The ecology and evolution of the symbiosis between the European bitterling, Rhodeus sericeus, and unionid mussels
PhD with Prof. John Reynolds
University of East Anglia, Norwich, UK
Bitterling are a group of fish in the family Cyprinidae, which spawn in the gills of freshwater mussels (Unionidae). Males defend territories around mussels, attract females to spawn and males can increase their reproductive success with alternative reproductive behaviours (Mills & Reynolds, 2003a). Bitterling show some obvious adaptations to spawning in mussels, such as the development of an ovipositor. Bitterling show preferences based on mussel ventilation rates, which may benefit the embryos through enhanced oxygenation (Mills & Reynolds, 2002a). Furthermore, recent studies have shown that the fish avoid species of mussels that have a high likelihood of ejecting their eggs prematurely.
This leads to the question of whether the interaction between bitterling and mussels could represent a case of coevolution, involving evolutionary responses by both species to selection imposed by the other. We found that host preferences by bitterling, both among and within mussel species, may indeed have evolved in response to differences in benefits for offspring survival (Mills & Reynolds, 2002b). There is no evidence yet for any benefits to mussels from receiving eggs (Mills & Reynolds, 2003b), whereas there are costs due to reduced ventilation rates when the gills contain bitterling eggs (Mills et al, 2005). Furthermore, sub-lethal effects of pollution on one species (mussels) can have lethal effects on another species with which it interacts (bitterling) (Mills & Reynolds, 2004).
While there are differences among mussel species and individuals in their tendency to reject bitterling embryos, these differences do not provide strong evidence for coevolution (Mills & Reynolds, 2003b). For example, they may reflect differences in host physiology such as ventilation rate and generalised responses to expelling objects from their gills. Therefore, while bitterling are well adapted for their obligate spawning relationship with mussels, it has been much more difficult to find evidence for adaptations by mussels for dealing with bitterling. This suggests that the bitterling-mussel interaction may not be driven strongly by coevolutionary dynamics.
