Invasional meltdown is a lovely phrase used to describe many species which help each others' colonising activity. Martin Reichard, Milan Vrtı'lek, Karel Douda and Carl Smith have had a long hard look at freshwater mussels and their fish partners in Prague and Brno Academies of Science and St Andrew's University, Scotland. Their research paper, An invasive species reverses the roles in a host-parasite relationship between bitterling fish and unionid mussels, has been published today in the Royal Society Journal Biology Letters.

The invasive species in this case is the recently-introduced (40 years ago) Asian mussel, Anodonta woodiana, whose larvae parasitise bitterling fish (Rhodeus amarus). These European fish use many Unionid mussels' gills as a safe haven for their vulnerable eggs. The European mussels are sadly unable to parasitise this fish with their own larvae (called glochidia). The reciprocal parasitism is rejected but the "new boy", Anodonta, is able to overcome rejection.

The evolutionarily naive fish can be assumed to be able to develop counter measures in the age-old war between parasite and host. The term "counter-adaptation" is used. When species are moved around as much as they are in modern trading conditions, these situations arise at many levels, and so it is worth considering how any parasite or predator, or even disease can be "countered."

Anodonta woodiana; Credit: USGS

In this case it is interesting to consider whether a new partner for the fish would suit the relationship better than the unrewarding present situation for the bitterling (or indeed the unrewarding past situation of the European mussels.) Note that the time for the bitterling to evolve any protection against the new mussel is 40 years, while the native mussels have had several centuries to evolve a preventative technique that disables fish egg development in their gills. In neither case has any success come about, but Asian species have many more mechanisms to confuse the issue.

Female fish oviposit directly into Unio spp. mussels, where the young fish develop at great cost to the mussels. Many mussels can eject the eggs and embryos of the Asian bitterling, Rhodeus amarus. But there are fifty related species, with the single European species now present there for as little as a few centuries. This leaves the west and centre of Europe with evolutionarily-naive mussels. An adaptive mussel (using counter-adapation) can also use the fish in this mutualistic variety of parasitism. Its glochidium larva could attach to several fish spp. in order to complete its development (ie. metamorphose). After brooding the female mussel discharges "ripe" glochidia, ready to attach and encyst within the fish's skin. How generalised or specialist this parasitism is varies. One clue is that in eastern Asia, there are many diverse Unionid (eg the genus Anodonta) mussel spp. and then here are those 50 spp. of bitterling.

The Asian Anodonta and the only European bitterling are investigated in this research paper. Many bitterling spp. use the Anodonta woodiana in its native range, but since the 1970s, it has not been parasitized by the European bitterling, except in Poland where the eggs were accepted, but the embryos rejected from the shells. Three approaches to the research were investigated:

[1] Population effects on the bitterling where it had both Anodonta woodiana (AW) and a native species, A. anatina (AA) to choose from. Outdoor pools were used and emerging young fish counted daily, but no bitterling emerged from AW pools, but where at least one AA mussel was present.

[2] The preferences shown for both spp. by the bitterling for oviposition using aquaria.

[3] The capacities of native mussels and A. woodiana to complete larval development on five different, widespread European fish spp.

The bitterling in experiment [3] were unsuitable for AA with only 4% metamorphosis, but 22% of the invasive AW developed successfully. The Polish example quoted above proved interesting. None of the experimental fish used AW at all, while the polish fish had used them, only to have the eggs rejected afterwards. The concern would be if the" new boy," Anodonta woodiana, out-competed the native AA species, leaving the bitterling with very little choice of egg sites. The AW arrives in Europe, commercially-imported on infected Asian carp.

The conclusions on such fascinating dynamics are that they mirror European history! There are Asian invaders, to which the natives must adapt. This freshwater arms race can be fast or slow, varying with the geography and resulting in outcomes all along the mutual/parasitic continuum. The temporary benefit for exploiters of naive hosts would seem to wear out after varying times. The authors claim that subsequent invaders may benefit from a "relaxed selection," where traits to defend against parasites have been lost.