Aren’t plants supposed be green?

One of my agenda items for spring break this week was to return to Elkhorn Slough and finish the hike that I started with my students a couple of weeks ago. I got out there only to be forcibly reminded that the visitor center, where the hike originates, is closed on Mondays and Tuesdays. Since I’d driven out there, I figured I might as well poke around the area and see what else would catch my eye. I ended up at Kirby Park, a public access area where kayaks put into the water. The tide was out when I arrived, shortly before noon, and the flats were occupied by foraging birds.

I was able to identify birds that forage in the water (avocets, northern shovelers, cormorants, and grebes) and birds that forage in mud (willets, marbled godwits, yellowlegs, and whimbrels), and there were others that I couldn’t see well enough to ID. I didn’t even really try with the gulls. I do know they weren’t either western or California gulls, but that’s about it. Someday I may be able to tackle the gulls, but with their multiple juvenile plumages they’re a notoriously tough group to figure out.

Many areas of Elkhorn Slough have been invaded by the Japanese mud snail Battilaria attramentaria. This snail was accidentally introduced into the area as tag-alongs on Asian oysters that were imported for mariculture. Battilaria aren’t very big, reaching lengths of about 30 mm, but they can occur in astounding densities. A researcher at the slough has documented how this invasive snail came to be so prevalent, and how it has affected the native California snail Cerithidea californica. From the boardwalk trail at Kirby Park I could look down and see many Batillaria in the exposed mud flat.

This isn’t a particularly dense group of Battilaria, either. Across the highway towards the ocean there are mud flats that, when the tide is out, appear to be carpeted with wood chips; all the “wood chips” are the shells of living or dead Battilaria.

One of the Slough inhabitants that I find very interesting is the plant Cuscuta pacifica, commonly referred to as marsh dodder. Dodder is a parasitic plant, and at Elkhorn Slough its main host is pickleweed (Salicornia pacifica). Pickleweed is a perennial succulent that dies back in the winter; it is now beginning to regrow into the mounds that will be the predominant plant in the salt marshes of the Slough.

The first time I saw dodder I thought that some clown had vomited a can of orange Silly String over the pickleweed. I still think that’s what it looks like:

One of the clues that something interesting is going on with dodder is the orange color. We are used to thinking of plants as being green, or at least green-ish, because they are photosynthetic. Dodder, on the other hand, is a parasite and lives off the tissues of its host; it therefore has no need for chlorophyll, the green molecule that captures light energy used to fix carbon into organic molecules. Looking more closely at the structure of dodder gives you an idea of how it makes a living:

Dodder consists primarily of orange tendrils that wrap around the host plant. The tendrils penetrate into the vascular tissue of the host and begin withdrawing phloem (the syrupy solution of sugars) from it. Once the dodder has established this internal connection with the host, its own roots die and the dodder becomes entirely dependent on the host. A single plant of dodder can send its tendrils around multiple host plants. From an evolutionary perspective it is impossible to believe that host plants such as pickleweed don’t have defenses against dodder. They may be able to repel the tendrils by producing noxious chemicals, but this is a topic that hasn’t been well studied. Somebody needs to fix that, as inquiring minds want to know.