Project Title:
Plastron respiration in ticks

Project Description (short):
A plastron is a type of physical gill consisting of a thin layer of air trapped by hydrophobic hairs or other cuticular projections (Schmidt-Nielsen 1997). Oxygen diffuses into this plastron which is linked to the tracheal system - a system of tubes that conducts air into the body. Arthropods that use plastron respiration can remain submerged for extended periods of time (hours to days). This study represents the first time that this respiratory mechanism has been investigated in ticks and will focus on the following : a) Confirming that the spiracles serve as a site for plastron respiration in submerged ticks, b) Modeling the efficiency of the spiracular plate as a plastron and c) relating models of plastron efficiency to actual metabolic demands of ticks while submerged.

Skills needed:
None specified, but a biology student with experience doing scanniggn electron microscopy would be beneficial.

Start Date:
January 2005

End Date:
May 2007 (or ongoing)

Mentors:
Prof. Laura Fielden (Biology), lfielden@truman.edu, Tel: 785-4624
Prof. Phil Ryan (Mathematics), pryan@truman.edu

Past Students:

• Bach Ha, 2005-2006 (Mathematics & Computer Science)
• Mark Thomas, 2005 (Biology, Metropolitan Community College)
• Susan Villerreal, 2005-2006 (Biology)
• Matthew Heinemann, 2006 (Biology)

Accomplishments:

• Susan Villarreal*, Bach Ha, Phil Ryan, Laura Fielden. Poster at The 53rd Annual meeting of the Entomological Society of America, 15-18 December 2005, Fort Lauderdale, FL
• Bach Ha, Susan Villerreal. Poster at Annual Joint Meeting of the American Mathematical Society and the Mathematical Association of America, Jan 2006, San Antonio TX
• Bach Ha, Susan Villerreal. Poster atAnnual meeting of the American Association for the Advancement of Science 16-20 February, St Louis, Missouri
• Bach Ha. Regional meeting of the Mathematical Association of America, 31 March – 01 April 2006, Colombia, MO
• Bach Ha, Susan Villerreal. 20th National Conference for Undergraduate Research, 6-8 April 2006, Ashville, NC
• Susan Villarreal*, Bach Ha, and Mark Thomas. "Plastron Respiration in Ticks." Truman Student Research Conference. April, 2006.
• Susan Villarreal*, Bach Ha, Phil Ryan, Laura Fielden. Poster at Annual Meeting of the Society for Mathematical Biology, August 2006, Raleigh, NC
• Fielden, L.J., Villarreal, S.M. and Knolhoff, L.M. 2006. Underwater survival in the Dog tick Dermacentor variabilis. Annual Conference 2006 for the society of Vector Ecologists, Anchorage, Alaska, USA.
• Ha, B., Bach, Heimann, M, Fielden L.J. and Ryan, P.2006. Modelling Plastron respiration in the Dog tick. Annual Conference 2006 for the society of Vector Ecologists, Anchorage, Alaska, USA
• The team (Fielden, Ryan, Bach and Matt Heimann) successfully submitted an APS general user proposal GUP-6390 entitled "Plastron Respiration in Ticks" requesting access for beam time. Worked at Argonne in collaboration with Dr Wah Keat –Lee and Dr Jake Socha . Dr Wah Keat-Lee is in charge of the XOR-32-ID beamline which has been used for many successful applications of live imaging work on the internal anatomy of insects.
• Ha, Bach and Heimann, Matthew. "Mathematical Modeling of Plastron Respiration in Ticks" Truman State University Student Research Conference. April 2007.

About Prof. Fielden:
She earned her BSc in Zoology and entomology from Rhodes University, Grahamstown, South Africa, and her Ph.D from University of Natal in Pietermaritzburg, South Africa in Zoology. She journeyed to the USA in 1995 to work on insect physiology as a postdoc at the University of Utah. After teaching at at Berry College in Georgia she came to Truman in 1999. Her research interests center on the biology and physiology of arthropod vectors of disease; specifically, ticks and fleas. She has an active research laboratory currently with five research students investigating different aspects of tick and flea physiology and ecology.

About Prof. Ryan:
He earned his BSc and MSc in Mathematics from the Australian National University, Canberra, Australia and his Phd from The University of California, Berkeley. After teaching at Case Western University in Cleveland he also came to Truman in 1999.

