PHOTOS: These Rare and Fascinating Sea Slugs are Found Right Here in the Philippines

PHOTOS: These Rare and Fascinating Sea Slugs are Found Right Here in the Philippines

Most land-dwellers will assume that their scuba diving friends descend below the waves to see big, impressive and maybe even scary animals: sharks, manta rays, massive groupers or maybe a dugong. But in reality, to many scuba divers, including both domestic afficionados of marine life as well as visitors from far-away lands, it’s the tiny sea slugs which are among the main attractions of Philippine waters.

A nudibranch named after the pioneer of Philippine marine science, Prof. Angel Alcala: Chromodoris alcalai

Pretty sea slugs

To begin with, a lot of sea slugs are really pretty. They impress with stunning colors, gaudy patterns and sometimes highly bizarre shapes, completely unlike the shapes of their cousins, the drab garden slugs. Many of the more unusual shapes come about because the sea slugs camouflage themselves to look like the soft corals they live on. A slug called Allen’s Miamira is one such specialist, which almost completely resembles the leathery soft corals it feeds on. I have had the chance to photograph this amazing animal in Anilao in Batangas.

ALSO READ

See Photos of the Turtles of Apo Island Up Close

These Christmas Tree Worms Add a Festive Spirit to Underwater Life

A big group among the marine sea slugs are the nudibranchs. What’s “nude” about them are their gills, which are exposed to the water, and not in a body cavity as they are in many other slugs. These are particularly sought out by scuba divers and underwater photographers. Besides their beauty, what impresses is their enormous biodiversity. The citizen science webpage iNaturalist lists an impressive 481 species of nudibranchs and 731 species of sea slugs for the Philippines.

Costasiella kuroshimae or “Sean the Sheep Sea Slug” re-uses plant cells

The extreme biodiversity is not that surprising, since the country is located in the coral triangle, which is the region of our planet with the most species of corals, fishes and other marine organisms. The location in the tropics, together with the unique geography featuring countless small islands and bays in the Philippines, Indonesia and Papua New Guinea make the coral triangle such a special place.

Many species of nudibranchs and other seas slugs seem to have arisen in evolution due to food specialization: They only munch on one type of sponge or soft coral.

Sea slug biology

There are some absolutely curious quirks in sea slug biology. These animals are hermaphrodites, meaning they are functional males and females at the same time. When two of them mate, each can fertilize the other. Nudibranchs and sea slugs usually lay long strands of eggs, which look like bands of silk caught on the reef. To come back to the aforementioned food specialization: there are nudibranchs which feed exclusively on the eggs of other nudibranchs.

Two sea slugs mating, seen during at night dive off Negros Island

Two very curious biological tricks performed by sea slugs are worth mentioning: The stealing of light-harvesting parts of algae, and the stealing of stinging cells of anemones.

Marine algae, like plants on land, use sunlight to chemically assemble sugars, in a process called photosynthesis. This is done in parts of their cells called chloroplasts. When you or I eat a salad, we simply digest the chloroplasts of the salad plant cells. Not so in some sea slugs, like Costasiella kuroshimae (seen in one of the photographs in this article). They keep the chloroplasts alive in their near-transparent bodies, and the plant cell parts keep making sugars from sunlight while in the body of the sea slug. It’s as if I ate a salad, and parts of the salad cells then migrate to my skin, coloring it green, and I can simply lie in the sun to feed myself from the photosynthesis of the salad cells. I’d get a tan and get fatter at the same time! What sounds like absolute science fiction in humans is reality in some sea slugs.

Jorunna funebris, a nudibranch which impresses with oddly patterned black blotches on its body. Seen in Zamboanguita, Negros, Island

The same principle also works with the stinging cells of anemones. Some nudibranchs feed on anemones but keep the cells alive which sting the prey animals of the anemones which they capture with their tentacles. Again, certain sea slugs use the cells of their lunch for their own purpose. A predator tempted to feed on the sea slug would get stung by the anemone’s re-used stinging cells, now incorporated into the sea slug body.

Not only are sea slugs very pretty animals, but seeking them out in the coral reefs and seagrass meadows of the Philippines is even more fascinating if you know about their unusual biology.

Sea slug names

A side note on the funny Latin names of the sea slugs. Animals known to humans have many names. Every language in the world has a word for “dog.” The situation is less straightforward for animals that were just discovered in the last few decades, like many of the sea slugs mentioned here. These have a “common name,” like “Sean the Sheep Sea Slug,” a name used casually by scuba divers. The tricky part is that these common names will be different in English, Tagalog, Japanese, German and any other language.

Miamira, a nudibranch which looks deceptively like the soft corals it lives on. Miamira alleni, seen in Anilao

Also, with the great number of known sea slugs, some will have no common name yet. Hence, scientists and educated divers use the “official,” or scientific names of the sea slugs. Like for all other animals, they have two parts, such as Jorunna funebris. The first part is capitalized and refers to the genus of the slug, while the second part is lowercase and describes the precise species. The two parts of the name are akin to a last and first name of a person—there are many people in one family and many sea slugs in one genus. The names are chosen by the scientists who first publish a precise description of the slug. Don’t be intimidated by scientific names.

Travel tips

While I have been a local of Negros’ diving hotspot Dauin for a few years now, and while Dauin has some nice sea slugs, the best spot for seeing these marine mollusks in the Philippines I have visited is Anilao in Batangas. Conveniently reached from Metro Manila by car or bus in a few hours (depending on where in Manila you start), Anilao was one of the first prominent scuba diving locations of the country.

Sea hares are among the larger sea slugs

Trinchesia yamasui, a particularly gaudily colored nudibranch

Note that some resorts in Anilao charge their dives per boat, and not per diver, so it makes sense to show up with a group of like-minded sea slug afficionados.

I had a great time diving with Aiyanar Resort a few years ago, and Crystal Blue resort also has an excellent reputation for dive guides who can help you find those rare sea slugs.