Sternal spines in penaeid postlarvae (Decapoda: Penaeidae): life-phase-specific and systematically significant?

Our ability to identify and discriminate postlarvae of penaeids below family level remains poor due to phase brevity and a lack of taxonomic characters. Whether sternal spines are unique and taxonomically significant to postlarvae has not been resolved. I describe number and placement of spines in Parapenaeus sp. Smith, 1885, and a specimen tentatively identified as Xiphopenaeus kroyeri (Heller, 1862) from the Gulf of Mexico; review information for penaeids worldwide; and evaluate the significance of sternal spines as a life-phase specific taxonomic character and to penaeid systematics. To date, sternal spines have been described for 14 of 32 genera and 26 species. Most taxa share one of two common sternal formulas: either 0 + 0 + 0 + 1 + 0, or 0 + 0 + 0 + 1 + 1. Only Metapenaeopsis Bouvier, 1905, and the tentative Xiphopenaeus kroyeri have a pair of spines on at least the first-two sternal plates, and only Metapenaeopsis and Litopenaeus Pérez-Farfante, 1969, contain members with different sternal formulas. I suggest that differences among taxa in shape of the sternal plates may be an unrecognized taxonomic character. Sternal spines are not life-phase specific and do not reflect lower-level systematic relationships within Penaeidae regardless of generic nomenclature applied. The unusual length, shape, and reverse orientation of the spine on plate five in species of Parapenaeus, and presence of an elongate ventromedian spine on one or more pleomeres in Parapenaeus and Funchalia Johnson, 1868, supports molecular and morphological data that Penaeidae may be paraphyletic. While generally ineffective as a stand-alone taxonomic character, differences in number, placement, and orientation of sternal spines, i.e., the 'sternal pattern'; knowledge of geographic distributions; and, perhaps differences in sternal plate shape should be included in the suite of characters used to discriminate and identify penaeids during the postlarva phase.