OCEAN CHRONICLES

Sea Lemon exposed with the tide © Chelsea Mathieson

SPECIES IN THE SPOTLIGHT SPECIES IN THE SPOTLIGHT THE SEA LEMON THE SEA LEMON Anisodoris Nobilis Anisodoris Nobilis

Josh McInnes

The Northeastern Pacific is home to countless marine organisms that capture our imagination, while leaving us hypnotized by their beauty. One such organism is the brightly yellow coloured sea lemon (Anisodoris nobilis), a species of nudibranch or “sea slug,” that earned its name from their bright yellow to orange body colouration that is shaped and looks like a lemon peel. Generally sea lemons are 10 cm in length, but some deep water specimens have been measured to 26 cm. Similar to most nudibranchs, sea lemons are dorsally flat with short rounded projections that have black blotches. In addition, sea lemons have forward facing tentacles or rhinophores, which are believed to be chemosensory devices aiding the “sea slug” in finding prey and mates. The posterior end of the sea lemon is dominated by a gill plume 1.0 cm in length. The gill plume aids the sea lemon as a respiratory structure, allowing for absorption of oxygen and secretion of carbon dioxide. The sea lemon can easily be confused with the similarly coloured Monterey Dorid (Archidoris montereyensis). However, in the sea lemon the blackish blotches do not extend or reach the tubercles.

Sea lemons are distributed from the cold temperate waters off Kodiak Island, Alaska south to the sub-tropical waters of Baja, California. They are commonly sighted in the low intertidal zone, but have also been viewed at depths of 230 m. Keen naturalists searching beaches are likely to find them in tide pools or clinging to exposed rock faces covered in kelp. Sea lemons can also be found in association with their main prey, which consists of several similarly coloured species of encrusting sponges. Their preferred prey is the breadcrumb sponge (Halichondria panacea). They use their radula, a conveyer of small tooth like structures, to rasp off sections of sponge that encrust rock.

The sea lemon, like all species of nudibranchs, is hermaphroditic-a specialized sexual characteristic enabling this ‘sea slug’ to be able to produce both eggs and sperm. When mating both sea lemons extend penises to exchange sperm and fertilize each other’s eggs. Sea lemons produce large light yellow coloured ribbon like structures, which contain as many as 2,000,000 eggs. However, less than 1% of the larvae will survive and reach adulthood.

Sea lemons have very few predators, and like most nudibranchs display bright colours and distinctive odors to repel potential predators. The odor of sea lemons has been likened to that of fruit. Researchers at California State University have demonstrated that sea lemons produce toxins by extracting mucus from outside glands of the animal, and testing it against potential predators. Their findings showed that several predators were successfully deterred.

Top: Dorsal view of the Sea Lemon (Anisodoris nobilis) Bottom: Mating Sea Lemons © Josh McInnes

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Sea lemons are highly abundant in most intertidal rocky ecosystems, and are able to handle a range of environmental stressors. Their main threats are likely due to habitat destruction and pollution. For example, harbours are areas that transform natural habitat to docks and fueling stations. Pollution in the form of pesticides and agricultural runoff are also likely to be detrimental to the species, but more research is needed to confirm this.

So next time you are down peering into a tide pool or diving at your favorite reef, be on the lookout for these incredible sea slugs!

Josh McInnes is a cetacean ecologist and research coordinator at MarineLife Studies in Monterey Bay, CA. He taught ocean sciences at the Bamfield Marine Science Centre from 2014- 2016. He lives in Victoria, BC.

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