Ochre sea star on kelp off the coast of California

© Ralph Pace/Minden Picture

A star is borne by seaweed

With a little 'kelp,' this starfish is enjoying a change from its typical view of craggy tidal beds. Both passenger and vehicle in this photo, taken off California's coast, play important roles in their ecosystems.

Canopies of underwater kelp forests create habitats that support many varieties of sea creatures. Distinguished by broad leaves and spherical gas bladders that make the seaweed float, kelp is also part of the diet for many organisms. 'Many organisms' includes us: Kelp-derived algin is an emulsifying and bonding agent that's added to various processed foods, including many ice creams (algin keeps the ice cream from crystalizing).

And despite appearances, our cute little guest star (an ochre sea star) is actually a terrifying killer if you happen to be a mollusk. Starfish like this one have the rare ability to wrench open a mussel 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. The sea star's role in maintaining diversity in its ecosystem makes it a keystone species.