New insight into pistol shrimp and goby association

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The mutualistic association with pistol (alpheid) shrimps has evolved twice within the goby lineage, according to a study published in the most recent issue of the journal Molecular Phylogenetics and Evolution.

Shrimp gobies are a group consisting of about 120 species in 12 genera that form mutually beneficial associations with shrimps.  

In the association, one or a pair of gobies occupies a burrow built and maintained by one or two Alpheus shrimps. In return for the shelter provided, the gobies act as sentinels for the shrimp, which have very weak eyesight.  

On sensing approaching danger, the goby warns the shrimp through a tactile system of tail flicks detected by the shrimp’s elongated antennae, and both shrimp and goby scuttle into the burrow for protection.

Christine Thacker, Andrew Johnson and Dawn Roje used the results of previous studies on goby phylogeny that suggested that the association with shrimps evolved more than once as a basis for their examination of the (molecular) phylogeny of the Indo-Pacific shrimp gobies.  

The authors focused their studies on shrimp gobies in seven genera (Amblyeleotris, Cryptocentrus, Ctenogobiops, Mahidolia, Stonogobiops, Tomiamichthys and Vanderhorstia).  

Sequencing three mitochondrial (cytochrome oxidase subunit I, NADH dehydrogenase subunits 1 and 2) and two nuclear genes (recombination activating gene 2 and rhodopsin) for 103 individuals representing 55 species (of which 27 were shrimp gobies), the authors obtained 4691 base pairs for their analysis.

After analysing their data, the authors found that the shrimp association has evolved twice within the shrimp gobies: once in a lineage comprising Amblyeleotris, Ctenogobiops, and Vanderhorstia and another in a lineage comprising Cryptocentrus, Mahidolia, Stonogobiops and Tomiamichthys.  

The authors hypothesise that the association has evolved more than once because of the high benefit to cost ratio for the goby and shrimp over a wide range of ecological conditions.  

In addition, the ease with which gobies and shrimps associate with other living organisms may have also played a role in the multiple evolution of this character.

The authors also discovered considerable cryptic diversity within the material they identified as Mahidolia mysticina.  

Three distinct lineages were obtained from specimens identified as this species that were caught in the same locality and were pairing with the same shrimp species.

For more information, see the paper: Thacker, CE, AR Thompson and DM Roje (2011) Phylogeny and evolution of Indo-Pacific shrimp-associated gobies (Gobiiformes: Gobiidae). Molecular Phylogenetics and Evolution 59, pp. 168–176.