Fact Sheet: Fish Anatomy WA Curriculum K-10 Science, SS Biology – ATAR, SS Biology – General, SS Integrated Science – ATAR, SS Integrated Science – General Region North Coast, Gascoyne Coast, West Coast, South Coast, Indian Ocean Territories Summary Fishes are a large and varied group of aquatic animals. Worldwide, there are over 32,000 described species, with over 4,400 in Australia (Australian Museum). They come in an amazing variety of shapes and sizes. All fishes have the following features in common: they live in water; have a backbone (vertebrate); and breathe using gills. Most fishes also: have scales; move using fins; and are cold blooded poikilothermic organisms. There are exceptions to these generalisations, e.g., some species of eels have no fins, lampreys and hagfish have no scales and some shark and pelagic predators are partially warm blooded. Fishes vary in size more than any other vertebrate groups. The world’s largest fish is the whale shark (Rhincodon typus), which can grow to 20 m long and a massive 34 t in weight. The smallest is arguably Paedocypris progenetica, part of the carp family, measuring only 7.9 mm long. Generated on 07/10/2021 https://marinewaters.fish.wa.gov.au/resource/fish-anatomy/ Page 1 of 18 Figure 1. The world’s largest fish – the whale shark (Rhincodon typus) There are three main groups (or classes) of fishes: 1. bony fish, 2. cartilaginous fish, and 3. jawless fish. Bony fish (Class Osteichthyes) Bony fish represent the largest and most diverse class of fishes, with well over 20,000 species. As the name suggests, bony fish have a skeleton made from bone. They possess true scales, a single pair of gill openings and an operculum that covers the gills. Most bony fish also possess a swim bladder to control their buoyancy. Most bony fish reproduce by external fertilisation, with the larvae developing outside of the parent’s body. Generated on 07/10/2021 https://marinewaters.fish.wa.gov.au/resource/fish-anatomy/ Page 2 of 18 Figure 2. A bony fish, the black trevally (Caranx lugubris) has a skeleton made of bone rather than cartilage (Image: © Shannon Conway) Cartilaginous fish (Class Chondrichthyes) Sharks, rays, skates and chimaeras belong to a class of fishes called Chondricthyes. All members of this class have skeletons made of cartilage rather than bone. These cartilaginous fish do not have typical fish scales; instead their skin is covered with tooth-like scales called denticles, making their skin extremely tough and abrasive. The gills are also different to bony fish – all cartilaginous fish have between five and seven gill slits. The teeth of cartilaginous fish are buried in their gums (rather than being attached to their jaws) and the teeth are constantly replaced. Cartilaginous fishes all reproduce by internal fertilisation, therefore producing small numbers of offspring. They either give birth to live young or lay a few large eggs. Males have claspers that are used to grasp females during mating and insert sperm into the female’s body. Generated on 07/10/2021 https://marinewaters.fish.wa.gov.au/resource/fish-anatomy/ Page 3 of 18 Figure 3. A cartilaginous fish, the black-tip reef shark (Carcharhinus melanopterus) has a skeleton composed of cartilage, as opposed to bone (Image: Ian Scott) Jawless Fish (Agnatha) Jawless fish are sometimes given the name Agnatha, which is Ancient Greek for no jaw. Among the most primitive of vertebrates, this group comprises lampreys and hagfishes. Around 50 species exist worldwide. Jawless fish have long bodies and look like eels. Their skeletons are made of cartilage, not bone. They have no scales. Lampreys are parasitic. Instead of jaws they have a sucking disc that they attach to their fish host and rasp away at the flesh.They are found in marine and freshwater. Hagfishes live only in marine waters. They are scavengers and feed on dead fish and other bottom-living animals using a similar method to lampreys. Generated on 07/10/2021 https://marinewaters.fish.wa.gov.au/resource/fish-anatomy/ Page 4 of 18 Figure 4. The pouched lamprey (Geotria australis) is native to rivers throughout the south west of Western Australia from Bunbury to Albany (Image: Neil Armstrong) ‘Fish’ or ‘fishes’? A group of fish of the same species are called fish. Two or more species of fish are called fishes. For example, a number of Australian herring swimming together is referred to as a school of fish. If you are referring to two or more different species, e.g., pink snapper and dhufish, the term fishes would be used. Figure 5. Fish or fishes? Left: A school of fish – Blue fusiliers (Caesio teres) (Image: Lynda Bellchambers). Right: Fishes – A goldspot seabream and multiple bigeye seabream (Image: Rachel Green). Generated on 07/10/2021 https://marinewaters.fish.wa.gov.au/resource/fish-anatomy/ Page 5 of 18 External Anatomy Figure 6. External anatomy features common to bony fish (Illustrations: © R. Swainston/www.anima.net.au) Body Shape Fish have adapted to live in an enormously wide range of aquatic habitats. The shape of a fish’s body tells us a lot about where it lives, how it feeds and, in particular, how it moves through the water. Benthic or bottom-dwelling fish (such as flounder and wobbegong) are generally flat in shape, so they can more easily camouflage with the bottom and ambush prey or avoid being eaten themselves. A flat pancake-like shape indicates the fish is able to stay very close to the sea floor. In contrast, fish that live in reef or coral crevices (for example, butterfly fish) have deep, flat bodies that are highly agile so they can move around without bumping into rocks and reefs. Long slender fish (such as eels and catfish (cobbler)) are able to hide under rocks and among coral, where shorter, stouter-shaped fish would have difficulty in going. Slow-moving fish with rounded bodies are often protected by spines or armour plating, and may also have poisonous flesh (such as blowfish and white-barred boxfish), owing to their lack of manoeuvrability. Fish with more elongated bodies, such as Western Australian salmon are able to swim very fast for a long time and thus have less need for any special body protection. Generated on 07/10/2021 https://marinewaters.fish.wa.gov.au/resource/fish-anatomy/ Page 6 of 18 Figure 7. Fish body shapes can be broken up into three distinct groups – extreme accelerating (for example mulloway); extreme cruising (such as tunas); and extreme manoeuvring (such as angelfish) (Illustrations: © R. Swainston/www.anima.net.au). Fins Fish possess fins to assist movement in the water. The pectoral and pelvic fins are paired fins – they are the same on both sides of the body. Pectoral fins can be used individually to manoeuvre the fish up, down and sideways. Together, these fins act as brakes and the fish can also use them to swim backwards. The pelvic fins are used for braking and steering. Fish also have single (unpaired) fins along the centre line, such as the dorsal (back) fins, anal fin and caudal (tail) fin. The dorsal and ventral (anal) fins play an important role by acting as Generated on 07/10/2021 https://marinewaters.fish.wa.gov.au/resource/fish-anatomy/ Page 7 of 18 stabilisers – without them the fish would roll over on its side. The fins of most bony fish can be folded flat against the body unlike sharks where the fins are rigid. The shape of the caudal fin plays an important part with the speed and strength of a fish’s forward movement (locomotion) (Figure 8). Figure 8. The tail or caudal fin is connected with the speed and strength of the fish’s forward movement and its shape plays an important part (Image: Department of Primary Industries and Regional Development). Generated on 07/10/2021 https://marinewaters.fish.wa.gov.au/resource/fish-anatomy/ Page 8 of 18 The majority of sharks have a heterocercal tail – that is, the upper and lower lobes are not the same size. The upper lobe is longer than the lower lobe which provides lift but is balanced by the pectoral fins. Scales Most bony fish have scales. The primary purpose of scales is to give the fish external protection from predators, as well as parasites and other injuries. The scales of bony fish are partly embedded in the skin, and the free parts overlap like tiles on a roof. Fish are usually scaleless when they hatch and develop scales during the first year. Figure 9. The overlapping scales of fish provide external protection. Fish scales come in all sorts of sizes and shapes, ranging from small in fast swimming pelagic species like tuna, to large in slow moving reef species like baldchin groper. Bony fish also have a very important mucus layer covering the body that helps prevent infection. Generated on 07/10/2021 https://marinewaters.fish.wa.gov.au/resource/fish-anatomy/ Page 9 of 18 Fishers should be careful not to rub this slime off when handling a fish that is to be released. Handling fish with wet hands or a wet rag will help to protect the mucus covering. Sharks have small tooth-like scales, called denticles, embedded in their skin. Lateral line All fish have a sense for which humans have no parallel. The lateral line is a sensory organ that runs along the sides of the fish’s body, under the skin. It consists of a series of tiny, sensitive cells called neuromasts, which are housed in mucus-filled canals. Small pores in the fish’s skin and scales allow vibrations in the water to pass through to the lateral line.