Practical Fishkeeping Diploma - Week 3, Day 4

Here's the last lesson of week three of the Practical Fishkeeping Aquarist Diploma. Next week, we cover disease management.

Week 3, Day 4: Fish shapes, mouths and reproduction

Body shapes

Fish have a body shape optimally evolved to the habitat they live in, and the type of lifestyle they pursue.

Fish that move about lots, or swim through moving water are usually a hydrodynamic fusiform or torpediform shape, tapering at each end and streamlined.

Fish that live in crevices, or explore confided spaces often have a cyclindrical or filiform (eel shaped) body. Smaller fish may even be vermiform (worm shaped) to help them bury in soft substrates.

Fish from slow habitats are sometimes compressed and laterally thin, such as Discus and Angelfish are. They do not need powerful bodies for endurance swimming, but are able to change direction quickly, evading predators.

Fish in torrential rivers are often ventrally flattened with flat bellies, to present as little resistance as possible to water as they hug to substrates. They may even have adapted fins or mouthparts that act as suction cups, helping secure them to rocks.

Fish that require sudden bursts of speed for hunting (such as Pike) have thin, arrow like bodies with their dorsal and anal fins set far back, for optimal acceleration. This is called a sagittiform body shape.

Fish mouths

Fish mouths are positioned according to where they feed from.

Terminal mouths are positioned at the front of the fish, facing straight ahead. A fish with such a mouth can eat from all levels, and often finds its food in midwater.

Superior mouths are upwards pointing, such as those on Mollies. Fish with superior mouths feed mainly from the surface and will struggle to eat food once it sinks down past them.

Inferior mouths point downwards and are slung underneath the fish, as in most catfish. Fish with an inferior mouth need to wait for food to reach the substrate before they can eat it. Some catfish have devised ways of swimming upside down in order to access food from the water’s surface.


Fish broadly fall into two reproductive categories: livebearers and egglayers.

Egg laying fish make up the majority of species. Most aquarium species are ovuliparous, which means that the female lays eggs that are then fertilised by the male outside of her body. With ovuliparous fish, the female will lay her eggs, and the male will then follow her, fertilising them by depositing his sperm straight on to them.

Less common are oviparous fish, where the eggs are fertilised inside the female and then laid afterwards. Some sharks and rays do this. 

Livebearing fish in aquaria are, with rare exceptions, ovoviviparous. Guppies, platies and mollies are all good examples, where the male fish fertilises the eggs inside the female, but she retains them inside her as they develop. Aside providing some oxygen for them, the female provides no further nutrients. The eggs feed on the yolks sacs they had as eggs, and when fully formed the female will release the swimming young directly in to the water to fend for themselves. Another word for this type of reproduction is ‘aplacental viviparity’.

Another, rarer type of livebearing in fish is viviparity, where eggs are fertilised internally, but then receive nutrients either as tissue or blood from the mother. Some halfbeaks, seahorse and marine sharks use this form of reproduction.

Fish can be either gonochoristic or hermaphroditic.

Gonochoristic fish are those with clearly defined males and females, and never change sex throughout their lives.

Hermaphroditic fish may switch sex at some stage of their life. This process is usually sequential, meaning that the fish starts life as one sex and later transforms into the other.

Fish that start their lives female and transform into a male are said to be protogynous. Many marine wrasses behave this way.

Fish that start their lives male and later transform to female are said to be protrandrous. Marine Clownfish are perfect examples of this.

As well as sequential hermaphrodites there are also synchronous hermaphrodites. These fish are rare, but can function as either sex at any time. A pair of synchronous hermaphrodite fish may bond, and play alternate sex roles from one spawn to the next.

Sexual characteristics

Fish will often display sexual differences between males and females.

In livebearing fish such as guppies, there are clear primary sexual characteristics – differences between the sexes that are directly involved in reproduction.

For example, in male guppies, the anal fin has developed into a reproductive organ known as the gonopodium. This tubular accessory can be physically aimed in many directions and is used to shoot packages of sperm directly at the vents of females. By comparison, the female guppy possesses an entirely normal, non-adapted anal fin, making the sexes easy to tell apart.

Many fish display secondary sexual characteristics. These are physical clues to a fish’s sex that play no direct role on the fertilisation process. Typical examples of secondary sexual characteristics include size differences, differences in fin shapes and differences in colours between males and females.

A good example of a secondary sexual characteristic can be seen in Swordtails. While female swordtails always have an ordinary, paddle shaped tail, the males develop huge extensions on their tails. Many commonly kept species, including the bulk of all cichlids and gourmais display some kind of secondary sexual characteristic.

What defines male and female?

The gender of a sexually reproducing organism is defined by the size of the sex cells. The organism with the larger sex cell is the female, while the smaller sex cells are those of the male. 

That’s it for week three. Next module: Disease management

Click here for next lesson: Stress and environmental diseases

How to gain your diploma: Once all the course modules and revision pages have all been posted online, we will open a link to a website that allows you to take your free online exam. If you pass the exam, you will digitally receive your very own Fishkeeping Diploma, to show that you have successfully completed the course, and which is yours to display on the wall near your aquarium, hang in your fish house — or keep somewhere safe where you can take it out and just look at it from time to time.

Note: The Fishkeeping Diploma is not a formal or accredited qualification and should not be confused with the type of diploma presented by colleges, universities and other educational establishments.

Further reading from our sponsors

Selecting fish for a freshwater community aquarium