The Biologist Is In: Convergence on the Seaside

Monday, March 9, 2015

Convergence on the Seaside

Cakile maritima flowers.

I found this plant growing on the beach on a trip to southern California two years ago. The four-petaled flower and shape of the seed pods told me it was in the family Brassicacea, like the wild radishes which are common in the region. The thick and succulent leaves, however, indicated this plant was unlike any plant in the family I had seen before. I collected a few seed pods, hoping to later identify the plant.

After periodic internet searches over the last two years, whenever I found myself thinking about the plant, I finally found an image of a flower with the right shape and color that also grows in beach habitats above the high-water line. I've identified the plant as Cakile maritima (European Sea-Rocket). It and the related C. edentula (American Sea-Rocket) and C. lanceolata (Coastal Sea-Rocket) have small seed pods which contain one or two seeds and are dispersed by floating in water. The pods and leaves differ in shape between the species, but they are otherwise very similar.

C. maritima seed pods.

The species in this genus are generally described as edible, with leaves tasting spicy like horseradish. The plant I found had leaves which tasted mostly sweet. This may be just because I chose young leaves to taste, or I might have lucked onto a plant that was mild instead of hot. Hopefully the seeds I've stored will germinate so I can find out. Either way, I see it as an interesting plant to develop for the salad garden.

Sea-Rocket is commonly described as edible, but it doesn't seem it was ever a major crop. I've found one reference to it being grown in a garden in 1596-1599, but it isn't clear if was grown for vegetable or botanical use.

Another interesting beach-side plant in the Brassicaceae is Crambe maritima (Sea-Kale). Both plants have succulent leaves and dry fruit that effectively float and disperse their seeds. I find interesting that the two species evolved convergently from related ancestors to similar forms in the shared environment of European coastlines.

Sea-Kale appears to have been commercially harvested in Roman times and was cultivated in Europe from around the 1600s, but went out of fashion around the time of WWII.

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