Purple Sea Stars and their cousins are no longer called “starfish” because they aren’t fish. They’re Echinoderms, related to sea urchins, sand dollars and sea cucumbers. While rebranding Pisaster ochraceus (the title they use on formal occasions) from starfish to sea stars has brought us closer to the awful truth, full disclosure requires renaming them again. So let’s call them what they are: the Purple Sea Monsters of the Pacific Northwest.

You may never have pictured these animals as monsters. In fact you’ve probably thought of them as lovely lilac rock decorations, or icons on holiday postcards, the very symbol of innocent summer days at the beach.

But you were wrong. These hideous things have five arms (or rays, as biologists call them). At the end of each ray is an eye. Well, they’re not full-on eyes; they’re “eye spots.” They’re hardly Nikon optics, but they can detect light or dark well enough to let the PSM (Purple Sea Monster) know whether it’s passing over or under something. But still, five eyes, where less greedy creatures limit themselves to a single pair.

The sea star’s body is leathery, with plates of calcium carbonate, and hundreds of tiny spines on the upper surface. It has no brain as such, but instead makes do with a nerve ring, which runs around its central disc and hooks up to nerves running down each ray. The downside to this decentralized management system is that PSMs rarely win Nobel Prizes. The upside is that a PSM is nearly indestructible. You may already know that if a sea star loses an arm, it can grow a new one. But did you know that if there’s enough of the nerve ring in the severed arm, it may grow a whole new sea star? Doesn’t that sound like something Ripley should nuke from orbit?

On its underside, the PSM waves hundreds of tube feet, which double as suckers. Appropriately for a monster, the sea star has no blood: it simply pumps sea water through its body to expand and contract the feet. These feet are also its primary sensing apparatus; while the PSM has cells all over its body that detect smells and tastes, they’re concentrated in the foot suckers.

One of the things those cells sense is the waste output from mussels. And this is where the monster part really comes in: If you’re a mussel, the sea star comes at you with terrifying speed  ̶  several inches a minute. (As an adult mussel, you spend your life cemented to a rock or to other mussels, so pretty much anything that can move is moving at terrifying speed.)

Once the PSM reaches you, it goes all Alien-facehugger, trying to pull your shells open with its multiple arms. As a mussel, you do have to open your valves slightly every so often to breathe, filter feed, and expel waste. Like taking a pee, it’s not something you can postpone forever. And it’s at this very personal and vulnerable moment that the PSM strikes: it levers the valve gap open further, then squirts part of its stomach inside your shells, and starts digesting you inside your own home. Worst. Houseguest. Ever.

In addition to their atrocious table manners, PSMs are neglectful parents. To see why, we must delve into their sordid sex life. To begin with, they’re not highly sexually dimorphic. For example, in many species of birds, the female dresses in quiet good taste while the male sports plumage that Adam Lambert would consider too campy. In contrast, there’s not much visible difference between girl sea stars and boy sea stars. In fact, you have to squint at their gonads with a magnifying glass to tell them apart. You might think that this similarity, combined with their poor eyesight, could make for awkward moments on the PSM dating scene: two sea stars bump into each other on the murky ocean floor, ten-armed groping ensues, followed by mutual embarrassment: “Oh… you’re a guy too?” But in fact sea star sex never involves much of a personal touch. Both genders take more of a spray-and-pray approach to reproduction, squirting millions of eggs and sperm into the open water, where they hook up more randomly than sophomores on spring break. Provided they don’t get eaten, those fertilized eggs grow through their larval stages into adulthood without any parental care.

Innocent cluster or wild orgy?

The closest sea stars might get to foreplay is clustering, where dozens of them pile up on top of each other. Even then, biologists aren’t certain whether this behaviour is to make sure that there will be a fruitful balance of eggs and sperm in the surrounding water or whether it’s a protective mechanism against overheating when low tide exposes them to air.

Beware: bleeding-heart marine biologists will try to persuade you that the Purple Sea Star is no monster at all. They’ll cite sciencey facts, like it being a keystone species. They’ll try to cast mussels as the villains, pointing out that when not kept in check by “our friends the sea stars,” mussels displace many other species in the intertidal environment and upset its ecological balance. Don’t be fooled. Next time you see a seagull or sea otter  ̶  about the only predators willing to choke down Purple Sea Stars  ̶  give’m a high five for keeping these monsters down to manageable numbers.