Superfamily Eusiroidea Stebbing, 1888
Eusiridae Stebbing, 1888: 949, 953; elevated to the rank of superfamily by Bousfield (1979). Calliopiidae G.O. Sars, 1893: 431; elevated to the rank of superfamily by Lowry & Myers (2013).
Eusiridae – Stebbing 1906: 338. — J.L. Barnard 1969: 5, 27, 33, 34, 46, 47, 82, 83, 84, 87, 88, 89, 90, 106, 109, 114, 159, 164, 167, 171, 172, 213, 233, 291, 389, 392, 393, 421, 457, 476, 478, 481. — J.L. Barnard & Karaman 1991: 19, 20, 21, 55, 58, 68, 69, 80, 82, 84, 91, 114, 117, 131, 132, 284, 378, 379, 391, 405, 413, 569, 644, 702.
Eusiroidea – Bousfield 1979: 349, 363; 1982: 263. — Bousfield & Hendrycks 1995: 4 (ubi syn.). — Verheye et al. 2016b: 314, appendices S1, S2 and S3.
Calliopioidea – Lowry & Myers 2013: 38 (in part).
The study of Verheye et al. (2016b) indicates that the superfamily Eusiroidea Stebbing, 1888 is a larger taxon than previously admitted. In their appendix S2, these authors include the following families in Eusiroidea: Acanthonotozomatidae Stebbing, 1906; Acanthonotozomellidae Coleman & J.L. Barnard, 1991; Amathillopsidae Pirlot, 1934; Bateidae Stebbing, 1906; Calliopiidae G.O. Sars, 1893 (excluding Cleippides Boeck, 1871 and Weyprechtia Stuxberg, 1880); Dikwidae Coleman & J.L. Barnard, 1991; Epimeriidae Boeck, 1871; Eusiridae Stebbing, 1888; Iphimediidae Boeck, 1871; Laphystiopsidae Stebbing, 1899; Pleustidae Buchholz, 1874; Pontogeneiidae Stebbing, 1906; Sanchoidae Lowry, 2006; Stilipedidae Holmes, 1908 (including the subfamilies Alexandrellinae Holman & Watling, 1983, Astyrinae Pirlot, 1934 and Stilipedinae Holmes, 1908); Thurstonellidae Lowry & Zeidler, 2008; Vicmusiidae Just, 1990; and the orphan genera Austroregia J.L. Barnard, 1989 and Chosroes Stebbing, 1888, which were formerly included within the Gammarellidae Bousfield, 1977.
The concept of Senticaudata Lowry & Myers, 2013 proposed by Lowry & Myers (2013) and adopted in major databases like the World Register of Marine Species (WoRMS, http://www.marinespecies.org/ accessed on 8 Mar. 2016) conflicts with that of Eusiroidea sensu Verheye et al. (2016b).A brief discussion on the Senticaudata issue has therefore to be given herein. Lowry & Myers (2013) erected the suborder Senticaudata for amphipods presenting a cluster of spines on the tip of the rami of uropods 1 and 2, postulating that this character state is a synapomorphy. Surprisingly, Lowry & Myers (2013) overlooked some ‘senticaudate’ taxa, e.g., Idunellinae d’Udekem d’Acoz, 2010, Pleustidae, Apherusa Walker, 1891, Halirages Boeck, 1871 and Paramphithoe Bruzelius, 1859). Most senticaudate families traditionally included in the Eusiroidea (the Calliopiidae and Pontogeneiidae) were transferred by Lowry & Myers (2013) to the superfamily Calliopioidea G.O. Sars, 1895 [sic] (real date of publication: 1893), i.e., the family Calliopiidae elevated to the rank of superfamily. The remaining eusiroid families were excluded from the suborder Senticaudata. Myers & Lowry (2013) also transferred the Cheirocratidae and Hornelliidae to the Calliopioidea, because the two latter families formed a clade with the Calliopiidae and Pontogeneiidae in their morphological phylogenetic tree based on the analysis of 41 characters. The molecular phylogenetic analyses of Verheye et al. (2016b), which focused on eusiroids, but also included a representative sample of other amphipods, did not support the validity of the suborder Senticaudata. Indeed, senticaudate taxa do not form a clade, suggesting that the distal ornamentation of uropods would be subject to homoplasy, as it is the case for many other characters in amphipods (e.g., J.L. Barnard & Drummond 1978: 7, 193; J.L. Barnard & Karaman 1984: 48; J.L. Barnard & Karaman 1991: 13, 58; Verheye et al. 2016b). In the 28S and 18S rDNA trees, Eusiroidea comprises senticaudate as well as nonsenticaudate taxa, and the families Calliopiidae (excluding Cleippides) and Pontogeneiidae are included in the superfamily, as it was the case in former classifications (e.g., Bousfield 1979, 1982; J.L. Barnard & Karaman 1991). The suborder Senticaudata was erected on the assumption that the senticaudate character state is a synapomorphy. However, the morphological phylogeny of the Senticaudata of Lowry & Myers (2013) does not test this hypothesis as it only includes senticaudate taxa. Moreover, the reliability of the nodes of Lowry & Myers (2013) is not assessed by any support values, e.g., bootstrap support or Bremer’s index. Additionally, it should be noted that the Cheirocratidae and Hornelliidae exhibit several important non-eusiroid character states. For example, they have a well developed accessory flagellum on antenna 1 as well as dorsal spines/setae on the urosome, which is never the case in Eusiroidea. Their position close to the Calliopiidae and Pontogeneiidae on the tree of Lowry & Myers (2013) might reflect poorly supported topologies and/or a subjective choice of characters used in their analysis.A direct observation of specimens indicates that the senticaudate and non-senticaudate character states are not always clear-cut and transitional dispositions are observed among dissimilar amphipods, suggesting multiple passages between the sentidaudate and non-sentidaudate character states. Finally, the polarity of the transition proposed by Lowry & Myers (2013): non senticaudate (plesiomorphic) to senticaudate (apomorphic) is another untested question. In conclusion, before its wide acceptance, the new classification system of amphipods based on the suborder Senticaudata should be evaluated by a combination of molecular and morphological investigations, without a priori assuming that the senticaudate character state is a synapomorphy.