ABSTRACT The excretion of nitrogenous waste products in the form of ammonia (NH 3 ) and ammonium (NH 4 + ) is a fundamental process in aquatic organisms. For mytilid bivalves, little is known about the mechanisms and sites of excretion. This study investigated the localization and the mechanisms of ammonia excretion in mytilid mussels. An Rh protein was found to be abundantly expressed in the apical cell membrane of the plicate organ, which was previously described as a solely respiratory organ. The Rh protein was also expressed in the gill, although at significantly lower concentrations, but was not detectable in mussel kidney. Furthermore, NH 3 /NH 4 + was not enriched in the urine, suggesting that kidneys are not involved in active NH 3 /NH 4 + excretion. Exposure to elevated seawater pH of 8.5 transiently reduced NH 3 /NH 4 + excretion rates, but they returned to control values following 24 h acclimation. These mussels had increased abundance of V-type H + -ATPase in the apical membranes of plicate organ cells; however, NH 3 /NH 4 + excretion rates were not affected by the V-type H + -ATPase specific inhibitor concanamycin A (100 nmol l −1 ). In contrast, inhibition of ciliary beating with dopamine and increased seawater viscosity significantly reduced NH 3 excretion rates under control pH (8.0). These results suggest that NH 3 /NH 4 + excretion in mytilid mussels takes place by passive NH 3 diffusion across respiratory epithelia via the Rh protein, facilitated by the water current produced for filter feeding, which prevents accumulation of NH 3 in the boundary layer. This mechanism would be energy efficient for sessile organisms, as they already generate water currents for filter feeding.

SUMMARY Direct measurement of predator feeding events would represent a major advance in marine trophic ecology. To date, devices available for empirically quantifying feeding in free-swimming fishes have relied on measuring stomach temperature, pH or physical motility, each of which has major practical limitations. We hypothesized that the considerable physical changes that occur in the stomachs of carnivorous predators during the processes of ingestion and digestion should be quantifiable using bulk electrical impedance measured across paired electrodes. We used a prototype archival data logging tag to record changes in impedance inside the stomachs of captive free-swimming tiger and sandbar sharks over multiple, successive feeding events. Feeding and digestion events produced characteristic changes in electrical impedance of the stomach contents, identifiable as five successive phases: (1) pre-ingestion (empty stomach), (2) ingestion, (3) chemical ‘lag’, (4) mechanical ‘chyme’ and (5) stomach emptying. The duration of the chyme phase was positively related to meal size.

SUMMARY The unique head morphology of sphyrnid sharks might have evolved to enhance electrosensory capabilities. The `enhanced electroreception' hypothesis was tested by comparing the behavioral responses of similarly sized carcharhinid and sphyrnid sharks to prey-simulating electric stimuli. Juvenile scalloped hammerhead sharks Sphyrna lewini and sandbar sharks Carcharhinus plumbeus oriented to dipole electric fields from the same maximum distance (approximately 30 cm) and thus demonstrated comparable behavioral-response thresholds (<1 nV cm -1 ). Despite the similarity of response threshold, the orientation pathways and behaviors differed for the two species. Scalloped hammerheads typically demonstrated a pivot orientation in which the edge of the cephalofoil closest to the dipole remained stationary while the shark bent its trunk to orient to the center of the dipole. By contrast, sandbars swam in a broader arc towards the center of the dipole. The different orientation patterns are attributed to the hydrodynamic properties of the cephalofoil, which enables the hammerheads to execute sharp turns at high speed. The greater trunk width of the sandbar sharks prevented them from demonstrating the same degree of flexibility. Therefore, although the sphyrnid head morphology does not appear to confer a greater sensitivity to prey-simulating dipole electric fields, it does provide(1) a greater lateral search area, which may increase the probability of prey encounter, and (2) enhanced maneuverability, which may aid in prey capture.