List the three groups of chordates with their characteristics.
List the general features of vertebrates.
Describe the classification of vertebrates.
Check Your Understanding
What is the function of the notochord in lower vertebrates?
What happens to the notochord in higher vertebrates?
Introduction
It is hard to believe that some of the organisms that are chordates are closely related to us and vertebrates like us - everything from fish to amphibians and reptiles, to birds and mammals. Chordates are a group of animals that includes the vertebrates, as well as several closely related invertebrates . Some chordates, as we will soon see, appear to be nothing more than animals resembling marine invertebrates, like the tunicates in Figure below. Chordates also include the lancelets, which appear as mostly featureless and simplified swimming animals (Figure below). What these all have in common, though, are certain characteristics appearing either in the larval or adult forms, and which we will explore further in the first section.
Vertebrates all have backbones or spinal columns as well as some other defining characteristics. About 58,000 species have been described and contain many familiar groups of large land animals.
Chordates
Chordates (phylum Chordata), including the vertebrates and several closely related invertebrates, are united by having, at some time in their life cycle, a notochord, a hollow dorsal nerve cord); pharyngeal slits (vertical slits in the pharynx wall, which help to filter out food particles); an endostyle (ciliated groove or grooves located in the pharynx), and a post-anal tail. The phylum is broken down into three subphyla: Urochordata (represented by tunicates), Cephalochordata (represented by lancelets) and Vertebrata (the vertebrates).
Urochordates have a notochord and nerve cord only during the larval stage and cephalochordates have a notochord and nerve cord but no vertebrae (bones in the backbone). In all vertebrates, except for hagfish, the notochord is generally reduced and the dorsal hollow nerve cord is surrounded with cartilaginous (made of cartilage, not bone) or bony vertebrae.
The urochordates consist of 3,000 species of tunicates, sessile (permanently attached) marine animals with sack-like bodies having thick membranes and siphons for water movement, and the cephalochordates which consist of 30 species of lancelets (burrowing marine animals).
The vertebrates encompass 57,739 species, including jawless and jawed examples.
The origin of chordates is currently unknown. The first clearly identifiable chordates appear in the Cambrian Period (about 542 - 488 million years ago) as lancelet-like specimens.
Figure 13.1
Tunicate colonies of (subphylum urochordata), showing oral tentacles at openings of oral siphons, which take in food and water, and expel waste and water.
Figure 13.2
(subphylum cephalochordates), perhaps the oldest known ancestor of modern vertebrates, resembled a living chordate, known as a lancelet, and perhaps swam much like an eel. is thought to have had a very primitive, proto-notochord. Its tentacles may be related to those in present-day hagfish, a jawless chordate.
What are Vertebrates?
Vertebrates, belonging to the subphylum Vertebrata, are chordates with a backbone or spinal column. Other characteristics are a braincase, or cranium, and an internal skeleton (the latter feature is present in all vertebrates except for lampreys). All vertebrates are most easily distinguished from all other chordates by having a defined head with pronounced cephalization. Cephalization is the concentration of nervous tissue towards one end of the organism. Vertebrates have sensory organs, especially eyes, concentrated at the front (anterior) aspect of the body. Why do you think this type of body design is an advantage?
Typical vertebrate traits include:
a backbone or spinal column
braincase
internal skeleton
defined head with pronounced cephalization
sensory organs, especially eyes
The vertebrate muscular system consists mostly of paired masses, as well as a central nervous system, partly located within the backbone, assuming the latter is present. Extant (living) vertebrates range in size from a carp species (Figure below), at as little as 7.9 mm (0.3 in), to the blue whale, as large as 110 ft (Figure below).
Figure 13.3
A species of carp, carp bream ().
Figure 13.4
An image of the blue whale, the largest living vertebrate, reaching up to 33 m (110 ft) long. Shown below it is the smallest whale species, Hector's dolphin (about 1.4 m (5 ft) in length), and beside it, a human.
Classification of Vertebrates
Vertebrates consist of both jawless and jawed varieties. The jawless vertebrates consist of more than 100 species including 65 species of hagfish, the conodonts, and the lampreys. The jawed vertebrates include over 900 species of cartilaginous fish, over 30,000 species of bony fish and over 18,000 species of tetrapods, or four-legged (or leg-like) vertebrates.
The bony fish are further divided into ray-finned and lobe-finned fish. The tetrapods consist of amphibians, reptiles, birds, mammal-like reptiles, and mammals.
Species of the Main Groups of Tetrapods Type of Tetrapod Number of Species
Amphibians 6,000
Reptiles 8,225
Birds 10,000
Mammal-like Reptiles 4,500
Mammals 5,800
Lesson Summary
Chordates are characterized by a notochord, pharyngeal slits, an endostyle, and a post-anal tail.
There are three main groups of chordates, including tunicates, lancelets and vertebrates.
