The year is 1970. A young Chou Loke Ming, with the help of his colleagues, finally wraps up several years of surveys along Singapore’s shores. The team had spent considerable time and effort combing through intertidal and subtidal habitats, including at several of the offshore islands including Pulau Satumu, which houses the iconic Raffles Lighthouse. The curious subjects of their study? Small, charismatic sea slugs called nudibranchs (or Nudibranchia) – their scientific name an amalgamation of the words nudus (naked) in Latin and bránkhia (gills) in Ancient Greek, referencing their exposed gill structures. Nudibranchs are commonly classified into two groups: dorids and cladobranchs. Dorids have smoother oval-shaped bodies, and most species feature a feathery branchial plume, their gills, near their anus. Cladobranchs, on the other hand, are distinguished by finger-like dorsal projections, or cerata, which they use for respiration and self-defence.

Chou, a professor of biological sciences at the National University of Singapore (NUS) who recently retired after an illustrious career spanning four decades, recounted an inventory of 53 nudibranch species his team had found in local waters. This pioneering checklist, accompanied by a series of primitive sketches, was published in the Malayan Nature Journal in 1970. This was a decade before the advent of modern DNA technology which is now widely used to help decipher new species. Over half a century has since gone by and today, over 150 nudibranch species have been recorded in Singapore waters. This seeming tripling of biodiversity could be attributed to several explanations, such as new encounters over the years due in no small part to burgeoning interest among recreational scuba divers, and discoveries of novel species. Another possible reason? The use of molecular research has increasingly enabled the discovery of species complexes – two or more distinct species previously assumed to be one and the same likely based on how similar they look.

As some of the most biodiverse habitats on Earth, ocean environments are rife with species complexes. Scientists estimate several tens of thousands of cryptic marine species are still yet to the unearthed. A phenomenon that drives cryptic speciation is phenotypic plasticity, which refers to an organism altering its phenotype (how it looks), behaviour and even gene expression, in direct response to the changing environment. This is commonly observed in corals and gastropods (slugs and snails), among other taxa. What can potentially occur is two populations of a similar species evolving distinct phenotypes that are specifically adapted to different conditions. Individuals from both populations may still interbreed and produce offspring which possess intermediate phenotypes that become challenging to classify. Over long stretches of time, hybrid populations may become reproductively isolated from their parent populations and evolve into genetically distinct entities, even though they may still resemble their parent species.

The naked eye thus often fails to delineate cryptic species solely based on how these animals look. Two nudibranchs, for instance, may feature matching colouration and external anatomies but there might very well be more than meets the eye. Taxonomists now integrate DNA barcoding with morphological analyses when trying to uncover new species or decipher cryptic ones. Doing so allows for an accurate characterisation and thus effective conservation of biodiversity in the ocean. Aptly known as ‘butterflies of the sea’, nudibranchs boast striking colours and intriguing behaviours. They have sparked considerable interest among taxonomists. The number of studies focused on the discovery and description of cryptic nudibranch species in recent years has been relatively extensive, as compared to their molluscan counterparts.