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The shape of fish

Posted on: August 24, 2012

Fish body shape comparison graphic

Fish body shapes give clues to how it moves through the water.

Did you know that the shape of a fish depicts where in the ocean it lives? Some fish are built for speed, others for manoeuvrability and some are built to travel long distances.

Demersal, or bottom-dwelling species, such as flounder and wobbegongs, are generally flat in shape. They do not have to be streamlined as they don’t tend to swim continuously, and being quite flat in shape allows them to stay close to the bottom and close to their food source.

Fish that live around reef areas (e.g. butterfly fish) have deep, flat bodies and are highly agile so they can move around easily.  Long, slender fish (e.g. moray eels) have the ability to hide under rocks and amongst coral.

Slow-moving fish with rounded bodies tend to have spines or armour plating, and many have poisonous flesh (e.g. blowfish) as forms of protection.  Fish with more elongated bodies (e.g. Australian herring) have the ability to swim relatively fast for long periods of time and thus don’t have the need for any special body protection.

The internal anatomy of a fish may also play a role in the fish’s survival.  Pelagic, or open water, fish such as tailor have a small swim bladder.  The swim bladder is an organ that the fish controls the amount of oxygen in, that enables the fish to control its ability to ‘float’ and ‘sink’ in the water column. Demersal species such as Western Australian dhufish have relatively large swim bladders to cope with large changes in pressure.  If you were to conduct a fish dissection with your students (see the Marine WATERs Lesson Plan: Dissect a Fish), the swim bladder, assuming it hasn’t been perforated, will be the air filled sac under the intestine. You can be assured it is quite safe to ‘pop’, in fact it is a bit of an anticlimax – no noise, no smell, no gooey stuff spurting out.

To find out more about fish shapes and how they aid a fish’s survival, check out the Marine WATERs Poster: Fish ‘Fiziks’.

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