When people go the the beach, they tend to be somewhat blinded by the density of biodiversity surrounding them. Sure, we’ll take note of the big jellies when they wash up on the beach, and they’ll probably see the gulls and pelicans drifting overhead, but we often tend to not notice the little things. The Sargassum swimming crab, Portunas sayi, is certainly one of those little things.
The Sargassum swimming crab is aptly named. It scratches out a living by drifting along with floating mattes of sargassum, that “seaweed” stuff you often find washed up on the beach. Sargassum isn’t really a weed at all; it’s not a plant. Instead, sargassum is a heterokont lifeform, more specifically a brown macroalgae. Floating about in the ocean, vast tracts of sargassum actually provide an entirely unique type of habitat for other organisms. Nudibranchs, shrimps, crabs… There are an array of species that are specifically adapted to living among the sargassum tangles. This crab, the Sargassum swimming crab, is one of them.
Of course, sargassum also has a habit of washing ashore when storms pass or when currents work against their favor. Along with the sargassum itself, so too do other organisms wash up on the shore. Though many of these organisms will “jump ship,” so to speak, and try to find another free-floating matte of sargassum, sometimes you can find these hitchhikers right there on the beach — beneath your feet.
As you can tell in these photos, the Sargassum swimming crab is fairly small (though this is a juvenile). They are also beautifully adapted for camouflage in the sargassum tangles. The coloring is spot-on, if you will. I find this species to be a perfect example of the phrase “hiding in plain sight.” That is indeed what this species does. It hides in plain sight, right beneath your feet, as you scamper into the surf zone for a day of swimming and sun burning.