Vertebrates are distinguished by having a backbone or spinal column.
Vertebrates are classified into two major groups: those without jaws and those with jaws.
Review Questions
What features characterize the chordates?
What are the main features of vertebrates?
The first clearly-identifiable chordates are lancelet-like (small, burrowing marine animals with a lancet shape) specimens. List three ways in which these first chordates could have evolved into a swimming-like animal.
Which two structures that all chordates possess sometime during their life cycle are used for food gathering, and how are these structures used?
Why do you think cephalization is not necessary in urochordates and cephalochordates? Explain how this is illustrated in tunicates.
Further Reading / Supplemental Links
http://www.ucmp.berkeley.edu/chordata/chordata.html
http://www.ucmp.berkeley.edu/vertebrates/vertintro.html
http://en.wikipedia.org/wiki
Vocabulary
cephalization
The placement of important sensory organs near or in the head.
cephalochordates
A group of chordates with a notochord and nerve cord but no vertebrae.
chordata
The phylum of chordates, containing the vertebrates and several closely related invertebrates.
cranium
The braincase or skull.
endostyle
A groove or pair of grooves having cilia; located in the pharynx; functions are to gather food particles and transport them along the digestive tract.
notochord
A hollow dorsal nerve cord.
urochordates
A group of chordates having a notochord and nerve cord present only during the larval stage.
vertebrata
The subphylum of vertebrates, distinguished by having backbones or spinal columns.
Points to Consider
The notochord’s stiffness in many chordates may have evolved to facilitate the effectiveness of swimming in S-shaped movements. Think about the advantages this may have for water-living vertebrates.
Unlike chordates with cephalization, cephalochordates (lancelets)have a mouth, but not a well-developed head, and have light-sensitive areas along their entire back, instead of at the anterior end of
the body.
How do you think cephalization could be an advantage in movement and feeding in fish?
Lesson 13.2: Fishes
Lesson Objectives
List the general traits of fish.
Describe the features of jawless fish.
List the general features of the cartilaginous fish.
Describe the features of bony fish and the significance of this superclass.
List some of the reasons why fish are important.
Check Your Understanding
What are the unique characteristics of vertebrates?
What are the two main groups of vertebrates?
Introduction
So what exactly is a fish? You probably think the answer is obvious. You may say that a fish is an animal that swims in the ocean or a lake. But there is lots more to fish than that. Fish are aquatic vertebrates, which through evolution became a dominant form of sea life and eventually branched to create land vertebrates. They have a number of characteristic traits and are classified into two major groups: jawless and jawed fish. Jawed fish are further divided into those with bones and those with just cartilage. Fish, in general, are important in many ways to humans - economically, recreationally and culturally. Perhaps you can think of some of these ways?
Characteristics of Fish
Fish are vertebrates that are typically ectothermic, are covered with scales, have jaws and have two sets of paired fins and several unpaired fins. A typical fish has a streamlined body that allows it to swim rapidly, extracts oxygen from the water using gills or an accessory breathing organ to enable it to breathe atmospheric oxygen, and lays eggs that are fertilized internally or externally (Figure below). Fish range in size from the 16 m (51 ft) whale shark to the 8 mm (just over ¼ of an inch) stout infantfish.
Traits of a typical fish include:
Vertebrate
Ectothermic
Scales
Jaws
Two sets of paired fins
Several unpaired fins
Streamlined body
Gills or an accessory breathing organ
Lays eggs that are fertilized internally or externally
Figure 13.5
The humphead or Napoleon wrasse (), showing some of the general traits of fish, including scales, fins and a streamlined body.
There are exceptions to many of these traits. For example, tuna, swordfish, and some species of sharks show some warm-blooded adaptations, and are able to raise their body temperature significantly above that of the water around them. Some species of fish have a slower, but more maneuverable, swimming style, like eels and rays (Figure below). Body shape and the arrangement of fins are highly variable, and the surface of the skin may be naked, as in moray eels, or covered with scales. Scales can be of a variety of different types.
Figure 13.6
One of the cartilaginous fish, a stingray, showing very flexible pectoral fins connected to the head.
Although most fish live in aquatic habitats, such as the ocean, lakes, and rivers, there are some that spend considerable time out of water. Mudskippers, for example, feed and interact with each other on mudflats for up to several days at a time and only go underwater when occupying burrows (Figure below). They breathe by absorbing oxygen across the skin, similar to what frogs do.
Figure 13.7
A mudskipper, shown on the mudflats, where it spends time feeding and interacting with other individuals.
Agnatha: Jawless Fishes
Agnatha is a superclass of jawless fish belonging to the phylum Chordata, subphylum Vertebrata (agnath means jawless). There are two extant (living) groups of jawless fish, the lampreys and the hagfish, with about 100 species in total. Although hagfish belong to the subphylum Vertebrata, they do not technically have vertebrae.
