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worms >Phylum Platyhelminthes > Class Turbellaria > Order Polycladida
Marine flatworms
Order Polycladida
updated Feb 14

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 common on all our shores. They range from tiny ones found under rocks to larger ones 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 busily slide about with elegant ruffles of their body edges, or even swim.

What are flatworms? 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 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 ear-like structures made out of folded edges of their bodies. They are not real tentacles like those of a snail.

Flat-footed advantage: Unlike bristleworms and earthworms which are segmented and belong to Phylum Annelida, flatworms are unsegmented and 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 diffused 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.

But being flat means most 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).

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. Flatworms may capture prey with their bodies or entangle it with slime. Some produce paralysing mucus and a few use their penis to stab their prey!

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.

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 large flatworms even have a sucker on their undersides to get a grip on the surface.

What eats them? Among their predators are tailed slugs (Family Aglajidae).

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, each flatworm tries to impregnate the other without itself being impregnated, as it involves more energy to produce eggs. This results in a sort of 'penis-fencing' when the two flatworms meet! Here's a video clip of 'penis-fencing' shared by Loh Kok Sheng.

Eggs are laid in gelatinous strings or masses. Some flatworms lay only a few eggs. Most flatworms do not have a free-swimming larval stage. Instead, miniature flatworms develop within the protective egg capsule. Here is a photo of a flatworm larva on Image Quest 3-D Marine Library

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.


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


Pseudo tentacles made out folded edges of the body margin. Mouth is on the underside.
Pulau Sekudu, Jul 05


Bigger flatworms may undulate the sides
of their bodies to 'swim'.

Pulau Semakau, Jan 05


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



Many flatworms are tiny.
This one was hardly 1cm long.

St. John's Island, Mar 05



Some may be nearly transparent!
Sisters Island, Feb 10


Mating in progress?
Beting Bronok, May 09


Order Polycladida recorded for Singapore
from Wee Y.C. and Peter K. L. Ng. 1994. A First Look at Biodiversity in Singapore.
*Ng, Peter K. L. & N. Sivasothi, 1999. A Guide to the Mangroves of Singapore II (Animal Diversity)
in red are those listed among the threatened animals of Singapore from 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.
+from our observation and identification based on Newman, Leslie and Lester Cannon. 2003. Marine Flatworms: The World of Polyclads.
**from WORMS

  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.
  Fine-lined flatworm
Silt flatworm
Olive flatworm
Orange-white black flatworm
Phlegm flatworm
White flatworm

  Family Euryleptidae
  +Maritigrella fuscopunctata (Punctuated flatworm)
+Maritigrella virgulata (Red stripe flatworm)

  Family Polycladida=**Callioplanidae
  Meixneria furva (DD: Data deficient)

  Family Prosthiostomidae
  Prosthiostomum trilineatum

  Family Pseudocerotidae
  +Acanthozoon sp. (Spotted black flatworm)

+Phrikoceros baibaiye (Little ruby flatworm)

+Pseudobiceros bedfordi (Persian carpet flatworm)
+Pseudobiceros damawan (Damawan flatworm)
+Pseudobiceros gratus (Brown striped flatworm) including Pseudobiceros cf gratus
+Pseudobiceros uniarborensis (Dawn flatworm)
+Pseudobiceros stellae (Starry flatworm)


+Pseudoceros sp. (Blue-lined flatworm)
+Pseudoceros bifurcus (Red-tipped flatworm)
+Pseudoceros felis (Feline flatworm)
+Pseudoceros indicus (Blue-spotted flatworm)
+Pseudoceros laingensis (Purple-spotted flatworm)
+Pseudoceros sapphirinus (Sapphire flatworm)
+Pseudoceros tristriatus (Triple-striped flatworm)

  **Family Limnostylochidae
  *Limnostylochus sp. (Red mangrove flatworm)

Acknowledgement
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.


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