Sea star adaptations – ventral view

Sea stars have many weird and wonderful adaptations - including some unusual internal systems. Click on any of the labels in this interactive to view short video clips or images to learn more.

Don’t forget to flip the sea star over and see what’s on top - the dorsal view!

Transcript

Hard body plates

Sea stars are invertebrates so they don’t have a backbone, but they do have a skeleton beneath their skin. This endoskeleton is made up of a complex network of hard bony plates made of calcium carbonate and held together by strong flexible tissues.

Acknowledgement: Suhailah J Nassar

Circulatory system

Sea stars have a very unusual circulatory system. They do not pump blood around their bodies. Instead, they use seawater and a complex water vascular system to keep things moving. Their tube feet, also used for movement, are an important part of this circulatory system. Sea stars have hundreds of tube feet on their underside.

Acknowledgement: Kathleen E. Conlan PhD

Stomach

A sea star has 2 stomachs, the cardiac stomach and the pyloric stomach. It can push the cardiac stomach out of its mouth, in the centre of its underside, to engulf prey or insert it into prey (between 2 shells, for example). The stomach then secretes a powerful digestive enzyme to break down the prey.

When the cardiac stomach comes back into the body, the food in it is transferred to the pyloric stomach.

Acknowledgement: University of Waikato

Obtaining oxygen

Sea stars don’t use gills or lungs to breathe. They rely on diffusion across surfaces in their body. For example, most oxygen is taken up from water that passes over their tube feet and papulae or skin gills. Skin gills are small projections near the base of the spines, usually on the topside.

Acknowledgement: University of Waikato

Tube feet

DR MILES LAMARE

They move relatively slow so we wouldn't necessarily see them swimming or see them moving around. But certainly if you were to watch them for long enough you would see they are very mobile, and they move around using special structures called tube feet, which are like little tentacles with little suckers on the end of them, and an individual starfish will have 2 or 3 hundred of those. So it’s got these little tentacles, which are on its underside, which it uses to hold on to the rock wall or the wharf piling or whatever and move around in a co-ordinated fashion.

Nerve net

DR MILES LAMARE

They don't have a central nervous system. They have something called a nerve net, which is basically just all their nerves are spread over their whole body. But they can still move in a co-ordinated directional manner. So they’ll have parts of their body which are detecting a stimulus, and they’ll move towards that or move away from it if it’s something they want to go to or something that’s annoying them.

Useful links

Most sea stars have eyes on the tips of their arms. Learn more about these ‘eye spots’ in this blog by Ed Yong.

To extend learners, look inside a sea star to see the inner workings of an animal very different from us in this animation, Sea star body plan.