Hemigrapsus Experiment

Hypotheses and Predictions: The authors wanted to test if it is possible to detect increased metabolic work in two estuarine species (Hemigrapsus oregonensis and H. nudus) and relate this function to increased osmotic work by measuring oxygen consumption with respect to changes in temperature, salinity, and the gradient, which they defined as the difference in concentration between blood and the external medium. The authors predicted “because of seasonal variation in salinity it would appear quite probable that Hemigrapsus would be a strong hyper-osmotic regulator, and as the osmotic gradient increased between blood and sea water, additional work would be necessary in order to maintain the gradient.”

Experimental Design: The independent variables
After a holding period of 18 hours for winter captured crabs and a period of 30 hours for summer captured crabs, all individuals were transferred to containers with experimental temperature and salinity levels to begin the experiment. To observe changes due to salinity concentrations over the 48-hour period, groups of crabs were exposed to one of the eight salinity treatments, and temperature treatments were held constant at 5°C or 15°C to resemble winter or summer field conditions, respectively. Temperature induced change occurred in 3 trials— 5°, 15°, or 25° C— which were exposed to increased salinity concentrations over the 48-hour period. Blood concentrations were sampled before the beginning of the treatment, and 3, 24, and 48 hours after the treatment to monitor rates of osmoregulation change within the crabs. These blood concentrations were compared to increasing salt concentrations to see if there was a relationship explaining blood concentration retention in the crabs. The summer and winter crabs were treated independently, and by
oregonensis and H. nudus are different interspecifically, and their ability to acclimate to temperature and salt gradients varies with season intraspecifically. For example, as the salinity gradient increased, decreases in temperature caused a sharp decrease in the blood gradient for H. oregonensis, but only a gradual decrease for H. nudus. Moreover, winter gradients by both H. oregonensis and H. nudus are greater than summer gradients, but H. oregonensis has a higher gradient at low and high salinities. Despite their differences, both species maintained hyper-osmoregulation with respect to changes in salinity and temperature gradients. Interestingly, increases in the blood gradient due to decreased salinity content resulted in a parallel increase in oxygen consumption, which cancels out any possibility of increased osmotic work detection, effectively annulling the author’s prediction of this