2.2. Experimental Design

This experiment was designed to determine if and how stocking density and environmental enrichment (provision of shelter), influenced the condition and survival of laboratory-kept crabs. Before the experiment began, crabs were removed from the holding tank, their carapace width measured and any appendage loss was noted. A coloured foam tag was secured using cyano-acrylate adhesive to the dorsal carapace surface and numbered for identification and tracking. Only crabs with a carapace width of 50–75 mm, possessing both claws, and with no more than two missing walking legs were used in experiments. Crabs were not separated by the colour of their underside (which can influence some physiological responses), but rather a continuum of colours from green through yellow-orange to red was used across all treatments [51,52].

Tagged crabs were placed into separate, plastic-coated, wire mesh cages (30 cm × 15 cm × 30 cm deep; 1 cm² mesh), stocked at one of three different densities in each cage: 1 tagged crab only (total = 1), 1 tagged crab with 3 additional unmarked crabs (total = 4) and 1 tagged crab with 7 additional unmarked crabs (total = 8). Furthermore, to determine if environmental enrichment affected the crab’s condition, cylindrical PVC shelters (10 cm diameter × 20 cm length) were added to half of the cages. In all, 144 tagged crabs were followed over a period of 27 weeks (24 tagged crabs in each of the six treatments: 1, 4, or 8 crabs with shelter and 1, 4, or 8 crabs without shelter). The additional unmarked crabs were not handled, but their survivorship was recorded and therefore acted as a control group for assessing mortality in response to handling. Unmarked crabs played no role in the assessment of vitality. Cages from all the 6 treatment groups were evenly distributed across three separate flow-through aerated seawater tanks (same water source, Volume = 3000 L, Temperature = 10–12 °C, Salinity = 31–32, natural day-night light cycle). Crabs were fed a mixed diet of fish, mussels and kelp to excess twice weekly. The tanks were drained and rinsed weekly to remove any uneaten food and debris. The experiment began in late September 2018 and was terminated in late March 2019: this ensured that the crabs would not moult during this time [53]. The crabs were checked twice weekly and any mortalities were recorded and removed. If any limbs were lost by the tagged crabs, the number and position of the lost limbs were recorded.