Bitterling are parasites not symbionts

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The Bitterling, which lays its eggs in the freshwater mussel is actually a parasite and not a symbiont, according to the results of a new study.

For decades, the Bitterling, Rhodeus sericeus, was believed to be a symbiont.

During the winter, the female Bitterling develops a very long ovipositor ("egg tube") which is used to insert its eggs into the freshwater Swan mussel, Anodonta cygnaea. The eggs hatch inside the mussel and are eventually released into the water when free-swimming.

The mussel's larvae, called glochidia, are parasitic and attach themselves to the sides of fish during their development, before dropping off and settling in the substrate to metamorphose into young mussels.

Now a new study by a team of scientists from the Academy of Sciences of the Czech Republic says that the relationship between the Bitterling and the mussel is not symbiotic, as previously believed.

Instead, their study showed that the Bitterling was actually a parasite of the mussel because it avoids the cost of infection by mussel larvae and imposes a direct cost on the mussels.

The team said: "Our experiments also indicate a potential coevolutionary arms race between bitterling fishes and their mussel hosts; the outcome of this relationship may differ between Asia, the centre of distribution of bitterling fishes, and Europe where they have recently invaded."

Reichard M, Ondrackov M, Przybylski M, Liu H, Smith C (2006) - The costs and benefits in an unusual symbiosis: experimental evidence that bitterling fish (Rhodeus sericeus) are parasites of unionid mussels in Europe. J Evol Biol. 2006 May ; 19(3): 788-96.