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worms > Phylum Platyhelminthes > Class Turbellaria > Order Polycladida
Marine flatworms
Order Polycladida
updated Feb 2020

if you learn only 3 things about them ...
They are fragile and tear easily. Don't touch!
They are fearsome predators and hunt and eat other animals.
Being really flat has some advantages. Can you think of some?

Where seen? Marine flatworms are ferocious predators that glide around the shores like liquid death. They are common on all our shores. Ranging from tiny ones found under rocks to larger monsters that roam out in the open. Some are brightly coloured and patterned, others blend with their surroundings. Flatworms are usually more active when it is dark when they skim the ground with elegant ruffles of their body edges, or even swim short distances in the water.

What are flatworms? Unlike bristleworms and earthworms which are segmented and belong to Phylum Annelida, flatworms are unsegmented worms belonging to the Phylum Platyhelminthes. 'Platyhelminthes' means 'flat worm'. There are about 18,500 species of flatworms, but only about 16% of these are free-living flatworms. Most members of this Phylum are internal parasites. These infest fish and other animals including humans; such as tapeworms and liver flukes.

Marine flatworms belonging to the Order Polycladida, however, are not parasites. They are free-living flatworms that earn an honest living by hunting down and eating other animals.

Sometimes confused with: nudibranchs and slugs. More on how to tell them apart.

Features: Most are about 1cm long or less, although some 'monster' species 8-10cm long are also commonly seen. There are also countless minute free-living flatworms that live among sand grains. These flatworms stick onto the sand grains with paired glands on their underside. One gland secretes a glue, and the other gland another substance to release the glue.

Flatworms are generally oval or leaf-shaped, some have highly ruffled edges. Some flatworms have tiny tentacles over their heads. Most of the commonly seen flatworms only have pseudotentacles on their heads, a pair of tiny ear-like structures made out of folded edges of their bodies. They are not real tentacles like those of a snail.

Liquid Death: Flatworms really very very flat. Usually less than 1mm thick! Being flat has its advantages. They can get into almost every kind of space: to hide or to get at their food. (see below for how some eat clams). Oxygen diffuses quickly across the skin and to all parts of the body. So a flatworm doesn't have a blood circulatory or respiratory system.

Nutrients also quickly diffuse from the central gut to the rest of the body, although larger flatworms may have a highly branched digestive system to bring food to the furthest reaches of the body. 'Polyclad' means 'many branches' referring to their branched digestive system.

Worm Slurpee: Being flat means most flatworms can't swallow their prey. Instead, the pharynx (a part of the gut) is pushed out through the mouth. The pharynx engulfs the prey outside the worm's body. Or digestive juices are injected into the prey and the resulting liquefied meal is then sucked up. Most flatworms don't have an anus and they spit out indigestible bits through the mouth. The mouth of a flatworm is on the underside of the body, in some, towards the centre or the back end of the body.

The skin of a flatworm is covered with cilia (tiny beating hairs). The swirling of their constantly beating cilia gives their group name 'Turbellaria' which means 'whirpool'. In bigger flatworms, the cilia are often only found on the underside. They also produce mucus that protects them from drying out or perhaps as protection from predators.

Flatworms have a central nervous system and a simple brain to co-ordinate their well developed muscular system.

Rainbow Worms: Flatworms come in a bewildering variety of colours and patterns. Some of these colours are due to the colour of the prey they have eaten, showing through their gut. Those with bright colours that contrast with their surroundings probably serve as a warning of their distasteful nature. Flatworms can contain powerful toxins. Others have colours and patterns that blend with the background. These and smaller ones are simply overlooked. Some flatworms mimic other animals such as nudibranchs (or perhaps visa versa).

Some may be colourfully patterned.
St John's Island, Nov 12

Some may be nearly transparent!
Sisters Island, Feb 10

Pseudo tentacles made out folded edges of the body margin. Mouth is on the underside.
Pulau Sekudu, Jul 05
What do they eat? Many flatworms are carnivores that prey on tiny animals (protozoa, copepods, worms) or feed on immobile animals such as bryozoans, ascidians and molluscs. Being flat, they slip easily between the shells of bivalves and some flatworms are considered pests of oyster farms. Some are scavengers, feeding on dead animals.

