Pyrazus ebeninus

Abstract: The value of adenylate energy charge as a biological indicator of the severity of departure from normal environmental conditions was examined in the gastropod Pyrazus ebeninus (Bruguiere, 1792) and the bivalves Anadara trapezia (Deshayes, 1840) and Saccostrea commercialis (Iredale and Roughley, 1933). Mean energy charge for each species decreased by 17% or more when salinity was reduced from 35‰ to 10‰. Changes with reduced salinity were also found in the concentrations of individual adenylates and several adenylate ratios other than energy charge. Energy charge was calculated from the relative concentrations of adenosine 5′-tri-, di- and monophosphate (ATP, ADP, AMP) in the columellar muscle of P. ebeninus and in the adductor muscle of A. trapezia and S. commercialis. Mean energy charge values for individuals in the low-salinity condition (10‰) were 0.61 for P. ebeninus, 0. 69 for A. trapezia and 0.53 for S. commercialis, compared with mean energy charge values in the control individuals (35‰ S) of 0.85 to 0.87 for P. ebeninus, 0.84 to 0.85 for A. trapezia and 0.64 to 0.76 for S. commercialis. The decrease in energy charge occurred within 24 h; no further change was found with exposure to low salinity for 48 h (A. trapezia) or no further change was found with exposure to low salinity for 48 h (A. trapezia) or 20 h (P. ebeninus, S. commercialis). Total adenylate concentrations (ATP+ADP+AMP) differed between the three species, with mean values (kg-1 wet weight) of 5.0 mmol in P. ebeninus, 3.4 mmol in A. trapezia and 3.0 mmol in S. commercialis. No trends with time were found in total adenylate concentrations in any species. The changes in energy charge that occurred were not consistent with the differing sensitivity of the three species to reduced salinity, and do not support the use of energy charge as an absolute measure of stress in molluscs generally.

Abstract: Manipulations of fully grown (>60.0 mm) detritivorous Sydney mud whelks Pyrazus ebeninus Brugiere 1972 suggest that the species serves as a 'trophic cul-de-sac', limiting flow of car- bon from producers to top consumers. However, although large P. ebeninus individuals experience negligible predation, smaller individuals that do not cause the same depletion of primary and sec- ondary producers may suffer predatory losses. To assess the role that predation on the small whelks might play in weakening the trophic cul-de-sac provided by larger conspecifics, we investigated size- and habitat-specific patterns of predation on P. ebeninus through a series of field and laboratory ex- periments. Field tethering of 3 size classes of snail indicated that irrespective of seasonal differences in predation intensity, small (30.1 to 40.0 mm shell height, SH) individuals experienced significantly greater predatory mortality (25% over 7 wk) than medium (50.1 to 60.0 mm SH: 7.5%) or large (70.1 to 80.0 mm SH: 2%) conspecifics. Predatory mortality was largely attributable to naticid predators, al- though several tethered snails were crushed, perhaps by elasmobranchs, toadfish or crabs. For some size classes there were differences in predatory mortality between low shore mudflats and higher shore mangrove forests, but the direction of these differences varied between autumn and spring, and the differences disappeared altogether when densities of predators were held constant, indicating that they were not driven by habitat per se. In laboratory experiments, the relative contributions of small, medium and large P. ebeninus individuals to the total prey consumed by the naticid gastropod Conu- ber sordidus Swainson 1821 were similar between choice and no-choice experiments, and even in the absence of small prey items, large P. ebeninus snails were not consumed. Thus, even when there is de- pletion of small size classes of P. ebeninus, it is unlikely that common benthic predators would con- sume sufficient numbers of large snails to prevent deleterious effects of this species on primary and secondary production. To the contrary, predatory mortality of small P. ebeninus individuals may en- sure that its populations continue to be dominated by the large, damage-causing size classes.