Adorable crayfish-dwelling flatworms

Opinion by Susan Lawler

I have written about the freshwater crayfish of Australia before in this column, but I have not told you that one of the special things about them is the animals that rely on crayfish.

There are two ways in which you might use freshwater crayfish to make a living: 1) you could reside in the burrows that they dig, and therefore use them as the architects of your world, or 2) you could reside on the crayfish themselves, in which case they are your world.

Burrows dug by crayfish are so valued by Australian fauna that the creatures that use them have their own name. The pholeteros are organisms as diverse as insect larvae, worms (oligochaetes and nematods), small crustaceans (copepods, ostracods and amphipods ), frogs, snakes and even small mammals.

Living on crayfish is a lifestyle that is also embraced by a range of invertebrates. Various worms and crustaceans live in the gill chambers, but the most visible of these are the flatworms called temnocephalans. Generally, they look like little hands with eyeballs.

Temnocephalans are misunderstood by fishers, who tend to see them as parasites, or “mites” infesting their food product. In fact temnocephalans are ectosymbionts: animals that occupy a host without causing it any harm. They may even play an important role in cleaning the crayfish of other more harmful inhabitants.

All Australian freshwater crayfish have a range of temnocephalan passengers. The two most common genera come in five or six legged forms. The legs of a temnocephalan are not really legs at all, but tentacles that are used to reach out and capture food. They eat small animals such as mosquito larvae. The crayfish provides a movable substrate that is always stirring up likely sources of food during its own foraging acivities.

Temnohaswellia is a genus with six tentacles and it is usually pale white, while the genus Temnosewellia have five tentacles and have a brown pigment.

I have an Honours student who has just completed a study on temnocephalans found on Murray crayfish, which included a laboratory test of tolerances to various chemicals. These little worms are remarkably robust and were able to survive for 60 days in jars sitting on the lab bench without food. I was not surprised to find that she became quite attached to her charges.

Still photos of these organisms do not do them justice, because their movement is so endearing. They can stretch out to long strings and move in an inch-worm fashion, or curl up into a ball and virtually disappear in the crevices of a crayfish carapace. They are difficult to remove from the crayfish, or even the side of a jar, which they adhere to with the use of a suction disc on their abdomen.

Here is a great video of Temnoseweillia fasciata on the crayfish Euastacus spinifer, a spiny crayfish from near Sydney. Watch how the temnos hold on with their suction disc and reach out with their tentacles. And no, they are not really coming to get you!

Eastern Australia is the centre of diversity for temnocephalans, where they are found almost exclusively on our freshwater crayfish. Several species can be found on a single crayfish, and crayfish will share several temnocephalan species. There are about 90 different species of temnocephalans known in Australia, and all of these would go extinct if the crayfish they rely on were to suffer a demise.

Conservation of freshwater crayfish, therefore, protects more than just one species. Dozens of other organisms are sharing their homes and even their skin.

The next time you are chasing yabbies, stop and take a closer look to see if those little bumps are actually curled up flatworms. Place them in water to see them unfold and be amazed.

First published on The Conversation on 21 November 2013.

Dr Susan Lawler is the Head of Department of Environmental Management and Ecology at La Trobe University in Albury-Wodonga. She writes a regular blog for The Conversation entitled This thing called life.

Image credit: Beverly van Pragh