Synalpheus

Although the genus Zuzalpheus was proposed for S. gambarelloides, S. brooksi, and their closest relatives, this has not been adopted and is not considered a monophyletic clade within the genus Synalpheus.^[2][3][4]

Over 160 species are recognised in the genus Synalpheus. For a complete listing, see List of Synalpheus species.

In the narrower sense, Synalpheus occur in the eastern Pacific where they are most plentiful and probably originated, and to a lesser extent in the Atlantic and the Indian Ocean; the species placed in Zuzalpheus occur mainly in the western Atlantic where their lineage probably originated, and to a lesser extent in the eastern Atlantic and Indian Ocean, and the eastern Pacific. It may thus be that the closure of the Isthmus of Panama in the Piacenzian (about 3 million years ago) was a key factor in separating the two lineages, as species referred to Synalpheus sensu stricto are most plentiful in the western Pacific.^[4]

The snapping behaviour of Synalpheus is rather well studied. In Synalpheus parneomeris, peak to peak source levels of 185–190 dB re 1 μPa at 1 m were measured, depending on the size of the claw.^[5]

The only known eusocial aquatic species occur within the genus Synalpheus. The species known to be eusocial are S. brooksi, S. chacei, S. elizabethae, S. filidigitus, S. rathbunae, S. regalis,^[2] S. microneptunus,^[6] and S. duffyi as well as potentially S. riosi.^[7] Eusociality has evolved at least three times within Synalpheus.^[3][8] It appears that there were multiple rapid radiations between 3 and 9 mya from which the ancestors of these eusocial species appeared.^[8] Eusociality is thought to have arisen due to competition for space, because among the species that host Synalpheus, empty sponges are rarely found.^[9] It also appears that kin selection was necessary for this evolution to occur because the only species in which eusociality has appeared are non-dispersing shrimp that hatch directly into crawling individuals.^[10] Until recently, eusocial species of Synalpheus have appeared in far greater abundance than, and appear to outcompete, less social species for space in sponges.^[9][11]