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Learning Goals
By the end of this reading you should be able to:
- Describe the difference between jawless and jawed fishes
- Discuss the distinguishing features of sharks and rays compared to other modern fishes
- Differentiate between the two groups of bony fishes
- Explain the evolution of bone in the fishes
Modern fishes include an estimated 31,000 species. Fishes were the earliest vertebrates, with jawless species being the earliest and jawed species evolving later. They are active feeders, rather than sessile, suspension feeders. Jawless fishes—the hagfishes and lampreys—have a distinct cranium and complex sense organs including eyes, distinguishing them from the invertebrate chordates.
Jawless Fish
Jawless fishes are craniates that represent an ancient vertebrate lineage that arose over one-half-billion years ago. In the past, the hagfishes and lampreys were classified together as agnathans. Today, hagfishes and lampreys are recognized as separate clades, primarily because lampreys are true vertebrates, whereas hagfishes are not. A defining feature is the lack of paired lateral appendages (fins). Some of the earliest jawless fishes were the ostracoderms (which translates to “shell-skin”). Ostracoderms were vertebrate fishes encased in bony armor, unlike present-day jawless fishes, which lack bone in their scales.
Myxini: Hagfishes
The clade Myxini includes at least 20 species of hagfishes. Hagfishes are eel-like scavengers that live on the ocean floor and feed on dead invertebrates, other fishes, and marine mammals. Hagfishes (Fig. 1) are entirely marine and are found in oceans around the world, except for the polar regions.
A unique feature of these animals is the slime glands beneath the skin that release mucus through surface pores. This mucus allows the hagfish to escape from the grip of predators. Hagfish can also twist their bodies in a knot to feed and sometimes eat carcasses from the inside out. The skeleton of a hagfish is composed of cartilage, which includes a cartilaginous notochord that runs the length of the body. This notochord provides support to the hagfish’s body. Hagfishes do not replace the notochord with a vertebral column during development, as do true vertebrates.
Petromyzontidae: Lampreys
The clade Petromyzontidae includes approximately 35–40 or more species of lampreys (Fig 2.). Lampreys are similar to hagfishes in size and shape; however, lampreys possess some vertebral elements. Lampreys lack paired appendages and bone, as do the hagfishes. As adults, lampreys are characterized by a toothed, funnel-like sucking mouth. Many species have a parasitic stage of their life cycle during which they are ectoparasites of fishes
Lampreys live primarily in coastal and fresh waters, and have a worldwide distribution, except for in the tropics and polar regions. Some species are marine, but all species spawn in freshwater. Eggs are fertilized externally, and the larvae distinctly differ from the adult form, spending 3 to 15 years as suspension feeders. Once they attain sexual maturity, the adults reproduce and die within days.
Lampreys possess a notochord as adults; however, this notochord is surrounded by a cartilaginous structure called an arcualia, which may resemble an evolutionarily early form of the vertebral column.
Review Question:
Review: Shared traits
Gnathostomes: Jawed Fishes
Gnathostomes or “jaw-mouths” are vertebrates that possess jaws (Fig. 3). One of the most significant developments in early vertebrate evolution was the development of the jaw, which is a hinged structure attached to the cranium that allows an animal to grasp and tear its food. The evolution of jaws allowed early gnathostomes to exploit food resources that were unavailable to jawless fishes.
Early gnathostomes also possessed two sets of paired fins, allowing the fishes to maneuver accurately. Pectoral fins are typically located on the anterior body, and pelvic fins on the posterior. The evolution of the jaw and paired fins permitted gnathostomes to expand from the sedentary suspension feeding of jawless fishes to become mobile predators. The ability of gnathostomes to exploit new nutrient sources likely is one reason that they replaced most jawless fishes during the Devonian period. Two early groups of gnathostomes were the acanthodians and placoderms, which arose in the late Silurian period and are now extinct. Most modern fishes are gnathostomes that belong to the clades Chondrichthyes and Osteichthyes (Fig. 4).
