Coast & Sea Areas

Wild Harris

Starfish and their Spiny Cousins

What do sea urchins and starfish have in common?

They are both Echinoderms. The name means 'spiny skin' and these animals have a hard outside protective skin which has spines which may be small (common starfish) or long (common sea urchin).

body of an echinoderm is normally divided into 5 parts. Common starfish have 5 arms, and sea urchins' bodies are divided into 5 as well (although some starfish have more than 5 arms - see below)

They all have tube feet. These are extendible little tubes, each with a sucker on the end, which the animal uses to move around and to grip solid objects. A sea urchin uses its tube feet to attach itself to the seabed or seaweeds, and a common starfish also uses them to grip the shells of a mussel to pull the shell open. Some starfish, such as the spiny starfish, have tube feet with little sets of jaws on the end which it uses to clean itself and to pick off small animals or plants which are trying to grow on its skin.

 common starfish skin.jpgCommon Starfish Close Up    (Photo: Paul Tyler)

A close-up of the spiny skin of a common starfish - not a very friendly place to try to set up home!

spiny starfish skin.jpgSpiny Starfish Close Up    (Photo: Paul Tyler)

The tiny pincers on the surface of this spiny starfish keep away unwanted guests

Some starfish have more than five arms - these include the 7 armed starfish and two varieties of sunstar.

luidia showing tubefeet.jpgSunstar    (Photo: Paul Tyler)

This seven armed starfish tastes the water and moves around using its tube feet Brittle stars and feather stars are also members of this group.

(Missing Image of Feather Star)

The feather-like arms of this echinoderm trap food particles. It has several short 'legs' that allow it to hold onto the seabed. Feather stars can also use their feathery arms to swim slowly through the water.

common brittle stars closeup.jpgBrittle Stars    (Photo: Sue Scott)

Brittle stars move their arms to crawl around instead of using tube feet. They also hold their arms aloft to catch passing food morsels.

urchin lots tubefeet.jpgSea Urchin     (Photo: Paul Tyler)

Sea Urchin A sea urchin's tube feet are arranged in 5 rows. The suckers at the end of the tube feet enable it to hold on and move around.

sea cucumber in crevice.jpgSea Cucumber    (Photo: Paul Tyler)

Sea cucumbers are the third kind of echinoderm. Many have a long body covered in spines which allow it to anchor itself in rock crevices or in sand. At one end is a ring of tentacles surrounding the mouth which are used to catch passing food items. This tree-like structure is actually the head end of a sea cucumber. The branches are tentacles that trap passing food and pass it to the central mouth. Most of the animal is wedged out of sight in a crack in the rock. Other sea cucumbers live buried in gravel, while some crawl around on rocks hoovering up all kinds of marine debris.

by Paul Tyler

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