Eyes Under Puget Sound: Critter of the month — the heart cockle

front view of cockle, a clam-like animal, and two side views, one held by a human hand

Left: Front view and side view of a small Clinocardium nuttallii specimen.
Right: Side view of Clinocardium nuttallii. Photo courtesy of Dave Cowles, wallawalla.edu.

What’s not to love about February’s adorable critter? With its many hidden talents, this charismatic bivalve proves that beauty isn’t just skin-deep.

Big-hearted

Heart with classifications: "Kingdom, animalia; Phylum, Mollusca; Class, Bivalvia; Order, Cardida; Genus, Clinocardium; Species, nuttalli.
The heart cockle is aptly named for its charming heart-shaped profile; the genus name, Clinocardium, is actually Latin for “sloped heart.” They are the largest cockles on the west coast, reaching almost 6 inches in length. Although they are considered a common species throughout their range of Alaska to California, we only collect them occasionally in our Puget Sound samples.

 

Quit playing games with my heart

We’re not sure how the heart cockle plays so “hard to get” with our benthic grab, but it certainly has plenty of tactics to evade potential predators. This clam species is famous for using its long, muscular foot to lurch away from danger like the creeping arms of the sunflower star, Pycnopodia helianthoides (see the cockle demonstrating this behavior on a different species of sea star in the video clip below). Each “hop” can cover several feet, making it one of the most mobile bivalves of its kind.
 

How is the heart cockle so good at telling when predators are close? The secret lies in its mantle, or internal body wall, which has a fringe of tentacles with sensory organs that are able to detect the scents of predatory sea stars.

Heavy heart

The heart cockle’s second line of defense is its thick, mottled shell. With 30 – 35 radial ribs that interlock perfectly for complete closure, the shell makes for strong armor against the powerful claws of red rock crabs. It also provides protection against a fall from a seagull’s beak onto the sharp rocks below.
 
If it can survive all that, a lucky cockle can live almost 20 years! Scientists use the growth rings on their shells to estimate their ages, much like the rings on a tree.
Cockleshell with small siphons along the edge. They look like fuzzy donuts.

A heart cockle pumps water through its siphons (incurrent siphon on top, smaller excurrent siphon below it). Image courtesy of Dave Cowles, wallawalla.edu.

Down the tubes

The heart cockle has to eat sometime! When it isn’t running for its life, you may find it lying on or just below the sediment’s surface with its short siphons protruding out of its shell, feeding on plankton. Siphons are hollow tubes that bivalves feed through. Water is drawn in though the incurrent siphon, filtered to obtain any tiny edible particles, then pumped out again through the excurrent siphon. A single cockle is a powerhouse of water movement, pumping over 2.5 liters of water per hour per gram of body mass!

The key to your heart

If you’re still not impressed by the heart cockle’s abilities, you might find it irresistible in another way – as a tasty morsel on your dinner plate! Although they are not commercially harvested, heart cockles make good targets for recreational shellfish diggers because they bury themselves very shallowly and are easy to dig by hand. When you go to enjoy your catch, don’t be surprised if you find a tiny crab inside; the pea crab, Pinnixa faba, loves the heart cockle too, and occasionally makes its home inside its protective, inviting shell.

Critter of the month

Our benthic taxonomists, Dany and Angela, are scientists who identify and count the benthic (sediment-dwelling) organisms in our samples as part of our 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 a Benthic 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 critter plays in the sediment community. Can't get enough benthos? See photos from our Eyes Under Puget Sound collection on Flickr.