Nitrogen excretion and osmotic balance

Abstract

Sea anemones lack specialized excretory structures and, like most aquatic invertebrates, excrete as their principal nitrogenous waste ammonia (which in the pH range prevailing in seawater and body fluids occurs primarily as the ammonium ion, NH₄ ⁺). Here, ‘ammonia’ is used generically, and ‘NH₄ ⁺’ specifies the ionized form. Zooxanthellae are involved in the recycling of ammonia. Some zooxanthellate and azooxanthellate species retain uric acid (a product of nucleic acid catabolism) and perhaps other purines in their tissues. Like most other marine invertebrates, sea anemones tolerate a degree of dilution of the external medium, but they do not regulate the osmotic concentration of their coelenteric fluid. This necessitates regulation of cellular volume, in part involving active control of the concentration of intracellular free amino acids (FAA). Both ecological and taxonomic trends are apparent in the composition and concentration of the FAA pool, which accounts for 4% to 14% of the total osmotic concentration (and up to 25% of the intracellular osmotic concentration) of the tissues in various species that differ in their ability to regulate tissue water content. Occupation of the intertidal zone subjects sea anemones to desiccating conditions and requires morphological, physiological and behavioural mechanisms to conserve water. Juveniles are more susceptible to dehydration than adult anemones: this apparently sets the upper limit of intertidal distribution and probably selected for viviparous reproduction in littoral anemones of the genus Actinia.