Flatworms on the hunt: Flatworms are quite adept hunters. Some have tentacles or pseudotentacles sense their surroundings. Others have sensory cells to detect water currents and chemicals released by potential food. A few also have balance sensors that tell them which way is up. Some have simple eye spots on their head or along their body margins. These don't form an image and only help flatworms detect movement and avoid the light.
Dawn flatworm (Pseudobiceros hancockanus)
Flatworms on the move: To move about, small flatworms secrete a mat of mucus and crawl on this mat with a dense layer of cilia on their underside. Bigger ones may swim by undulating the sides of their bodies. Some flatworms even have a sucker on the underside to get a grip on the surface.

The mouth of a flatworm may be towards
the middle of the underside of the body.
Pulau Hantu, Jan 06

Eating Yellow clustered bead ascidians?
Changi, Jun 08

Bigger flatworms may undulate the sides
of their bodies to 'swim'.
Pulau Semakau, Jan 05
Flatworm babies: Marine flatworms are hermaphrodites, that is, each flatworm has both male and female reproductive organs. When two flatworms meet, they exchange sperm. Some species simply insert their needle-like penis anywhere in the body of the partner. This is not surprisingly called 'hypodermic impregnation'.

In yet other species, as two flatworms attempt to use each one's penis to stab the other, they appear to be 'penis-fencing'. It is believed that each flatworm tries to impregnate the other without itself being impregnated. In one study, penis fencing was observed to be just a mating ritual and not necessary for insemination, not always aggressive, and could also result in eventual reciprocal insemination. 

Eggs are laid in a mass attached to a hard surface, in batches of hundreds of eggs. The eggs hatch into free-swimming larvae which disperse and undergo metamorphosis into the adult form. Some like the Blue-lined flatworm and Purple-spotted flatworm displayed long-term parental care.

Two worms 'penis fencing'.
Beting Bronok, Jun 17

'Penis fencing'
Terumbu Berkas, Jan 10

Photo shared by Loh Kok Sheng on his flickr.

'Penis fencing'
Terumbu Bemban, Jun 10
Photo shared by Loh Kok Sheng on his blog.
Here's a video clip of 'penis-fencing' shared by Loh Kok Sheng.

Penis fencing in flatworms from Loh Kok Sheng on Vimeo.

Fragile worms: Flatworms are very delicate and tear easily when handled. So please avoid touching them.

Status and threats: None of our flatworms are listed among the threatened animals of Singapore. However, like other creatures of the intertidal zone, flatworms are affected by human activities such as reclamation and pollution. Trampling by careless visitors, and overcollection of their food source can also have an impact on local populations.

Unidentified flatworms on Singapore shores

Possibly Stylochid
Punggol, Sep 14
Photo shared by Loh Kok Sheng on his blog.

Stylochid 8
Seletar, Jan 11
Photo shared by Loh Kok Sheng on flickr

Pseudocerotidae 2
Chek Jawa, Jan 14
Photo shared by Loh Kok Sheng on his blog.

St John's Island, Mar 13
Photo shared by Loh Kok Sheng on his blog.

Pseudobiceros hymanae
Seringat Kias, Aug 12
Photo shared by Loh Kok Sheng on his blog.

Pseudobiceros hymanae
Terumbu Raya, Feb 23
Photo shared by Jianlin Liu on facebook.

Pseudoceros cf cruentus
Sentosa, Dec 18
Photo shared by Loh Kok Sheng on facebook.

Pseudoceros cf cruentus
East Coast Park, Nov 21
Photo shared by Jianlin Liu on facebook.

Tytthosoceros sp. 2
Pulau Hantu, Oct 14
Photo shared by Loh Kok Sheng on flickr.

