The giant Madagascan hissing cockroach, Gromphadorhina portentosa,and the dermanyssid mite, Gromphadorholaelaps schaeferi, have evolved a very close partnership. The mites spend their entire life cycle on their host and when the host dies so do its mites. Some female mite foundresses do move to adjacent unoccupied hosts but with this unusually heightened degree of host permanency the mites are entirely dependent on their host for resources. However, they do not parasitise the roaches. Instead they evolved ptyalophagy– feeding on their host's saliva – clustering between the host's legs to feed on saliva and food debris. The mites can also absorb water vapour via their oral cavity and cluster around the roaches' spiracles,which release humidified air. In return the mites clean food debris from their hosts, and their predatory nature is seen as an underlying reason for the mite being the only species that lives on G. portentosa. Jay Yoder and colleagues set out to investigate the water requirements of this mite that evolved to live exclusively on the hissing cockroach.
In a series of elegant experiments the Ohio-based team recently demonstrated the close correlation between G. schaeferi's water balance strategies and the water levels in the microenvironment provided by G. portentosa's bodies by constructing a water balance profile for each life cycle stage. They determined survival requirements, developmental shifts in water balance and the humidity levels of the environment provided by the hosts.
Measuring desiccation tolerance (minimum amount of body water required) and desiccation resistance (retaining water) the team collected mites at all life stages, weighed them, placed them in 0% humidity at 25°C, weighed the mites at regular intervals and monitored the mites' survival rates. Well-hydrated mites had body water contents ranging from 62% (larvae) to 75%(adults) and could withstand water losses of 17% in adults up to 24% in protonymphs. Mites lost water ranging from 2.8% h–1 (larvae)to 0.3% h–1 (adults), limiting survival to 7–56 h.
The team also determined the mites' ability to absorb water vapour by exposing them to humidities ranging from 75% to 100%. They found that protonymphs required an atmosphere with 77% and adults 93% humidity to absorb water vapour, while larvae required 100% humidity to prevent water loss.
Finally, assessing whether the mites drink to replenish water loss, the team gave the mites access to free water and roach saliva (both stained with Evans blue so that the team could track whether the mites were drinking). Interestingly, the mites did not drink free water at any life stage; however,all stages, except the larvae, eagerly drank roach saliva.
Yoder's results show that species that thrive in dry environments have low water loss rates whereas species with high water loss rates require moist habitats. Gromphadorholaelaps schaeferi are extremely vulnerable to desiccation and have high rates of water loss, especially the larvae. Lacking functional mouthparts, the larvae neither drink nor absorb water vapour but avoid desiccation by progressing rapidly to the protonymph stage 6–8 hours after birth. To remain in balance with their environment, protonymphs absorb water vapour at lower humidities whereas adult mites reduce their water loss rates to survive. Throughout all life stages G. schaeferirequire large amounts of fluid and their refusal of free water for roach saliva clinches their dependence on G. portentosa. By providing an ideal and stable moisture-rich microhabitat, meeting all of the mites' water and nutritional requirements, each individual hissing cockroach thus effectively becomes a self-contained mite ecosystem.
