Lesson 2 FISHES Aim Describe the distinguishing characteristics of all major groups of fishes. WHAT IS A FISH? There are two common definitions to the term fish. One follows the Greek word Ichthyos and includes Lampreys and Hagfish, and other follows the Latin word Pisces and excludes them. The part of Zoology that studies fishes is called Ichthyology. The plural form Fishes in contrast to Fish means many species of fish. eight percent of fish are marine. The fishes were the first vertebrates, appearing more that 500 million years ago. They belong to the Subphylum Vertebrate, along with amphibians, mammals, reptiles and birds. They probably evolved from an invertebrate chordate not much different from the tadpole larvae of sea squirts that still inhabit oceans (their larvae are free-swimming and exhibits chordate characteristics: it has a notochord, a dorsal nerve cord, pharyngeal slits, and a post-anal tail). Fish have evolved and developed to now cover a huge range of habitats in the ocean and freshwater ecosystems. Fishes are the most abundant vertebrates in terms of species and individuals. Fifty All fishes are water animals in all stages of their life cycle. They breath with gills (all the other vertebrates have lungs with only a few exceptions in Amphibians). Fishes have a two-chambered heart and are poikilothermal (i.e. they lack the ability to control constant body temperature, therefore it varies with the environment). Approximately 20,000-22,000 living species of fishes are currently recognised. DIVERSITY Fishes can be divided into 3 major groups: Superclass Agnatha (jawless fishes) Class Chondrichthyes (cartilaginous fishes) Class Osteichthyes (bony fishes). SUPERCLASS AGNATHA Class Cyclostomata (Lampreys and Hagfish) These are the most primitive fish living today. They have the main characteristics of a vertebrate, that is a cranium to protect a brain and paired organs of sight. They lack jaws though and feed by the aid of a round mouth-like suction cup with horny teeth. The body is cylindrical and elongate like that of eels or snakes. They do not have paired fins as found in cartilaginous or bony fish. Other distinguishing characteristics: Skin is smooth, lacking scales. They typically have a single nostril. They have gill slits. The cartilaginous skeleton (not bones, but a softer material called cartilage like sharks) is essentially a reinforced notochord.
Subclass Cephalaspidomorpha ( Lampreys) Belong to the Order Petromyzontia. They have seven pairs of gills, each pair opens separately. They have a pineal eye. The sexes are separate. They attach themselves to the skin of fish by their mouths, and then rasp through the skin of the host. Lampreys are found in most temperate regions and are predominantly freshwater fishes. Some may move to the sea as adults and breeding takes place in freshwater. Some of the lampreys are edible and have some commercial value. The largest species is about 1m long. There are about 30 species. Subclass Myxinoidea ( Hagfishes) Belong to the Order Myxiniformes. These are marine animals. They may be hermaphroditic or two sexed. They have between 5 and 16 pairs of gills. They lack a pineal eye. They have an internal nasal opening. Hagfishes attack dying or dead fish and sometimes eat them from the inside out. They are usually active by day and burrow in silt tunnels on the sea bottom at night. The largest species is up to 80 cm. There are 20 species known. They are mostly known for attacking bait or fish on fishing line and nets. All other vertebrates are members of the Superclass Gnathostomata. The most important evolutionary development common to all the other fishes was the enlargement and adaptation of the first gill arch to function as jaws. The Gnathostomata differ from the Agnatha vertebrates in having a vertically biting device, the jaws, which consist of a variety of exoskeletal grasping, crushing, or shearing organs, i.e. the teeth, and jaw bones. Among recent vertebrates, the gnathostomes include jaw bearing fishes (sharks, rays, chimaeras, ray-finned fishes, lobe-finned fishes) and land vertebrates (Tetrapods). Gnathostome fish have jaws and retain gills as adults, and have paired appendages (unlike the Agnathans). SUPERCLASS GNATHOSTOMATA Class Chondrichthyes (Sharks, Skates and Rays, Chimaeras) also known as Elasmobranchia This is another ancient group of fish which are cartilaginous. This means they have a skeleton made of cartilage, a material lighter and more flexible than bone. We have cartilage in our nose and ears. This group is more advanced than the cartilaginous Agnathans as they possess moveable jaws usually armed with well-developed teeth (i.e sharks). The mouth is often ventral (underneath the head) and they have rough sandpaper-like skin (made up of tiny scales). These scales are placoid - dentine plates covered by enamel, similar to teeth, and often called denticles. The Chondrichthyes have also developed paired lateral fins for efficient swimming. Some other distinguishing characteristics include: Notochord is reduced but is present in an adult. Teeth rows are replaceable. They lack a swim bladder. Its function is partly carried out by an oily liver. Marine fishes, with only few species able to tolerate fresh water. Balance of water and salt in organism is maintained by unusually high concentration of urea in blood. All contemporary species have internal fertilisation. Male intromittent organs called claspers are developed on pelvic fins. They have 5 to 7 gill slits on each side of the body behind the head (Chimaeras have only 4).
