Ochre sea star on kelp off the coast of California

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 (Pacific United States), 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 often added to ice cream to keep it from crystallising).

And despite appearances, our cute little guest star (the 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 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 them a 'keystone species'—or critical top predator.