Halearcturus, Poore, 2015

Poore, Gary C. B., 2015, Halearcturus, a new genus of Antarcturidae Poore, 2001 (Crustacea: Isopoda: Valvifera) with a key to genera of the family, Memoirs of Museum Victoria 73, pp. 13-18 : 14-15

Halearcturus gen. nov.

Zoobank LSID. http://zoobank.org/ urn:lsid:zoobank.org:act:80AB03F2-0DAD-4205-9831-B8F30D3D0757

Type species. Arcturus serrulatus Whitelegge, 1904 , by monotypy and original designation (masculine).

Diagnosis. Body weakly geniculate between pereonites 4 and 5, anterior pereon slightly elevated. Head with pair of submedian spines between eyes; pereonites with supracoxal spines on all pereonites, with paired sublateral tubercles, without paired submedian spines, without middorsal spines; pleonite 1 fused to remaining pleotelson; pleotelson with 2 middorsal tubercles near apex, without paired submedian spines, with pairs of sublateral tubercles, with marginal lateral projections, without prominent paired posterior sublateral spines more dominant than other spines, without prominent medial posterior spine dorsal to margin. Antennal flagellum of 2 articles, first about third as long as peduncle article 5, second quarter length of first. Pereopods 2–4 with regular row of long setae along flexor margins of merus-propodus, second row on mesial face. Pereopods 2–4 unguis short, curved. Pleopod 1 of male exopod groove opening distolaterally on margin, scarcely produced at opening. Pleopod 2 of male, appendix masculina acute, at least 1.5 times as long as endopod. Uropod without exopod. Oostegites 2–4 supported by ventral coxal processes, largest on pereonite 4; oostegite 5 absent, without ventral coxal processes.

Etymology. For Herbert Mathew Hale (1895–1963) whose contributions significantly expanded knowledge of marine isopods in southern Australia, combined with Arcturus , a commonly used genus stem.

Composition. Type species only.

Remarks. Several characters set this genus and its only species apart from other antarcturids. Halearcturus serrulatus is unique in the family in having an antennal flagellum of one major article plus a short curved tapering second article; all other genera have either three or four articles, or about nine of similar lengths, plus a terminal one. The genus is unusual in lacking a uropodal exopod but may not be unique in this regard. The exopod is absent also in ‘ Microarcturus’ digitatus Nordenstam, 1933 , now accepted as Mixarcturus digitatus ( Nordenstam, 1933) ( Poore, 2003) , but not in M. abnormis ( Kussakin, 1967) , type species of the genus. The exopod was said to be absent in Antarcturus usitatus Schultz, 1978 although it is present in all other species of the genus.

Halearcturus has unique complex ornamentation of plate-like tubercles and lacks both a strong medial posterior spine and a pair of prominent sublateral spines on the pleotelson. Abyssarcturus Kussakin & Vasina, 1995 , Globarcturus Kussakin & Vasina, 1994 and Tuberarcturus Brandt, 1990 (and some members of the loosely defined Fissarcturus Brandt, 1990 ) share this pleotelsonic spination but all three have very different patterns of body ornamentation. Abyssarcturus and Globarcturus lack pigmented eyes and have a 4-articled antennal flagellum. Abyssarcturus has a setiform unguis on pereopods 2–4. Tuberarcturus has middorsal tubercles on the posterior margins of pereonites 1–4 and a 9-articled antennal flagellum. Spinarcturus Kensley, 1978 and Thermoarcturus Paul & Menzies, 1971 have simple pleotelsonic sculpture but both genera (one species each) are covered with a mat of fine setae and lack the pairs of long ‘filter-setae’ on pereopods 2–4 ( Wägele, 1987) which are usual in Antarcturidae .