Starfish are fascinating animals which are part of a larger group called ‘echinoderms’, meaning ‘spiny-skinned’. The group includes some very spiny species like sea urchins, sea cucumbers and brittle stars.
Starfish do not always have five arms, some species have more. For example the common sun star and the seven armed starfish, both found in British waters and the crown of thorns starfish found in tropical waters.
There are around 1,500 different species of starfish in the world and around 32 species in British waters.
Most starfish release eggs and sperm into the open water, where fertilization occurs. Larvae then drift freely in the sea as plankton allowing wide dispersal. One of the most beautiful and interesting larvae is that of the seven armed starfish, which resembles a shooting star. When ready to settle, the larvae split into two sections. A small orange starfish, which sinks to the seabed and a transparent, jelly-like section, which continues on its way, surviving for over a month before eventually dying. The cushion Starfish is different as it lays its eggs, on the shore, which develop into tiny starfish.
- If a starfish loses an arm, it can grow another. Some starfish shed arms which are damaged or to escape predators.
- Starfish use seawater to transport oxygen around their bodies and ‘breathe’ through their feet.
- Some starfish eat with their stomachs outside their body.
“How do they move?”
Starfish move using small, hollow tube legs. Each has a small sucker at the end and is controlled by muscles and a complex system of hydraulic tubes.
“What do they eat?”
Many species of starfish are predatory, meaning they feed on other animals. The spiny starfish will even hunt and eat other starfish!
“How do they feed”
The common starfish feeds on mussels and other similar animals by prizing open their shell with their strong arms. Once the mussel becomes weak and opens its shell very slightly, the starfish’s stomach is inserted into the mussel’s shell where it digests the soft flesh inside (quite a gruesome way to go!).
“What is the biggest?”
The largest starfish in the world is the sunflower star (Pycnopodia helianthoides) found in the north pacific it can measure 40 inches across and weigh up to five kilograms (11 pounds). It has around fifteen to twenty four legs.
“Where do they live?”
Starfish are marine species and are found around the world’s oceans from small tidal rock pools to the deep depths of the ocean.
“How long do they live?”
They can live for a variety of different ages some live for around five to thirty years depending on species.
Some species commonly found around the British Isles
Common Starfish (Asterias rubens)– Common in the Eastern Atlantic, it often feeds on mussels and can occur in huge numbers.
Cushion Starfish (Asterina gibbosa)- Often found when rock pooling.
Sand Star (Astropecten irregularis)- it burrows under sand, hunting small animals.
Spiny Starfish (Marthasterias glacialis)- A large starfish which sometimes eats other starfish