A prawn goby (Cryptocentrus cinctus) peers out of its hole, acting as a sentinel for its mutualistic partner, an Alpheid shimp 

Goby fish (Gobiidae) are small bottom dwelling fish that live in a variety of aquatic environments. They are a common member of reef ecosystems throughout the tropics, and many species form symbiotic relationships with alpheid shrimps, commonly known as pistol or snapping shrimp (belonging to the family Alpheidea). Using their claws as shovels, alpheid shrimps dig and sustain burrows in the sand for both the goby and the shrimp to live in, providing shelter and refuge from predation. In return, the goby acts as a sentinel to the poorly sighted alpheid shrimp, warning their partner of nearby predators or intruders. When foraging or excavating outside of their burrows, shrimps maintain antennal contact with their partners tail. In doing so, the goby is able to communicate with the shrimp using a series of tail flicks, providing the shrimp with a signal to return to their burrow when danger approaches. As both species benefit from this relationship, it is an example of a mutualistic symbiosis.

The evolution of such a specialised tactile communication method would likely suggest that this symbiosis has existed for a significantly long duration. During this time, a number of complex behaviours and interactions have developed as part of the goby-shrimp symbiosis. For example, gobies have demonstrated their importance in providing food to their partner shrimps. Some species have been observed to collect food items and bring them back to their burrows for the shrimp to feed on. Others have purposefully laid droppings inside their burrows rather than away from the burrow when their partner shrimp is able to utilise these droppings as a food source. Shrimps have been observed to clean their partner gobies, and feed on the parasites that are removed during cleaning. In certain relationships, shrimps have also demonstrated their importance in building and maintaining breeding chambers for their partner gobies. Female gobies enter these chambers to mate with resident males, laying their eggs inside the buried chambers where males then provide paternal care to the developing offspring

As a result of such complex interactions that exist between alpheid shrimp and goby fish, these individuals provide model organisms to investigate symbiotic relationships, and the many interactions that are associated with mutualism. Furthermore, as these species spend the majority of their time in close proximity to their burrows, this has often allowed for observations to be easily made in situ while snorkelling or diving. Such limited movement away from their burrows has also resulted in their ease of use during laboratory research.

Several studies have been conducted using goby fish that live in symbiosis with alpheid shrimps, investigating a range of topics. Results from these have often helped us in understanding the important interactions that occur between species within an ecosystem. These studies have also provided valuable insight into the evolution of symbiotic relationships, as well as fitness benefits and costs that are associated with different types of symbiosis.

Many mutualistic relationships are of an obligate nature, including the majority of goby-shrimp associations, whereby survival of both members would be significantly compromised were the relationship to be no longer maintained. Understanding the elements of, and interactions that occur as part of obligate symbiosis is essential in order to minimise threats that may affect these vitally important relationships. Furthermore, individuals living in mutualistic symbiotic relationships also share important interactions with other species living within an ecosystem, and so the breakdown of certain mutualisms may have significant direct and indirect effects to a number of organisms throughout the reef.

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