extensive challenge, thus maintaining the ranges with which we are most familiar.

Traditional groups

In this sense, the traditional division consists of: Urochordata, Cephalochardata, Myxini, Petromyzontida, Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves and Mamalia.

The first two groups, the urochordates and the cephalochords, are known as protochordates and acraniates.

All the remaining groups belong to Vertebrata and Craniata. Myxini and Petromyzontida belong to Agnatha, while the rest belong to Gnathostomata (this last classification takes into account the presence or absence of a mandible).

Tetrapoda includes amphibians, reptiles, birds, and mammals. Finally, the representatives of Amniota are reptiles, birds, and mammals. In general, these groups make up the traditional classification of the Phylum Chordata.

Habitat

Chordates have managed to encompass a number of extraordinary habitats. Urochordates and cephalochordates live in marine environments.

Vertebrates, meanwhile, have a wider range. Amphibians - in part - reptiles and mammals live in terrestrial environments. Birds and bats have managed to colonize the air; while some mammals, the cetaceans, returned to the water.

Reproduction

Urochordates are the chordates with the broadest reproduction pattern. These organisms exhibit sexual and asexual reproduction. The species are usually hermaphroditic and fertilization is external. The gametes come out through the siphons, and upon fertilization, the new individual develops into a larva.

Cephalochordates have external fertilization and the sexes are separated. Thus, males and females release their gametes into the ocean. When fertilization occurs, a larva is formed, similar to the juvenile form of urochordates.

Vertebrates reproduce predominantly sexually, with a series of strategies that allow the multiplication of individuals. Both variants of fertilization are present - internal and external.

Nutrition and diets

The nutrition of the two basal groups of chordates - sea squirts and cephalochordates - are fed by a filtration system in charge of capturing suspended particles in the marine environment.

On the other hand, mixins are scavengers - they feed on other dead animals. Lampreys, in contrast, are ectoparasites. Using a complex suction cup mouthpart, these animals can adhere to the body surface of other fish.

However, the juvenile forms feed by sucking the mud, rich in nutritious organic debris and microorganisms.

An evolutionary innovation that determined the fate of the group was the appearance of the jaws. These appeared as a modification of the development pattern of the anterior cephalic region.

This structure made it possible to expand the range of prey consumed by these dams, as well as being much more efficient in trapping potential prey.

As for vertebrates, it is virtually impossible to generalize the trophic habits of their members. We find from carnivorous, filter, hematophagous, frugivorous, herbivorous, insectivorous, nectarivorous, granivore, folivorous, among others.

Breathing

Respiration in sea squirts occurs through the propulsion of water. These have structures called siphons through which they can circulate and go through the gill slits.

In cephalochords, respiration occurs in a similar way. These animals constantly circulate the water in a current that enters through the mouth and exits through an opening known as the atriopore. This same system is used for feeding the animal.

In vertebrates, the respiration systems are much more varied. In aquatic forms, fish and related, the process of gas exchange occurs through gills.

In contrast, landforms do so by means of lungs. Some species, such as salamanders, lack lungs and perform the exchange using only the skin.

Birds have an adaptive modification that allows them to meet the energy demands of their expensive means of locomotion: flight. The system is extremely effective, and is made up of bronchi connected to air sacs.

Evolutionary origin

Fossil record

The first fossil found in the record dates from the Cambrian period, about 530 million years ago.

Despite the fact that most of the members of the group are characterized mainly by a hard-boned skeleton, the ancestors of the group were soft-bodied - therefore, the fossil record is particularly scarce.

For these reasons, information on the origin of chordates is derived from anatomical evidence from current chordates and from molecular evidence.

Ancestral Vertebrates: Key Fossils

Most of the fossils dating back to the Paleozoic are ostracoderms, a species of fish-shaped organism without jaws. Some outstanding fossils are Yunnanozoon, an individual reminiscent of a cephalochord and Pikaia is a famous representative of the Burgess Shale, it is 5 centimeters long and ribbon-shaped.

Haikouella lanceolata has been key in the process of elucidating the origin of vertebrates. Approximately 300 fossil individuals of this species are known, reminiscent of today's fish. Although they do not have signs of vertebrae, they have all the characteristics of chordates.

Protostomes or deuterostomes?

The evolutionary origin of chordates has been a subject of heated discussion since the time of Charles Darwin, where the focal point of research was establishing relationships between groups of living organisms.

At first, zoologists speculated the possible origin of the chordates starting within the lineage of the protostomates. However, this idea was quickly discarded when it became clear that the characteristics they apparently shared were not homologous.

At the beginning of the 20th century, the discoveries of developmental patterns in animals made the relationship with chordates and other deuterostomized animals evident.

Garstang hypothesis

In the course of biological evolution, chordates went two separate ways - very early in that process. One led to sea squirts and the other to cephalochordates and vertebrates.

In 1928, the British ichthyologist and poet Walter Garstang proposed a very imaginative hypothesis, which involves processes of heterochronies: changes in the synchrony of the developmental processes.

For Garstang, the ancestor of the chordates could be an ancestral individual similar to the sea squirts in this juvenile that retained its larval characteristics. This very avant-garde idea is based on the fact that the juvenile sea squirts present in a very conspicuous way the five diagnostic characteristics of the chordates.

According to the hypothesis, at a crucial moment in evolution, the larva could not complete the metamorphosis process and move into an adult, sessile tunicate. Thus, the hypothetical larva with reproductive maturity arises. With this event, a new group of animals appears with the ability to swim freely.

Garstang used the term pedomorphosis to describe the retention of juvenile characters in the adult state. This phenomenon has been reported in various current groups of animals, for example, in amphibians.

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