Symbiosis in Pistol Shrimp and Goby Fish
How The Behavior Impacts The Organisms' Reproductive Fitness
From the perspective of this behavior’s adaptive value, this partnership is hugely effective at increasing the lifetime reproductive success of both organisms involved. The risk of predation is greatly reduced for both the shrimp and the goby, though the shrimp may benefit more in this respect, since it relies on the goby’s more effective eyesight to watch for predators while foraging for food outside of the den—a regularly occuring behavior that would leave the shrimp an easy meal for any predator, if not for the goby’s vigilance. So the shrimp's ability to survive improves, clearly increasing its potential for reproductive successs. However, the goby reserves its own benefits from the coalition. In addition to providing a safe space to sleep in at night without fear of predators, the shrimp’s burrow also provides a functional space for the goby to breed (examples of burrow structures in figure below). The goby’s mating behavior involves a prolonged “dance” in which both partners stimulate the other before breeding. This requires a sufficient area free from predators for a prolonged period of time, and the shrimp provides it, in addition to maintaing the burrow to keep it from collapsing during the event (Duerbam 2013).
In addition, both the shrimp and the goby increase the other's access to food. Gobies have been documented sifting through the sediment that the shrimp digs up and filtering organisms to eat (Karplus 2011). Also, at least on species of Alpheid shrimp forages while cleaning its partner, eating organisms right off of the goby's back (Karplus 1987). So it seems logically sound and straightforward to assert that the symbiosis of the pistol shrimp and the goby increases lifetime reproductive fitness for both partners, since both receive protection from predators, increased access to food, and the goby receives a safe and convenient location for mating.
Above: The three different burrow structures for goby/shrimp pairs (Karplus 1987).