Ochre sea star on kelp off the coast of California, USA

© Ralph Pace/Minden Picture

Hitching a ride. The underwater forests supporting sea life

This little starfish is hitching a ride on kelp, a type of large brown seaweed which grows in underwater forests close to coastlines. And both have a crucial environmental role to play. Kelp's broad leaves and floating gas bladders are important habitats for a huge variety of sea creatures as well as being food for many organisms (including us: Kelp-derived algin is often added to ice cream). They also help capture carbon and reduce coastal erosion.

And, despite appearances, this little ochre sea star is actually a terrifying killer if you happen to be a mollusc. Starfish can wrench open mussels just wide enough to insert a stomach-like organ that digests the bivalve's body right inside the shell. Gross, but necessary: A landmark study showed that mussel populations grow to invasive levels in areas where even a few ochre sea stars have been removed, making these starfish a keystone species - or critical top predator.