Review Question:
Quick Review: Jaws and Paired fins
Chondrichthyes: Cartilaginous Fishes
The clade Chondrichthyes is diverse, consisting of sharks, rays, and skates, together with sawfishes and a few dozen species of fishes called chimaeras, or “ghost” sharks.” Chondrichthyes are jawed fishes that possess paired fins and a skeleton made of cartilage. This clade arose approximately 370 million years ago in the early or middle Devonian. They are thought to be descended from the placoderms, which had skeletons made of bone; thus, the cartilaginous skeleton of Chondrichthyes is a later development. Parts of shark skeleton are strengthened by granules of calcium carbonate, but this is not the same as bone (Fig. 5).
Most cartilaginous fishes live in marine habitats, with a few species living in fresh water for a part or all of their lives. Most sharks are carnivores that feed on live prey, either swallowing it whole or using their jaws and teeth to tear it into smaller pieces. Shark teeth likely evolved from the jagged scales that cover their skin, called placoid scales. Some species of sharks and rays are suspension feeders that feed on plankton.
Sharks have well-developed sense organs that aid them in locating prey, including a keen sense of smell and electroreception, with the latter perhaps the most sensitive of any animal. Organs called ampullae of Lorenzini allow sharks to detect the electromagnetic fields that are produced by all living things, including their prey. Electroreception has only been observed in aquatic or amphibious animals. Sharks, together with most fishes and aquatic and larval amphibians, also have a sense organ called the lateral line, which is used to detect movement and vibration in the surrounding water, and is often considered homologous to “hearing” in terrestrial vertebrates. The lateral line is visible as a darker stripe that runs along the length of a fish’s body.
Sharks reproduce sexually, and eggs are fertilized internally. Most species are ovoviviparous: The fertilized egg is retained in the oviduct of the mother’s body and the embryo is nourished by the egg yolk. The eggs hatch in the uterus, and young are born alive and fully functional. Some species of sharks are oviparous: They lay eggs that hatch outside of the mother’s body. Embryos are protected by a shark egg case or “mermaid’s purse” that has the consistency of leather. The shark egg case has tentacles that snag in seaweed and give the newborn shark cover. A few species of sharks are viviparous: The young develop within the mother’s body and she gives live birth.
Rays and skates comprise more than 500 species and are closely related to sharks. They can be distinguished from sharks by their flattened bodies, pectoral fins that are enlarged and fused to the head, and gill slits on their ventral surface. Like sharks, rays and skates have a cartilaginous skeleton. Most species are marine and live on the sea floor, with nearly a worldwide distribution.
Review Question:
Osteichthyes: Bony Fishes
Members of the clade Osteichthyes, also called bony fishes, are characterized by a bony skeleton. The vast majority of present-day fishes belong to this group, which consists of approximately 30,000 species, making it the largest class of vertebrates in existence today.
Nearly all bony fishes have an ossified skeleton with specialized bone cells (osteocytes) that produce and maintain a calcium phosphate matrix. This characteristic has only reversed in a few groups of Osteichthyes, such as sturgeons and paddlefish, which have primarily cartilaginous skeletons. The skin of bony fishes is often covered by overlapping scales, and glands in the skin secrete mucus that reduces drag when swimming and aids the fish in osmoregulation. Like sharks, bony fishes have a lateral line system that detects vibrations in the water.
All bony fishes use gills to breathe. Water is drawn over gills that are located in chambers covered and ventilated by a protective, muscular flap called the operculum. Many bony fishes also have a swim bladder, a gas-filled organ that helps to control the buoyancy of the fish.
Bony fishes are further divided into two extant clades: Actinopterygii (ray-finned fishes) and Sarcopterygii (lobe-finned fishes). Actinopterygii, the ray-finned fishes, include many familiar fishes—tuna, bass, trout, and salmon, among others. Ray-finned fishes are named for their fins that are webs of skin supported by bony spines called rays. In contrast, the fins of Sarcopterygii are fleshy and lobed, supported by bone . Living members of this clade include the less-familiar lungfishes and coelacanths.
