Organisms often hitch rides on other organisms (see here and here for some recent examples). They do this for a variety of reasons. Some need to conserve energy. Others prefer to get to their destination in a more efficient manner (maybe it’s to get away from a predator). For the parasitic water mite, Partnuniella thermals, it’s because they need a host to survive.
These mites live in the hydrothermal areas of Yellowstone National Park, specifically on algal-bacterial mats. The mites use brine flies (Diptera: Ephydridae) as hosts. In Yellowstone, there are three large species of brine flies that all feed and lay eggs on the hydrothermal mats. Ephyrdra thermophila live in the acidic thermal areas, Ephydra bruesi thrive in low-productive alkaline areas, and Paracoenia turbida inhabit the highly productive alkaline pools (Collins 1977). All of them seem to be vehicles for mites.
It’s actually the mats that drive the relationship between parasite (mite) and host (fly). When the mats are young and thin, they are not accessible to the flies because they are covered by hot water. As time passes and the mats grow and exceed the the water level and divert water flow. The mats then cool to below 40˚C as they are exposed to the air. This cooling gives the flies just enough time to land, feed and lay eggs. As the larvae hatch they immediately begin feeding on the mats, which causes the mats to loose their thickness and dip back below the water. Adults or un-emerged larvae that over stay their welcome on the mats are washed out. This provides a continuous cycle of growth and regrowth for the mats, but also requires a window of perfect timing for the flies.
Like the flies, the mites also lay their eggs in the mats. These eggs hatch into parasitic larvae that strategically jump onto the bodies of flies as they walk by. The flies may, in fact, groom themselves and remove the mites. Occasionally, though, a mite will remain attached and will feed on the fly until the fly dies. The mite will then detach, enter the water and later emerge as a nymph, where it will feed on fly eggs as an adult.The unique thing about this mite’s behavior is that it is only parasitic during the larva stage (see figure above). Some of the mite’s closest relatives have extended their parasitic lifestyle into the nymph stage. Others have completely done away with the parasitic lifestyle. Biologists believe the explanation of this particular mite’s behavior lies in the hot spring environment (Collins 1975).
Other mites that extend their parasitic lifestyle into the nymph stage must attach to adult flies and feed on the flies for several weeks. Partnuniella uses flies of all ages. Therefore, it may not have multiple weeks to feed. In fact, this mite only stays attached to the host fly for 5-6 days.
The greatest risk of death in Partnuniella’s life cycle occurs during the transition from egg to parasitic larva (again, see the above figure). As much as 90% of the larvae never attach to a host fly and, eventually, die. So, why not abandon parasitism altogether? Partnuniella needs the host fly as an energy source and as a means of dispersal. Without the ability to disperse, the mite would go extinct. Adult mites as well as nymphs feed on fly eggs. Since these fly eggs are only laid on the mats that are above water at a particular time and place, the parasitic mite larvae need to get to those spots. In an unstable environment like hot springs where the water flow is not predictable, dispersal is important.
This still does not explain how Partnuniella, as a population, deals with the loss of 90% of its larvae. How does it survive from generation to generation? Adult mites have the remarkable ability to store large amounts of energy for long periods. Collins (1975) showed the adults can survive for 3 months without food in a laboratory. This fits their lifestyle since the exposed mats (and feeding areas) are constantly changing. Storing energy over long periods allows them not only to survive as individuals, but also to reproduce continuously over long periods. The more they reproduce the more likely some larvae will survive.
Through years and years of adaptations, natural selection has orchestrated this relationship in such a way that the parasitic water mite is perfectly suitable to its environment. It’s usually the smallest organisms that have some of the coolest and most interesting interactions. Unfortunately, it’s this world that usually goes unnoticed.
Collins, N. C. (1977). Mechanisms Determining the relative Abundance of Brine Flies (Diptera: Ephydridae) in Yellowstone Thermal Spring Effluents. The Canadian Entomologist, 109: 415-422.
Collins, N. C. (1975). Tactics of Host Exploitation by a Thermophilic Water Mite. Miscellaneous Publications, Canadian Entomological Society, 9: 250-254.