A Successful Partnership

            There are many wondrous adaptations that have evolved in the coral reef ecosystem. One in particular is the symbiotic relationship between the Banded or Yellow Watchman Goby (Cryptocentrus cinctus) and the Tiger Pistol Shrimp (Alpheus spp.). There are three type of symbiosis: commensalism, parasitism, and mutualism. Commensalism is an association between two species where one organism benefits without harming the other. Parasitism involves an association where one organism benefits to the detriment (harm) of the other. The symbiotic relationship demonstrated by the shrimp goby and pistol shrimp is classified as mutualism. Mutualism is a type of symbiosis where two (or more) organisms from different species live in close proximity to one another. They rely on one another for food, protection, or other life functions. Both members of this partnership derive some benefit from this relationship. In the case of the shrimp goby and pistol shrimp, the shrimp goby provides security while the pistol shrimp maintains their shared burrow home.

            The shrimp goby and pistol shrimp live in the sandy sediments that surround reefs, where there is lots of food, but little protection from predators. Because the pistol shrimp is virtually blind, it is especially vulnerable to predation above the surface and finds it necessary to live underground to provide it some protection. The pistol shrimp excavate burrows and tunnels in its shallow muddy or sandy bottom habitat. The shrimp goby, characteristically being small, also finds life precarious (dangerous) aboveground. They can perceive predators but may not always have a place to conceal themselves from them. Undoubtedly, at some point in their evolution, the goby and pistol shrimp lived separate lives. Then by chance, a goby and a pistol shrimp discovered that living together gave them a better chance of survival. Natural selection reinforced this partnership and today the lives of shrimp and goby are absolutely intertwined. This mutualistic relationship provides the pistol shrimp with early warning of predators and the shrimp goby with a place to hide.

            The burrows and tunnels constructed by the pistol shrimp require continuous maintenance. The entrance to the burrow home is typically 1 inch in diameter and the tunnel length is usually several feet long. The pistol shrimp can be observed removing sand and debris out of the tunnel. Furthermore, the shrimp can lift twice its weight with its enlarged snapping claw. While the shrimp maintains the burrow and tunnels, the goby is diligently providing security. Occasionally, the goby will dart out to get food or filter the sand through its gills. The pistol shrimp when out will always maintain contact with the goby by placing an antenna on the tail of the goby. The goby and shrimp will develop signals over time from a caution twitch to a panic thrash. When this happens the shrimp will always enter the tunnel first. The time recorded for both to hide is about 1/10th of a second.

            Gobies belong to the Gobiidae family, and make up the largest family of marine fishes and consist of over 200 genera and 500 Indo-Pacific species. While gobies are relatively small fish, usually reaching a size of only three inches in an aquarium, some can reach a length of over 20 inches in the wild. These fish have long, tubular bodies, blunt heads, most have two dorsal fins and all but a few species lack gas bladders. Gobies are found throughout the world in tropical and temperate waters. They inhabit coral reefs, rocky areas, and lagoons. Also, they reside in brackish and freshwater habitats. Most Gobies are found in pairs or small groups, and are often associated with crustaceas or sessile invertebrates. Some Gobies spend their lives with pistol shrimp, sharing a burrow; other species of gobies live on the branches of sea fans, sponges, or live corals. A small number of Gobies also act as cleaners, picking parasites and dead skin from larger fish.

            The Palau Aquarium currently exhibits the Yellow Watchman Goby with its pistol shrimp symbiont or partner in the Symbiotic Relationships Exhibit. Also featured in this exhibit are the Anemonefishes and Yellow Coral Gobies.

 
Banded or Yellow Watchman Goby (Cryptocentrus cinctus)

References:

Herre, A. W. C. T.  1936. Eleven new fishes from the Malay Peninsula.  Bull. Raffles Mus. 5-16

Myers, R. F. 1999. Micronesian Reef Fishes: A Comprehensive Guide to the Coral Reef Fishes of Micronesia, pp 239, Coral Graphics, Guam

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