Every month we showcase a relationship between two or more species or groups of species that can be considered a mutualism: a relationship where both members benefit. This month’s mutualism is between several species of North American crayfish and a group of cleaner-worms!
The two groups engage in a cleaning symbiosis, where the worms keep the carapace (the outer shell of the crayfish) and particularly the crayfish's gills clean of parasites, detritus, dead tissue, and epibionts (non-parasitic animals that grow on other animals). Cleaning symbioses are considered one of the quintessential classic mutualisms: the client species gets parasites removed while the cleaner species is fed.
Until recently, many of these cleaning relationships were considered static. No matter what state (e.g, age, sex, health) the hosts and the cleaners were in, the relationship is positive and both benefit. Recent work has shown that that isn’t the case. The relationship is highly dependent on the state of the participants.
For example, work by Kaitlin Farrell (Appalachian State University) has shown that the relationship between the crayfish and worms changes as females become sexually active. Worms preferentially inhabit the same surfaces that would be used to brood eggs and young on the female. This is certainly a conflict of interest for female crayfish trying to maximize the size and success of their brood. Kaitlin and collaborators showed that reproducing females have reduced numbers of worms as well as worms are relegated to the sides of the females. Female crayfish are known to extensively clean underneath their tail (where the eggs are held) prior to egg-extrusion and this may be the mechanism for the reduction in the number of worms. Furthermore, after extruding eggs, female crayfish fan their eggs with extensions of the tail, called pleopods, which may inhibit worms from moving back underneath the tail as well as knock off any leftover worms. These results suggest that the reproductive state of females modifies the nature of this normally mutualistic relationship such that mutual benefits may temporarily reduce. The scientific paper publishing these results recently came out in the journal Southeastern Naturalist and can be accessed here.
Another researcher from Appalachian State University (ASU), Michael Thomas, has found that the species of cleaner-worm on an individual crayfish changes as the crayfish get older. Young crayfish actively removed one species while allowing the other species to maintain a population on their carapace. As crayfish got older (and larger), they stopped manipulating their worm communities and the worms that were originally removed, when the crayfish were small, became the dominant species. The change in community structure has been attributed to intraguild predation (eating of potential competitors). That’s one way to get rid of the competition! Adult crayfish don’t seem to care which species succeeds. They don’t control the population of the two competing species and, regardless of which species is dominant, the large crayfish grow quicker with worms present. The competitive nature of the two worm species and their different relationships with different sized crayfish has some interesting implications on mutualism theory. In some life-stages, these worms aren’t beneficial at all, but as the crayfish grows the relationship becomes beneficial. The relationship changing as the crayfish ages makes sense. Crayfish molt frequently and there is relatively little time between molts when they are young. Every time a crayfish molts, the outer layer of the gills and its outer shell are replaced by a newer, cleaner, and slightly larger version. Since the worms are gill-cleaners and the crayfish are molting frequently, they don't need the cleaner worms, but as they age and the time between molts (intermolt) increases, worms become a necessity to keep their gills clean. Thomas’ thesis on this interesting relationship can be accessed here.
Researchers at ASU and Virginia Tech have also shown that an intermediate level of worms is the best amount. Too few worms and the benefit isn't measurable; too high and the worms turn parasitic. Instead of cleaning the gills of the crayfish, they start eating the tissue and the blood! A worm has to eat after all. Crayfish have been found to actively decrease levels of worms, knocking them off and sometimes consuming them when numbers get too high.
Certain species of North American crayfish don't harbor cleaner-worms and actively reject any amount of worms on their body. They don't need them, as they produce chemicals in their blood that reduce colonization of bacteria on their gills (the main job of the worms). Without the need to have their gills cleaned, the relationship would almost certainly turn parasitic. The crayfish who associate with worms don't have these antimicrobial compounds in their blood and rely on the worms for gill cleaning. The freeing up of energy, that would've had to have been invested into antimicrobial compounds, can now be used for growth and reproduction.
Don’t forget to check out more mutualisms of the month and all the other great articles on FTDM! Stay hungry!