Review Question:
The Evolution of Bones in Fishes
The evolution of bone begins in fish, as they were the first vertebrates. The first bone to evolve in the early fish is different from the bone found in later vertebrates. Bone or osseous tissue is a connective tissue that constitutes the skeleton, internal or external, of vertebrates. Components of bone are the mineralized portion, hydroxylapetite ( a mineral formed from calcium phosphate), collagen fibres that support the formation of mineralized bone, and some vascular tissue that supplies blood to the living cell component of bone. Calcification is the process of deposition of mineral salts on the collagen fiber matrix that crystallizes and hardens the tissue. The process of calcification only occurs in the presence of collagen fibers.
The bone of early vertebrates is of 2 main types, cellular and acellular. The structure of these bone types consists of the same basic components, the only difference is that there are spaces in cellular bone for the osteocytes, the bone-forming cells that occur throughout the bone. The earliest forms of acellular bone have a tendency to be laminated, forming in successive layers that were deposited by the dermis. Some early agnathans have an acellular bone that is called aspidin. Acellular bone is also found in the layered bones of more advanced fish. In placoderms, a group of gnathostomes most closely related to sharks, an acellular bone is found had in bony plates around the edge of their mouths that acted as teeth (this was not the origin of teeth, however).
In the earliest fish most of the skeleton is formed of bone produced by the dermis, hence its name dermal bone. Dermal bone composed all the externally visible bones of the head and trunk and the scales and biting surfaces inside the mouth. There were other types of specialized bone that developed in various early osteichthyans. These bone types are characterized by the complexity, nature, and composition of the external layers of the dermal bones, as well as their growth processes and whether or not resorption was involved in their development.
Among other bone types found in early vertebrates are perichondral bone (a thinly laminated acellular bone) and endochondral bone. Perichondral bone is often found surrounding soft tissue that passes through cartilage, as can be found in the placoderms with cartilage braincases where the nerves and arteries passing through the braincase wall can have perichondral bone surrounding them. The endogirdles that support the pelvic and pectoral fins also have perichondral bone surrounding them.
Endochondral bone forms around a cartilage precursor. These bones make up much of the internal skeleton in reptiles and mammals and form the arm and leg bones of the appendicular skeleton. Endochondral bone first evolved as a specialized feature in some groups of gnathostomes (osteichthyans and acanthodians).
As well as these basic tissue types, there are many types of bone involved in dental tissues that evolved in fish and continued on to the higher land animals. Chondrichthyans have a mostly cartilaginous skeleton, but in the dorsal fin spines, they have a type of dentine with a thin enameloid layer. Nearly all the fossil parts of sharks are a similar type of dental tissue, including the placoid scales covering the body that are similar to small teeth. In many early agnathans, there are layers of dentine covering the dermal bone.
Review Question:
Summary
The earliest vertebrates that diverged from the invertebrate chordates were the jawless fishes. Fishes with jaws (gnathostomes) evolved later. Jaws allowed early gnathostomes to exploit new food sources. Agnathans include the hagfishes and lampreys. Hagfishes are eel-like scavengers that feed on dead invertebrates and other fishes. Lampreys are characterized by a toothed, funnel-like sucking mouth, and most species are parasitic on other fishes. Gnathostomes include the cartilaginous fishes and the bony fishes, as well as all other tetrapods. Cartilaginous fishes include sharks, rays, skates, and ghost sharks. Most cartilaginous fishes live in marine habitats, with a few species living in fresh water for part or all of their lives. The vast majority of present-day fishes belong to the clade Osteichthyes, which consists of approximately 30,000 species. Bony fishes can be divided into two clades: Actinopterygii (ray-finned fishes, virtually all extant species) and Sarcopterygii (lobe-finned fishes, comprising fewer than 10 extant species but which are the ancestors of tetrapods).
End of Section Review Questions:
REVIEW: Chondrichthyes
Thinking about it