More than 160 species of sea slugs have been recorded in Singapore's waters, according to The Straits Times, with a dedicated group of hobbyists leading the effort to document them. The creatures inhabit environments ranging from coral reefs south of the mainland to the mangroves at Kranji. Some of these animals have yet to be formally classified by science and remain listed as undescribed species.
Why it matters
The records compiled by volunteers are helping to build a fuller picture of marine diversity in a country where formal scientific study of these animals has been limited. The Straits Times reports that while researchers around the world are cataloguing sea slugs and examining their characteristics for biomedical purposes, dedicated local studies remain few. The hobbyists' work has therefore become a meaningful source of data for the Lee Kong Chian Natural History Museum, which holds many of the records they have submitted.
The timing of this documentation effort carries additional weight. In August, Singapore announced plans for a new industrial island, referred to as Western Island, which would involve reclaiming several southern islands. Pulau Hantu, one of the southern islands and a well-known dive site, may form part of that cluster.
A doughnut-shaped find with global appeal
Off the coast of Pulau Hantu, divers can encounter a small marine slug that looks like a group of frosted doughnuts. Known as the doughnut doto (Doto greenamyeri), it is among the species recorded in Singapore.
Sea slug enthusiast Toh Chay Hoon told The Straits Times that the creature's appeal has drawn underwater photographers from far afield. Before it was discovered in Singapore in the 2010s, she said, divers would travel to Bali specifically to photograph it. "Singapore is privileged to be one of the locations that can see this," she said, according to the newspaper.
Toh, 49, runs the @seaslugsofsg Instagram account and has authored numerous records for the natural history museum. Her interest began in 2003, when she was training as a guide at Chek Jawa on Pulau Ubin and another person pointed out a dotted nudibranch (Jorunna funebris) to her.
Over the years that followed, she learned that locating these animals depends on knowing what they eat. As she put it to The Straits Times: "You can find them where their food is." The dotted nudibranch, which resembles an Oreo cookie, is typically spotted feeding on a blue sponge. The sargassum sea slug (Crosslandia sp.), meanwhile, is camouflaged against the seaweed from which its common name derives.
Toh, a senior executive at the National University of Singapore, estimates that Pulau Hantu alone hosts about 100 sea slug species, roughly 60 per cent of those recorded nationwide. She attributed part of that richness to the island's popularity as a dive destination, which makes the animals easier to locate.
Hobbyists fill a research gap
Fellow enthusiast Liu Jianlin has also contributed several records, motivated by a desire to learn and to add to marine science. The 36-year-old researcher at the Institute of Mental Health has surveyed Singapore's shores since his undergraduate days at NUS and has bought more than 10 books on sea slugs, the most expensive of them a vintage volume costing $200.
His interest has taken him as far as Australia, where he found himself searching for sea slugs at 3am during winter. Reflecting on the possible redevelopment of the southern islands, he told The Straits Times: "Every time I go, I start to have a new sense of appreciation."
The animals themselves are frequently mistaken for one another, despite their striking appearance. Sea slugs are soft-bodied, spineless and coated in slime; most have no shell and depend on chemical defences instead, and they move slowly. Most possess horn-like sensory organs on their heads, called rhinophores, which help them detect food.
Nudibranchs, a subset of sea slugs, are distinguished by exposed gills, among other traits. They are often confused with ribbon-like marine flatworms, which lack respiratory systems and body cavities, move faster, and sense their surroundings through folds on the edges of their bodies rather than gills. The two groups differ in fundamental ways even where their colours and patterns appear similar.
These sea slugs can be as small as a jellybean, and their bright coloration and unhurried movement make them a favoured subject for scuba divers seeking an easy photographic target.
What happens next will depend partly on how the Western Island reclamation proceeds and whether the southern islands that host these populations are affected. For now, the volunteer record-keepers continue surveying Singapore's shores and subtidal waters, adding entries to a catalogue that scientists have so far been slow to build themselves.
