Austroniscus brandtae Kaiser, Stransky & Brix 2023, n. sp.

Austroniscus brandtae Kaiser, Stransky & Brix n. sp.

(Figures 7–16)

Type fixation: Holotype, adult male, SMF 57927 (Voucher No. R4 B), 2.7 cm, designated here.

Material examined: see Tables 1 and 2.

Etymology: The new species (brandtae, Latin genitive, feminine) is named in honour of Angelika Brandt, doctoral mother of three of the authors, for her commitment and achievements in deep-sea isopod research and thanks to her as a pathfinder to the authors on their way into deep-sea science.

Distribution: Abyssal and hadal of the Puerto Rico Trench, 4,552 –8,338 m.

Diagnosis: Pereonite 1 lateral margins frontally directed. Pereonite 1 coxae each with a spine-like frontally directed appendix, tipped with a small spine-like seta and clearly visible in dorsal view, almost reaching second article of the antennula; pereonites 1–4 anterolateral margins each with a spine-like seta on apex; pereonite 1 anterolateral margins each with 2 spine-like setae in female. Rostrum with 2 slender setae in female. Antennula with 7 articles in male and with 6 articles in female.

Description of holotype (SMF 57927) and paratype male (SMF 57929): Habitus (Figs7A, 10, 11)dorsoventrally considerably flattened and broadened, body length 2.5 pereonite 2 width; pereonites 2–7 coxae not visible in dorsal view; pereonites 2–7 and pleotelson of similar width; pereonites 1–4 with strongly frontally directed lateral margins, tipped with a small spine-like seta apically. Pereonite 1 narrowest, length 0.1 width; coxae each with a spine-like frontally directed appendix, tipped with a small spine-like seta and clearly visible in dorsal view, almost reaching second article of the antennula. Pereonite 2 width 1.2 pereonite 1 width, length 1.8 pereonite 1 length; pereonites 2 and 3 of similar length; pereonite 4 longest, length 3.0 pereonite 1 length. Pereonite 5–7 of similar length, 2.5 pereonite 1 length; pereonite 5 anterior margin straight; pereonites 6–7 anterior margins convex. Pleotelson length 0.3 body length, 0.6 width; width 1.3 pereonite 1 width, posterior margin semi-circular, anterior margin slightly concave. Uropod length 0.4 pleotelson length, projecting beyond posterior margin.

Cephalothorax (Figs 7A, 10A, 11A) free, length 0.5 width. Rostrum well developed. Cephalothorax anterior margin slightly concave, posterior margin slightly rounded, antennae inserting frontolaterally in a deep fold between rostral crest and anterolateral, triangular projections; each fold with a single robust seta medially.

Antennula (Figs 7D, 11A) length 0.2 body length, with 7 articles (article 6 damaged, article 7 broken off in paratype); first article ovoid and broadest, length 1.1 width, with 1 broom seta, 1 slender simple seta and 1 robust unequally bifid seta distally; second article length 1.2 article 1 length, 4.7 width, with 4 long broom setae and 2 simple setae distally; article 3 length 0.4 article 1 length, 2.5 width, with 1 simple seta distally; article 4 length one-third article 1 length, twice width, with 1 small broom seta laterally; article 5 length about 0.6 article 1 length. Antenna (Fig. 7B), drawn in situ; only peduncular articles 1–4 present; articles 1–4 short; article 2 with a small robust seta distally; article 4 with 1 robust spine tipped with a small seta distally.

Left mandible (Fig. 8E) palp well developed, consisting of 3 articles, extending beyond distal tip of incisor. Palpal article 1 length 0.7 article 2 length; article 2 with 2 simple setae medially, with a row of fine setules laterally; terminal article length about one-third article 2 length, with several fine setules ventrally, and 3 robust setae terminally. Incisor process with 6 distal teeth and 1 subdistal tooth. Lacinia mobilis with 4 teeth. Setal row with 9 robust setae, dentation decreasing, seta size increasing proximally. Molar process triangular, with 7 long, serrate setae distally.

