Exploring Our Fluid Earth

Introduction to Phylum Chordata

The phylum Chordata consists of animals with a flexible rod supporting their dorsal or back sides. The phylum name derives from the Greek root word chord- meaning string. Most species within the phylum Chordata are vertebrates, or animals with backbones (subphylum Vertebrata). Examples of vertebrate chordates include fishes, amphibians, reptiles, birds, and mammals. A modern human—one species of mammal—is a familiar example of a chordate. However, we share this phylum with two groups of invertebrates as well. Tunicates (subphylum Urochordata) and lancelets (subphylum Cephalochordata) are the only invertebrate groups within the phylum Chordata. Figure 3.97 shows some invertebrate representatives of the phylum Chordata.

Fig. 3.97. (A) A colony of blue bell sea squirt tunicates (Clavelina moluccensis; subphylum Urochordata), East Timor

Image courtesy of Nick Hobgood, Wikimedia Commons

Fig. 3.97. (B) A floating colony of tunicates called salps, Oregon

Image courtesy of Oregon Department of Fish and Wildlife

Fig. 3.97. (C) A bioluminescent floating colony of tunicates called pyrosomes, East Timor

Image courtesy of Nick Hobgood, Wikimedia Commons

Fig. 3.97. (D) A lancelet (Branchiostoma lanceolatum; subphylum Cephalochordata)

Image courtesy of Hans Hillewaert, Wikimedia Commons

Tunicates (subphylum Urochordata) are filter-feeding animals with sac-like bodies. Most tunicate species are sessile (Fig. 3.97 A). They can live either in large colonies (Fig. 3.97 A) or as solitary individuals (Figs 3.1 and 3.97 B). Some species float freely as plankton with jelly-like bodies in large colonies (Fig. 3.97 B and Fig. 3.97 C). Lancelets (subphylum Cephalochordata) are arrow- or spear-shaped animals that live in shallow marine habitats, often partially buried in the sediment (Fig. 3.97 D).

At first glance it’s hard to imagine that a tunicate and a fish are related, but the lancelets look very much like fish, and all chordates have a set of features lacking in all other animals (Fig. 3.98). Chordates also have many of the most derived features we have already examined in the other phyla, such as complete digestive systems with coeloms, complex nervous systems, and respiratory and circulatory systems for efficient gas exchange.

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Fig. 3.98. Generalized diagram of chordate features

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Image adapted from Piotr Michał Jaworski, Wikimedia Commons

All chordates share the following common features that are unique to the group:

1. The notochord is a stiff but flexible rod of cells and connective tissue (from the Greek root words noto meaning back and chord meaning string) that gives the phylum its name, Chordata. In some chordates the notochord is a major support structure (Fig. 3.98). In fishes, amphibians, reptiles, birds, and mammals, the notochord is present only in the embryo. Because bony, segmented vertebrae replace it during later stages of development, these animals are assigned to the subphylum Vertebrata. Tunicates and lancelets are examples of invertebrate chordates since they have a notochord, but it does not develop into a vertebral column.
    
2. Pharyngeal slits are in the pharynx, the region of the digestive tract just behind the mouth (Fig.3.98). In some chordates, such as tunicates, these slits filter food from the water. In other chordates, such as fish, they are respiratory structures. In humans they appear only in the early embryo as a few indentations, not as open slits. They are therefore called pharyngeal clefts in a human embryo.
    
3. The dorsal hollow nerve tube lies above the notochord (Fig.3.98) and sends branches of nerve tissue into muscles and other organs. As the nerve tube grows, its walls thicken, almost eliminating the central hollow space. A dorsal hollow nerve tube is a common feature of all chordates, including humans, in both embryonic and mature stages.
    
4. All chordates have a post-anal tail that is located posterior to the anal opening (Fig.3.98). In humans, our tail is reduced to a small nub of bone (the tailbone) that does not protrude significantly outside the body.

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Fig. 3.99. (A) Botrylloides violaceus

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Image courtesy of U.S. Geological Survey (USGS)

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Fig. 3.99. (B) Lollipop tunicate (Nephtheis fascicularis)

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Image courtesy of Nick Hobgood, Wikimedia Commons

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Fig. 3.99. (C) Unidentified green colonial sea squirt with one shared atrial siphon per colony

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Image courtesy of Nick Hobgood, Wikimedia Commons

Fig. 3.99. (D) Perophora namei colonies grow in a long slender row.

