A Re-examination of the Excretion of Nitrogen by Terrestrial Isopods

In 1950 Dresel and Moyle published a paper concerning the nitrogenous excretion of amphipods and isopods, upon which most subsequent authors of text-books have relied when discussing protein metabolism and nitrogen excretion in these groups of animals (Prosser & Brown, 1961; Lockwood, 1963). The major findings by Dresel & Moyle pertinent to our discussion were the following: (1) Isopods are predominantly ammonotelic, releasing 10-30% of their ammonia in gaseous form, the major portion dissolved in faeces and urine. (2) Terrestrial isopods excrete much less nitrogen than related fresh-water and marine species. The authors concluded that ' in this group, adaptation to terrestrial conditions has been attended by a general suppression of nitrogen metabolism rather than by a transformation of ammonia to other, less toxic products'. There are two reasons why these conclusions have to be challenged and why it is necessary to re-examine the problem of nitrogen excretion in terrestrial isopods. The first reason is the recent discovery that gaseous ammonia may constitute an ' even more important end product of nitrogen metabolism in terrestrial invertebrates than would be expected on the basis of the so-called 'Needham's rule' (Needham, 1938; W. C. Sloan (personal communication)*; Speeg & Campbell, 1968; Hartenstein, 1968). The second reason is that an inverse relationship seems to exist in isopods between activity and protein metabolism (Wieser, Schweizer & Hartenstein, 1969), so that measuring the end products of the latter during periods of feeding activity—as did Dresel and Moyle—may not give a true picture of the levels of nitrogen turnover in these animals.