Cryosignum incisum ( Richardson, 1908 )

Just, Jean & Wilson, George D. F., 2021, Redescriptions and new species in the ‘ Austrosignum-Munnogonium’ complex sensu Just & Wilson (2007), mainly from the Southern Hemisphere (Crustacea Isopoda: Paramunnidae), Zootaxa 4952 (3), pp. 401-447 : 410-412

Cryosignum incisum ( Richardson, 1908)

( Fig. 6 )

Austrimunna incisa Richardson, 1908: 152 , Fig. 9 .

Austrosignum incisa .— Menzies, 1962: 50; Wolff, 1962: 252, 287.

Paramunna incisa .— Vanhöffen, 1914: 450; Stephensen, 1927: 358; Nierstrasz, 1941: 291.

Cryosignum incisum .— Just & Wilson, 2007: 3, tab.1.

Type fixation. Lectotype, ♂, MNHN IS 3057 .—Here designated.

Type Locality. Wiencke Island , Antarctica .

Material examined. Lectotype, subterminal ♂, 2.7 mm. Antarctica, Palmer Peninsula , Wiencke Island, 64°30’S, 63°25’W, “ Austrimunna incisa Richardson ; Type!; 11° 822-Drague 25m; (2-5-05); Museum Paris; Ile Wiencke; Mission Charcot; 1906”, MNHN-IU-2019-3351.

Other material (all AM). Antarctica, Palmer Peninsula, Anvers Island , Arthur Harbor (less than 25 nautical miles from type locality): R/ V Alpha Helix stations [Rich] by M.D. Richardson, Van Veen Grab (0.067m ²) in silty sand, 7-9 Feb 1971. West of Palmer Station , 64°46.43’S 64°04.37’W, depth 15 m: Rich 10-2, ♀, P.84808; Rich 10- 3-1, ♀, P.84809; Rich 10-4, ovigerous ♀, P.84810; Rich 10-5, preparatory ♀, P.84811. Northwest of Amsler Island , 64°45.75’S 64°05.83’W, depth 18 m: Rich 12-3-1,T ♂, P.75060. South of Amsler Island , 64°46.05’S 64°04.92’W, depth 23 m: Rich 13-1, juv ♂, illustrated, P.84812; Rich 13-1, juv ♂, P.84813; Rich 13-2, 5 inds, P.84814; Rich 13-3, 22 inds, P.84815; Rich 13-3-1, ovigerous ♀, dissected, illustrated, measured, P.84816; Rich 13-3-1,T ♂ “A”, il- lustrated, P.84817; Rich 13-3-1,T ♂ “B”, illustrated, measured, P.84818; Rich 13-3-1, subterminal ♂, P.75058; Rich 13-3-1, 1 ind, P.75065; Rich 13-3-1, 2 inds, P.75373; Rich 13-5, subterminal ♂, P.84819. South of Amsler Island , 64°46.07’S 64°04.92’W, depth 30 m: Rich 14-5-1, subterminal ♂, P.84820. Hero Inlet , 64°46.55’S 64°03.53’W, depth 5-7 m: Rich 7-1-1,T ♂, 2 ovigerous ♀♀, P.75061; Rich 7-2, 7 inds, P.75059; Rich 7-2-1, 2 inds, P.75062; Rich 7-4, 2 ♀♀, juv ♂, P.75063. Hero Inlet , 64°46.55’S 64°03.53’W, depth 5 -7 m; Rich 7-4, 31 inds; Rich 7-5-1, subter- minal ♂, illustrated, P.84821; Rich 7-5-1, 11 inds, P.75057

Description. Body ovate (except in terminal males); width 0.62 length in ovigerous ♀, width 0.41–0.47 length in male (lectotype, T ♂ AM P.84818), widest in male and in female at pereionite 3, (enlarged pereionite 1 of terminal males wider).

Head longer in male than female, length in male 0.46 and 0.49 width (T ♂, lectotype ♂), length in females 0.33 width (ovigerous ♀); length posterior to eyestalks in female 0.44 anterior length (ovigerous ♀), length posterior to eyestalks in male 0.65–1.1 anterior length (lectotype ♂, T ♂ AM P.84818). Frontal margin slightly convex in middle, straight laterally, with angular lateral margins adjacent to antennae. Eyestalks prominent, in female length 2.5 width (ovigerous ♀) , in male length 3.0 width (lectotype ♂, T ♂), apex rounded, shaft before apex constricted proximally, long axis angling forward at approximately 30° in female (ovigerous ♀), long axis angling forward 40–45° in male (lectotype ♂,T ♂), becoming sinuous in terminal males.

Pereionite 1 of terminal male greatly enlarged; sagittal length in male 1.6–2.3 pereionite midline length (lectotype ♂, T ♂), in female 1.3 pereionite midline length (ovigerous ♀); pereionite 1–7 dorsal surface with transverse ridges (low, rounded); pereionite lateral margins coxae visible in dorsal view (T ♂ coxa fused to pereionite 1); pe- reionite 1–7 lateral margins not projecting (T ♂ projecting forward), rounded.

Pleon length 1.5 width in female (ovigerous ♀), 1.4–1.5 width in male (T ♂, lectotype ♂). Pleonite 1 width 0.97–1.0–1.3 distance between uropods (lectotype ♂, T ♂, ovigerous ♀), length 0.4 width (lectotype ♂, ovigerous ♀). Pleotelson laterally rounded but with weak inflection between proximal and lateral margins (less so in T ♂), dor- sal surface smooth; length of pleonite 1 in ventral view 0.08 total pleotelson length; posterior margin apex broadly rounded to narrowly rounded (T ♂, ovigerous ♀), evenly curving into lateral margin; in female forming 70° angle (ovigerous ♀), in male forming 85–90° angle from vertex to uropod insertions (T ♂, lectotype ♂).