The sharks and rays belong to the subclass Elasmobranchii and the chimaeras (also known as ratfishes and ghost sharks) to the subclass Holocephali. There are worldwide around 547 species of sharks and rays. Superorder Selachii Sharks Selachians are characterized by a fusiform body and five to seven pairs of lateral gill slits. There are eight orders and about 350 species in the world. One hundred and sixty six species, approximately half of the world sharks, occur in Australian waters. They have long, streamlined bodies, two dorsal fins, a large tail, small eyes and two ear holes. The skin is covered in minute placoid scales, with an outer enamel layer and an inner pulp layer. This gives protection and aids in locomotion by providing a firm base for muscle attachment. They are very well adapted for fast swimming and predatory feeding. Black tipped reef shark Carcharhinus melanopterus (Fam. Carcharinidae) Sharks have specialised types of teeth, depending on their feeding habits: pavement teeth, found in rays and some sharks (eg. Port Jackson Shark Heterodontus portjacksoni). These are flat grinding plates used to crush molluscs and crab shells. awl teeth long, tapering teeth with a very sharp point, used for grasping prey before swallowing it whole, eg. Mako (Isurus oxyrinchus) and Grey Nurse Sharks (Carcharias taurus) triangular teeth these are flat and triangular with serrated edges, used for cutting and shearing large chunks of flesh, eg. Hammerheads (Sphyrna spp), White Sharks (O. Lamniformes, i.e. Carcharodon carcharias) Teeth are set loosely in the jaws, and there may be many rows of teeth, but only the front row is used. The other rows grow to replace worn or damaged teeth in the front row. Sharks have changed little through time because they are so successful as they are. Buoyancy Unlike bony fishes, sharks do not have a swim bladder that regulates buoyancy. Many sharks have large amounts of low density oil in the liver, which aids floatation. They swim by bending their bodies as they push forward with their tails. The pectoral fins are rigid, allowing very little movement, except to regulate direction and maintain stability. As water flows over the fins, lift is created which stops the shark from sinking. Feeding All sharks are predators, using highly developed senses of smell, hearing and sight. They can detect minute vibrations and low frequency sound waves through sensory pits in the skin and Ampullae of Lorenzini in the snout, which they use to hunt prey. Most sharks eat fish, but some species eat plankton (the whale shark and the basking shark are filter feeders) or invertebrates such as crabs and worms. Most sharks hunt alone, but some hunt in groups. Some sharks (the catsharks, epaulettes and leopards) have modified teeth. They use grinding plates to crush their prey (similar to the sting rays). Reproduction All males possess claspers, paired sex organs at the base of the pelvic fins. The claspers are used to transfer sperm to the female where it fertilises developing eggs. Some sharks lay eggs, which are encased in a leathery shell and deposited on the seabed, but the offspring of most sharks develop inside the mother and are born live. Respiration Most sharks swim continuously, forcing water through the mouth, over the gills, and through the gill slits. Oxygen is absorbed in the process. Those sharks that need to keep moving to ensure they
obtain enough oxygen will die from suffocation if they stop (this accounts for the majority of the big predators caught in shark nets being brought up dead). Not all sharks need to swim however to obtain the oxygen they need. Some bottom dwelling species (dogfish, carpet sharks. Etc), pump water by rhythmically contracting the muscles that control the inlet and outlet valves of the gills. This is done whilst lying on the bottom of the ocean. Superorder Batoidea - Rays and skates Batoids include stingrays, electric rays, skates, guitarfish, and sawfish. They have a dorsoventrally flattened body with expanded pectoral fins fused to the head. All batoids have five pairs of ventral gill slits. The gills and mouth are underneath the body. They breathe primarily