Mixarcturus digitatus (Nordenstam, 1933)

Poore, G. C. B., 2003, Revision of Holidoteidae, an endemic southern African family of Crustacea, and re-appraisal of taxa previously included in its three genera (Isopoda: Valvifera), Journal of Natural History 37, pp. 1805-1846 : 1839-1842

Mixarcturus digitatus (Nordenstam, 1933)

(figures 14, 15)

‘ Microarcturus ’ digitatus Nordenstam, 1933: 167–171 , pl. 2 figures 15, 16, text-figure 39a–e. Mixarcturus digitatus: Brandt, 1990: 97 ; Wägele, 1991: 173–174, figure 65.

Material examined

Nordenstam (1933) listed 11 specimens all from stn 34 of which two, a male of 12.5 mm and a female of 9 mm, were designated syntypes. Today the material in SMNH (‘typsaml 796’) consists of four complete animals (5.8–11.4 mm) and 32 slides from at least two males, two females and one juvenile. The carcases of none of these survive. It is possible to identify among the slides a pleopod 1 of a male of 12.5 mm and six slides of a female with marsupium. These are the presumptive syntypes. An additional ‘immature specimen’ of 3.2 mm from stn 17 no longer exists .

A male was selected from complete specimens and dissected for illustration. Supplementary illustrations were made from Nordenstam’s slides of unidentified individuals, including a pleopod 1 from the male syntype.

Figured specimen: South Georgia, mouth of Cumberland Bay, 54°11∞ S, 36°11∞ W (Swedish Antarctic Expedition stn 34), 252–310 m (male, 9.5 mm).

Description of figured male

Body of male. (Figures in parentheses from Nordenstam’s slides.) Length 9.5 mm. Head with eyes lateral, pigmented. Major dorsal spination arranged in submedial, dorsolateral and lateral rows of erect spines. Head with pair of small anterolateral spines and two submedial pairs. Pereonites 1–4 each with one submedial, one dorsolateral and one lateral pairs of spines; pereonites 5–7 each with one dorsolateral pair of spines; pleonite 1 with one sublateral and one lateral pair of spines; pleonites 2 and 3 each with one submedial and one strong backwardly directed lateral pair of spines; remaining pleotelson with two submedial, three dorsolateral and two lateral pairs of spines, plus two subapical mid-dorsal spines. Coxa of pereopod 4 with posteroventral spine; coxae 5–7 with two spines, more posterior larger.

Limbs of male. Antenna 2 peduncle 64% of total body length; ratio of articles 3–5 and flagellum, 10:14:17:14 (10:14:16:14); ratio of flagellum articles 100:43:50:7 (100:47:65:7), the last a small claw; antenna 2 peduncle with two dense rows of setae along lower margin, flagellum with shorter setae.

Mandible and maxillae typical of Antarcturidae . Maxillipedal endite truncate, with transverse rows of three setae on posterior face and eight (five) distally plus longitudinal row of four along mesial margin; palp articles 1–5 with ratio of lengths 31:53:100:81:34 (33:51:100:79:33); articles 1–5 with 0, 11, 15, 13, 5 (0, 7, 22, 16, 8) medial and posterior facial setae, respectively; article 3 the widest, article 4 70% (73%) its greatest width, article 5 50% (60%) width of article 4.

Pereopod 1 carpus with small posterodistal spine; propodus elongate, width 34% length; dactylus (including unguis) 85% propodus; ischium–propodus with long simple setae along and near posterior margin, others laterally; propodus with row of lateral facial setae; dactylus widest at mid-point, apically rounded, lateral, posterior and mesial face densely setose; unguis about tenth length of body of dactylus; secondary unguis almost as large.

Pereopod 2 with anterodistal spine on basis–merus; ratio of lengths carpus–dactylus 100:134:150; dactylus tapering, curved; unguis 13% length of dactylus; secondary unguis absent; basis–dactylus with setae along posterior margin, in ca ten pairs of long setae accompanied by smaller setae on carpus–dactylus. Pereopod 4 with anterior spines on basis; anterodistal spine on ischium–merus; ratio of lengths carpus–dactylus 100:89:81; propodus curved proximally; dactylus tapering; unguis 12% length of dactylus; secondary unguis shorter (aberrant in 9.5 mm figured male); basis–dactylus with setae along posterior margin, in pairs of long setae accompanied by smaller setae on carpus, less abundant on propodus and dactylus. Pereopods 5–7 progressively shorter, anterior spine on basis, anterodistal spine on ischium–merus; ischium–propodus each with 3, 2, 4 and 6 (2, 3, 3, 7) posterior robust setae, respectively.

Penial plate tapering to slightly notched apex.

Pleopod 1 exopod 90% length of endopod; endopod with six (five) plumose setae on distal margin; groove on posterior face opens on lateral margin 90% way along, beyond which is a doubly lobed setose margin; lateral margin with 16 marginal simple setae. Pleopod 2 exopod with 40 lateral and distal plumose setae; endopod barely exceeding exopod, with 25 distal and mesial setae; appendix masculina 1.4 times length of endopod, with acute apex.

Uropod peduncle with longitudinal row of five spines; exopod absent; endopod triangular, with only minute setae.

Remarks

Mixarcturus digitatus differs from M. acanthurus in not having the well-developed pleotelsonic projection but the two are similar in the details of the distolateral apex of the endopod of the male pleopod 1. Both are bilobed and setose (cf. Monod, 1925: figure 28D) and this is a probable generic character. M. abnormis does not have a dorsally tuberculate pleotelson.