Physiological Observations on Water Loss and Oxygen Consumption in Peripatus

Investigating onychophoran gas exchange and water balance as a means to inform current controversies in arthropod physiology

“…Using flow-through respirometry, Woodman et al (Woodman et al, 2007) reported a WLR of 61.33 mg h -1 (20°C, mass=295 mg, flow rate of 100 ml min -1 ) for Euperipatoides rowelli, similar to the rate we documented. Thus, in keeping with claims in the literature (Morrison, 1946;Woodman et al, 2007), the Onychophora can be considered a group with high WLR, or 'extremely mesic'.…”

“…By contrast, the SMR of P. capensis was significantly higher than the values estimated from the scaling relationships of 'anomalous' arthropods, which have typically low SMR (Lighton and Fielden, 1995;Klok et al, 2002;Terblanche et al, 2004). For example, the SMR values for a 488 mg animal derived from the tick (Lighton and Fielden, 1995), scorpion (Lighton et al, 2001), centipede (Klok et al, 2002) and whip-spider (Terblanche et al, 2004) (Morrison, 1946), Peripatus acacioi (0.50 μW mg -1 ) (Mendes and Sawaya, 1958) and Euperipatoides rowelli (1.27 μW mg -1 ) (Woodman et al, 2007), the basal phylogenetic position of Onychophora suggests that the low metabolism of 'anomalous' arthropods is a derived condition. Further, explicit comparative studies incorporating information on life-history strategies of the species used in such comparisons should be undertaken to assess the extent to which low metabolic rates are indeed associated with or constrained by variation in life histories (Lighton and Fielden, 1995;Reinhold, 1999;Lighton et al, 2001).…”

“…Peripatopis capensis lies well above the regression lines (3ϫ95% CI) for both mesic and xeric insects, indicating that for its SMR, it has a much larger WLR (76.3 mg h -1 at 25°C) than insects inhabiting a wide range of environments. Although comparisons with other Onychophora show that P. capensis has a lower WLR than Epiperipatus brasiliensis (Peripatidae; 222.2 mg h -1 at 24°C, 788 mg, placed in a dry-air container), Oroperipatus corradi (Peripatidae; 167.5 mg h -1 at 24°C, 423 mg, dry-air container) (Morrison, 1946), and a Peripatopsis spp. (263.3mgh -1 at 30°C, 303 mg, with 27.5% RH and 7 m s -1 air flow) (Manton and Ramsay, 1937), different S. Clusella-Trullas and S. L. Chown methods and the likelihood of unaccounted activity in the previous studies may explain the differences.…”

“…Despite the frequent reports that onychophorans are highly susceptible to desiccation, few studies have quantified water loss rates (WLR) within the group and undertaken comparisons among species or with other taxa (Morrison, 1946;Woodman et al, 2007). One reason may be the difficulty of estimating area-independent WLR or cuticular permeability (Loveridge, 1980) of onychophorans given that they readily change body shape, confounding estimates of body surface area.…”

“…MongeNájera, 1995;Sunnucks et al, 2000;Reinhard and Rowell, 2005). The few studies of onychophoran metabolism and water balance undertaken to date (Manton and Ramsay, 1937;Morrison, 1946;Bursell and Ewer, 1950;Mendes and Sawaya, 1958;Woodman et al, 2007) suggest that their gas exchange is mediated via a simple open tracheal system with large numbers of non-closable spiracles, and limited or no branching (Lavallard and CampigliaReimann, 1966;Bicudo and Campiglia, 1985). In consequence, information on gas exchange and water balance in the Onychophora would substantially inform debates about the likely basal pattern of gas exchange in the arthropods, baseline expectations for cuticular versus respiratory water loss, whether low metabolic rates in ticks, scorpions, centipedes and whipspiders are a basal condition, and perhaps also the extent to which low critical oxygen partial pressure in insects (reviewed by Hoback and Stanley, 2001;Schmitz and Harrison, 2004;Harrison et al, 2006) can be considered derived.…”