Symbiosis in Pistol Shrimp and Goby Fish
The Mechanistic Explanation Of How An Organism's Structure Functions
Little is known about the neural or biochemical cues that cause an independent Alpheus shrimp to identify and pair with a goby, but the mechanisms that underlie their behavior as a pair are more thoroughly documented. There are two important behaviors that facilitate communication between them:
Tail Flick Behavior
The goby exhibits a characteristic tail flick behavior when it spots a potential threat. The shrimp’s antennae, which are kept in constant contact with the goby when outside the burrow, receive this signal, which elicits a flight response in which the shrimp retreats to the burrow under critical circumstances. The goby is able to communicate the level of danger through a series of tail flicks, in which increasing numbers of tail flicks signal more danger, so that a shrimp is more likely to flee under a longer series. By doing so, the shrimp is able to become cautious under low-danger conditions (in which the threat is further away, etc.), and be alerted of immediate threats when they approach. The tail flick behavior is triggered upon visual recognition of a potential predator or an organism that presents a threat in another way. Studies have shown that small fish do not cause the goby to flick its tail, but medium to large fish do. Even if the medium fish is not a predator, the tail flick response is still triggered if it presents a possible obstacle in the escape path to the burrow entrance.
Above: Warning signals given by gobies in the presence of a predator Parapercis hexophthalma (top line) vs. an empty box (middle line), and means and standard deviations (bottom line) (Karplus 1987).
Shrimp Antennae Contact
The quick transmission of the goby’s signals are mediated by the shrimp’s antennae. When outside the burrow, the shrimp maintains contact with the goby. This constant contact allows the shrimp to keep from being separated from its partner, whose eyesight it relies upon. The antennae also allow the shrimp to sense the goby’s movement in space, and are capable of differentiating between the goby’s normal movements and its characteristic tail flick. Contact with the antenna also make the goby aware of the shrimp’s presence, helping determine if shrimp-specific behaviors such as the tail flick are necessary.
Above: Daily activity rhythm of pistol shrimp in the Red Sea (Karplus 1987).