Blue crabs have a complex life-cycle (Fig. A5) in which zoea must develop in the high salinity ocean waters before returning to the lower salinity waters of an estuary to complete their life-cycle. Growth in crabs can be very rapid, with sexual maturity reached in as short a time as 1 to 1.5 years. Conceptually, individual crab model consist of three primary parts. The first part involves algorithms which control crab movement, updating and crab-crab interactions (Appendices A.5.2-A.5.4). The second part of the model focuses on energy balance which encompasses crab feeding, egestion, metabolism, movement, molting and egg production (Appendices A.5.5-A.5.12). The final facet of the model deals with rules for crab spawning, larval development, recruitment back into the estuary, and sources of crab mortality (Appendices A.5.13-A.5.14).
Each individual crab maintains a set of state variables (energy balance, location, size, rate of movement, etc.) that are controlled and influenced by these sub-models. When a crab is updated (Appendix A.5.2), these state variables values are recomputed. This is accomplished by first determining whether the crab interacted with another crab and was killed or died from other causes (Appendix A.5.3 and A.5.14). If the updated crab did not die during the time period, it is then determined whether the crab has gained enough mass to start molting (Appendix A.5.11) which involves, among other things, altering the crab's metabolic rate until molting finishes. Next the crab's energy balance is computed by determining the amount of food ingested given the way it forages (Appendix A.5.6 and A.5.7), egested (Appendix A.5.8), metabolic costs under its current environmental conditions (Appendix A.5.9), and movement costs (Appendix A.5.10). Based on the crab's overall energy balance, the amount of excess energy is either devoted to growth (or loss if negative) if the crab is not an adult, while excess energy is devoted to reproduction if the crab is a mature female (Appendix A.5.12) and the crab spawns once it has produced sufficient eggs and the environmental conditions are adequate. The larvae develop and are instantiated into the model estuary upon reaching the equivalent of the 7th instar (Appendix A.5.13).