When we returned in the rains of early summer to collect our keys and move in, we found that the thaw had revealed our forest sanctuary to be a mosquito-infested swampy wetland, complete with this welcome sign:
Ticks? In a wetland? You wouldn’t think ticks would do so well in the water… But think again.
Ticks, like spiders, are arachnids. They’re known for feeding on the blood of other animals, but in reality, they only do so three times in their life (once in each life stage). Between those three blood meals, they can live for months or even years without feeding. Their extremely low energy metabolism (only about 10% that of insects and spiders), waterproof “skin”, and cold hardiness give them the ability to hunker down and ride out a range of weather conditions. And anyone who has tried swimming or doing laundry as a means of getting rid of ticks knows they can withstand those pressures too.
|Photo of ticks |
in a jar by
If ticks can live underwater for nearly two weeks, this suggests that either 1) they are holding their breath and do not need much oxygen or 2) they can somehow get oxygen out of the water. The researchers tested this by having another set of ticks in water-filled jars, but this water had less oxygen in it. These ticks survived underwater for only 7 days on average, which shows us that the ticks are getting and using oxygen from the water.
So how do air-breathing ticks get oxygen from water? Ticks, like most insects and many spiders, have a breathing system called a tracheal system. This is a system of branching gas-filled tubes that provide each cell in the body with access to the environmental gas and the oxygen it contains. The tracheal system connects with the environment through openings in the body wall called spiracles. This is a great system for breathing air, but not for breathing water. So many water-breathing insects have developed an additional system called a plastron. A plastron is typically a region of the body near the spiracles that has lots of tiny water-repelling hairs that trap air around the opening of the animal’s breathing system. As the animal uses up its body’s oxygen, the oxygen levels in this trapped air space decline and oxygen then diffuses from the water around the animal into this trapped air space. Once the oxygen in the plastron air space is restored, the animal can breathe it with its air-breathing tracheal system. It works kinda like a perpetually refilling oxygen tank for SCUBA divers (if something so cool even existed).
|The picture on the left shows a wood tick under the microscope. Notice the large spiracles behind the hind legs. If you don't see them there, check out the red spots on the sketch on the right. Images by Laura Fielden-Rechav.|
The researchers were curious whether wood ticks have a plastron system to help them survive underwater for long periods of time. Ticks have two spiracles located behind their hind legs (which means they breathe with their rear end, not with their face – This is why they can bury their face into another animal to eat a blood meal without suffocating). The researchers wanted to test if these spiracles may each have a plastron. A plastron system relies on water surface tension that is generated by the water-repelling hairs, so if you do something that reduces this surface tension, it should block a plastron from being able to function. Rubbing alcohol breaks surface tension. So the researchers submerged three different groups of ticks in water: one group was just submerged as-is; a second group had one spiracle wetted with alcohol before they were submerged in water; and a third group had both spiracles wetted with alcohol before they were submerged in water. The group with no alcohol on either spiracle lived several days longer than either of the groups with alcohol.
When the researchers looked at the ticks under a high-powered microscope, they did not see little hairs that are typical of plastrons, but they did see lots of pores that open into structured pockets of airspace. This structure could produce the surface tension needed for a plastron system to work. So based on their evidence that ticks with spiracles wetted with alcohol don’t live as long in water and the microscope images showing these air pockets, it looks like air-breathing wood ticks can survive for long periods of time underwater by breathing with a unique plastron system never before seen in another species.
Before you get too grossed out the next time you pull a tick off your dog, take a minute to appreciate the incredible ability of this hardy critter.
Want to know more? Check this out:
Fielden, L.J., Knolhoff, L.M., Villarreal, S.M., & Ryan, P. (2011). Underwater survival in the dog tick Dermacentor variabilis (Acari:Ixodidae) Journal of Insect Physiology, 57, 21-26 DOI: 10.1016/j.jinsphys.2010.08.009