Maxillula (Fig. 8C) outer endite length about 4.0 width, with 13 strong spine-like setae and 10 simple setae distally, with several simple setae of varying length laterally; inner endite width 0.5 outer endite width, with several fine setae distally and laterally. Maxilla (Fig. 8B, D) mesial endite as long as middle endite, with several setae distally; lateral endite and middle endite each with 4 strong setae distally. Maxilliped (Fig. 8F) with 2 long retinacula. Epipod smooth, triangular, slender, length 4.2 width, reaching mid of palpal article 3. Palpal article 1 short, width 2.5 length, with several small setae laterally; article 2 length 2.3 article 1 length, width 1.1 length, with several small setae and with 2 simple setae laterally; article 3 length 1.7 article 1 length, width 1.5 length, with 5 robust sensory setae and 1 simple seta distally; article 4 length 1.2 article 1 length, width 0.4 length, with a distal projection exceeding tip of article 5, with 7 long, slender setae distally; article 5 length 0.5 article 1 length, width 0.3 length, with 3 slender setae terminally. Endite distal margin with some robust, dentate setae, lateral margin with several fine setae laterally.

Pereopod I (Fig. 7G) more robust than pereopod III; basis length 6.0 width, with 1 simple seta dorsally and 2 simple setae ventrally; ischium length about half basis length, 3.1 width, with 1 simple seta distodorsally, and 2 simple setae ventrally; merus length 0.5 ischium length, 1.6 width, with 2 robust setae distodorsally, and 2 simple setae of varying size distoventrally; carpus length 2.4 merus length, 5.2 width, with 1 simple seta dorsally, 1 somewhat longer simple seta distodorsally, with 3 unequally bifid setae and 3 somewhat longer simple setae ventrally; propodus length 0.6 carpus length, 3.3 width, with 2 simple setae dorsally, with 2 somewhat longer simple setae distodorsally, with 2 simple setae ventrally and 2 robust unequally bifid setae distoventrally; dactylus length 0.6 propodus length, 4.4 width, with 2 slender setae medially, with 1 small simple seta distroventrally; unguis length 0.2 dactylus length, with 2 long, slender setae between unguis and ventral claw.

Pereopod II only described in female paratype.

Pereopod III (Fig. 9B) basis length 4.8 width, with 1 simple seta dorsally, with 3 simple setae ventrally and 1 somewhat longer simple seta distroventrally; ischium0.7 basis length,3.6width,with1robust simple seta dorsally,with 1 simple seta and 1 unequally bifid seta ventrally; merus length 0.4ischium length, 1.7width, with2 robust simple setae distodorsally,with 1somewhat longer simple seta distoventrally; carpus length 4.6merus length,7.7 width, with4setae (1 broken off, 1simple,2 unequally bifid) dorsally,with a row of 9stout unequally bifid setae ventrally; propodus length 0.9 carpus length, 9.4 width, with 4 simple setae of varying size dorsally, with 4 stout unequally bifid setae and 1 small simple seta ventrally, with 3 setae (1 unequally bifid and 2 simple) distoventrally; dactylus length 0.3 propodus length, 5.0 width, with 1 simple seta dorsally; unguis length 0.3 dactylus length, with 2 long slender seta underneath unguis.

Pereopod IV (Fig. 9C) basis length 5.3 width, with 3 simple setae dorsally, with 2 simple setae ventrally and 1 somewhat longer simple seta distoventrally; proximal part of ischium slightly damaged, length 0.6 basis length, length 3.3 width; merus length 0.4 ischium length, length 1.4 width, with 3 simple setae distodorsally, with 1 somewhat longer simple seta distoventrally; carpus length 5.7 merus length, length 8.0 width, with 3 simple setae dorsally, with a row of 8 stout unequally bifid setae ventrally; propodus length 0.8 carpus length, length 10.0 width, with 4 setae (2 broken off, 1 long, 1 small simple) dorsally, with 6 setae (4 unequally bifid, 2 broken off) ventrally; dactylus length 0.3 propodus length, length 4.5 width, with 1 simple seta medially; unguis length 0.2 dactylus length, with 1 slender seta underneath unguis.

