﻿New subgeneric names for the most commercially important shrimp genus Penaeus Fabricius, 1798 (Crustacea, Decapoda, Penaeidae)

﻿Abstract Although a recent comprehensive molecular phylogenetic study on Penaeus Fabricius, 1798 reinstated a single genus for these economically important shrimps, several clades in the molecular phylogenetic tree do not have formal names. Subgeneric names are given herein to five of these clades if Penaeus is to be split. A key to the subgenera in Penaeus is also provided.


Introduction
The most comprehensive study to date on the phylogenetic relationships amongst the members of the genus Penaeus s.l. Fabricius, 1798 was by Yang et al. (2023), which suggested that a single genus should be reinstated for these commercially important shrimps. Their study also proposes that if those molecular clades revealed in the phylogenetic tree of Penaeus s.l. (Yang et al. 2023: fig. 3) are recognized as taxonomic groups, the use of subgenera is preferable; the use of this rank would also reduce confusion and maintain stability for non-taxonomists who use the name.
In their phylogenetic study, Yang et al. (2023: fig. 3) showed that up to 11 subgeneric-level clades can be recognized. While many of these clades have been named in the past, five of them, however, remain un-named. In the interest of nomenclatural stability and consistency in discussing their systematics, I here propose to apply formal names for them. This action is justified especially if the peculiar taxon Marsupenaeus Tirmizi, 1971, which has a very specialized pouch-like thelycum, is to be maintained.
A key to these 11 subgenera is also provided even though all important characters used have already been proved to be neither synapomorphic nor evolutionary informative in Yang et al. (2023).

Systematic account
Penaeus ( Remarks. Although this is the nominotypical subgenus of Penaeus, it is unusual in lacking an epipod on the fifth pereiopod; the subgenus contains only two of the 32 recognized species in the genus.

Remarks.
Amongst the members of Penaeus, only this subgenus has a geographical distribution in the eastern Atlantic and the Mediterranean. This subgenus is also unique in the genus by having a long cervical carina which has the dorsal end almost reaching the dorsal carapace. Remarks. This subgenus together with Penaeus (Litopenaeus) are often called the American Penaeus. Morphologically these two subgenera are markedly different from each other and had long been thought to be evolutionary far apart (see Burkenroad 1934;Kubo 1949;Pérez Farfante 1969;Dall et al. 1990;von Sternberg and Motoh 1995;Pérez Farfante and Kensley 1997;von Sternberg 1997). They are, however, very closely related genetically (see Yang et al. 2023). At present only one morphological character, the sixth abdominal somite with dorsolateral sulcus, is found to separate the American Penaeus from other congeneric species. Recent molecular analysis has suggested that P. (Farfantepenaeus) notialis, originally described as a subspecies of P. (Farfantepenaeus) duorarum, may not be distinct at the species level (Timm et al. 2019 Remarks. This taxon corresponds to "gen. nov. 5" in the 11-genus scheme of fig. 3 in Yang et al. (2023). This subgenus is unusual in the genus in inhabiting deeper waters (see Chan 1998). It is also unique amongst the "grooved" species by completely lacking a median sulcus on the postrostral carina. Remarks. This taxon corresponds to "gen. nov.1" in the 11-genus scheme of fig. 3 in Yang et al. (2023). Morphologically this subgenus is similar to Penaeus (Litopenaeus). Other than having different types of thelycum, these two subgenera can be distinguished by the body coloration [banded in Penaeus (Eopenaeus) subgen. nov. but not banded in Penaeus (Litopenaeus)] and the development of the dorsolateral sulcus on the sixth abominal somite [weak to distinct in Penaeus (Litopenaeus) but completely absent in Penaeus (Eopenaeus) subgen. nov.]. Pérez Farfante (1969) and Pérez Farfante and Kensley (1997) also pointed out that there are differences in the shape of the petasma between these two subgenera, with the ventral costa reaching or not reaching the distal margin of the lateral lobe in Penaeus (Eopenaeus) subgen. nov. and Penaeus (Litopenaeus), respectively.
Etymology. The name Ischiopenaeus alludes to the presence of a strong ischial spine at the first pereiopod in this subgenus of Penaeus.
Species included. Penaeus (Ischiopenaeus) longistylus Kubo, 1943 Remarks. This taxon corresponds to "gen. nov. 4" in the 11-genus scheme of fig. 3 in Yang et al. (2023). This subgenus differs from almost all the non-American "grooved" species in the first pereiopod bearing a strong ischial spine (vs. small to absent). Another non-American "grooved" species with a strong ischial spine at the first pereiopod is P. (Altiopeneaus) marginatus, which lacks a median sulcus on the postrostral carina and generally has two ventral rostral teeth. Thus, the enigmatic Melicertus similis Chanda & Bhattacharya, 2002 described from the Andaman Sea likely represents juveniles of P. (Ischiopenaeus) longistylus as its original description and figures (Chanda and Bhattacharya 2002: figs 1, 6) indicated the presence of postrostral sulcus, only one ventral rostral tooth and the first pereiopod bearing a strong ischial spine. The "absence" of lateral spines on the telson in Melicertus similis is likely evidence that Chanda and Bhattacharya's (2002) material are juveniles (total length including rostrum less than 80 mm) as juveniles of Penaeus generally have the lateral spines on the telson rather small and can be easily detached or overlooked.
Etymology. The name Oleopenaeus (from the Latin olea for olive coloured) refers to the more or less uniform greenish-yellow body coloration of this group of Penaeus shrimps.
Species included. Penaeus (Oleopenaeus) hathor Burkenroad, 1959, Penaeus (Oleopenaeus) latisulcatus Kishinouye, 1896, Penaeus (Oleopenaeus) plebejus Hess, 1865. Remarks. This taxon corresponds to "gen. nov. 3" in the 11-genus scheme of fig. 3 in Yang et al. (2023). Except for the shape of the thelycum and body coloration, this subgenus is morphologically very similar to Penaeus (Marsupenaeus) (see Chan1998; Tsoi et al. 2014). The thelycum is of the normal closed type in Penaeus (Oleopenaeus) subgen. nov. but pouch-like in Penaeus (Marsupenaeus). With regards to the colour in life, the body is not banded in Penaeus (Oleopenaeus) subgen. nov. but is covered with thick cross bands in Penaeus (Marsupenaeus). The taxonomic status of P. (O.) hathor is still uncertain if it merely represents a subspecies of P. (O.) latisulcatus or even a synonym of the latter, as both morphological and genetic differences between these two taxa are rather minor (Holthuis 1980;Miquel 1984;Chan 1998;Ma et al. 2011;0 (Olivier, 1811). Remarks. This taxon corresponds to "gen. nov. 2" in the 11-genus scheme of fig. 3 in Yang et al. (2023). Mophologically, including coloration, this subgenus is extremely similar to Penaeus (Marsupenaeus) (see Yu and Chan 1986;Chan 1998) and such close affinity is also supported by the molecular data (Yang et al. 2023: figs 2, 3). Penaeus (Plagosopenaeus) subgen. nov. only differs from Penaeus (Marsupenaeus) in lacking lateral spines on the telson (vs. bearing three pairs of lateral spines), the thelycum not pouch-like and the last transverse band on the sixth abdominal somite not interrupted (Chan 1998).