Project Description (long):
Purpose: This research proposes to determine the mechanism by which ticks are able to survive submersion under water. Our preliminary research has shown that unfed adult ticks (Dermacentor variabilis) can remain submerged in water for up to three weeks. Their survival is dependent on oxygen since they soon die if placed in deoxygenated water. Ticks have two large spiracles for external gas exchange. These spiracles are covered with a sieve plate in which there are numerous small holes or aeropyles. This sieve plate could serve as a plastron (Woolley, 1972). A plastron is a type of physical gill consisting of a thin layer of air trapped by hydrophobic hairs or other cuticular projections (Schmidt-Nielsen 1997). Oxygen diffuses into this plastron which is linked to the tracheal system - a system of tubes that conducts air into the body. Plastron respiration has evolved independently in several groups of tracheate arthropods including, insects (Hadley 1994) and mites (Hinton 1971, Krantz 1974) and Arachnids (Hebets and Chapman, 2000.) . Arthropods that use plastron respiration can remain submerged for extended periods of time (hours to days). This study represents the first time that this respiratory mechanism has been investigated in ticks. Extreme longevity is one of the reasons why ticks are important vectors of disease since they have the capability to outlive their hosts and thus serve as disease reservoirs. This research will contribute to our understanding of why ticks are amongst the most long-lived of arthropods. The ability of ticks to survive submersion must be critical for their survival under circumstances of heavy rainfall and/or periodic flooding.

Objectives: The proposed research will focus on the following

• Do the spiracles serve as a site for plastron respiration in submerged ticks.
• Model the efficiency of the spiracular plate as a plastron
• Relate models of plastron efficiency to actual metabolic demands of ticks while submerged.

Research design: Initial experiments will be conducted on unfed adults of the dog tick, Dermecentor variabilis and later extended to other species of ticks. Colonies of Dermacentor variabilis, D. albipictus, Amblyomma americanum and A. maculatum are maintained on site at Truman.

Do the spiracles serve as a site for plastron respiration in submerged ticks? Survival with a non-functional plastron. Plastrons can be made non-functional with water of low surface tension (Hebets and Chapman, 2000). Ticks will be submerged in water with different surface tensions. Ticks will also be submerged in water with their single pair of spiracles sealed with wax. Reduced survival will indicate that submerged ticks do rely on gas exchange uptake across the spiracles as their primary source of oxygen uptake as opposed to diffusion across cuticle covering the general body surface.

Model the efficiency of the spiracular plate as a plastron.

(i) Determination of the airwater interface. The respiratory efficiency of the plastron is in part determined by the area of air-water interface in relation to the respiring biomass of the animal (Hinton, 1971). Calculation of the interface will necessitate estimates of the dimensions of the pores in the spiracular plate using scanning electron microscopy (specimen preparation according to Hebets and Chapman 2000) and surface imaging.

(ii) Determination of the pressure differences across the air-water interface. For a plastron to function effectively, the pressure difference across the air-water interface must be maintained through the entire plastron . In insects, oxygen tension within the plastron varies with distance from the spiracles according to this function , n(x1) = i0 x12/Dh)1/2 , derivation of which is outlined in detail in Crisp and Thorpe (1947.), where i0 is the invasion coefficient of,oxygen, x1 is the greatest distance of the plastron from the tissues, D is the diffusion constant of oxygen in the plastron space, and h is the thickness of the plaston. We will develop a model of oxygen tension differences within the tick plastron based on the pertinent features of the tick spiracular plate. This model will be used to predict how effective the tick plastron is in gas exchange with the aqueous environment.

Relate models of plastron efficiency to actual metabolic demands of ticks while submerged.

(i) Metabolic requirements of submerged ticks in comparison to nonsubmerged ticks. A Gilson respirometer will be used to determine oxygen uptake in individual ticks at various intervals of submergence (1, 5, 10 and 15 days). These determinations will be compared to measurements of metabolism of inactive ticks in air using a computerized flow through respirometry system to see if there is a reduction in oxygen demand during submersion and b) to quantify oxygen requirements while submerged, to relate these oxygen requirements to the efficiency of the plastron

(ii) Interspecific comparisons. After initial studies on D. variabilis, comparisons will be made with other species of ticks. Tick spiracular morphology shows considerable interspecific variation, so much so that it is used for a taxonomic feature (Yunker et al 1986). These comparisons will relate variation in spiracular morphology to plastron efficiency and metabolic requirements while submerged.

This research will (1) confirm the spiracular plate as the site of plastron respiration in ticks. 2) Develop a model for plastron efficiency and 3) test this model on different species of ticks which show variation in spiracular morphology. This study represents the first ever investigation into the role of plastron respiration in ticks.