Pereopod V only described in female paratype.

Pereopod VI (Fig. 9D) basis broken off. Ischium length 3.0 width, with 1 unequally bifid seta distodorsally and 1 simple seta ventrally; merus length half ischium length, length 1.8 width, with 2 simple setae of varying size distoventrally, with 1 long simple seta dorsally; carpus length 3.7 merus length, length 8.2 width, with 4 slender unequally bifid setae and 1 small simple seta dorsally, with 1 long and 1 short simple seta distodorsally, with 8 unequally bifid setae ventrally; propodus length 0.8 carpus length, 10.0 width, with 3 simple setae dorsally, with 5 setae (2 broken off, 2 unequally bifid) ventrally, and 3 setae (1 unequally bifid, 2 slender simple) distoventrally; dactylus length 0.3 propodus length, 4.5 width, with 1 simple seta medially; unguis length 0.3 dactylus length, with 3 slender setae underneath unguis.

Pereopod VII (Fig. 9E) basis length 4.5 width, with 2 simple setae dorsally, 1 small simple seta medially, with 4 simple setae ventrally. Ischium length 0.6 basis length, length 3.3 width; merus length 0.4 ischium length, length 1.7 width, with 2 simple setae distodorsally, with 1 simple seta (broken off) distoventrally; carpus length 3.7 merus length, length 6.1 width, with 4 slender setae (3 simple, 1 broken off) dorsally, with 2 simple setae medially, with 2 setae (1 stout unequally bifid, 1 broken off) dorsally, and 2 unequally bifid setae distoventrally; propodus length 0.9 carpus length, length 11.3 width, with 3 simple setae dorsally, with 2 setae distally, with 6 unequally bifid setae ventrally; dactylus length 0.3 propodus length, length 4.5 width, with 1 robust seta medially; unguis length 0.6 dactylus length, with 1 slender seta between unguis and claw (damaged).

Pleopod I (Fig. 7E) length 2.3 proximal width; distal projection width 0.5 proximal width, lateral margins straight; lateral lobes rounded; distal margins semi-circular, with eight simple setae of varying length each. Pleopod II (Fig. 7F) sympod length 3.0 width; lateral margin rounded, with 8 setae (2 simple, 6 broken off); endopod inserting 0.3 from distal tip of sympod; stylet length 0.8 sympod length, slightly curved, distal end not extending beyond distal tip of sympod; exopod short and rounded, inserting 0.1 from distal tip of sympod.

Uropod (Figs 7H, 11C) biramous; sympod trapezoid, length about twice width, with 2 simple setae laterally (1 broken off), with 3 simple setae distally (2 broken off); exopod length 0.8 sympod length, length 3.8 width, with 3 setae terminally (broken off); endopod length 1.2 exopod length, length 4.5 width, with 1 simple seta laterally, with 6 long simple (3 broken off) setae terminally.

Differences in paratype female (SMF 57930): Habitus (Figs 12A–B, E, 14) 1.9 cm long. Pereonite 1 anterolateral margins each tipped with 2 small spine-like setae apically; pleotelson length 0.2 body length, length 0.4 width.

Cephalothorax (Figs 12A, 14A, 15A) rostrum well developed with 2 small simple setae dorsally. Antennula (Fig. 12C) with 6 articles; first article length 1.5 width, with 3 broom setae of varying size and 1 unequally bifid seta distally; second article length 6.3 width, with 4 long broom setae and 1 simple seta distally; article 6 length 0.3 article 1 length, length 5.0 width, with 1 aesthetasc and 3 simple setae terminally.

