Sunflower star: an amalgam of superlatives - part 2

Part 2 of the sea stars of Vancouver Island features one specie: the sunflower star, Pycnopodia helianthoides. With more in information about biology, behaviour and morphology.

It is the most impressive sea star of British Columbia. In my post of October 21, 2008 I described it as massive and it is. This one (top) looks like the beheaded Medusa. 

It can reach a diameter of approximately 100 cm, so it is the largest sea star on earth, albeit not volume-wise.

I saw a lot of sunflower stars, but not in numbers as described and shown on echinoblog.blogspot.com: in 2010 huge numbers of sunflower stars were seen in British Columbia. I can not remember seeing more than 20 at any dive. For more information have a look at this very informative and extensive blog about echinoderms. 

The sunflower lives from the intertidal zone - nearly always subtidal - to 120 meters deep. One specimen was found at 435 meters deep, but that's probably an error (Lambert, 2000).

One of the days at Rock Bay it was extreme low tide. To my surprise this sunflower star didn't bother to move to deeper water, although it can walk quite swiftly. 'It may be the largest and fastest seastar in the world. It can move up to 3 meters per minute, and has been known to travel at least 3 km' (Cowles, 2005). 

But maybe there is no reason for such behavior. Apart from the Alaskan king crab and Solaster dawsoni, the northern sun star, it hasn't any enemies. In the Netherlands gulls frequently swallow starfish like Asterias rubens (NL: gewone zeester) (in WoRMS: Echinaster sepositus, a mistake?), but they are relatively small. A sunflower star this size is too big to swallow.

Where as some sea stars have a typical habitat, the sunflower star is not picky. Sand, gravel and mud bottoms, rocky coasts, among seaweeds, on pontoon bridges.

I once picked up a sunflower star to show it to my wife. It was heavy, I think about 3 to 4 kg. I turned it upside down to prevent the tube feet to stick to my glove. To my surprise it still sticked: with its pedicellaria. These are pincerlike appendages with which it can clean its back against on growing seaweeds and barnacles. And as it turned out to defend itself. In Barnes (1968): 'pedicellaria are used for protection and to capture small animals.'

The sunflower star is a voracious predator: on bivalves, snails, chitons, urchins, other sea stars, sea cucumbers, crabs and it also scavenges on dead animals. So almost anything this carnivore can lay its arms on...

These brittle stars, Ophiura species, were fleeing from the approaching sunflower star. Normally they are not quick enough to escape. But in this instance the sunflower star was not hungry (enough) and let them escape.

This sunflower star excavates bivalves for food by transporting sediment particles. It can evert its stomach, but if the prey is small enough (not to difficult when you are that big) it swallows it whole.

The shell was already cleaned out.

This lined chiton, Tonicella lineata, in the arms of a sunflower star should be terrified.

Colour variations.

Like lava.

The skeleton of sea stars consists of multitudes of small calcareous plates called ossicles that move with one another, forming flexible joints. In the sunflower star they are not connected to each other, hence the flexibility of this sea star. If you take a sunflower star out of the water its weight will easily tear it apart, so you have to support it well.

The fingerlike projections, the papulae, are thin-walled and are extensions of the coelom (the internal body cavity) that protrude between the calcareous plates. They are in short its gills.

The sunflower star has a lot of spines, which are hidden in the rounded cushion like appendages (one in each). I know they appear when you disturb the sea star. But ever so often you can see the spines when there is no visible reason for 'agitation'.

Its numerous (up to 15.000) tube feet.

Young specimen. They start with five arms and gradually grow more arms.

And more.

You don't know how they increase their arms in number? Like this!