Eyes Under Puget Sound: Critter of the month – Common Sun Star

Crossaster papposus; photo courtesy of Neil McDaniel,
http://www.seastarsofthepacificnorthwest.info/index.html

Getting ready for the Aug. 21 solar eclipse? Well, this month’s critter was born ready. The Common Sun Star goes by many common names – Rose Star, Spiny Sun Star, Snowflake Star – but no matter what you call it, there’s nothing common about Crossaster papposus. We think it is out of this world!

Total eclipse of the star

With its bright sun-like appearance, the Common Sun Star is one of the more beautiful creatures in Puget Sound. Individuals can be all one color but generally vary in color and pattern, with some featuring pink, white, orange, and yellow rings.

A star is born

Common Sun Stars can grow to an impressive 14 inches or 34 centimeters across – larger than a dinner plate – but they also grow slowly, taking 10 years to reach maximum size. As small juveniles, they prefer shallow subtidal habitats; as adults they migrate to deeper waters, down to 1,200 meters in depth. This explains why we rarely encounter them while sediment sampling, although they occur from Alaska to Puget Sound – as well as the north Atlantic coast).



Two Common Sun Stars with different color patterns; photo
courtesy of Dave Cowles at https://inverts.wallawalla.edu/

Well-armed

The Common Sun Star has more arms than the typical five-armed sea star that's familiar to most of us. Resembling the sun for which the family Solasteridae was named, it has many pointed ray-like arms originating from a broad central disk. Some sea stars have as few as eight or as many as 16 arms. For this species, the magic number of arms is almost always 11. It, too, can regenerate, or grow back, missing or damaged arms if the central disc remains intact.

Close up of pseudopaxillae on top surface of a
preserved Crossaster papposus specimen.

A spine in the sand

Even if the Common Sun Star didn’t have a unique shape and color, it has plenty of distinct features that taxonomists like us can use to identify it. Its dorsal – or top – surface is covered with little hedgehog-like bundles of spines called pseudopaxillae. The mouth (located on the animal’s underside) is bare and surrounded by long spines, and there are two rows of sucker-tipped tube feet running down each arm. It also lacks pedicellariae, tiny pincer-like defense organs that many other echinoderms possess. 



Common Sun Star goes into attack mode;
photo courtesy of Neil McDaniel
 http://seastarsofthepacificnorthwest.info/index.html

Here comes the sun (star)

A dominant and agile predator, the Common Sun Star scoots across the sediment at 70 centimeters a minute, using sensory chemoreceptors to “smell” when a potential prey animal is near. While the Common Sun Star wouldn’t quite beat the Sand Star (Luidia foliolata) in a race, it has several other advantages over its speedy relative. By standing on the tiptoes of its tube feet, it can make itself tall enough to cover and engulf its prey.

Can you put your stomach outside your body to eat a big meal?  Well this star can. It has an eversible stomach, which means it can turn its stomach inside out, shooting it out through its mouth. This gives the Common Sun Star the ability to handle the larger prey items it likes to munch such as sea urchins, other sea stars, and clams.  When it appears with a big hump, you know it’s enjoying a big meal.

A brighter future?

In 2013, scientists in Washington State discovered sea stars that appeared to be wasting away and dying from a mysterious disease. We now know that Sea Star Wasting Syndrome is caused by an ocean virus, and it has wiped out millions of sea stars on the Pacific coast. Although Crossaster papposus is not one of the species hit the hardest, a few cases of wasting Common Sun Stars have been documented in British Columbia, and they are listed as “likely affected.” Keeping our fingers and arms crossed that sea star populations will rebound and thrive on our coast once more.

Critter of the Month
Our benthic taxonomists, Dany Burgess and Angela Eagleston, are scientists who identify and count the benthic (sediment-dwelling) organisms in our samples as part of Ecology’s Marine Sediment Monitoring Program. We are tracking the numbers and types of species we see in order to understand the health of Puget Sound and to detect any changes over time.

Dany and Angela share their discoveries by bringing us Critter of the Month. These posts will give you a peek into the life of Puget Sound's least-known inhabitants. We'll share details on identification, habitat, life history, and the role each species plays in the sediment community. Can get enough benthos? See photos from our Eyes Under Puget Sound collection on Flickr.

Critter of the month: The dumbbell worm

By Dany Burgess & Angela Eagleston, marine sediment monitoring taxonomists

Ventral (bottom) view of Sternaspis affinis with anterior end retracted

Welcome to our first edition of the Eyes Under Puget Sound Critter of the Month series. Our taxonomists are excited to shed light on the lives of some of Puget Sound's least known creatures: benthic invertebrates.

Benthic invertebrates are the tiny water animals that live in the soil at the bottom of Puget Sound. "Benthic" comes from the term benthos which means the community of organisms which live on, in, or near the seabed, also known as the benthic zone.

These mysterious benthic invertebrates often resemble something straight out of a science fiction novel that comes from a planet far away in space, but in reality they live in our own backyard.

Call me dumbbell worm, mud owl, or gooseberry worm, just don't call me late for dinner

Known to our scientists as Sternaspis affinis Stimpson, 1864, the dumbbell worm has many names. It has been nicknamed the "mud owl" because of two colorful protective shields, called the "caudal shield", that cover its gills when its head is buried in the soil searching for tasty edible specks of organic matter. The shields look so much like eyes that scientists felt that the worms looked like tiny owls in the soil.

The caudal shield and "branchiae", or gills, (see diagram below) were originally believed to be located at the head end, leading to the genus name Sternaspis, which is Greek for “breast-shield”. The actual head end includes a mouth and rows of thick hooks used for burrowing into the mud, and is completely retractable into the worm’s body. On the rear end, the long gills are exposed to the water column to help them pull oxygen from the water.