Dataset: Blue crab activity
Deployment: Griffen_2014

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.

We assessed individual crab aggression levels daily to determine if diet influenced behavior. To reduce biases in behavior originating from previous feeding history we did not begin measuring behavior until two weeks after capture. Behavior was measured by slowly lowering a metal prong into each container, stopping approximately three cm from the mouth of the crab, and observing the crab response. The container sides were opaque to help prevent the crab from reacting to stimuli outside of the container and the observer was careful to never appear directly over the container. Crab behavior was categorized as aggressive if the crab approached or raised its chelipeds towards the prong, while stationary crabs or crabs that moved away from the prong were labeled as docile. Crab behavior was measured once each day between the hours 1200–1330 prior to feeding to help control for any behaviors associated with the crab circadian rhythm and to prevent changes in behavior associated with consuming food. Individual crab aggression was expressed as the proportion of days it acted aggressively.