Fish Biology and Anatomy of Marine Wildlife on Tobago Shores

Fish are aquatic vertebrates with fins for appendages. They 'breathe' by means of gills. Fish make up the largest of the vertebrate groups with over 20,000 species. They can be found in a great variety of lakes, streams, oceans and estuaries.

Fish are divided into two groups based on the composition of their skeletons. The sharks, skates and rays are cartilaginous fish. Their skeletons are made of cartilage, the same flexible material in human ear lobes and nose. The majority of fish have skeletons made of bone and are called bony fish.

• External Anatomy

  You can tell a bony fish from a cartilaginous fish without looking at its skeleton. The way the fish looks on the outside gives you many clues.
  

  Fish Anatomy Picture showing:
  caudal fin, lateral line, dorsal fin, eye,mouth, operculum, pectoral fin, pelvic fin, scales, anal fin of a fish.
  

  Shark Anatomy Picture showing:
  
  caudal fin, dorsal fin, lateral line, eye, mouth, gill slits, pectoral fin, pelvic fin, anal fin.
  

  Scales:

  
  Most fish have scales. In sharks the scales are called dermal denticles. They are tiny tooth-like structures in the skin. They give the shark's skin a smooth appearance that feels like sandpaper. Bony fish scales are made of bone and look like the shingles on a roof.

  Fins:

  
  Fins move, stabilise and sometimes protect the fish. A fish may have paired fins (pectoral and pelvic fins), and unpaired fins (anal, caudal, and dorsal fins). Some fish do not have all of these fins, and their placement shows great variability. The very flexible fins of most bony fish have visible supporting rays and spines. The skeletal supports of cartilaginous fish fins are not visible, and these fins are fairly stiff.

  Gills:

  
  Oxygen enters the bloodstream at the gills. The gills are feathery structures found along the sides of the head. The gills of a healthy fish are bright red due to the large amount of blood present. In bony fish the gills are usually covered by a bony plate called an operculum. In sharks there are five to seven gill slits on the sides of the head which allow water to pass out of the gill cavity.

  Eyes:

  
  The eyes of most fish are well developed. Most sharks have pupils that dilate and constrict and they have an eyelid that closes from the bottom upward. Bony fish eyes lack both of these characteristics.

  Mouth:

  
  Sharks' mouths are located on the underside of the head. In bony fish the location of the mouth varies.

• Body Shapes

  
  The way a fish looks is a good indicator of how it 'makes a living.' Body shape, mouth location and size, tail shape and colour can reveal a lot about a fish's lifestyle.

  Fusiform

  
  Fusiform, or streamlined, fish like the barracuda or jack are capable of swimming very fast. They usually live in open water.

  Laterally compressed

  
  Fish that are laterally compressed (flattened from side to side) usually do not swim rapidly (some schooling fish are an exception). However, they are exceptionally manoeuvrable. Many, like the angelfish, are found near coral reefs. Their shape allows them to move about in the cracks and crevices of the reef. A flounder is a laterally compressed fish that lies on its side on the bottom. Both eyes migrate to the left or right side early in development.

  Depressed

  
  Depressed fish (flattened from top to bottom), like stingrays, live on the bottom

  Eel-like

  
  Eel-like fish have a snake-like body shape. The electric eel and moral eels are good examples of fish with this body shape.

  Others: Many fish like the boxfish and porcupinefish do not fit into any of these categories. They are slow swimmers with special protective mechanisms.

• Tails

  The shape of the tail can be an indicator of how fast a fish usually swims.

  Crescent-shaped

  

  Forked:

  
  Fish with forked tails, like the striped bass, are also fast swimmers, though they may not swim fast all of the time. The deeper the fork, the faster the fish can swim.

  Rounded:

  
  Fish with a rounded or flattened tail are generally slow moving, but are capable of short, accurate bursts of speed. The mummichog has a tail with a rounded end.

• Mouths

  
  The location and size of the mouth can be a good indicator of the food a fish eats and where it lives. Fish with large mouths generally eat large food items like another fish; however, the whale shark eats very small organisms which it strains from the water with its huge mouth. Fish with small mouths eat small food items: small crustaceans or molluscs; and, fish with tiny mouths eat tiny things like zooplankton.

  Terminal:

  
  Terminal mouth located on the end of the head. These fish chase and capture things, like the tuna, or pick things, like the butterfly fish.

  Up-Pointing:

  
  A fish with an up-pointing mouth has a longer extending lower jaw. The mouth opening is toward the top of the head. The tarpon has this kind of mouth. They tend to feed near the surface.

  Sub-Terminal:

  
  A sub-terminal mouth is on the underside of the head. Fish with this type of mouth usually feed on the bottom. The bonefish has a sub-terminal mouth.

  Specialised:

  
  Some fish have specialised mouths. The seahorse has a tiny mouth at the end of a straw-like snout that is used to 'slurp' zooplankton.

• Colour

  Red:

  
  Red is a common colour in fish. You might think that red fish would be very easy for a predator to find. However, most fish that have this coloration live in dark or deep water, or are nocturnal (active at night). In deep water red light is filtered out quickly so red is a good camouflage. At night red-coloured objects appear grey. The squirrel fish has this kind of coloration.

