There are more than 2000 species of sea stars worldwide, but around the Discovery Islands, it’s the bright pink and purple hues of the ochre sea star (pisaster ochraceus) that really stand out.
Many wonder why a sea star that’s usually purple is known as the ochre sea star. In truth, not all are shades of purple or pink, they also come in oranges and browns. Why the variety? There’s actually no scientific consensus, but one 2006 study found sea stars that feed mainly on mussels more likely to be shades of orange, while those in areas with fewer mussels and more barnacles, (such as the Discovery Islands) more likely to be purple or pink. It’s probable that the settler who coined the name “ochre sea star” just happened to be in an area with lots of mussels!
Ochre sea star colour variation
One name that’s not correct is “starfish”. Sea stars aren’t fish, and apart from living in the ocean, they share little else with their watery chordate friends. Sea stars belong to the phylum “echinodermata”, which roughly translates to “hedgehog skin” and includes sand dollars, sea cucumbers, and sea urchins. In addition to boasting some sort of spiny(ish) surface, echinoderms are most recognizable by their five part radial symmetry. Check it out the next time you pick up a sea urchin or sand dollar; you should see five distinct rows of muscle or tube feet.
If you’ve ever played with a sea star, you’re likely aware of the incredible holding power of those tube feet. While the feet themselves are moved by a water-vascular system (think water pressure), their incredible holding power is generated by a temporary glue secreted from the end of each tube foot. Sea stars leave behind a bit of that glue with each step, meaning that the seafloor is actually covered in tiny invisible sea star footprints!
In addition to locomotion, tube feet play a major role in the grotesquely fascinating ochre sea star feeding process. With their incredible strength, sea stars can easily pry open bivalves such as clams and oysters, insert their stomach, and begin to digest the bivalve within its own shell. Keep this in mind the next time a sea star asks to come by for dinner! Not known for their discerning tastes, ochre sea stars also feed
on barnacles, anemones, urchins, and even other sea stars. So voracious are their appetites that some organisms have developed escape mechanisms specifically to defend against sea star attacks. Most dramatic is the swimming anemone, which detaches at the first touch of a tube foot and awkwardly “swims” to a new home.
Ochre sea stars help to control sea urchin populations
Like many other ocean dwellers, sea stars utilize broadcast fertilization for reproduction. Females and males release sperm and eggs (one female can produce as many as 40 million eggs!), and the resulting larvae drift for several months before the few survivors settle to the seafloor. Sea stars can famously also reproduce asexually. Although rare, a perfectly cut in half ochre sea star can grow into two genetically identical stars, but it’s far more common to see a sea star regrow a limb that has been chewed off by a hungry seagull. Sometimes the process gets messed up and extra limbs result; we once found a seven legged sea star!
Although juvenile sea stars fall prey to many organisms, fully grown ochre sea stars have very few natural predators, and their population is most likely kept in check by food and habitat limitations. So why have we seen record sea star die-offs in recent years? The answer is a double edged sword: ocean acidification and disease.
You may have heard of ocean acidification. Increasing atmospheric C02 is absorbed into ocean waters, causing them to become more acidic. This is not a slow change; the acidity of the ocean has increased by a staggering 25-30% since before the industrial revolution. This is disastrous for ocean health: it dissolved the calcium-carbonate shells and skeletons of sea stars and countless other ocean organisms. This slows or even reverses growth, and increases their disease susceptibility.
Once common, a sunflower sea star is now a rare sight for us.
Rising atmospheric C02 levels also increase ocean temperatures, and create perfect conditions for warmth-loving microbes that attack sea stars. These are believed to have caused the 2014 “plague” in which most sea star species on the coast were reduced to a fraction of their original populations. Such diseases are naturally occurring, but there has never been a recorded die-off as severe as 2014. Some populations such as the ochre sea stars are bouncing back, while others such as the sunflower sea star are now considered regionally extinct (we still see a couple every year here in the Discovery Islands).
Equally disturbing have been the ecological effects of the disease. Sea stars are considered a keystone species in the marine environment for the role they play in keeping other species’ populations under control. Indeed, with reduced sea star numbers, we’ve seen a massive increase in red sea urchin populations. While beautiful, these fellow echinoderms have happily munched most of the kelp forests and the critical habitat they provide. Some point to the adaptability and quick reproduction rate of sea stars as reasons why they may ultimately survive and restore balance to the ecosystem. Others cite out of control climate change and residual pockets of disease as reasons to be less optimistic. One thing is certain; we humans have put sea star existence in a perilous place.
For many, the ocean is a far away sheet of water. Look below the surface though, and you’ll see a beautiful and diverse web of connections every bit as vital as those of the Amazon. So please spare a thought for our ocean friends the next time you gas up your car, purchase a plane ticket, buy something you don’t really need, and especially when you vote. Sea stars need all the help we can give them.