Sea Slugs – Butterflies at the edge

As most folks know intuitively, edges are the most diverse areas in nature. In a mature redwood forest it is often profoundly still; likewise in the nearby scrub. Yet where the forest meets the chaparral, there is an increase in life and in action. At this interface there exists a greater variety of habitats and niches — opportunities for plants and animals to exploit. Biologists have coined the word “ecotone” to describe this location.

The tidepool area along the coast of California is one of the richest ecotones in the entire world. It is an edge- the edge between the North American continent and the Pacific Ocean. The cornucopia of intertidal life here is due to several factors: upwelled, nutrient-rich water which fertilizes the top layer of the sea and fuels a prodigious food web; an absence of killing frost and destructive winter ice; summer fog which protects the sensitive plants and animals from desiccation; and finally, an inexplicable dearth of herbivorous fishes which allows for the prolific growth of marine plants and a subsequent increase in food and habitat.

Of all the beautiful creations seen in these tidepools, those I hold most dear are the sea slugs. Slugs!!!! What an ignoble name for such delightful creatures. I prefer to call them the “butterflies of the sea.” They come in a diverse array of forms and colors. They are iridescent blue, shocking pink, red and black-striped, orange with purple spots, or even transparent. Some are tufted with large plumes; others are warty and smooth. Because they are small and not good for human consumption they are often overlooked.

Sea slugs are mollusks (the snails, clams, squid, etc.) and belong to a sub-category called the nudibranchs. They have no shell and their gills are exposed. All are carnivorous and feed on hydroids, anemones, tunicates and sponges. We have over 50 species in the Bay area, making ours one of the richest faunas in the world.

June is spawning time for many nudibranchs. They come close to shore and mate and lay eggs. All are hermaphroditic; they possess both male and female sexual organs. Hermaphroditism is common to many invertebrates. When I ask people to explain the benefits of hermaphroditism they often reply, “You don’t need to go to singles bars,” or “it is easy to find a meaningful relationship,” or “you’re not obligated to converse afterwards.” However, most hermaphrodites cannot self-fertilize. The real advantage of hermaphroditism is that slow moving animals have a low rate of encounter but every slug you meet is always the right sex.

May, June and July are the best months for tidepooling. There are usually no storms. The tides are extremely low and often occur in the morning before the wind begins to blow and one’s coffee wears off. One should try to arrive at least two hours before the low tide in order to follow the receding water out. Be sure to dress very warmly and count on wet feet. Exercise caution at all times. The edge can be hazardous to your health. But the best way to see the intertidal zone is in the company of an experienced prober (me).

Click here: Footloose Forays – for complete details of educational trips into the natural world personally directed and guided by Michael Ellis.



Posted on

August 4, 2009