Nannoniscus Sars 1870

Genus: Nannoniscus Sars, 1870.

Nannoniscus Sars, 1870: 164; Hansen, 1916: 87–89; Gurjanova, 1932: 51; Menzies, 1962b: 133; Birstein, 1963: 78; Siebenaller & Hessler, 1981: 241; Kussakin, 1999: 68; Wilson, 2008: 13; Saetoniscus Brandt, 2002: 11.

Type species: Nannoniscus oblongus Hansen, 1916.

Species included (see also Table 3): Nannoniscus acanthurus Birstein, 1963, Nannoniscus aequiremus Hansen, 1916, Nannoniscus affinis Hansen, 1916, Nannoniscus analis Hansen, 1916, Nannoniscus antennaspinis Brandt, 2002, Nannoniscus arcticus Hansen, 1916, Nannoniscus arctoabyssalis Just, 1980, Nannoniscus australis Vanhöffen, 1914, Nannoniscus bidens Vanhöffen, 1914, Nannoniscus bidens sensu Brandt, 1992, Nannoniscus brenkei Kaiser, Brix & Jennings sp. nov., Nannoniscus camayae Menzies, 1962, Nannoniscus caspius Sars, 1899, Nannoniscus cristatus Mezhov, 1986, Nannoniscus detrimentus Menzies & George, 1972, Nannoniscus hilario Kaiser & Kihara sp. nov., Nannoniscus inermis Hansen, 1916, Nannoniscus laevis Menzies, 1962, Nannoniscus laticeps Hansen, 1916, Nannoniscus magdae Kaiser, Brix & Jennings sp. nov., Nannoniscus menoti Kaiser, Janssen & Mohrbeck sp. nov., Nannoniscus menziesi Mezhov, 1986, Nannoniscus meteori (Brandt, 2002), Nannoniscus minutus Hansen, 1916, Nannoniscus muscarius Menzies & George, 1972, Nannoniscus oblongus Sars, 1870, Nannoniscus ovatus Menzies & George, 1972, Nannoniscus pedro Kaiser, Brix & Kihara sp. nov., Nannoniscus perunis Menzies & George, 1972, Nannoniscus plebejus Hansen, 1916, Nannoniscus profundus Svavarsson, 1982, Nannoniscus reticulatus Hansen, 1916, Nannoniscus simplex Hansen, 1916, Nannoniscus spinicornis Hansen, 1916, Nannoniscus teres Siebenaller & Hessler, 1981.

Diagnosis (modified from Siebenaller & Hessler, 1981: 241; Wilson, 2008: 14): Pereonal tergites projecting laterally from pereopodal coxae; pereonites 6–7 dorsal articulation absent medially. Pleotelson distinctly shorter than pereonites 5–7 combined. Antennula with 5 segments, distal article bulbous, article 4 distal margin with ventromedial angular projection. Mandible with 3-segmented palp. Pereopods I–II equally robust. Uropods biramous or rarely uniramous.

Distribution: Known records from the Arctic, Atlantic, Pacific and Southern oceans and the Caspian Sea, although likely to be globally distributed. Although few Nannoniscus species are described from the continental shelf (≥ 75 m), they occur mainly at slope and abyssal depth, with two species recorded from the hadal Zone (N. ovatus Menzies & George, 1972 and N. perunis Menzies & George, 1972; Table 3).

Remarks: Species described herein were assigned to Nannoniscus due to the following characters: antennula article 4 distal margin with ventromedial angular projection, antennula terminal article 5 bulbous, pereopods 1 and 2 equally robust, lack of ventral articulation between pereonites 6 and 7. However, the genus Nannoniscus, thus far, is largely defined by a combination of plesiomorphic characters, such as uropods inserting posteroventrally close to the anus (Wilson, 2008), defining the family Nannoniscidae, as well as synapomorphic characters, such as a bulbous terminal article of the antennula, a specialized antennula article 4 and fusion of pereonites 6 and 7 that characterize a cluster of nannoniscid genera containing Nannoniscus, Nannonisconus Schultz, 1966, Nymphodora Kaiser, 2009, Rapaniscus Siebenaller & Hessler, 1981 and Regabellator Siebenaller & Hessler, 1981. Wilson (2008) states that the broad body form with laterally projecting pereonite tergites is present in all Nannoniscus species. While this is true for some species (e.g. the type species N. oblongus), others have a slender body (body length> 4.5 times pereonite 1 width) with lateral tergites that extend only slightly, if at all (e.g. N. ovatus, N. perunis, N. menziesi, N. meteori and species described herein). Overall, the genus comprises species with diverse morphologies mostly referring to the shape of the pleotelson and the presence of a ventral spine on the female operculum and/or pereonite 7. While N. oblongus possesses a ventral opercular spine, there are several species, where a spine is overall absent (e.g. N. aequiremis, N. arctoabyssalis, N. cristatus, N. inermis), or one occurring on the seventh pereonite (N. australis, N. minutus, N. muscarius, N. spinicornis, N. reticulatus, N. plebejus, N. affinis, N. profundus, N. caspius). In N. reticulatus, ventral spines are present both on the female operculum and the seventh pereonite. The presence or absence of a ventral opercular spine has been found a useful character to separate the nannoniscid genera Ketosoma and Thaumastosoma Hessler, 1970 (Kaiser et al., 2018). Equally, the position of ventral spines on pereonites 6 and 7 represents an apomorphy of Regabellator. In contrast, in Rapaniscus species, similar to Nannoniscus, the position of the ventral spines is variable, present on either pereonite 7 [Rapaniscus crassipes (Hansen, 1916), Rapaniscus dewdneyi Siebenaller & Hessler, 1981] or the operculum (Rapaniscus multisetosus Brandt, 2002).

Further differences exist in the presence or absence of the uropodal exopodite among Nannoniscus species; most species within the genus possess biramous uropods, while a lack a uropodal exopod is reported for two species (N. ovatus and in one new Nannoniscus species described below). Presence of uniramous or biramous uropods has been used as a segregating character to define genera within the munnopsid subfamily Ilyarachninae (Merrin, 2007); however, there are several genera (e.g. within Desmosomatidae, Paramunnidae), where both character states occur (Just & Wilson, 2007; Brix & Bruce, 2008; Kaiser & Marner, 2012 and discussion therein). Nannoniscus species described below show a gradual reduction of the exopodite (well-developed vs. minute vs. absent), thus, at least in Nannoniscus, the presence or absence of the uropodal exopodite represents a valuable character at the species, but not at the generic level [see also Brix & Bruce (2008) for desmosomatids].

Siebenaller & Hessler (1981) and Brandt (2002) already discussed the likely paraphyly of Nannoniscus particularly referring to “odd” species such as N. muscarius (with a strongly produced coxal spine) and N. ovatus (= uniramous uropods). However, these species are notsingle occurrences but representative for this heterogeneous group. Up to now there has been no rigorous phylogenetic assessment of all Nannoniscus species, and it is not the purpose of the present study to address this issue. Nevertheless, as a prelude, the position of N. coalescus (Menzies & George, 1972) is discussed. The species had been first described as Desmosoma coalescum in the family Desmosomatidae and was later transferred to Nannoniscus by Siebenaller & Hessler, 1977 due to the bulbous terminal article of the antennula as well as fusion of pereonites 6 and 7. Our morphological analyses of the holotype alongside line drawings made by Menzies & George (1972: p. 9.48) suggest the species belongs to Rapaniscus owing to a broadened pereopod I carpus bearing several long robust setae (Siebenaller & Hessler, 1981). Therefore, N. coalescus is herein transferred to Rapaniscus.