Marine flatworms

What are flatworms?

The flatworms – Phylum Platyhelminthes – comprise a very diverse group of worms, with over 10,000 species described. Included within this large phylum are parasitic flatworms, such as tapeworms and liver flukes. Unlike many other kinds of worms, such as earthworms, flatworms do not have a body cavity containing organs – instead the body is a solid mass of tissue. There is no circulatory system either, and flatworms rely on diffusion to convey nutrients and waste products between cells. That is why flatworms are so flat and thin: no part of the interior of the body is far from the surface.

Flatworm, Pseudoceros lividus

Flatworm Pseudoceros lividus
Photographer & Source: John Chuk

Where are flatworms found?

On land, the most frequently seen kinds of flatworms are called triclads (or Tricladida). Triclads are usually brownish, less than a few centimetres long, and are flat unsegmented worms found in moist places and in fresh waters. Although triclads also occur in the sea, by far the most common marine flatworms belong to a different group, called polyclads (or Polycladida). Triclads and polyclads are all free-living (ie they are not parasites). Polyclads are found in most marine habitats, usually on the sea floor among algae, corals, or on rocky reefs. However, they are often hidden in tiny crevices and can be hard to find.

Polyclad identification

Complete description and identification of a polyclad is a difficult and complex process. Living polyclads are very fragile and require special handling and preservation. The internal anatomy (arrangement of reproductive and other organs) differs considerably between different types of polyclad. Although these internal differences are fundamental to flatworm classification, they can only be deduced by making a series of very thin serial sections of specially preserved specimens. These cross-sections reveal the structure of internal organs, and many of them in combination can be used to draw a diagram of the otherwise mysterious interior of a flatworm. Think of taking a series of very thin slices through your garage to try and find the lawnmower and you get an idea of how difficult this can be, and of the advantage for flatworm taxonomy of any short-cuts, such as colour photography.

Ultimately dissections and sectioning of specimens by experts is still necessary to confirm identifications, and to describe new species. But in many cases different species of polyclad are so distinctively patterned that it should be possible to learn to tell some species apart simply from photographs.

Polyclads in Australia

We do not yet know how many flatworm species occur in Australia, nor how many remain undescribed by scientists. Many colourful species occur on coral reefs in tropical regions, and are illustrated in the publications of Leslie Newman and Lester Cannon (see Further reading). Much of what we know about flatworm biology has been gleaned from observing the fauna of tropical waters. Museum Victoria and ReefWatch Victoria have initiated a photographic atlas project and we hope to begin to make similar discoveries in the poorly known flatworm fauna of the Bass Strait region in southern Australia (see Internet resources).

Bizarre behaviour

The world of a flatworm must be a violent existence. They are voracious predators which attack and digest prey with eversible (able to be turned inside out) mouthparts (called a pharynx). Oysters are a favourite food, but no doubt there are many prey preferences and other associations with invertebrates that are yet to be discovered. Many flatworms are strikingly coloured, are toxic to other invertebrates, or mimic other invertebrates (or all three). Sex in flatworms is bizarre. Flatworms are hermaphrodites (each worm has both male and female reproductive systems) and sex may involve "penis fencing" whereby each worm tries to spear sperm into the other. Sort of like a "pin the tail on the donkey" game, albeit with significant family planning consequences.

All these details, and much more besides, are elaborated in a fascinating and beautifully illustrated book recently written by Leslie Newman and Lester Cannon titled Marine Flatworms – the World of Polyclads (see Further reading).

Flatworm, family Stylochidae

Flatworm, family Stylochidae
Photographer & Source: John Chuk

Further Reading

Cannon, L. R. G. 1986. Turbellaria of the World. A Guide to Families and Genera. Brisbane: Queensland Museum. [A short technical overview of free-living flatworms of the world.]

Edgar, G. J. 2000. Australian Marine Life. Revised edition. Reed: Kew, Victoria. 544 pp. [A compendium of photographs of marine life, especially of southern Australia. Includes four polyclads.]

Newman, L. J. & Cannon, L. R. G. 2003. Marine Flatworms - The World of Polyclads. Melbourne: CSIRO Publishing. [Highly recommended]

Newman, L. J. & Cannon, L. R. G. .2005. Fabulous Flatworms - A Guide to Marine Polyclads. Melbourne: CSIRO Publishing. [CD-ROM interactive guide]

Prudhoe, S. 1982. Polyclad flatworms (Phylum Platyhelminthes). In S. A. Shepherd & I. M. Thomas (Eds), Marine Invertebrates of Southern Australia Part 1 (pp. 220-227). Adelaide: Handbooks Committee of the South Australian Government. [A short chapter describing 16 species from southern Australia. Hard to find – try a library.]

Prudhoe, S. 1985. A Monograph on Polyclad Turbellaria. London and Oxford: British Museum (Natural History) and Oxford University Press. [A technical treatise, only for the dedicated researcher.]

Comments (16)

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Delaney Slade 24 March, 2011 12:51
how old do they liv 2
Discovery Centre 7 May, 2011 12:36
Hi Delaney, the lifespan of flatworms may vary from one species to another & also depend on whether the specimens are in the wild or kept in captivity and tends to measured in months rather than years.
Esperado 4 August, 2011 20:13
Hi, How many different species are there in the world? and can they cross breed?
Discovery Centre 8 August, 2011 15:47

Hi Esperado - good question. This is not a field where we have that type of highly specialist knowledge; our Marine Invertebrates department suggest you might want to pose this question to an expert such as Dr. Leigh Winsor at JCU via

Hope this helps!

kassy 12 October, 2011 11:31
wow i have a project but it doesnt have the info i need
sarah 12 January, 2012 12:08
omg same...i have been lokking everywhere and cant find the stuff i need!
lauren 19 February, 2013 06:32
whats another god site for invertabres
mehtevas 7 January, 2014 04:07
What do they taste like?
Encien 3 February, 2014 13:15
Can 2 different patterned flatworm reproduce with each other?, or do they have to be the same pattern?.
Discovery Centre 22 February, 2014 16:15
Hi Encien, unfortunately we don’t have any flatworm experts at MV. However, from what we understand, flatworms can exhibit considerable variation in external morphology. Therefore it would be possible to reproduce, within the limits of their variation. However, if the colour patterns are very different, this may be an indication that they are different species, and therefore unable to reproduce.



emma 15 January, 2015 01:08
how many colours can flatworms be/have?
Discovery Centre 20 January, 2015 12:29

Hi Emma,

As stated above, we don’t have any flatworm experts at MV so we are unable to answer your question. The best we can offer is that you do an image search on flatworms in order to get an idea of their variety.

Alain-Pierre Sittler 19 October, 2015 00:46
It seems that Leslie J. Newman was passed away in 2015. :-/ is it wright ?
Childman 16 December, 2015 05:12
Hello I like flatworms in many places
Tarun Das 4 April, 2016 23:18
how to study the spatial distribution of polyclads
shreya 11 June, 2017 18:19
how fast do they move
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