Abstract
BackgroundThe hippolytid genus Lysmata is characterized by simultaneous hermaphroditism, a very rare sexual system among Decapoda. Specialized cleaning behavior is reported in a few pair-living species; these life history traits vary within the genus. Unfortunately, the systematics of Lysmata and the Hippolytidae itself are in contention, making it difficult to examine these taxa for trends in life history traits. A phylogeny of Lysmata and related taxa is needed, to clarify their evolutionary relationships and the origin of their unique sexual pattern. In this study, we present a molecular phylogenetic analysis among species of Lysmata, related genera, and several putative hippolytids. The analysis is based upon DNA sequences of two genes, 16S mtDNA and nuclear 28S rRNA. Phylogenetic trees were estimated using Bayesian Inference, Maximum Likelihood, and Maximum Parsimony.ResultsPhylogenetic analysis of 29 species of Lysmata, eight genera of Hippolytidae and two genera of Barbouriidae based on a single (16S, 28S) and combined gene approach (16S+28S) indicates that three groups of Lysmata differentiate according to antennular morphology: (1) Lysmata, having a multi-segmented accessory branch, (2) Hippolysmata (prior to Chace 1972), with a one-segmented accessory branch, and (3) a third group of Lysmata outliers, with one-segmented unguiform accessory branch, and close affinity to the genera Exhippolysmata and Lysmatella. The monophyly of the clade bearing a multi-segmented accessory branch is robust. Within the short accessory branch clade, species with specialized cleaning behaviors form a monophyletic clade, however, the integrity of the clade was sensitive to alignment criteria. Other hippolytid and barbouriid genera used in the analysis are basal to these three groups, including one displaying simultaneous hermaphroditism (Parhippolyte). The two barbouriid species occur in a separate clade, but among hippolytid taxa.ConclusionsThe data support the historical morphological division of Lysmata into clades based on accessory branch morphology. The position of the "cleaner" shrimps, indicates that specialized cleaning behavior is a derived trait. The topologies of the cladograms support the monophyly of the barbouriids, but do not support their elevation to familial status. Taxa ancestral to the genus Lysmata display simultaneous hermaphroditism, suggesting that this life history trait evolved outside the genus Lysmata.
Highlights
The hippolytid genus Lysmata is characterized by simultaneous hermaphroditism, a very rare sexual system among Decapoda
Phylogenetic analyses of 16S (Figure 1), 28S (Figure 2) and the concatenated data sets (16S+28 S; Figure 3) overall supported the historical division of the Lysmata based on antennular accessory branch morphology
The low clade values supporting the basal position of L. olavoi with respect to species possessing a short accessory branch reflect the uncertainty of its placement in the phylogenetic tree (Figure 1)
Summary
The hippolytid genus Lysmata is characterized by simultaneous hermaphroditism, a very rare sexual system among Decapoda. The hippolytid shrimp genus, Lysmata (Risso, 1816), has attracted the attention of biologists for several decades. Members of this genus are small caridean shrimp and occur in tropical to warm temperate marine coastal waters worldwide. Among confirmed euhermaphroditic Lysmata species, all individuals pass through a functional male phase early in life [4,5,6,7]. This is the impetus of the term “protandric simultaneous hermaphroditism” or “PSH” The early male phase contributed to the mistaken impression that Lysmata species were protandric hermaphrodites [1,2,3]
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