Isopoda

Lang, Birgit & Russell, David J., 2022, Excretion of nitrogenous waste by soil fauna and assessment of the contribution to soil nitrogen pools, Soil Organisms 94 (2), pp. 69-83 : 75-76

2.6 Isopoda

Woodlice, isopods of the suborder Oniscidea , are crustaceans which made the evolutionary transition to a partly or completely terrestrial life. This required morphological, ecological and behavioral adaptations of reproduction, respiration and excretion, as well as a protection to counter evaporative water loss ( Hornung 2011). The transition to terrestrial life is generally assumed to be accompanied by ureotely and uricotely (i.e. excretion of uric acid), but terrestrial isopods have retained ammonotely with gaseous ammonia as the main excretory product ( Carefoot 1993). Hoese (1981) proposed that ammonia is excreted by the maxillary glands (nephridia) into the water-conducting system, where it is transported to the ventral pleopods and is volatilized during the transport ( Greenaway 1991). Kirby & Harbaugh (1974) identified both the telson and the head as sites of ammonia release, but the telson was found to play the major role. In accordance with Kirby & Harbaugh (1974), Wright & Odonnell (1993) found the pleon fluid to be more important for ammonia excretion as it showed a higher concentration of ammonia than the maxillary urine and the haemolymph. The importance of the maxillary glands for nitrogen excretion is thus questioned (Wright & O’Donnell 1993).

As a large amount of water is needed to dilute ammonia to subtoxic concentrations, isopods face the problem of concomitant water loss. A solution may be to excrete ammonia during periods of high relative air humidity or during phases of active water vapor absorption ( Wright & Odonnell 1993). For instance, isopods may excrete ammonia during daytime when they are inactive and hide in moist places ( Kirby & Harbaugh 1974, Wieser et al. 1969, Wieser & Schweizer 1970, Wright & Peña-Peralta 2005). This means that nitrogenous waste has to be stored in a non-toxic way until its release. As potential storage compounds, several amino acids such as glutamine, glutamate, glycine and arginine have been reported ( Wright et al. 1994, 1996). Interestingly, diel patterns of ammonia excretion are reported for experiments where animals were exposed to constant light or darkness ( Kirby & Harbaugh 1974, Wieser et al. 1969, Wieser & Schweizer 1970), but not when animals were kept under natural diurnal photofluctuations ( Wieser 1972, Wieser et al. 1969).

Quantitative measurements of ammonia excretion are available for several species and range from 0.02 to 1.2 µg N mg-1 day-1 with a mean excretion of 0.109 µg N mg-1 day-1 ( Tab. 6). Ammonia excretion differs between species and seems to be higher for males than for females ( Wieser 1972). Further influences on ammonia excretion are the feeding status of the animals and environmental temperature. Animals that are allowed to feed are reported to excrete less ammonia in comparison to starving animals ( Wieser et al. 1969, Wieser & Schweizer 1970). Temperature affects ammonia excretion with a higher excretion at increased temperatures ( Wieser 1972). There are, however, exceptions such as Oniscus asellus Linnaeus, 1758 where no influence of temperature was found ( Wieser 1972). Furthermore, ammonia excretion was found to vary seasonally with higher excretion in spring and summer than in autumn or winter ( Wieser et al. 1969, Wieser & Schweizer 1970). The cause for this pattern is unclear, but it is speculated to reflect seasonal changes in metabolic rate or dietary nitrogen ( O’Donnell & Wright 1995).