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
used in this study were the temporal season the crabs were captured in (summer or winter), the temperature experimental gradient (5°, 10°, 15°, 20°, and 25° C), the salinity (medium) experimental gradient (6, 12, 25, 75, 100, 125, 150, and 175% sea water), the two Hemisgrapus species, and time (0, 3, 24, and 48 hours after crab exposure to treatment). The dependent variables used in this study were the changes in osmotic regulation in the crabs (which looked at the blood concentration as a percentage of sea water) and the relationship of the gradient between blood and external sea water to medium concentrations. Once the laboratory received the animal specimens, they placed each animal in a 75% seawater medium to standardize all individuals to the same salinity level, but placed different groups of crabs in treatments of varying experimental temperatures to permit thermal acclimation.
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
species.

Summarized Results: The osmotic responses between H.
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
system.

Significance: By measuring physiological preferences of species within the same geographic area, biologists can infer which habitat qualities are required to provide ample space and resources, especially for species of concern. This study was able to show H. nudus individuals are more likely to inhabit higher intertidal regions due to their lack of salinity preference during winter conditions and less-permeable exoskeleton. Conversely, H. oregonensis individuals are more likely to inhabit lower intertidal regions due to their affinity for 75% seawater and are more likely to competitively drive H. nudus away from this specific resource. During the summer months, both species have a high gradient, which allows niche overlap seasonally.

Critique: There are some critical flaws in the experimental design; namely, the author failed to mention if they repeated or replicated any of their methods. Did the author repeat this study more than once to confidently assess if temperature affects osmoregulation in the pattern he recorded? What if there was an extraneous variable he did not account for in one group of crabs? Without replication, it is difficult to surmise if this is the definitive result of this experiment. Furthermore, the author never mentions the total sample size used for either seasonal or species treatment. There is a note that the containers had approximately 30 crabs for each treatment, but this is not standardized across treatments, which could result in varying standard errors (that he did not provide). Additionally, there is no control with respect to a standard temperature and salinity concentration, which would have been useful in a comparison to see how crabs normally behaved to changes in the treatments.

Questions: Could the amount of crabs in a treatment container influence the rate of osmoregulation (i.e. more individuals taking in higher oxygen concentrations in a small environment)? Did the author keep track of which individuals he sampled blood from, or did he randomly choose 10 to 15 individuals each time? Why did the author keep the animals in the dark and with no food? Could this treatment induce physiological stress or other factors that could have affected the results of the study?