The genus Synalpheus is a species-rich group of snapping shrimps (Alpheidae) common to coral-reef habitats worldwide. The informal “gambarelloides group” (Coutière, 1909; Dardeau, 1984) is a monophyletic clade (Morrison, et al., 2004; Hultgren, et al., 2014) of approximately forty-five currently known species of Synalpheus that live symbiotically within sponges and are mostly restricted to the tropical West Atlantic. Sponge-dwelling Synalpheus species exhibit a range of social systems, from the family’s ancestral condition of pair-living, to social groups with varying numbers of queens and workers (Duffy, 1996a; Duffy & Macdonald, 1999; Duffy, et al., 2000; Duffy, 2003; Duffy, 2007). This social diversity is evident in the distribution of social structures and patterns of reproductive skew among species of Synalpheus, which are qualitatively similar to those observed across the entire range of social vertebrate and invertebrate taxa. Eusociality has evolved independently multiple times within Synalpheus (Duffy, et al., 2000; Morrison, et al., 2004). Thus, this socially diverse group - including the only known eusocial species from the marine realm - offers a unique opportunity to study the evolution of sociality in the sea. SOCIAL DIVERSITY How Common is Sociality in Shrimps? The Crustacea is one of the most phylogenetically, morphologically, and ecologically diverse groups of organisms in the marine realm, with over 50,000 species living in nearly every conceivable ocean habitat (Martin & Davis, 2001). Crustaceans also exhibit a wealth of interesting behavioral variation, including a range of social systems as diverse as in their terrestrial relatives (Duffy & Thiel, 2007). Although crustacean social behavior has been less studied than that of insects or vertebrates, group living has been documented in a wide range of terrestrial, freshwater, and marine species (Linsenmair, 1987; Shuster & Wade, 1991; Diesel, 1997; Duffy, 2010). Crustacean social behavior has reached its apex in the diverse shrimp genus Synalpheus. Shrimp in the genus Synalpheus belong to the snapping shrimp family Alpheidae, whose common name derives from an enlarged claw - used primarily for communication, aggression, and defense against predators, conspecifics, and heterospecifics - that “snaps” upon closing to produce a powerful jet of water (Nolan & Salmon, 1970; Versluis, et al., 2000).
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