New research on polyclad flatworms is anything but flat

Different species of flatworms possess rich physical markings. (Photo: Rene Ong and Samantha Tong)

Detracting from the generalised impression of typical worms with long cylindrical tube-like bodies – polyclad flatworms are a unique group of free-living marine flatworms that often boast vibrant colours and patterns on their bodies. 

True to their name, these flatworms are usually less than a few millimetres thick, and can be found in a variety of marine habitats including mudflats, rocky shores, seagrasses and coral reefs. With their multi-coloured, multi-patterned bodies, they are a vibrant addition to marine ecosystems, adorning reefs, seagrass habitats and shores with a burst of colour. However, these attractive creatures have rarely received the attention they deserve, and many aspects of their biology and ecology are poorly studied.

NUS Tropical Marine Science Institute Research Assistant Ms Samantha Tong collaborated with Ms Rene Ong to correct the lack of data and taxonomic work regarding polyclad flatworms found in Singapore waters. Up until 1903, only eight species of polyclads had been formally recorded in Singapore. The Comprehensive Marine Biodiversity Survey conducted in 2010 initiated the large-scale collection of more specimens. Following an increased interest and knowledge of local polyclads, more sampling and collection trips were conducted.

The two researchers have since established that Singapore has an impressive 128 species of polyclad flatworms, many of them possibly new to science. Though Singapore may be a small country, its unique geography situates its waters at the crossroads of the Indian and Pacific Oceans, and unsurprisingly, its biodiversity of polyclad flatworms is far from poor. 

Pseudobiceros sp. 1_960x540.jpg

This flatworm with a beautiful broad black marginal band is from the genus Pseudobiceros and is possibly new to science. (Photo: Rene Ong)

These recordings only form the beginnings of a preliminary checklist, and many aspects of polyclad flatworms’ biology and ecology still elude researchers today. The gaps in knowledge include their diets, distribution and abundance in Singapore. To date, some species have yet to be assigned a scientific name. 

To learn more about the fascinating marine creatures, Ms Tong and Ms Ong investigated the reproductive biology of polyclad flatworms, covering the core aspects of reproductive effort, behaviour, parental care and embryonic development. Sixteen species of flatworms from the family Pseudocerotidae were documented and presented for the first time. Their findings were published in the journal Invertebrate Biology on 27 June 2020. 

Although all studied species underwent similar embryonic stages, there were observable differences in the developmental times, brood sizes, parental care duration, embryo and larvae colours. 

The two researchers described a notable trend in the genus Pseudoceros, which hatched as reddish-purple pigmented larvae, regardless of their initial embryo colour. In Pseudoceros laingensis, the initial embryo colour was greyish yellow but reddish-purple speckles appeared during development, and eventually hatched as reddish-purple larvae. The other studied genera developed brownish-orange speckles and hatched as brownish-orange larvae. 

Another interesting observation was that the egg capsules of the genus Pseudobiceros have a pointed operculum, whilst the other genera produced eggs with smooth opercula.

Ms Ong explained, “We are interested as to why some flatworms such as those from the genus Pseudobiceros have such different, pointed operculum. As the species does not display much parental care for their young, the evolutionary design of the operculum could be a defence mechanism to protect against potential egg predators”.


These four images are of the egg operculum of two species of flatworms observed under microscopes. The top two images show the pointed egg operculum of Psuedobiceros stellae while the bottom two images show the smooth egg operculum of Acanthozoon sp. (Photo: Samantha Tong)

A deeper understanding of the embryonic and reproductive characteristics of these flatworms can serve as additional features to complement the classification of polyclad genera or species.  

Like many other marine creatures, the survival and continuity of flatworms can be affected by human activities such as reclamation and overfishing. Careless trampling and increased pollution can also have an impact on local populations. 

“There is much we still do not know about marine flatworms, and it will remain that way until and unless researchers, and people, care enough to learn and discover more about these beautiful creatures. With more studies on flatworms, which will hopefully inspire even deeper research, we will be able to further appreciate these animals we share the planet with, and find ways to protect them and their natural environment,” said Ms Tong.