through spiracles, large openings to the mouth and gill cavity behind the eyes. There are four orders and about 470 species. Rays live in shallow water around sandy sea bottoms and seagrass beds. Some live in the open ocean. The largest ray is the Manta ray, which grows to over 8 m wide. Most species are less than 2 m wide. Skates and Rays are similar in appearance (see Figure 1) and feeding but rays possess a whip-like tail and stinging spines. Some skates, although lacking the stinging spine, may have sharp horns on their dorsal surface and tail. Figure 1: Morphological differences between the Stingray and Skate. Habitat Rays and Skates vary widely in their use of habitat. The larger Atlantic Manta can be found both close to the surface of the ocean and at depths of up to 120m. Other Rays, such as the Eagle Rays can be found both in marine environments and in estuaries and mangroves. Skates are generally restricted to marine environments but can move towards the coastline and further out to sea at different times of the year. They can also be found at varying depths within the ocean. Locomotion and protection Some rays swim by flapping their pectoral fins up and down like wings; others ripple the edges of their pectoral fins, from front to back. Like sharks, they have flexible, cartilaginous skeletons. They have a very good sense of smell and can detect electric signals given off by other fish. Many species the eagle, bat, and cow-nosed rays have a whip-like tail usually equipped with stinging spines at the base. Poison glands produce venom that can cause serious wounds to anyone who steps or falls on them. Electric rays have special organs on either side of their head that produce electricity. They can deliver shocks to stun fishes they eat and shun predators.
Feeding Rays and Skates are generally similar in their feeding habits. Many stingrays cover themselves with sand as camouflage to capture prey. They feed on clams, crabs, small fishes, and other small animals that live in sediment. Their teeth have been modified into grinding plates that crush the prey. The Manta Ray, like the largest sharks, feed on plankton and small fish. Reproduction Skates lay their eggs in leathery cases, whereas rays give birth to live young. Subclass Holocephali Chimaeras Unlike other cartilaginous fishes, chimaeras have a gill cover that covers four pairs of gill slits. Teeth are in the form of tooth plates that are slowly replaced. The first dorsal spine is erectile. The skin is naked. Males have a clasping organ on head. They can grow to 1.5 m/4.5 ft. Most chimaeras are deep-water fish. Subclass Holocephali contains only one order Chimaeriformes with three families and about 25 species. Family Chimaeridae (chimaeras, or short nose chimaeras, or ratfishes). The snout is bluntly rounded. Large eyes, small mouth. Two genera living at different depths in all oceans. Family Rhinochimaeridae (longnose Chimaeras). Greatly elongated snout. Deep water dwellers. Family Callorhynchidae (ploughnose Chimaeras). Four species with heterocercal tail. Snout pointed downwards. Up to one meter long. Live in shallow waters off South America, Southern Australia and Africa, and New Zealand. SELF ASSESSMENT Perform the self assessment test titled test 2.1 If you answer incorrectly, review the notes and try the test again. CLASSIFICATION OF BONY FISH All bony fish fall within the Class Osteichthyes. Bony fishes have evolved the most complex head skeleton of all other classes, with complete dermal (skin) skeletons of small to medium size bones (scales). They have internal gills in one chamber that are protected by a bony operculum. The internal skeleton is composed, at least partly, of true bones. Fins have rays, sometime spiny. Most species have a swim bladder. The salt and water balance is maintained by kidneys. Fish of this class inhabit both seas and fresh waters. Many species migrate between seas and rivers in their life cycle. This group has enormous commercial importance around the world. This is the largest class of vertebrates. Of the 25 000 fish species most are bony. Ninety eight percent of all fishes are bony this is almost half of all