Jonathan Wright – Research Interests

Comparative and evolutionary physiology of land isopods

Comparative physiology provides a powerful tool with which to study processes of evolution: how organisms adapt to differing environments over evolutionary time, how specific organ systems evolve, and the significance of both preadaptation and design constraints in shaping adaptive radiations. One of my major research interests is the physiology of the land isopods (sub-order Oniscidea) and how particular systems have been modified in the evolutionary transition from an aquatic to a terrestrial habit. The sub-order Oniscidea is particularly suitable for comparative studies of this kind, comprising several independently derived terrestrial lineages with transitional habitat types broadly represented in the extant fauna. Their terrestrial adaptations include a capacity for active water vapor absorption (WVA), volatilization of ammonia as the major route for eliminating nitrogenous waste, and sequestration of blood electrolytes enabling osmoregulation during dehydration. By comparing the physiology of these processes within and between lineages, it is possible to build a picture of parallel adaptive solutions to common problems, and see how mechanisms are derived from an ancestral aquatic plan. For example, WVA involves pleopodal salt secretion to generate colligative lowering of vapor pressure. Available evidence indicates that pleopodal salt secretion originally served in aquatic osmoregulation, with secretion of more concentrated ‘pleon fluid’ initially evolving in intertidal genera as a means of countering a dietary salt-load. From this preadaptive condition, it is then a relatively minor evolutionary step to retain hyperosmotic fluid in the pleon for condensing water vapor from high humidities.

My lab is currently conducting studies in several aspects of isopod physiology. Undergraduate students at Pomona College have developed or collaborated in several projects and are co-authors in several published papers. Projects include the cellular mechanisms of branchial salt secretion; the role of the maxillary glands in counteracting an osmotic water-load; the tolerance of brooded eggs to changes in PNH3, salinity and pH; calcium uptake by embryos and the early stages of cuticle mineralization; metabolism and the significance of cuticle hydration and lung development for resting metabolic rate, critical PO2 and metabolic scope; the functions urate storage and mobilization; and the mechanism by which ammonia is volatilized from the gill surface. Several of these topics have formed the basis for experimental senior theses. For a list of recent undergraduate theses conducted in my lab, see the courses page.

Water vapor absorption (WVA)

One of the more remarkable adaptations seen among terrestrial arthropods is the capacity of some species to absorb water vapor from sub-saturated humidity. Well-known examples include mites and ticks, mealworms and some other beetle larvae, psocopterans (booklice), lepismatids (silverfish) and terrestrial isopods. By allowing water replenishment in the absence of liquid water, WVA enables many species to exploit very xeric habitats and dry food materials, and some, including silverfish, flea larvae, booklice and dust mites, are significant domestic pests. Physiological studies of WVA allow us to determine the critical humidity requirements of different species, but also offer the possibility of identifying processes that can serve as models for biomimicry.

Recent collaboration with Dr. Peter Westh at the University of Roskilde, Denmark, has utilized the ultra-sensitive microcalorimetry to demonstrate a capacity for WVA in the minute penicillate millipede, Polyxenus lagurus. This species has an uptake threshold humidity of 85% and WVA shows saturation uptake kinetics with increasing ambient RH. By quantifying water absorption and desorption from voided fecal pellets, we were able to show that the rectal water activity closely approximates this uptake threshold, indicating a rectal uptake site. This is further supported by earlier anatomical studies of Schlüter and Seifert (1985) who showed that the Malpighian tubules lie closely apposed to the rectal wall, forming a ‘cryptonephric system’. This represents a closed evolutionary parallel to the similar system of mealworms which also serves in WVA.

The water vapor uptake mechanisms of isopods, mealworms, and probably Polyxenus, depend on elevated solute concentrations to lower the vapor pressure of water at the condensing surface. Such colligative processes are unlikely to explain all mechanisms of WVA, however. Salt solutions based on the major inorganic ions — NaCl and KCl — saturate at activities of 0.75 (85% RH) and 0.85 respectively. Lower uptake thresholds may employ organic solutes such as trehalose or glycerol that can be accumulated to much higher concentrations. But it is unclear whether more impressive vapor-absorbers could maintain the extremely large osmotic gradients required to explain colligative uptake. In particular, some lepismatids and psocopterans have uptake thresholds below 50% RH, which would require an osmotic gradient in excess of 55 osmoles kg-1. Explaining a mechanism for WVA in these groups is the focus of an ongoing collaboration with Peter Westh.

Literature Cited:

Schlüter, U., and Seifert, G. 1985. Functional morphology of the hindgut-malpighian tubule-complex in Polyxenus lagurus (Diplopoda; Penicillata). Bijdr. dierkd. 55, 209-218.