General experimental methods – We conducted an experiment to determine the effect of food type (algae vs. animal) and amount of food consumed on the energetic state of the Asian shore crab. We chose to use the most commonly consumed and preferred food types in invaded habitats. For animals, this is the mussel Mytilus edulis, and for algae it is the abundant red alga Chondrus crispus. We collected female crabs from Odiorne Point, NH, USA in early May 2011. To standardize reproductive state of experimental crabs as far as possible, we only collected gravid females. Crabs were taken immediately to the University of South Carolina where they were held at elevated temperatures (20°C, which falls within the range of natural summer temperatures for seawater at the site where these crabs were collected, but is higher than temperatures that commonly occur) for 5 d to stimulate egg hatching. All crabs used in the experiment had therefore recently produced and released a clutch of eggs.
The experiment was conducted in a closed, recirculating aquarium (salinity 34 psu) at water temperatures that mimicked summer temperatures on the New Hampshire coast (15°C), and with a 18 h light: 6h dark cycle. Each crab was housed in its own experimental chamber (1 l volume). Each chamber was supplied with water so that flow occurred through each chamber within the recirculating system. Chambers were designed to capture all feces and unconsumed food particles so that these could be precisely quantified for each crab. Algae fed to crabs during the experiment were collected at the same time and place as crabs. However, given the scarcity of mussels at the site at the time of collection, we purchased live mussels from Cape Cod Shellfish & Seafood (Boston, MA) and froze them until they were used in the experiment. Only mussel mantle tissue was used. No crabs died or molted during the 10 wk experiment.
Feeding and consumption – Each crab was fed a constant diet 3 times per week throughout the experiment. These experimental diets differed in the total amount of food offered at each feeding (0.1, 0.2, 0.4, 0.8 g wet weight) and the percent of that food that was mussel or algae (ratios used were 0:100, 25:75, 50:50, 75:25, 100:0), yielding a total of 20 different quantitative × qualitative diet combinations. Crabs were fed on Monday, Wednesday, and Friday throughout the 10 wk experiment and were given 24 h to consume the provided food before any uneaten food was removed, dried at 70°C for 48 h, and weighed (mussel and algae separately) to determine the precise amount of each food type remaining. Initial dry weight of food offered was calculated from measured wet weights using the percent water content of each of the different food types (mussel: 82.5 ± 1.6%; algae: 70.0 ± 0.3%; mean ± SD), determined independently using the mean difference in mass of wet and dry replicate samples (n = 30) of each food type. Additionally, a nonconsumption control (mussel tissue and algae submerged within the experimental chamber, but not accessible to a crab) was included on each feeding day to determine any nonconsumptive changes in food mass.
Absorption – After removing uneaten food in each feeding cycle, crabs were then given 24 h for gut clearance before being fed again (except on weekends where food was removed on Saturday and feeding did not occur again until Monday). Experimental chambers were designed with a mesh subfloor so that feces fell through to a collection chamber where they were isolated from the crab to prevent reingestion. Feces were collected by filtration onto a Whatman Qualitative No. 1 filter, and were weighed after drying for 24 h at 70°C. We used the ratio of the dry weight of feces produced and the dry weight of food consumed as the absorption efficiency (termed Assimilation in the data file).
Metabolic rate – In the 9th week of the experiment we also measured the metabolic rate of each crab to determine whether diet-induced differences in metabolic rate could be responsible for any differences in physiological condition seen at the end of the experiment. Rates were measured using a YSI dissolved oxygen meter (model no. 52CE) in a closed volume chamber. Measurements were made after the gut clearance period (so at least 24 h after crabs had eaten). Crabs were placed into a closed chamber filled with water at the same salinity and temperature as the experimental water. They were allowed 15 min after introduction into the respirometry chamber to acclimate before the initial oxygen reading was taken. The final reading was then taken 15 min later and the metabolic rate was calculated as mg O2 consumed per gram dry weight of crab per hour. Following the metabolic rate measurement, the crab was then returned to its experimental chamber and the regular feeding schedule continued as des cribed above.
Physiological condition – At the conclusion of the experiment, each crab was dissected and the hepa topancreas was removed. The hepatopancreas and the rest of the crab were then each dried for 72 h at 70°C and weighed. As a second estimate of physiological condition, we measured the percent lipid of the hepa to pancreas tissue. Bulk lipids were extracted using a modified Folch method where chloroform is replaced with hexanes.
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