Exercise and emersion in air and recovery in seawater in the green crab (Carcinus maenas): Effects on nitrogenous wastes and branchial chamber fluid chemistry.

At low tide, the green crab, which is capable of breathing air, may leave the water and walk on the foreshore, carrying branchial chamber fluid (BCF). N‐waste metabolism was examined in crabs at rest in seawater (32 ppt, 13°C), and during 18‐h recovery in seawater after 1 h of exhaustive exercise (0.25 BL s−1) on a treadmill in air (20°C–23°C), or 1 h of quiet emersion in air. Measurements were made in parallel to O2 consumption (ṀO2), acid‐base, cardio‐respiratory, and ion data reported previously. At rest, the ammonia‐N excretion rate (ṀAmm = 44 µmol‐N kg−1 h−1) and ammonia quotient (AQ; ṀAmm/ṀO2 = 0.088) were low for a carnivore. Immediately after exercise and return to seawater, ṀAmm increased by 65‐fold above control rates. After emersion alone and return to seawater, ṀAmm increased by 17‐fold. These ammonia‐N bursts were greater, but transient relative to longer‐lasting elevations in ṀO2, resulting in temporal disturbances of AQ. Intermittent excretion of urea‐N and urate‐N at rest and during recovery indicated the metabolic importance of these N‐wastes. Hemolymph glutamate, glutamine, and PNH3 did not change. Hemolymph ammonia‐N, urea‐N, and urate‐N concentrations increased after exercise and more moderately after emersion, with urate‐N exhibiting the largest absolute increments, and urea‐N the longest‐lasting elevations. All three N‐wastes were present in the BCF, with ammonia‐N and PNH3 far above hemolymph levels even at rest. BCF volume declined by 34% postemersion and 77% postexercise, with little change in osmolality but large increases in ammonia‐N concentrations. Neither rapid flushing of stored BCF nor clearance of hemolymph ammonia‐N could explain the surges in ṀAmm after return to seawater. Research Highlights: In addition to continuous ammonia‐N excretion, intermittent urea‐N and urate‐N excretion are important components of the nitrogen metabolism of the green crab when in seawater. Excretion of all three continues during air exposure, but at reduced rates. After both exercise in air or quiet emersion in air, crabs exhibit elevated hemolymph ammonia‐N, urea‐N, and especially urate‐N. Branchial chamber fluid (BCF) volume is reduced, but its osmolality barely changes; its ammonia‐N levels increase above those in the hemolymph, while urea‐N and urea‐N do not. Upon return to seawater, massive, short‐lived increases in ammonia‐N excretion occur, but these do not appear to result from either flushing of BCF or clearance from the hemolymph. [ABSTRACT FROM AUTHOR]