Phylogeny and taxonomy of the enigmatic genus P etalidium ( D ecapoda, S ergestidae), with biological remarks

P etalidium ( D ecapoda: D endrobranchiata: S ergestidae) is one of the least known genera of pelagic decapods. On the basis of collections taken during Dana Expedition 1920–22 and 1928–30, we revise the morphology of the genus. To understand the taxonomic status and position of P etalidium within other sergestid genera, we used 150 morphological characters (127 binary, 23 multistate). Three recognized species of P etalidium and eighteen type species representing all other genera of S ergestidae were included as terminals. Characters were polarized using L ucifer typus ( L uciferidae: S ergestoidea) and A risteomorpha foliacea ( A risteidae: P enaeoidea) as the outgroups. Phylogenetic analysis revealed monophyly of P etalidium comprising three species: P etalidium foliaceum   B ate, 1881; P etalidium obesum ( K røyer, 1859); and P etalidium suspiriosum   B urkenroad, 1937. Phylogenetic analyses also revealed possible monophyly of the clade including 15 new genera of the former S ergia and S ergestes and the position of L ucifer (family L uciferidae) within the clade S ergestidae. These results highlight necessity of future phylogenetic analyses of the remaining genera L ucifer , A cetes , P eisos , and S icyonella involving all species of these genera and promise interesting changes for the classification of S ergestoidea. Morphological trends within S ergestoidea are discussed, and an emended diagnosis of P etalidium and diagnoses of the species it includes are given. All species are illustrated and biology of the genus is discussed. Keys to species of P etalidium based upon new information are presented. © 2015 The Linnean Society of London