Tytthosoceros sp. 2
Pulau Hantu, Oct 14
Photo shared by Loh Kok Sheng on flickr.

Pseudobiceros hymanae
Cyrene, Feb 20
Photo shared by JIanlin Liu on facebook.

Eurylepta sp.
Berlayar Creek, Feb 20
Photo shared by Loh Kok Sheng on facebook.

Maritigrella sp
Terumbu Pempang Tengah, Nov 18
Photo shared by Gina Tan on facebook.

Terumbu Selegie, May 24
Photo shared by Vincent Choo on facebook.

Pseudoceros cf stimpsoni
Changi, Nov 20
Photo shared by Loh Kok Sheng on facebook.

Acanthozoon/Thysanozoon sp. 2
Pulau Hantu, Aug 14
Photo shared by Loh Kok Sheng on his blog.


Thysanozoon nigrum

Big Sisters Island, Feb 21
Photo shared by Marcus Ng on facebook.

Thysanozoon nigropapillosum

Big Sisters Island, Feb 21
Photo shared by Joleen Chan on facebook.

Thysanozoon nigropapillosum
Seringat-Kias, Oct 17
Photo shared by Jianlin Liu on facebook.

Sisters Island, Feb 13
Photo shared by Loh Kok Sheng on his blog.


Pulau Semakau North, Jul 15
Photo shared by Loh Kok Sheng on facebook.

Pulau Hantu, May 09
Photo shared by Loh Kok Sheng on his blog.

Pulau Salu, Apr 21
Photo shared by Jianlin Liu on facebook.

Pseuobiceros bajae
(ID by Rene Ong).
Terumbu Pempang Laut, Aug 21
Photo shared by Marcus Ng on facebook.

Planocerid 7
Cyrene Reef, Jun 11
Photo shared by Loh Kok Sheng on his blog.

Order Polycladida recorded for Singapore
from Rene S.L. Ong and Samantha J.W. Tong. 29 October 2018. A preliminary checklist and photographic catalogue of polyclad flatworms recorded from Singapore.
+Other additions (Singapore Biodiversity Record, etc)

  Flatworms seen awaiting identification
Species are difficult to positively identify without close examination.
On this website, they are grouped by external features for convenience of display.
  Phlegm flatworm

  Family Callioplanidae
  Meixneria furva (DD: Data deficient)

  Family Euryleptidae
  cf. Acerotisa sp. 1

Cycloporus venetus
sp. 1

Eurylepta aurantiaca

sp. 1
Eurylepta sp. 2
Eurylepta sp. 3
Eurylepta sp. 4
Eurylepta sp. 5
Eurylepta sp. 6
Eurylepta sp. 7
+Eurylepta sp. 8

Maritigrella fuscopunctata
(Punctuated flatworm)
Maritigrella virgulata (Red-lined flatworm)

  Family Gnesiocerotidae
  Gnesioceros cf. sargassicola

  Family Limnostylochidae
  Limnostylochus sp. (Red mangrove flatworm)
Limnostylochid 1
Limnostylochid 2

  Family Pericelidae
  Pericelis byerleyana

  Family Planoceriade
  Planocerid 1
Planocerid 2
Planocerid 3
Planocerid 4
Planocerid 5
Planocerid 6
Planocerid 7

  Family Prosthiostomidae
  cf. Enchiridium sp.

Prosthiostomid 1
Prosthiostomid 2
Prosthiostomid 3
Prosthiostomid 4

  Family Pseudocerotidae

Acanthozoon sp. 1 (Spangled flatworm)
Acanthozoon sp. 2

Acanthozoon/Thysanozoon sp. 1
Acanthozoon/Thysanozoon sp. 2

Nymphozoon bayeri (Bayer's flatworm)
Nymphozoon orsaki (Orsak's flatworm)

