Kaylee Peoples, of Balclutha Primary School, asks :-

How do starfish breathe, eat and move?

Mike Barker, a marine zoologist at the University of Otago, responded.

Starfish like most of the animals they are closely related to (sea urchins, brittle stars, sea cucumber and sea lilies) breath by absorbing oxygen directly from seawater. Starfish have a complicated internal circulation system of vessels filled with water that is very similar to, although not exactly the same, as seawater. This is circulated very slowly around the body, not by a heart but by millions of tiny beating hairs called cilia that line the inside of the walls of these vessels.

Because the body of most starfish is quite thick and rigid and oxygen does not pass easily through it, there are small bulbs that have a very thin wall (like tiny balloons) that protrude from the inside of the starfish into the surrounding seawater. This allows carbon dioxide to pass out into the seawater and oxygen to pass in a process called diffusion.

Starfish move using tube feet. There are thousands of these long tubes each of which has a sucker on the end that are used to hold the starfish onto the bottom and also move it along. The feet are also in close contact with the seawater and also help it breath by allowing the easy diffusion of gasses. The arms (5 in many starfish varieties) can also be used to help move the starfish along by pushing on the bottom.

Starfish feed in a very peculiar way. Most animals take food into their stomachs through the mouth but starfish are able to push the whole stomach out of the mouth and use it like a giant bag to surround the prey and release digestive juices which dissolve the prey directly. The thin stomach can also be pushed through the tiny gap between the two shells of a mussel or cockle (one of their favorite prey)and eventually absorb the meat from the inside. This may take several hours.