In general, seaweed shouldn’t move. Sway? Sure, but crawl? Slither? No, no. Yet there, inside the tank of sea grapes (Caulpera lentillifera), a green, globular, mass was climbing the glass. This looked like sea grapes … but the factor of motility had my biology senses tingling.
We’ve been testing a range of different culture conditions for Caulerpa lentillifera – a green seaweed, the last several months. Caulerpa, or sea grapes is not yet a widely-farmed seaweed, but we believe that it could have great culinary appeal. This “vegan caviar” pops in your mouth, with a burst of ocean flavor. The thallus (or ‘leaf’, if you will) of sea grapes is a cluster of small, green vesicles, called ramuli, that look like tiny grapes.
What we observed in our tank was akin to watching a bunch of grapes slinking down a trellis. A closer examination revealed a slug-like body beneath the bunch of greenery, and two antennae probing the waters as it slithered forward. Known to science as Sacoproteus smaragdinus, these sea slugs appear similar to nudibranchs, such as the famous “Spanish dancer”, but are only distantly related. The Sacoproteus genus was named after the Greek sea god, Proteus, who could change his shape at will. What an apt name for such a clever algae mimic!
As a gastropod (“stomach-foot” in Latin), our S. smaragdinus literally walks all over its lunch. Most of the species of Sacoproteus seek out just a single Caulerpa species. The slugs sniff out their beloved sea grapes by perching on their sluggy haunches and waving their bodies through the water, hoping a wafting scent of Caulerpa leads them to the vineyard. Each species of slug has a uniquely-shaped tooth adapted to pierce the ‘bubble’ of its preferred species of sea grape.Once they’ve located their lunch/lounge, their special tooth pokes the grape, sucks it dry and stores the green pigments in their bulbous backs as the perfect disguise.
Larval Sacoproteus are planktonic, and drift through the oceans with the currents, until they sense the Caulerpa species of their choice. This can then trigger metamorphosis, and they settle down to a more sedentary life. We suspect that this is probably how this slug “showed up” in our tank; by drifting in through our seawater intake line.
Keeping tabs on our new-found friend is challenging; it’s more like “newly-lost”. Peering through a tank full of green bubble-algae is like one long, losing game of hide and seek, with a slug that Nicholas Paul (an expert on seaweed and algae at Australia’s University of the Sunshine Coast) says, “may be the best example of an animal masquerading as a plant that we have.”
More: Would you like further reading? See the National Geographic article: “Stunning new sea slug species looks just like seaweed”