We collected 60 mature fully intact female Callinectes sapidus (carapace width = 12.5 – 16.5 cm) using baited crab traps from the National Estuarine Research Reserve, North Inlet (33°20’N, 79°10’W, Georgetown, South Carolina). Crabs were obtained during early May 2014 over the course of a week, one month prior to the peak spawning season. We started the experiment in blocks (5 total) as crabs were captured so that no crabs were held longer than 24 hours before commencing the study and being fed.
Crabs were transported to the Baruch Institute wet lab (situated on North Inlet) where they were placed into individual plastic containers (length 29.8 cm, width 19.7 cm, height 20.3 cm) submersed within seven flow-through tanks supplied with seawater directly from North Inlet. Water temperature matched environmental conditions and varied between 25.4 – 34.5 °C throughout the experiment. Individual containers were filled with a 1.5 cm layer of sediment collected from the field and continuously received water at a rate of ~1.3 L/min. Once a week the containers were cleaned with an aquarium vacuum and the substrate layer was replenished with new sediment. This sediment was provided because sediment is required for development of normal egg masses in this species. Sediment may also have served as an additional source of food. Crabs that died before the end of the experiment were frozen and stored at -20 °C for later dissection. Any egg masses produced by the crabs were stored in the freezer for later analyses. The experiment was terminated after 12 weeks, on 30-Jul-2014.
Throughout the duration of the experiment, crabs were fed either exclusively ribbed mussels (Geukensia demissa), mummichogs (Fundulus heteroclitus), or seaweed (Ulva lactuca), with all crabs having access to sediment. Because consumers are known to compensate for low-quality diets by increasing the amount of food consumed we fed crabs either a satiating amount of food (4 ribbed mussels, 25.2 g mummichog, 3.7 g seaweed) or approximately one-quarter this amount (1 ribbed mussel, 5.8 g mummichog, 1.3 g seaweed). The quantities of food offered depended on food type. Food was weighed to the nearest 0.0001g using an analytical scale (HR-120) While mummichog weight corresponded to the average weight of the soft tissue within 4 or 1 mussels, seaweed weight related to the volume of 25.2 or 5.8 g of mummichog because U. lactuca is substantially less dense than mummichog and the amount of food blue crabs can consume is dependent on their stomach capacity. Thus, this study had a 3x2 factorial design with ten crabs randomly assigned to each of the six different experimental diets.
Crabs were fed a constant experimental diet every other day and any excess food was removed after 24 h. Food was collected from North Inlet daily and, to prevent decomposition, was never refrigerated longer than 48 h prior to use. Mussels were cracked open prior to being fed to the crabs in an effort to make handling effort more similar across food types, and only soft tissue weights of mussels were used.
At the end of the experiment, crabs were dissected and the primary energy storage organ of crabs, the hepatopancreas, was removed to assess the relative physiological condition of individuals. The hepatopancreases were dried separately to constant weight at 70°C using a gravity convection oven (VWR 414004-562), and weighed. The lipid content of the hepatopancreases were determined using this protocol. All weights in the protocol were measured to the nearest 0.00001 g using an analytical balance (Mettler Toledo XS205 Dual Range).
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