Image courtesy of Nick Hobgood, Wikimedia Commons

Exploration of oceanfront beaches or docks will likely lead to discovering tunicates attached to rocks or pilings. Tunicates may be solitary or colonial. Many solitary tunicates look like miniature vases with two necks. Some tunicates can reach 10 centimeters (cm) in length (Fig 3.101 B), but colonial tunicates are often much smaller (Fig. 3.99). Some colonial tunicates attached to rocks and pilings resemble gelatin embedded with stars (Fig. 3.99 A). The point of each star is a tunicate, and the center of the star is the common pore through which they all exhale. Individual tunicates in a colony are very small, but the colony itself can be very large (Fig. 3.97 A). Some planktonic kinds of colonial tunicates called salps form huge floating tubes of tunicates, all embedded in the wall of the tube with their mouths pointing outward (Fig. 3.97 B). These colonies can grow to over 10 meters in length.

When solitary tunicates are disturbed, they squirt out jets of water, earning their nickname “sea squirts.” Many solitary tunicates are dull gray, brown, or black. Colonial tunicates can be more colorful, some bright red or yellow, others emerald green. Some tunicates are eaten by fish and molluscs, but in many species the body contains high concentrations of poisonous substances such as the element vanadium (V) and the compound sulfuric acid (H₂SO₄). In others, spiny calcareous spicules cover the outer surface. The poison and spines protect the animals from predators. Tunicates have no known commercial value, and in some areas have become an invasive biofouling organism.

Tunicates have a notochord only during their larval stage. The notochord supports the whole tail of the larva but just part of its body. This arrangement gives the subphylum its name, Urochordata (Greek root words uro meaning tail and chord meaning string). The tunicate larva is a bilaterally symmetrical, free-swimming animal that looks like a tadpole or a lancelet (Fig. 3.100). To swim, it wiggles its tail by contracting muscles on either side of its notochord. An opening on top of the body, called the oral siphon, leads to the pharynx. A second hole on top of the body, the atrial siphon, leads to the atrium, which surrounds the pharynx. The pharynx has pharyngeal gill slits. The tunicate cycles water through the oral siphon and down the pharynx, through the pharyngeal slits and into the atrium, then out the atrial siphon. The ventral surface of the pharynx has a long row of cells that form an endostyle. The cells of this digestive structure have tiny cilia that constantly sway back and forth, sweeping food particles toward the stomach opening. The endostyle also secretes mucus, which traps small particles of detritus and plankton.

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Fig. 3.100. Diagram of the internal anatomy of a swimming tunicate larva

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Image courtesy of Jon Houseman, Wikimedia Commons

In some species of tunicates, the larval stage does not last long, and the atrial and oral openings do not develop fully. These larvae do not eat until they are attached to a substrate. Only then do their siphons open. The larva swims feebly as a planktonic organism for just a few hours or days, then attaches itself headfirst to the substrate with a set of adhesive projections (Fig. 3.100). As it slowly grows into an adult, its body changes drastically. It loses its tail, its notochord, and most of its nerve chord. Its two siphons and its pharynx, pharyngeal slits, endostyle, and atrium change size and shift position.

Fig. 3.101. (A) Diagram of the internal anatomy of an adult solitary tunicate

Image courtesy of Jon Houseman, Wikimedia Commons

Fig. 3.101. (B) Gold-mouth sea squirt (Polycarpa aurata), an example of an adult solitary tunicate

Image courtesy of Nick Hobgood, Wikimedia Commons

Adult tunicates have a tunic or outer wall covering their exterior surfaces. The tunic can be thin, clear, and gelatinous in some species (Fig. 3.97 A). It is thick and leathery in texture in other species (Fig. 3.101 B). An adult tunicate gets oxygen and food by taking water through its oral siphon into its pharynx—a sac with slit-like holes that looks like gauze under a microscope. The inside of the pharynx has a coating of mucus, secreted by the endostyle. As water flows into the pharynx, the pharyngeal slits take up oxygen and filter food particles in the form of plankton and detritus, which stick to the mucous coating. After filtering, the water flows into the atrium and out the atrial siphon. In this way, some species of tunicates filter more than 100 liters of water in a single day. The food particles trapped in the mucous layer then move along the network of slits into the digestive tract. After digestion, wastes leave via the anus into the atrium. At regular intervals the tunic contracts, squirting water out through the atrial siphon and clearing the atrium of water and particles of feces.

