Purple Sea Star (Pisaster ochraceus)

One thing I absolutely loved to do as a kid was wander around rocky shores, looking for any form of life. I’d lift up whatever rocks I could to search for crabs, I’d run from tidal pool to tidal pool to peer at starfish and sea anemones. I love living in Ontario, but one thing I desperately miss is the ocean. So today I’m going to going to write about a species of sea star, to fully embrace my nostalgic mood.

Probably the most common starfish I encountered as a child were large purple ones. Unsurprisingly, these are known as purple sea stars, but confusingly are also known as ocher sea stars, since not all of them are purple. They can also be orange, yellow, red or brown. That being said, most of these guys are purple, so I’ll refer to them as purple sea stars. They have five legs, each of which can be from 10 to 25 cm long. Purple sea stars are covered in very small spines, which are no bigger than 2 mm in height.

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An example of some colour variants in purple sea stars. Image credit: D. Gordon E. Robertson

Purple sea stars live in the Pacific Ocean, from Alaska to California. Depthwise, they can be found from 90 m to the low-tide zone. They are commonly found along the coast, on mussel beds and rocky shores. Special suckers on their feet allow purple sea stars to cling to rocks with great force, so they can withstand violent waves.

Despite their harmless looks, purple sea stars are quite vicious eaters. They feed mainly on mussels, but will also eat chitons, limpets, snails, barnacles, and sea urchins. They will swallow their prey whole if they can, grabbing their meal with their tubular feet and then everting their stomach onto their prey. The digestive enzymes in the starfish’s stomachs will then liquify the victim, so it can be easily ingested.

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A purple sea star. Image credit: D. Gordon E. Robertson via Wikipedia

Mussels try and defend against starfish by clamping their shells together very tightly, but sea stars are patient hunters. Mussels need to open their shells slightly, both to breathe and to feed. The starfish then use their strong feet to pull the mussel shell further open, and insert part of their stomach into the shell, digesting the poor mussel inside. Purple sea stars have quite large appetites, with one sea star being able to eat eighty Californian mussels a year (a California mussel can reach lengths of 20 cm, so they aren’t small).

Purple sea stars breeding from May to July, with both sexes releasing gametes into the water. If all goes well, sperm and eggs meet in the ocean,  and the fertilized eggs can drift around until they hatch. Sea stars progress through a number of larval stages, and at this stage of their life the little starfish are filter feeders, relying on plankton to sustain them. Purple sea stars live to be at least four years of age, and may live much longer.

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I decided to try something a bit more abstract for this week’s drawing. I had fun with all the shapes!

Purple sea stars aren’t just pretty; they are an extremely important species. Thanks to their diet, purple sea stars help keep mussel populations under control. A loss of only a few purple sea stars leads to a drastic increase in mussel populations. When sea stars are present, the intertidal ecosystem is diverse, instead of becoming dominated by mussels. So next time you see a sea star, remember that they help keep our oceans wonderfully diverse!

Cover image source

Common Sunstar (Crossaster papposus)

I grew up on the west coast of Canada, so I spent my fair share of time on the ocean, on beaches, and rummaging through tidal pools for interesting sea life. One group of animals I remember most were starfish, which always impressed me for a number of different reasons. They were: a) very small and thus extremely cute, or b) very large and therefore impressive because of their sheer bulk or c) brightly coloured, which was very pretty. I also remember seeing a sunstar on a kayak trip. It was huge – I think about two feet in diameter, and had a ton of legs. I was fascinated, and today I’m going to honour my childhood interest in sunstars by blogging about them.

Common sunstars belong to the family Solasteridae, which is comprised of a number of different species of sea star. I’m pretty sure what saw was a common sunstar, because they’re found widely in the northern Atlantic and Pacific oceans. They prefer ocean floors that are rocky, or comprised of gravel or loose sand. Though I usually associate sea stars with tidal areas, sunstars can be found anywhere from the low tide line to over 300 meter deep waters.

A sunstar showing off its many legs and bright colours. Image credit: Sue Scott

A sunstar showing off its many legs and bright colours. Image credit: Sue Scott

Apparently my estimate about the size of the sunstar I saw as a child was probably blown out of proportion, as common sunstars only grow up to 30 centimetres in diameter. I was probably just impressed by its legs and inflated its size in my mind. Common sunstars are really quite beautiful, with red colouring on top, and bands of white, pink, yellow or dark red on its legs. They have a large middle disc with relatively stubby legs, of which there can be up to fourteen. Can you imagine having fourteen arms? Think of how many things you could do at once!

Have you ever wondered how starfish breed? I hadn’t, until today. They do it the same as us, with sexual reproduction. Except they fertilize their eggs externally, with the female laying eggs and the male depositing his sperm onto them. Juvenile sunstars spend their time in waters with soft sediment bottoms until they are five centimetres in diameter, at which point they move to shallow water with rocky bottoms. As they grow, they move deeper and deeper into the ocean.

Contrary to what you might think, sunstars are not sedentary animals that sit around waiting for food to come to them. They are hunters, and are often the dominant predator in their ecosystems. Common sunstars can cover over five meters in just 12 hours. When they find prey, they pull in their sensory feet, then push out tubular feet, which pushes the sunstars above their prey, allowing them to swallow the victim whole. If the animal is too big for the sunstar to swallow whole, the sunstar just covers the animal with its body, and then ejects its stomach to engulf the prey. They should make a movie about giant killer sunstars. I bet it would be really gross.

Sunstars also possess the ability to regrow limbs, as long as a part of the central disk remains intact. Which definitely makes a killer sunstar movie an interesting possibility. Perhaps I should write to Hollywood…

Cover image source: http://www.habitas.org.uk/marinelife/species.asp?item=ZB1490