What is a fish?

A fish is an ectothermic aquatic vertebrate animal that breathes by means of gills; have fins used for movement and stability; most have scales. The study of classifying and biology of fish is the "Ichthyology".

Taxonomic classification is based on ancestral relationships and on their feeding and breeding behaviour.

3 main groups:

- JAWLESS Fishes (Agnatha): no jaw or scales; cartilaginous skeleton with several gill openings.

- CARTILAGINOUS Fishes (Chondrichthyes): Skeleton made of gristle or catilage with scales buried within their skin; gill openings.

- BONY Fishes (Osteichthyes): Skeleton of bone; thin scales; 1 single gill opening.

EXTERNAL ANATOMY

The body is made of 3 parts: head + trunk + tail

head : nostrils, mouth, eyes, gill covers

trunk : since the gill until the anus

tail : caudal peduncle and tail fin (caudal fin)

ECOLOGICAL ROLE

In any aquatic system, the energy enters in the form of sunlight and organic matter from plants and animals in the catchment, forming the nutrients for developing bacteria, algae, higher plants. Within this system, they are the primary producers upon which the rest of life in the aquatic environment depends.

These organisms provide food source for insects, snails and fish that are herbivores and detritivores (labeos, redbreasted tilapia) : primary consumers in in the aquatic web.

Carnivores and scavengers (crab, predatory insects, fish,...) feed on the primary producers and primary consumers.

They are the secondary consumers, which are in turn eaten by larger and more specialised top fish predators (tigerfish, african pike, bass).

The top predators will feed on any suitable prey species from any level in the pyramid. They are preyed by terrestrial creatures (birds, crocodiles, humans).

TYPICAL AQUATIC FOOD PYRAMID

REPRODUCTION AND GROWTH

The sexual reproduction in freshwater fish is one of 2 types:

1. OVIPAROUS (egg-producing): Most fish are oviparous and have male and female organs in separate individuals, with external fertilization and internal development o the eggs and embryos.
2. OVOVIVIPAROUS (live egg-producing): These fish practice internal fertilization and bear their young alive after the development  in the ovarian cavity of the mother. Ex: Guppy; Swordtails.

Oviparous: reproduce by means of eggs that develop and hatch outside the mother's body.

Ovoviviparous: reproduce by means of eggs that develop within the female, deriving some nutrition from her but remaining encased within an egg membrane.

Diapause: period during which the metabolism of some animals including fish or insects slow down, temporarily suspending their body development and growth. Such periods are linked to seasonal or environmental changes.

BREEDING BEHAVIOUR

Freshwater fish display elaborate form of territorial and ritualised courtship dance. Males establish and defend breeding territory to which females are attracted.

Some scatter eggs without preparing area for spawning, whereas others select sites (crevices...). Others construct simple or elaborate nests.

Exemples:

• Bass males clear suitable area and guard brood by himself, allowing females to go off and mate with other males.
• Banded tilapia and other cichlids pair up for breeding and excavate a depression for spawning, after which the male and female tend the brood.
• Red-breasted tilapia excavates a large nest with separate brood tunnels for the eggs and larvae.
• African pike and Blackspot climbing perch construct floating nests of froth or foam while guarding the nest from below.

GROWTH IN FISH

It is temperature dependent. Fish that live in warmer region grow quite quickly in summer when the temperatures are high and there is abundance of food. They stop growing during colder winter months. Seasonal growth is like 3 rings, is reflected in annual rings in scales.

Once reached reproductive maturity, they continue to grow. The lifespan is from 1 to 20 years. Large fish average is from 5 to 10 years. Smaller fish (Killifishes, Topminnows) is from 1 to 2 years. Minnows is from 3 to 6 years whereas Yellowfish, Carp, Tench, Eels is more than 10 years, some until 20 years.

SIZE

The size is determined by genetics, age, waterbody size, food availability, general ecological and other biological factors.