Tunicates have a circulation system with a heart and a network of blood vessels. When the heart contracts it pumps blood through the blood vessels and cavities that distribute nutrients throughout the body. The tunicate’s heart reverses the direction of flow every few minutes for reasons not yet well understood. In most chordates, the blood flows through the vessels in one direction because of the action of valves in the heart. In the adult tunicate, the dorsal hollow nerve tube of the larva becomes reduced to a cluster of nerve cells, called a ganglion, located between the siphons (see Fig. 3.103 in Activity: Tunicate Life History). Nerve branches control the movement of the siphons and the movement of the material on the pharyngeal slits toward the digestive tract. Colonial tunicates form clusters in several different patterns. Figure 3.99 shows some examples of colonial sea squirt tunicates. In one pattern a single tunic surrounds several individuals (Fig. 3.99 B). In another pattern the individuals form a circle and share one atrial siphon (Fig. 3.99 C).

Not all invertebrate chordates are sessile as adults. Lancelets are small streamlined animals rarely longer than five centimeters. Although they can swim, they live mostly on or in sandy and muddy bottoms of warm, shallow ocean waters (Figs. 3.97 D and 3.102). Their outer body surface is so nearly transparent that an observer can clearly see their internal organs. Lancelets feed on microscopic algae and tiny suspended particles of decaying food matter. During the day they hide in the sand or mud, exposing just their mouths for feeding. At night they may wiggle out and swim about. Lancelets are important as a human food source in southeastern China, where they are collected and sold in large quantities.

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Fig. 3.102. Internal organs of a cephalochordate lancelet (Branchiostoma sp.) (A) Buccal cirri (B) Gill bars (C) Gonads (D) Antiopore (E) Ventral Fin (F) Anus (G) Intestine (H) Dorsal fin (I) Tail (J) Notochord (K) Nerve cord

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Image courtesy of Jon Houseman and Matthew Ford, Wikimedia Commons

Lancelets display chordate features in both the larval and adult stages. In adults the notochord runs the full length of the body almost to the tip of the front end (Fig. 3.102). For this reason scientists assign lancelets to another subphylum called Cephalochordata (from the Greek root words cephalo- meaning head and chord meaning string). Muscles are arranged in V-shaped segments, much as they are in fish, along the entire body on either side of the notochord. When the muscles contract, they pull the notochord from side to side, producing a wiggly swimming motion.

The food-filtering apparatus of lancelets works much as it does in tunicates, but it is arranged differently (Fig. 3.102). The mouth has two sets of tentacles that trap and capture large food particles. The mouth opens into a large pharynx that has slits along both sides. A ventral endostyle secretes mucous material that coats the inside of the pharynx. Water containing suspended food particles moves through the mouth opening into the pharynx and filters through the slits. The filtered water passes into the atrium surrounding the pharynx and out the atrial siphon on the ventral side of the body. Food and mucous material move into the digestive tract, where the small filtered particles are digested and absorbed. The indigestible remains pass out of the body through the anus behind the atrial opening on the ventral side.

The lancelet’s circulatory system has the same basic plan as a fish’s, but instead of a heart a lancelet has a large blood vessel in the pharynx ventral to the endostyle. This vessel has muscular walls that pulsate to move the blood. The blood flows through a series of vessels in the pharyngeal arches up to a dorsal blood vessel and then to the other organs of the body. The nervous system has some similarities to a fish’s but also some differences. Following the chordate body plan, the dorsal hollow nerve cord runs the length of the body above the notochord. But there is no anterior enlargement of the nerve tube that could be called a brain. The nerve tube divides into segmental branches that control the contraction of the segmental muscles.