Phrikoceros baibaiye (Vermillion flatworm)
Phrikoceros sp. 1
Phrikoceros sp. 2

Pseudobiceros bajae
Pseudobiceros bedfordi (Persian carpet flatworm)
Pseudobiceros damawan (Damawan flatworm)
Pseudobiceros flowersi
Pseudobiceros fulgor (Lightning flatworm)
Pseudobiceros hancockanus (Dawn flatworm)=Pseudobiceros uniarborensis
Pseudobiceros hymanae
Pseudobiceros stellae (Starry flatworm)
Pseudobiceros sp. 1
Pseudobiceros sp. 2 (Brown-stripe flatworm)
Pseudobiceros sp. 3
Pseudobiceros sp. 4
Pseudobiceros sp. 5 (Halloween flatworm)
+Pseudobiceros sp. 6

Pseudoceros bifurcus (Racing-line flatworm)
Pseudoceros caeruleocinctus (Sapphire flatworm)=Pseudoceros sapphirinus
Pseudoceros concinnus (Blue-lined flatworm)
Pseudoceros cf. cruentus
Pseudoceros duplicinctus
Pseudoceros indicus (Blue-dot margined flatworm)
Pseudoceros laingensis (Purple-spotted flatworm)
Pseudoceros meenae (White flatworm)
Pseudoceros microcelis
Pseudoceros paralaticlavus
Pseudoceros cf. prudhoei (Silver-lining flatworm)
cf. rubronanus
Pseudoceros rubrotentaculatus (Ocher-striped flatworm)
Pseudoceros scintillatus (Scintillating flatworm)
Pseudoceros cf. stimpsoni

+Pseudoceros susanae
Pseudoceros sp. 1 (Marbled flatworm)
Pseudoceros sp. 2
Pseudoceros sp. 3 (Yellow-line flatworm)
Pseudoceros sp. 4 (Braided-line flatworm)
Pseudoceros sp. 5
Pseudoceros sp. 6
Pseudoceros sp. 7
Pseudoceros sp. 8
Pseudoceros sp. 9
Pseudoceros sp. 10
Pseudoceros sp. 11
Pseudoceros sp. 12
+Pseudoceros sp. 13
+Pseudoceros sp. 14

Thysanozoon lagidium
Thysanozoon nigropapillosum
Thysanozoon nigrum
sp. 1

Tytthosoceros lizardensis (Olive flatworm)
Tytthosoceros sp. 1
Tytthosoceros sp. 2

Pseudocerotid 1
Pseudocerotid 2
Pseudocerotid 3
Pseudocerotid 4
Pseudocerotid 5
Pseudocerotid 6

+Pseudocerotid 7
+Pseudocerotid 8
(Feline flatworm)

  Family Stylochidae Silt flatworm
  Stylochid 1
Stylochid 2
Stylochid 3
Stylochid 4
Stylochid 5
Stylochid 6
Stylochid 7
Stylochid 8

  Not assigned to family
  Acotylea 1
Acotylea 2
Acotylea 3
Acotylea 4
Acotylea 5
Acotylea 6
Acotylea 7
Acotylea 8
Acotylea 10
Acotylea 11
Acotylea 12
Acotylea 13
Acotylea 14
Acotylea 15
Acotylea 16
Acotylea 17
Acotylea 18
Acotylea 19

With grateful thanks to Leslie H. Harris of the Natural History Museum of Los Angeles County for comments on and identifying some of these flatworms.

Grateful thanks to Rene Ong for sharing details and identifying the flatworms on this page.


