Stars in the Sea


Vermilion Star, Mediaster aequalis, at the Vancouver Aquarium

My earliest memories of seastars* are the times my parents drove my sister and me down to Sea World, and next to the stingray feeding tank, there were touch tide pools. We could carefully pet their backs (aboral side) with a pinky finger, and if we were lucky, a worker would pluck one off of the rocks lining the side and hand us it on our flat, outstretched palms. It tickled, its arms wiggling only a little bit in protest at this far-too-often injustice.

*The scientific community is working away from names for invertebrates that include “fish,” so starfish = sea stars and jellyfish = jellies.

The seastars always seemed so fragile; I gathered this from the way we were taught to hold them, as well as the way the arms would snap off of my dead one at home that came from the local craft store. It wasn’t until my summer course in Mexico that I realized they aren’t as breakable as I thought. I tugged them off of barnacles with ease as my class snorkeled, grabbing a rock with one hand while craddling the star with another as the waves swung my legs about.


Oral side of a Pyramid sea star

In fact, sea stars are incredibly resilient. There are a few species, such as the Blue Sea Star, Linckia laevigata, that actually leave arms behind on purpose; they just creep away while one stays locked put. Eventually, an entire individual will grow from this one piece.

Sea stars use structures called tube feet (the circles in the grooves of the Vermilion star, top) to move as well as anchor themselves in place for protection against strong waves as well as predators. These act like suction cups, and the sea star draws in water first through a madreporite, a spot you’ll always see on their tops, off center and between two arms. That water then travels through the stone canal, the ring canal, the radial canals, and the lateral canals (sounds like Venice!) to connect to each tube foot, which then uses a series of muscles to stick each one to the rock and create suction.

On this Pyramid sea star, the madreporite is the brown spot.

On this Pyramid sea star, the madreporite is the brown spot.

Now, sea stars come in many flavors. All invertebrates from the phylum Echinodermata (includes sea stars, sea urchins, sea cucumbers, sand dollars, and brittle stars) have pentaramous radial symmetry, which means that things come in multiples of five, like arms in sea stars, rows of tube feet in sea cucumbers, and the petaloids of sand dollars.

Five petaloids on a dead sand dollar

Five petaloids on a dead sand dollar

Some sea stars have way more than the typical five arms we automatically picture. When we snorkeled at Los Islotes in the Sea of Cortez, a California sea lion rookery, we saw a crown-of-thorns for the first time. A few of the girls plus one of my professors and I were letting the current pull us down alongside the island, snapping pictures and maneuvering around with a flick of our fins. I had just passed over a very shallow part and turned back to get a closer look at some coral when, for some reason, I turned my head to the left to find a large male sea lion had clambered in a few feet away and was now staring at me! I cursed the lack of peripheral vision inherent with a mask and got out of there ASAP. My heart still pounding (those guys are waaay bigger than you would think, and nasty too during breeding season), I almost missed the spiky new thing right underneath me. In fact, I didn’t really take notice of it until Shauna, a classmate, found me a few minutes later and asked to borrow my camera to take a picture of it. Ladies and gentlemen, the crown-of-thorns:

Crown-of-thorns, Acanthaster ellsii

Crown-of-thorns, Acanthaster ellsii

Another common feature up and down the Sea of Cortez is the Gulf sun star, with an astounding 25 arms! The color of these individuals varied from beach to beach; most were a mottled mossy green, baby pink, and mustard yellow,with this nice salmon-y underside. They were often found wrapped around the corner of a rock, so when plucked off,  they even doubled as hats!

Oral side of a Gulf sun star

Oral side of a Gulf sun star


4 thoughts on “Stars in the Sea

  1. Awesome post! Sea stars are one of my favorite marine creatures. Whenever my family and I go to the San Juan Islands, we always see lots of them (especially purple ones!) I can’t believe that you saw a Crown-of-Thorns, how cool!

    • I’m super jealous of your sea star sighting excursions; about the coolest thing my family can see back home in Utah is the occasional dead raccoon on the side of the road! I just got a Pacific Northwest marine species photo guide last weekend, and now I can’t wait to go tidepooling.

  2. Such beautiful photos, Kaitlyn! Is there a trick to taking good photos underwater? And I didn’t realize sand dollars were related to sea stars. Such interesting information, as well as interesting stories.

    • I would definitely recommend a waterproof camera, instead of doing what I did and putting a normal digital camera in a case. The buttons are much easier to deal with, fog isn’t an issue, and there’s probably a setting made just for underwater photography. It’s also important to get pretty close and use a macro setting, since light doesn’t travel through water very far. The hardest part for me is to anchor myself so that pictures don’t come out blurry. For every good photograph, I have about a dozen wonky ones. When diving below the surface, it’s close to impossible to stay at the same depth, and there are beaches where each wave will easily drag you in five feet and then pull you back as much. My best advice is to wedge an arm or leg against a hardy rock and ignore the barnacle cuts 🙂 I’m sure professional photographers go to much greater lengths to get their best shots!

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