Pereopod I (Fig. 13C) distal part of basis slightly damaged, length 5.0 width, with 1 simple seta dorsally and 2 simple setae ventrally; ischium length 2.8 width, with 2 simple setae ventrally; merus length 0.6 ischium length; carpus length 2.5 merus length, length 5.0 width, with 3 simple setae dorsally, with 2 unequally bifid setae and 1 long simple seta ventrally; propodus length 0.6 carpus length, length 4.0 width; dactylus length 0.7 propodus length, length 4.0 width, with 3 slender setae medially; unguis with 2 long, slender setae underneath unguis. Pereopod II (Fig. 13D), basis length 4.8 width, with 2 broom setae and 1 simple seta dorsally, with 1 long simple seta distoventrally; ischium length 0.6 basis length, length 3.2 width, with 1 long simple seta and 1 unequally bifid seta dorsally; merus length 0.6 ischium length, length 2.0 width, with 1 long simple seta distodorsally, with 1 long simple seta distoventrally; carpus length 3.2 merus length, length 6.4 width, with 4 setae (all broken off) dorsally, with a row of 5 unequally bifid setae ventrally; propodus length 0.8 carpus length, length 8.3 width, with 3 setae (1 broken off, 1 long simple, 1 short simple) dorsally and 3 unequally bifid setae ventrally; dactylus length 0.3 propodus length, length 4.0 width, with 1 simple seta dorsally, with numerous small setae, membranously embedded ventrally; unguis length 0.4 dactylus length, with 2 slender setae (1 broken off) underneath unguis.

Pereopod III (Fig. 13E) only basis present; length 6.3 width, with 3 simple setae and 1 long broom seta dorsally.

Pereopod IV (Fig. 13F) basis length 4.1 width, with 2 simple setae and 2 long broom setae dorsally, with 2 simple setae ventrally and 1 simple seta (broken off) distoventrally; ischium with 1 small simple seta distodorsally, with 3 simple setae (all broken off) ventrally; merus with 1 long simple seta distodorsally; carpus with 3 setae (2 simple, 1 broken off) dorsally, with 3 stout unequally bifid setae ventrally, with 1 simple seta distoventrally propodus with 1 simple seta (broken off) dorsally, with 3 stout unequally bifid setae ventrally; unguis with 2 slender setae between unguis and ventral claw.

Pereopod V (Fig. 13G) basis length 4.7 width, with 2 simple setae (broken off) dorsally, with 1 long simple seta ventrally; ischium length 0.7 basis length, length 3.3 width, with 2 long simple seta dorsally, with 1 simple seta ventrally; merus length about half ischium length, length 1.5 width, with 1 long simple seta distodorsally, with 1 small simple seta distoventrally; carpus length 3.1 merus length, length 5.6 width, with 1 simple seta and 1 unequally bifid seta dorsally, with 1 long simple seta distodorsally, with 1 simple seta medially, with 2 simple and 2 unequally bifid setae ventrally; propodus about as long as carpus, length 9.7 width, with 2 simple setae dorsally, with 4 unequally bifid setae ventrally; dactylus length 0.3 propodus length, length 4.5 width, with 1 robust seta medially; unguis length 0.4 dactylus length, with 2 slender setae between unguis and ventral claw.

Pereopod VI (Fig. 13H) basis length 5.0 width; ischium length 0.8 basis length, with 3 setae (2 unequally bifid setae, 1 broken off) dorsally, with 3 setae (1 simple, 2 broken off) ventrally; merus with 3 simple setae distodorsally, with 1 simple seta distoventrally; carpus with 3 simple setae dorsally, with 2 simple setae ventrally and 1 stout unequally bifid seta distoventrally; propodus with 1 simple seta distodorsally, with 3 unequally bifid setae ventrally; dactylus with 3 simple setae and 1 robust seta medially; unguis with 1 slender seta between unguis and ventral claw.