Antennula articles 1–2 combined extending beyond eyestalk apex, article 1 shorter than article 2, inflated, broader than 2; article 2 distal margin with cluster of 5 penicillate setae; articles 4–6 of subequal length, all shorter than 3. Antenna article 3 male much more robust than female, tubular, width 0.48–0.45–0.61 length (ovigerous ♀, lectotype ♂, T ♂), article 5 distinctly longer than article 4 (ovigerous ♀); flagellum with 10 articles (ovigerous ♀), proximal article distinctly longer than more distal articles, proximal article 2.1 length of second article (ovigerous ♀).

Mandible molar process distally wedge-shaped, triturative surface sloping; with anterior spine, distinctly projecting medially from gnathal surface; with distinctly projecting spine on posterior margin of gnathal surface; mandible palp present.

Pereiopod I coxa without projections(ovigerous ♀) or with anterior projections (T ♂), anterior margin simple (T ♂, ovigerous ♀), in terminal males fully fused to pereionite 1; basis anterior margin anterior margin smooth, length 3.7–3.6–4.5 width (lectotype ♂, ovigerous ♀, T ♂); pereiopod I carpus oval, distal width 0.48–0.52 posterior margin length (T ♂, ovigerous ♀), posterior margin with 2 robust setae, with 1 denticle proximal to each robust seta; propo- dus narrowing distally to insertion of dactylus, with 2 robust setae. Pereiopod III basis anterior margin smooth.

Male pleopod I lateral lobes moderately projecting from midlateral margin, width about 0.3 distance to midline (lectotype ♂); lateral lobes distal sublobe partly overlapping proximal sublobe; distal projection length 0.32 pleopod total length, forming acute angle, with pointed apices. Female operculum distal part tapering with concave distolateral margins, width 0.69 length.

Uropods dorsal and directly adjacent to lateral margin of pleotelson; protopod present but not exposed, (may be exposed in decalcified specimens).

Size. Largest ♀ 2.1 mm (ovigerous); largest ♂ 2.7 mm (lectotype).

Distribution. Antarctica, Palmer Peninsula, Wiencke Island and Anvers Island, 5– 30 m.

Remarks. Richardson (1908) reported 2 males and 2 females of Cryosignum incisum ( Richardson, 1908) but did not designate a holotype. We have examined one male and one female from this syntype series. The male (somewhat decalcified but otherwise in good condition) appears to be the specimen illustrated by Richardson (1908, fig. 9). We have designated this specimen as lectotype. The female is an undescribed species of Austrosignum sensu stricto; it has been removed from the syntype series. The remaining two syntypes (now possible paralectotypes), if still in existence, need to be examined to ascertain their status.

A study of a collection of males from Arthur Harbor, Anvers Island, shows that the lectotype is in a subterminal stage of development. The more extreme terminal males of this species show the similar forms as in C. lunatum ( Hale, 1937) . We have used these non-type specimens to score females and male variation.

We have not seen a female Cryosignum incisum ( Richardson, 1908) from the type collection, but females from the Anvers Island collection have a more compact pereion and a more angular pleotelson than males. This species is quite similar to other described species of the genus although it can be distinguished by these features: the pleotelson is laterally rounded but with small inflection between proximal and lateral margins; the frontal margin of the head is nearly straight laterally and has angular margins adjacent to the antennae; the eyestalk shaft is constricted proximally so that the tips are broader than the rest of the shaft. The other three species C. lunatum , C. latifrons and C. nordenstami lack an inflection between lateral and proximal margins of the pleotelson; the frontal margin of the head is broadly rounded and lacks angular margins adjacent to the antennae, and the eyestalks shafts are approximately parallel. Brandt (1999) described a male of this genus from the South Shetland Islands as “ Paramunna lunata ”, which we subsequently excluded from that species ( Just & Wilson, 2007). Now that we have seen male specimens of Cryosignum incisum from Anvers Island (nearby and west of Wiencke Island, the type locality), Brandt’s specimen appears to be related to this species although we cannot be certain how close without inspection of the specimen. Recently Zemko et al. (2017) reported a single specimen as Cryosignum incisum from Admiral Bay, South Shetland Islands.

The Anvers Island collection shows important variability in several features. The pleotelson has an ontogenetic change from a more angular pleotelson posterior margin (vertex) in females and juvenile males to a more rounded form in mature males. This can also be seen to a lesser degree in C. lunatum . In addition, the preservation of the specimens appears to affect this feature as well. The lectotype male has a rounded vertex and an exposed uropodal protopod similar to a decalcified terminal male from the Anvers collection while another male in better condition had a rounded angular vertex and a retracted protopod. Therefore, in Cryosignum at least, these features need to be used with caution, especially where one of the sexes is missing.

Figures

FIGURE 6. Cryosignum incisum (Richardson, 1908). Lectotype ♂, MNHN IS 3057. au, antennula; cvv, head ventral view; p1, pereiopod I; ur, uropod; plt vv, pleotelson, ventral view.

FIGURE 9. Cryosignum sp., ♂. p1, pereiopod I; pl1, pleopod I; up, uropod. Scale for habitus, 0.5 mm.