The largest freshwater fish species in South Africa are the Vundu and the Sharptooth Catfish, until 1.5 meters and 50/60 kg or more.

The smallest freshwater fish are the Upjaw barb, the Sicklefin Barb, the Sand Catlets :20 mm.

FEEDING BEHAVIOUR

They spend more time and energy eating, or searching for food than anything else.

They are grouped into:

• carnivores
• herbivores
• filter-feeders
• omnivores
• scavengers
• parasites

Most fish cannot chew their food because it would block the current of water that flows across gills. Some eels have molar-like teeth in their jaw for crushing their preys. Others are able to grind their food making use of pharyngeal teeth in throat. Most carnivorous swallow prey whole, using pointed teeth in their jaws and on roof of the mouth to grip their prey. When the mouth of large mouthed fish is opened, a negative pressure is created that causes the victime to be swept into the mouth.

What the fish eat is reflected in their feeding anatomy:

• predators: large protruding mouth with shap pointed teeth.
• omnivores: may have multi-cusped teeth.
• herbivores: teeth suitable for biting or nipping plants.
• Mormyrids (Snoutfish): small mouth with small teeth for extracting insects from soft sediments.
• Catfishes: large mouth with sensory barbells suited for feeding on a range of live or inactive food.
• Bottom feeders may scrape algae from rocks.

LIPS:

Fish with sharp lips scrape food from firm surfaces, grubbers have large soft lips.

Fish with thin lips are predatory fish. Lips cushion teeth allowing them to be exposed while feeding.

TEETH:

The teeth may occur along the jaws, in mouth and throat of some fishes. Teeth in mouth and throat serve the function of crushing and masticating the food before it is digested in gut.

The Cyprinids (Barbs, Yellowfish, Labeos) do not have teeth in their jaws but have very well developed teeth in throat.

Their fifth gill arches are modified to form strong teeth-bearing bones that grind food against a padded plate at the base of the skull.

In Cichlids, it is also modified and joined, forming a triangular bone with teeth that operate in conjunction with 2 teeth-bearing plates in palate.

The replacement of fish teeth takes place several times during their life cycle.

LOCOMOTION:

The movement in fish varies from a very flexible side-to-side snake-like head and tail movement, to those fish that propel themselves only by means of caudal tails movement (fast moving fish). The mechanism for propulsion comes from the musculature of its trunk and tail. Muscles that are called myomeres.

The larger the fish is the faster it is able to swim:

•  a 30 cm fish can swim at around 10 km/h
• a Barracuda(fastest fish) at 43 km/h

FISH SENSES

EYES SIGHT

• a fish lens is rounded which will reduce the distortion of the image
• they have a 3-D vision
• no moveable eyelids
• unble to vary size of pupils
• some have colour vision, but sharks, rays see only in black & white.
• the sight is useful for detect predators, search for food, maintain cohesion between schooling fish and it is a visual communication and mating behaviour.
• the eye placement depends on where the fish occur in aquatic habitat.

SMELL

It is highly refined, used to detect food, locate familiar areas, used for communication through pheromones.

TASTE

They have specialised cells concentrated around the mouth, lips and barbels (Catcfish, Cyprinids) to detect chemical elements in water.

HEARING

They have 1 pair of inner ears serving as organs of balance and hearing. The sound is carried through the water much easier than through the air.

PIGMENT AND COLOUR

• dull in colour: camouflaged in tune with the environment.
• bright colours: warning that they have poisonous defence mechanism.
• translucent or transparent: larvae, small fish.
• cryptic: blend into environment.
• counter-shaded: upper part dark, lower part bright
• some can change colour (Cichlids) for communication, especially during breeding season.
• colour is due to chromatophores in dermal layer of skin that contains orange, red, yellow and black pigment.
• the colour change is possible over both short and long term periods due to cells responding to both nerve and hormonal controls.
• Silvery sheen of many fishes is due to crystals contained in the reflective cells.