vertebrates! Between 75 and 100 new species are described every year. CLASS OSTEICHTHYES The Class Osteichthyes is divided into two subclasses (sometimes three or even four): Subclass Choanichthyes or Sarcopterygii - (lungfish) The subclass is divided into two orders: Order Crossopterygii, the coelacanth Order Dipnoi or Dipneusti, the lungfishes The Choanichthyes or Sarcopterygii are fleshy finned fish. This includes ancient species including lung fish and coelacanths. They are often referred to as a class of their own but for this classification we will group them with the Osteichthyes. They are also bony fish and are consider
ancestors of the land vertebrates. They are characterised by internal nostrils, fleshy fins called lobe fins, and cosmoid scales. Class Actinopterygii ray-finned fish These are ray finned fish. The vast majority of fish living today fall within this group. The Class Actinopterygii is divided into two subclasses and one Infraclass (being discussed taxonomically today): Sub Class Chondrostei. These include a number of orders of Fossil Fish (no longer living) as well as a small number of living orders including Acipenseriformes (the Sturgeons ). Infra Class Holostei. These include a number of orders of Fossil Fish (no longer living) as well as a small number of living orders including Lepisosteiformes (garpikes) and Amiformes (bowfins). Sub Class Neopterigii, which includes the InfraOrder Teleostei. This is the largest infra order and is broken into several superorders, which in turn are subdivided into many different orders. These include Herrings, Trout, Perch, Scorpionfish, Boxfish, Anglerfish, Cod, Eels and many others. (See Lesson 3 for explanation of Infra-order and Infra-phylum) Here are some of the many orders in existence: Subclass Chondrostei Order Acipenseriformes - Sturgeons and Paddlefishes Large freshwater and anadromous (fish that spend all or part of their adults lives in saltwater and return to fresh to breed) fishes with long snout, mouth underneath. Tail heterocercal. Northern Hemisphere only. Skeleton is only partly bony. Fishes with many primitive features. Many are popular edible fish. Eggs used for caviar. About twenty five species separated into two families: Sturgeons (family Acipenserida) with no scales but five rows of scutes. Include the largest freshwater fish Beluga Husso husso (up to 9 m long). Mostly bottom feeders or can feed on other fish. Paddlefishes (family Polyodontidae). Naked skin without scutes, snout elongated into a beak. Only two contemporary species plankton feeder Polyodon spatula form North America and Psephurus gladius from China feeding on fish and growing up to 7 m. Subclass Neopterigii Order Semionotiformes- Garfish North American fresh water fishes with heavy ganoid scales. Tail is slightly heterocercal. Many fossils of different Gars were found, but only seven species have survived to our days. Order Amiiformes - Bowfin Smooth cycloid scales, slightly heterocercal tail, elongated dorsal fin. The swim bladder can function as a lung and helps survival in low oxygen water condition. There is only one living species out of this once large order. Amia calva is a freshwater North American fish up to 20 cm long. Infraclass Teleostei Order Osteoglossiformes - Bony tongues Rather ancient group of fresh water fishes mostly from tropical areas of the Americas. Large sculptured scales, head covered with bony plates. Swim bladder can serve as an additional respiratory organ in some species. About a hundred known contemporary species separated into four or six families. The best known fish of this order is the 4 m long Arapaima gigas from Amazon River. Order Mormyriformes - Elephant fishes African fresh water fishes. Mostly small, although there are species up to 1.5 m long. Some of them have an elongated trunk-like snout. They have electrical organs derived from caudal muscles. An electrical field generated by this organ helps them to navigate in dark or murky water. Probably
