Dendrocoelum lacteum

Dendrocoelum lacteum is one of only 12 species of freshwater triclads found in Britain and Ireland.

The Tricladia are named as such due to the three main branches of the alimentary canal. Tricladia are a sub-order of the Turbellaria which sits within the phylum Platyhelminthes. Turbellaria are a globally widespread group, with new species continuing to be described. They also occur in such large numbers in some lakes that they are considered a significant components of aquatic communities.

Due to their dorso-ventral flattening, Triclads are commonly referred to as "flatworms". The adults range in size from approximately 8-15mm in length. All except one of the 12 British & Irish (B&I) species are normally found in the surface waters of lakes and streams, where they feed on a variety of prey species, including isopods (woodlice etc.), gastropods (snails and slugs) and oligochaetes (worms). Each species has its own realised niche though, which has evolved to allow coexistence. For example, D. lacteum feeds mostly on isopods, while Dugesia polychroa prefers snails and Polycelis nigra and Polycelis tenuis favour the oligochaetes.

Illustrations of 11 of the 12 species of triclad that have been found in Britain & Ireland.
Taken from "A key to the freshwater triclads of Britain & Ireland"
by T.B. Reynoldson and J.O. Young.

British freshwater triclads can be categorised into two groups: those that are found in lakes, and those that inhabit streams, however this distinction is not absolute. When they are found in lakes, triclads prefer sheltered, stony shores and are most abundant in shallow waters up to a depth of 30-35cm.

On the continent, stream-favouring species are spatially separated downstream, with Crenobia alpina (and sometimes Phagocata vitta) found near the source, with Polycelis felina occurring further downstream, followed by Dugesia gonocephala (not recorded in B&I). A similar pattern of distribution can also be seen in Britain, with P. nigra inhabiting the lower reaches of streams and rivers, however C. alpina and P. felina can cause complications as both occur along the length of short streams (although there is often spatial separation related to stream gradient).

P. nigra/tenuis (left) & D. lacteum (right) 

Triclads are interesting in terms of their reproduction, because different species reproduce either sexually, asexually (by fission & regeneration) or both! Asexual reproduction is though to have evolved secondarily in the Dugesiidae and Planariidae families, to allow reproduction even when food is limiting. Another interesting thing is that pseudogamic reproduction is common among several species. This is where sperm is necessary to stimulate egg development, but no male chromosomal material is incorporated into the egg nucleus!

Of the lentic species (those living in still waters such as lakes), there are two main types of life histories: D. lacteum, Bdellocephala punctata and Planaria torva are 'annuals' that die after their spring breeding season, meanwhile P. nigra, P. tenuis, D. polychroa and D. lugubris don't die after breeding. The latter group exhibit iteroparity, breeding numerous times during their lifespan, while the former group exhibit semelparity, which is where they breed once and then die. No one really knows why these two life histories have evolved, but there are many different hypotheses, including a relation to their feeding strategies or a reduction in interspecific competition.

Dendrocoelum lacteum showing the 2 'eyes' in the top left and the internal
structures throughout. Specimen collected from the River Yare.

According to the 2009-2011 Biodiversity Audit, 5 species of triclad have been recorded on the UEA campus. One of these is Dendrocoelum lacteum, a pale and fairly large species of freshwater triclad, and one that I recently found a specimen of in a still part of the River Yare. It is a lentic species which ranges between 14-25mm in length. In the UK, it is quite common and is most frequently found in productive lakes where the calcium concentration is at least 10mg per litre. D. lacteum predominantly feeds on Asellus isopods and the triclad's distribution is closely linked with that of its prey, providing D. lacteum with a "food refuge". In order to capture intact prey, D. lacteum moves faster than the Planariid species, and has a shallow anterior pseudo-sucker. Its distribution in Britain is mostly curtailed by interspecific competition and has not yet spread into northern Scotland or the western islands.

Further reading:

  • "A Key to the Freshwater Triclads of Britain and Ireland with notes on their ecology" by T.B. Reynoldson and J.O. Young - https://bit.ly/2FcoUVV

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