In addition to the absence of jaws, Agnatha are characterized by absence of paired fins, the presence of a notochord both in larvae and adults, and seven or more paired gill pouches. The branchial arches (a series of arches that support the gills of aquatic amphibians and fishes) lie close to the body surface.
Agnatha have a light sensitive pineal eye (an eye-like structure that develops in some cold-blooded vertebrates) and do not have an identifiable stomach. They reproduce using external fertilization. They are ectothermic, have a cartilaginous skeleton, and a heart with two chambers.
Many agnathans from the fossil record were armored with heavy bony-spiky plates. The first armored agnathans - the Ostracoderms – were precursors to the bony fish and hence to the tetrapods, including humans.
What advantages would the advent of jaws have for fish? Such an adaptation would allow fish to eat a much wider variety of food, including plants and other organisms. In the next two sections you will be introduced to two groups of fish with jaws: those with a cartilaginous skeleton and those with a bony skeleton.
Cartilaginous Fishes
The cartilaginous fishes, or Chondrichthyes, are jawed fish with paired fins, paired nostrils, scales, two-chambered hearts, and skeletons made of cartilage rather than bone. The approximate 1,000 species are subdivided into two subclasses: Elasmobranchii (sharks, rays and skates) and Holocephali (chimaera, sometimes called ghost sharks). Fish from this group range in size from the dwarf lanternshark, at 16 cm (6.3 in), to the whale shark, up to sizes of 13.6 m (45 ft) (Figure below).
Figure 13.8
One of two male whale sharks at the Georgia Aquarium. Whale sharks are the largest cartilaginous fish.
Animals from this group generally have ratio of brain weight to body size that is close to that of mammals, and about ten times that of bony fishes. One of the explanations for their relatively large brains is that the density of nerve cells is much lower than in the brains of bony fishes, making the brain less energy demanding and allowing it to be bigger.
Since they do not have bone marrow (as they have no bones), red blood cells are produced in the spleen, in special tissue around the gonads, and in an organ called Leydig’s Organ, only found in cartilaginous fishes. The tough skin of this group is covered with dermal teeth, or placoid scales, although they are mostly lost in adult Holocephali, making it feel like sandpaper. It is assumed that their oral teeth evolved from these dermal teeth, which migrated into the mouth.
The sharks, rays and skates are further broken into two superorders, one containing the rays and skates, and the other containing the sharks (Figure below). There are eight orders of sharks within the superorder. They are distinguished by such features as:
Number of gill slits
Numbers and types of fins
Type of teeth
Body shape
The sawsharks, with an elongate, toothed snout used for slashing the fish that they eat.
The bullhead sharks, with teeth used for grasping and crushing shellfish.
Carpet sharks with barbels
Nocturnal habits
The groundsharks, with an elongated snout.
The mackerel sharks, with large jaws and ovoviviparous reproduction, where the eggs develop inside the mother’s body after internal fertilization, and the young are born alive.
Figure 13.9
A spotted Wobbegong shark (), at Shelly Beach, Sydney, Australia, showing skin flaps around the mouth and cryptic coloration.
Bony Fishes
The Osteichthyes, or bony fish, contain almost 27,000 species, which are divided into two classes: the ray-finned fish (Actinopterygii) and the lobe finned fish (Sarcopterygii). Most bony-fish belong to the Actinopterygii; there are only eight living species of lobe finned fish, including the lungfish (Figure below) and coelacanths (Figure below).
Figure 13.10
One of the only eight living species of lobe finned fish, the lungfish.
Figure 13.11
One of the eight living species of lobe finned fish, the coelacanth.
The vast majority of fish are osteichthyes, and this group is the most various of vertebrates, making them the largest group of vertebrates in existence to
day. They are characterized by a relatively stable pattern of cranial bones, and the head and pectoral girdles (arches supporting the forelimbs) are covered with large dermal bones (bones derived from the skin). They have a lung or swim bladder, which helps the body create a neutral balance between sinking and floating, by either filling up with or emitting such gases as oxygen; have bone fin rays (jointed, segmented rods) supporting the fins; have an operculum (a cover over the gill), which helps them to breathe without having to swim; and are able to see in color, unlike most other fish.
One of the best-known innovations of this group is the ability to produce endochondral or “replacement” bone, by replacing cartilage from within, with bone. This is in addition to the production of perichondral or “spongy bone.” The effect is to create a relatively lightweight, flexible, “spongy” bone interior, surrounded by an outline of dense bone. This is still much heavier and less flexible than cartilage.
The ocean sunfish is the most massive bony fish in the world, up to 3.33 m (11 ft) in length and weighing up to 2,300 kg (5,070 lb) (Figure below). Other very large bony fish include the Atlantic blue marlin, the black marlin, some sturgeon species, the giant grouper and the goliath grouper. In contrast, the dwarf pygmy goby measures only 1.5 cm (0.6 in).
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