because of the use of this complicated method of navigation, the brain is highly developed and reaches as much as 1/80 of body mass that is similar to birds and mammals. About 110 contemporary species are recognised. Order Perciformes Ray-finned Fishes This is the largest order of fish as well as being the largest order of vertebrates. It includes around 40% of all bony fish and contains at least 7000 species. The species in this order vary greatly in size and can be found in most aquatic environments. Their name comes from the Greek terms perch + form. They are typically defined by the divided dorsal and anal fins. Their diet can vary from algae, plankton and smaller fish. Order Clupeiformes - Herrings and Anchovies A very large family of mainly marine fishes with huge economical importance. Lateral line canals are present on the head only. Round cycloid scales easily come off. Elongated compressed Emperor angelfish (Pomacanthus imperator) from the Family Pomacanthidae, Order Perciformes, the largest of the fish orders. body, symmetrical tail fin, no spines present, always one rather short dorsal fin, low positioned pectoral fins, pelvic fins far back on the belly. Most species have sharp scutes along the rear part of belly forming a keel. About 300 species separated into four families. Family Clupeidae (herrings) has about 190 species, mostly marine, but with some anadromous and fresh water representatives. Family Engraulidae (Anchovies) has about 100 marine species. Both herrings and anchovies (which are smaller) are abundant in oceans and constitute the majority of commercial catches. The other two families are small and less important. Order Elopiformes Tarpons, Tenpounders, and Bonefishes Herring-like fishes with slender body, cycloid scales and a deeply forked symmetrical tail. Two families Elopidae and Megalopidae with one genus each. Order Anguiliformes Eels This rather diverse order contains serpent-like fishes. Pelvic fins are absent, long dorsal fin, symmetrical tail fin. No spines in fins. Skin is naked or has small cycloid scales. Mainly marine fishes with a few catadromous and freshwater representatives. Feed on fish or bottom animals. Mostly bottom dwellers. About 600 species separated into about twenty, sometimes more, families. Some species, like European eel and American eel, are commercially important. EXAMPLES OF FAMILIES OF BONY FISH Family MURAENIDAE (Moray Eels) Family ANGUILLIDAE (Freshwater Eels) Family CLUPEIDAE (Herrings, Sardines, Pilchards) Family SALMONIDAE (Salmon and Trout) Family PLOTOSIDAE (Eel Tailed Catfish) Family ANTENNARIDAE (Anglerfish) Family HEMIRAMPHIDAE (Garfish, Halfbeaks) Common sea dragon Family TRACHICHTHYIDAE (Roughies, Sawbellies) Phyllopteryx taeniolatus Family BERYCIDAE (Red Snapper) (Fam. Syngnathidae) Family ZEIDAE (Dories) Family SYNGNATHIDAE (Pipefish, Pipehorses, Seahorses, Sea Dragons) Family SCORPAENIDAE (Scorpion fish) Family PLATYCEPHALIDAE (Flathead) Family SERRANIDAE (Rock Cod, Sea Perch) Family TERAPONTIDAE (Grunters) Family DINOLESTIDAE (Long Finned Pike) Family SILLAGINIDAE (Whiting) Family POMATOMIDAE (Tailor, Bluefish) Family CARANGIDAE (Trevally)
Family SPARIDAE (Bream) Family HAEMULIDAE (Sweetlip) Family CHAETODONTIDAE (Butterflyfish) Family POMACENTRIDAE (Damselfish) Family MUGILIDAE (Mullet) Family LABRIDAE (Wrass) Family ODACIDAE (Weed Whiting, Rock Whiting) Family BLENNIDAE (Blennies) Family GEMPYLIDAE (Gemfish) Family SCOMBRIDAE (Tuna, Mackerel) Family BOTHIDAE (Left Eyed Flounder) Family PLEURONECTIDAE (Right Eyed Flounders) Family SOLEIDAE (True Soles) Family MONACANTHIDAE (Leatherjackets, Filefish) Family ARACANIDAE (Temperate Boxfishes) Family OSTRACIIDAE (Tropical Boxfish Cowfish, Trunkfish) Family TETRAODONTIDAE (Toadfish, Pufferfish) BIOLOGICAL CHARACTERISTICS OF BONY FISH Body shape Is related to the lifestyle. Fast swimming fish (eg. Tuna, Marlin, and Mackerel) have streamlined bodies. Laterally compressed bodies are better for swimming around kelp beds or reefs. Flat bodies (eg. Flounder) are ideal for living on the bottom of the sea bed. Body shape may also help with camouflage (eg. Pipefish live in sea grass which has a similar shape to the fish s body). Slow moving fish (eg. Blennies and Sculpins) have irregular growths on body that contribute to camouflage. Colouration Colour is often used for