Pereopod VII (Fig. 13I) basis length 6.6 width, with 1 simple seta dorsally, with 3 simple setae (1 broken off) ventrally; ischium with 1 simple seta dorsally; merus with 1 simple seta distodorsally, with 2 simple setae distoventrally; carpus with 2 simple setae dorsally, with 2 setae (1 broken off, 1 simple) ventrally; propodus with 2 slender simple setae dorsally, with 3 setae (2 unequally bifid setae, 1 simple seta) ventrally; unguis length 0.3 dactylus length.

Operculum (Fig. 12D, 14B, 15C) length 1.4 width; lateral margin slightly rounded, posterior margin almost straight, with several (> 20) simple setae. Pleopod III (Fig. 12G) sympod length 1.3 width, length 0.7 endopod length; exopod 1.3 endopod length, length 3.0 width, tapering distally, with numerous short simple setae laterally, 2 somewhat longer simple setae distally and 1 long slender simple seta terminally; endopod length twice width, with 3 long plumose setae distally, distal end strongly rounded. Pleopod IV (Fig. 12H) sympod rectangular, length 0.7 width, about 0.6 endopod length; exopod slender, about as long as endopod, length 8.6 width, with several thin setules laterally and 1 long plumose seta distally; endopod almost triangular, strongly tapering distally, distal margin rounded, length 2.3 width. Pleopod V (Fig. 12I) small oval lobe, without setation, about as long as pleopod IV; length 2.5 proximal width, width tapering towards distal end.

Uropods (Fig. 12F) biramous; sympod trapezoid, length 1.5 width, with 1 simple seta laterally, with 4 simple setae (2 broken off, 1 long, 1 short) distally; exopod as long as sympod, length 4.0 width, with 5 long simple setae of varying size terminally; endopod length 1.3 exopod length, length 4.3 width, with 2 seta (1 broken off) laterally, with 7 simple setae of varying size terminally.

Description of Manca (SMF 57944): Habitus (Figs 16A–C) 1.1 mm long. Body dorsoventrally flattened, body length 3.2 pereonite 2 width. Coxae not visible in dorsal view, coxae of pereonite 1 without spine-like appendix. Pereonites 2–4 of similar width, 1.1 pereonite 1 width. Pereonites 2–4 with frontally directed anterolateral margins, pereonite 2 tipped with 1 small spine-like seta apically. Pleotelson length 0.3 body length, length 0.5 width; width 0.9 pereonite 1 width; posterior margin strongly rounded, uropods missing. Cephalothorax (Figs 16A, C) free, length 0.6 width. Rostrum poorly developed. Cephalothorax anterior margin straight, posterior and lateral margins slightly rounded. Antenna inserting anterolaterally in a deep fold. Antennula 0.3 body length.

Remarks. Austroniscus brandtae n. sp. is most similar to the Southern Ocean species A. chelus Kaiser & Brandt, 2007, and A. obscurus Kaiser & Brandt, 2007 in having two spine-like frontally directed appendices extending from the coxae of pereonite 1. The new species, however, can be clearly distinguished from these species by the following characters: A. brandtae n. sp. with 2 robust spine-like setae on pereonite 1 anterolateral margins in female (1 in A. chelus and A. obscurus); with 2 slender setae on the rostrum in female (absent). Furthermore, the new species differs from A. chelus as follows: male antennula with 7 articles (6).

Notably, pereopods of Austroniscus specimens were covered with epibiont ciliates (exemplary illustrated in Fig 13G, see also Fig. 14). These have been previously found in deep-sea isopods (Ólafsdóttir & Svavarsson 2002) and also specimens in the family Macrostylidae from the hadal PRT (Kniesz et al. 2018). Epibionts indicate an epibenthic lifestyle as they are shaved off the body by digging (Ólafsdóttir & Svavarsson 2002); Kniesz et al. (2018) therefore suspected a different lifestyle between male and female for the macrostylid species examined, as these were populated by epibionts to different degrees. Although we did not assess the epibiont infestation quantitatively, there seemed to be little difference between the sexes, suggesting a similar epibenthic way of life and thus capability to disperse.