Those who know me are aware of my strong interest in the seaweeds, this diverse group of marine algae found in the numerous habitats along the Cape’s coastline. There is no outdoor activity that I enjoy more than scanning the Cape’s beaches, marshes, and rock jetties in search of rare specimens of green, brown, or red algae.
One of my favorite seaweeds is the strangely shaped and somewhat elusive brown alga, Leathesia difformis, commonly called the sea potato. It’s one of the easiest seaweeds to identify, characterized by being hollow, spherical, and rubbery. It goes by other descriptive names as well, including rat’s brain and sea cauliflower. It is approximately three to five inches in size, depending on its age. Normally it is found washed up on the beach during the warmer summer months. It disappears in the winter as it enters its microscopic stage. Its range is quite extensive, and can be found from Canada to the Carolinas. There are many locations on Cape Cod where the sea potato can be spotted.
This alga is a member of the brown algae phylum, the Phaeophyta. Its color is due to the pigment fucoxanthin which masks the chlorophyll pigments that enable it to make its own food through the process of photosynthesis. While the sea potato may be unfamiliar to most, it belongs to the same group of algae that includes the common rockweeds and kelps. These species are recognized by the presence of air bladders which allows them to float, thus maximizing their growth and ability to make sugars. Other brown algae species form long, slimy filaments, such as the notorious mung which clogs fishermen’s nets in the lower Cape area.
The sea potato is epiphytic, which means that it grows on other seaweeds, particularly the common red alga, Irish moss (Chondrus crispus). I have also collected it growing on eel grass (Zostera marina). However, it is typically seen isolated from its host when found on a beach. I am always searching for it when I’m beachcombing, but sometimes with little success. In fact, it appears when and where I least expect it.
My most successful collecting trip occurred at the barrier beach system of South Cape Beach in Mashpee. I took my coastal ecology class from the Cape Cod Community College, and we found dozens of specimens at one time. The round, yellowish-brown sea potatoes are sometimes seen there tangled with other plants and shells in the wrack line. However, there have been many visits in which our search came up empty. After a series of trips to this beach without finding any samples, I offered an automatic “A” on a field report to any student who could locate a sea potato. I should have inspected the beach first. We weren’t there for a minute before each student came up to me with a handful of sea potatoes. Grades were good for that class.
We collected a few samples of these spherical algae and took them back to the classroom lab for closer examination. The students commented on the gelatinous and spongy nature of the algae. I was pleased to see the level of interest and enthusiasm in my students as they discovered something different and unusual. We cut small, thin sections of the seaweed, and looked at them under a microscope, revealing a mass of thread-like filaments. Because of its round shape, it was impossible to make a pressed specimen for the herbarium, so I preserved
it in a jar of formalin. Unfortunately, some of the yellowish-brown pigment faded when treated this way.
Sometimes I think that my students get a great deal of pleasure out of seeing my reaction to a successful “seaweed hunt”. Once I took a couple of my Sandwich High School students to Town Neck Beach where we quickly found several sea potatoes. I was literally jumping up and down with excitement as I had never found this species at that location before. I explained the unique biology of the sea potato to the two kids, and told them how lucky they were to see this. Out of the corners of my eyes, I could see them rolling theirs.
It’s amazing how I can go exploring for many weeks to several locations all around the Cape and never see this special seaweed. And then, taken by complete surprise, one or more will appear along the shore. This happened to me recently at Cadet Beach at the Massachusetts Maritime Academy in Buzzards Bay. The sea potato remains elusive just enough to keep up my interest, and it’s always a joy when I encounter this beautiful and unusual plant of the sea.
Copyright Gil Newton 2009 Thanks to Chris Dumas for logo image.