camouflage, particularly in tropical species. Deep water fish often use colour for camouflage. Many fish can rapidly change colour by contracting or expanding pigment in special cells called chromatophores. Some chromatophores contain special crystals that reflect light giving the fish s surface a shiny appearance. Colour change may be for any of several reasons, including: camouflage, mood or reproduction. Open water fish and shallow water predators are rarely very colourful. Locomotion Swimming is used to find mates for reproduction; chase or find food; escape danger; to obtain oxygen. Cartilaginous fish need to swim to move water through gills from which they obtain oxygen. Muscle used for swimming comprises a large percentage of fish body weight (eg. In Tuna, up to 75%). Bony fish have a swim bladder that provides buoyancy (Cartilaginous fish do not have this; hence they need to use pectoral fins to provide lift as well as thrust). Bony fish generally have greater manoeuvrability than cartilaginous fish. Feeding Both sharks and many bony fish are carnivores. Bony fish tend to hunt prey smaller than themselves: however, sharks will take bites out of things larger than themselves. Bony fish are diverse in what they feed on, however different species have evolved specific preferences, e.g. Some prefer sponges, others sea urchins; some prefer sea weeds and others filter plankton for food.
Digestion In general, all fish share certain types of digestive parts including a stomach, intestine, liver, kidney and anus. Bony fish do have a more complex digestive system than cartilaginous fish. Carnivorous fish have a shorter, straight intestine. Fish that eat more difficult to digest plant material have a longer, coiled intestine. Nervous system Most fish have a well developed sense of smell (particularly strong in sharks). Olfactory sacs are located on both sides of the head. These open to the outside through nostrils. Taste buds allow determination of various chemical stimuli. These may be located not only in the mouth, but also on the lips, fins and skin. Taste buds can also be found on whisker-like structures called barbels. Bony fish rely on vision more than cartilaginous fish. Fish have a unique sense organ called a lateral line This involves a series of small canals running along the body (in the skin, bone or cartilage). These canals open to the outside usually through visible pores. The lateral line detects vibrations of sound or water movement. Sound waves can also be detected by inner ears located beside the brain, just behind the eyes. Behaviour Due to their well developed nervous system, fish are able to respond to their environment in complex ways. Different types of complex behaviours are characteristic of different types of fish e.g. territorial behaviour, schooling, migration, reproductive behaviour. TERMINOLOGY Learn the terms in the following diagram and list. Adipose Fin a fin composed of fatty, gelatinous matter, unlike a bony fin Operculum - bony movable structure to cover gills Scutes bony plates or armour found on some fish. Branchial -related to the gills
SELF ASSESSMENT Perform the self assessment test titled test 2.2 If you answer incorrectly, review the notes and try the test again. SET TASK 1. Visit an aquarium supply shop, Marine Park, fish retailer, or other facility where you can observe fish. If your mobility is restricted or you are unable to locate such a facility, look at the web site of an aquarium, and see what diversity of fish is to be seen on that web site. Write down the scientific names (if available - otherwise common names/breeds) of a variety fish which you find. Try to find representatives from at least three different orders. Observe these fish (and/or anything written about them) for at least 5 minutes, taking note of the characteristics that differentiate each from other types of fish. 2. Research the anatomical characteristics of one species of fish. You may do this in any way you wish (e.g. Internet searches, books, interviewing experts, or even obtaining a fish perhaps from a fishmonger, and dissecting it). ASSIGNMENT Download and do the assignment called Lesson 2 assignment.