A review of the families and genera of the hyperiidean amphipod superfamily Phronimoidea Bowman & Gruner, 1973 (Crustacea: Amphipoda: Hyperiidea) Author Zeidler, Wolfgang text Zootaxa 2004 2004-07-14 567 1 66 journal article 4802 10.11646/zootaxa.567.1.1 173cf168-6357-4b76-955f-7b523590ff1d 1175­5334 5259734 41C7D868-7BD9-46F4-94F1-EBEA427E2836 Genus Themisto Guérin 1825 Remarks This genus has provided much taxonomic confusion in the past. Its status has been in question being known alternately as Euthemisto or Parathemisto , sometimes with subgenera. Bowman et al . (1982) restored the genus Themisto , which up until that time was considered a junior homonym of the nudibranch Themisto Oken, 1815 ; a work rejected for nomenclatural purposes by the ICZN (1956). The uncertainty of the generic status combined with several ill­defined species, subspecies and varieties has made specific determination very difficult, despite the efforts of Bowman (1960) , Sheader and Evans (1974) and Schneppenheim and Weigmann­Haass (1986) . Vinogradov et al . (1982) recognise six species, but Schneppenheim and Weigmann­Haass (1986) demonstrated that northern hemisphere material, previously identified with T. gaudichaudii Guérin, 1825 , is a separate species, T. compressa Goës, 1865 . In view of the past confusion, and recent studies by Schneppenheim and Weigmann­Haass (1986) , a new key to species is provided to assist future workers. The holotype of T. gaudichaudii was discovered recently in the Guérin­Méneville collection (No. 438), in the ANSP ( Zeidler 1997 ). The correct citation for the original description of this species has been confused in the past with some authors citing Guérin, 1828 (eg. Schneppenheim & Weigmann­Haass 1986 ; Spamer & Bogan 1992 , 1994 ). This has arisen because, although the genus and species was first described in 1825, Guérin (1828) more or less repeated his description in a separate memoir introducing them as new, and providing figures of the type. All species of Themisto are mainly free­swimming, and can occur in large numbers, particularly in colder waters where, like krill, they play a significant role as food for plankton­feeding predators. Although considered mainly free­living, species of Themisto are known to be associated with medusae and salps ( Madin & Harbison 1977 ; Laval 1980 ), and specimens of T. australis have been collected from Salpa fusiformis from Tasmanian waters. Additional biological and ecological information is given by the following; Bary (1959) , Kane (1963) , Siegfried (1965) , Gray (1967) , Evans (1968) , Semenova (1974) , Sheader (1975 , 1977 , 1981 , 1990 ), Sheader and Evans (1975), Williams and Robins (1981) , Bowman et al . (1982), Hiroki (1988) , Corey (1990) , Semura et al . (1991) , Percy (1993) , Colombo and Vinas (1994) , Koszteyn et al . (1995) , Condon and Norman (1999) and Vinogradov (1999b) . Themisto has a bi­polar distribution with species restricted to the colder waters of the Arctic and Antarctic regions and occasionally venturing into cool­temperature waters. Species : as in the following key. Key to the species of the genus Themisto 1. Pereopod 5 not longer than P6 or P7 ........................................................................... 2 ­ Pereopod 5 much longer than P6 or P7 ........................................................................ 5 2. Maxilliped without row of setae on distal margin of basal plate (arctic and subarctic of Atlantic) ............................................................................. T. abyssorum ( Boeck, 1871 ) ­ Maxilliped with row of setae on distal margin of basal plate ..................................... 3 3. Pereopods 5 & 6; dactylus smooth. Antenna 1 of female hooked, stout (colder waters of Australia and New Zealand ) T. australis ( Stebbing, 1888 ) ­ Pereopods 5 & 6; dactylus with setae at base. Antennae 1 of female straight, tapering (North Pacific) ............................................................................................................. 4 4. Antennae 2 of females longer than A1. Adult females reach 9–17 mm in length. Pereopod 3 with 6–8 (usually 7) robust setae on posterior margin of carpus ...................... .......................................................................................... T. japonica ( Bovallius, 1887 ) ­ Antennae 2 of females equal in length to A1. Adult females reach 5–9 mm in length. Pereopod 3 with 3–5 (usually 4) robust setae on posterior margin of carpus ................ ........................................................................................... T. pacifica ( Stebbing, 1888 ) 5. Pereopod 7 usually longer than P6, but shorter than P5. Gnathopod 2; merus with relatively straight distal margin,anterodistal corner produced into small tooth(North Atlantic) ................................................................................................. T. compressa Goës, 1865 ­ Pereopod 6 & 7 subequal in length. Gnathopod 2; merus with rounded distal margin 6 6. Pereopods 3–7; dactylus with setae on basal part (arctic and subarctic) ........................ ....................................................................... T. libellula (Lichtenstein in Mandt, 1822 ) ­ Pereopods 3–7; dactylus smooth (colder waters of southern oceans) ............................. ......................................................................................... T. gaudichaudii Guérin, 1825