Themisto Guérin 1825

Zeidler, Wolfgang, 2004, A review of the families and genera of the hyperiidean amphipod superfamily Phronimoidea Bowman & Gruner, 1973 (Crustacea: Amphipoda: Hyperiidea), Zootaxa 567, pp. 1-66 : 27-28

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