  • Samantha Jia Wen Tong and Rene S.L. Ong. Mating behavior, spawning, parental care, and embryonic development of some marine pseudocerotid flatworms (Platyhelminthes: Rhabditophora: Polycladida) in Singapore. 27 June 2020. Invertebrate Biology.
  • Seven new records of marine polyclads for Singapore.Rene Ong, Loh Koh Sheng, Ng Boon Leong, Tan Tsu Soo & Toh Chay Hoon. 30 April 2020. Singapore Biodiversity Records 2020: 43-44 ISSN 2345-7597.
  • A marine flatworm, Pseudobiceros hymanae, at Cyrene Reef. Jianlin Liu and Rene S. L. Ong. 27 March 2020. Singapore Biodiversity Records 2020: 28 ISSN 2345-7597
  • Rene S.L. Ong and Samantha J.W. Tong. 29 October 2018. A preliminary checklist and photographic catalogue of polyclad flatworms recorded from Singapore. Nature in Singapore 2018 11: 77–125.
  • Rene S. L. Ong, D. Marcela Bolaños and Samantha J.W. Tong, 19 June 2018. New records of marine flatworms (Platyhelminthes: Polycladida: Cotylea) from Singapore. Nature in Singapore 2018 11: 53–62.
  • D. M. Bolaños, B. Q. Gan & R. S. L. Ong. 29 Jun 2016. First records of pseudocerotid flatworms (Platyhelminthes: Polycladida: Cotylea) from Singapore: A taxonomic report with remarks on colour variation. The Raffles Bulletin of Zoology Supplement No. 34: 130-169  Pp. 130-169.
  • Rene Ong, Samantha Tong & Teresa Stephanie Tay. 13 November 2015. Marine flatworms at Seringat Kias. Singapore Biodiversity Records 2015: 182-184
  • Chim C. K., R. S. L. Ong & Gan B. Q. Penis fencing, spawning, parental care and embryonic development in the cotylean flatworm Pseudoceros indicus (Platyhelminthes: Polycladida: Pseudocerotidae) from Singapore. 10 July 2015. The Comprehensive Marine Biodiversity Survey: Johor Straits International Workshop (2012) The Raffles Bulletin of Zoology 2015 Supplement No. 31, Pp. 60-67.
  • Tan Yee Keat. 26 June 2015. Apparent mimicry of marine flatworm and nudibranch: Marine flatworm, Pseudoceros sp.; Nudibranch, Gymnodoris impudica. Singapore Biodiversity Records 2015: 85-86.
  • Newman, L.J. & Cannon, L.R.G. 1997. Nine new species of Pseudobiceros (Platyhelminthes: Polycladida) from Indo-Pacific. The Raffles Bulletin of Zoology 45(2): 341-368.
  • Newman, L.J. & Cannon, L.R.G. 1995. Colour pattern variation in the tropical flatworm, Pseudoceros (Platyhelminthes: Polycladida), with descriptions of three new species. The Raffles Bulletin of Zoology 43(2): 435-446.
  • Newman, L.J. & Cannon, L.R.G. 1998. Pseudoceros (Platyhelminthes: Polycladida) from the Indo-Pacific with twelve new species from Australia and Papua New Guinea. The Raffles Bulletin of Zoology 46(2): 293-323.
  • Davison, G.W. H. and P. K. L. Ng and Ho Hua Chew, 2008. The Singapore Red Data Book: Threatened plants and animals of Singapore. Nature Society (Singapore). 285 pp.
  • Newman, Leslie and Lester Cannon. 2003. Marine Flatworms: The World of Polyclads. CSIRO Publishing. 97pp.
  • Humann, Paul and Ned Deloach. 2010. Reef Creature Identification: Tropical Pacific New World Publications. 497pp.
  • Kuiter, Rudie H and Helmut Debelius. 2009. World Atlas of Marine Fauna. IKAN-Unterwasserachiv. 723pp.
  • Gosliner, Terrence M., David W. Behrens and Gary C. Williams. 1996. Coral Reef Animals of the Indo-Pacific: Animal life from Africa to Hawaii exclusive of the vertebrates Sea Challengers. 314pp.
  • Allen, Gerald R and Roger Steene. 2002. Indo-Pacific Coral Reef Field Guide. Tropical Reef Research. 378pp.
  • Edward E. Ruppert, Richard S. Fox, Robert D. Barnes. 2004.Invertebrate Zoology Brooks/Cole of Thomson Learning Inc., 7th Edition. pp. 963
  • Pechenik, Jan A., 2005. Biology of the Invertebrates. 5th edition. McGraw-Hill Book Co., Singapore. 578 pp.
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