  Countershading

  
  Many fish are dark on top and light on the bottom. Most of these fish are found in open ocean. When seen from above they 'disappear' by blending in with the dark colour of the depths or the bottom. From below the light belly blends into the sky above. The tuna is countershaded. Seabirds like puffins are also countershaded.

  Disruptive Coloration

  
  This is a form of camouflage. The patterns and lines break up the outline of the fish or help it to blend into the background. The brightly patterned fish of coral reefs blend in with the corals despite their brilliant colours. The moorish idol exhibits disruptive coloration.

  Eye Spots

  
  Eye spots are a form of mimicry. The eye spot, usually found near the tail, draws attention away from the real eye which is a target that a predator might strike. The eye spot may cause the predator to attack the wrong end and allow the fish to escape alive.

  Warning Coloration:

  Many fish use bright colours. to 'advertise' the presence of poisonous spines or some other defensive mechanism. The nave surgeonfish has two bright orange spots near the base of the tail that advertise the presence of razor sharp spines.

  Camouflage:

  Many fish have colours. or patterns that match their backgrounds. The flounder is a camouflaged fish. It can even change colour to match different backgrounds.

• Life Support Systems

  Fish need oxygen to survive. Since they do not have lungs they get their oxygen with gills. Oxygen in water passing across the membranes of the gills enters the fish's blood by diffusion. The oxygen that fish 'breathe' is dissolved in the water. It enters the water at the surface by diffusion and as a by-product of photosynthesis by aquatic plants.

  Temperature and salinity are also important. Most fish are adapted to a narrow temperature range. Rapid temperature changes may cause death. Most fish are also restricted to a narrow range of salinity. Freshwater fish placed in salt water, or saltwater fish placed in fresh water may die. Some fish are adapted for life in estuaries where the salinity fluctuates and others, such as salmon or striped bass, are able to migrate between salt and fresh water.

• Senses

  Fish have a number of senses that help them survive in their environments. Humans share some of them, but others are found only in fish.

  Sight:

  Most fish have well developed eyes which are located on the side of the head. This positioning allows the fish to see in every direction. Fish that are colourful probably have colour vision. Nocturnal fish have large eyes that help them see in the low light.

  Smell and Taste:

  Fish can smell things in the water with two blind sacs called nares. Nares are similar to our nostrils, except fish cannot breathe through their nares. A fish can taste with taste buds in the lining of its mouth and gills. Some fish have feelers, like the 'whiskers' on a catfish, called barbels, which are covered with taste buds. Hearing: Fish have ears but you cannot see them. Their inner ears are well developed and pick up sound waves in the water through the fish's body.

  Lateral Line:

  
  The lateral line system helps the fish feel movements in the water. The line, actually a row of tiny holes in the skin, begins behind the gill cover and runs along the side of the body to the tail. Tiny hairs in the lateral line system are sensitive to vibrations. This system helps fish swim in schools, avoid predators and find food.

  Electricity:

  Some bony fish and sharks have special pores on the head that allow them to detect electrical currents. This sense aids them in navigating or finding prey in dark or muddy water.

• Fish Behaviour

  Behaviour is the action of a fish in response to its environment including other animals. The most interesting and sometimes the most obvious behaviours involve interactions with other fish.

  Migration:

  Many species of fish migrate during their life cycle. Salmon and some species of eel make long migrations to spawn. This reduces competition for food and space between adults and young fish. Other fish migrate to escape cold temperatures or to find food.

  Schooling:

  Many fish gather in groups called schools. A school is a group of fish of the same size and species moving in the same direction. Schooling may increase the chances of finding food, avoiding predators and finding a mate. It may also make swimming easier. Sight is important in keeping the school together. The lateral line may also help.

  Aggression:

  Aggressive behaviour is an interaction between two fish of the same species or different species. It is usually associated with reproduction and defense of feeding territory. It includes posturing, direct attacks and ritualistic displays such as fin flaring and changes of colour During the breeding season some fish protect spawning territories.

  Resting:

  Fish don't sleep like people, but some fish spend a good portion of their day or night resting. Coral reef fish that are active during the day spend the night in caves and crevices that nocturnal (night active) fish hide in during the day. This may reduce competition, conserve energy and keep the fish safe from predators.

  Communication:

  Fish communicate with one another in many ways. Senses play a part in communication. Visual communication is important to most fish. Body movements, postures, colours., colour patterns and light are the primary means of visual communication. Sound is also used for communication. Sounds are produced by grinding teeth, rubbing body parts together and with the swim bladder, an air sac inside the body used to regulate buoyancy. Drum, croakers and toadfish get their names from the sounds they produce with their swim bladders. Fish also communicate by producing chemicals called pheromones that other fish can smell. A few fish that live in very muddy water where visual communication is impossible use electricity to communicate.

  Cleaning:

  
  Some fish are cleaners. They pick parasites from other fish. Cleaning fish, such as wrasses, are brightly coloured or patterned. They establish cleaning stations that other fish approach in order to be cleaned. Look for fish in a funny position or holding their fins out. They are signalling that they are ready to be cleaned.