Xenosinelobus balanocolus Chim & Tong, 2019

Kong, Chim Chee, 2024, A synopsis of the Tanaidacea (Crustacea: Peracarida) of Singapore, with a review of tanaidacean diversity in Southeast Asia and the South China Sea, Zootaxa 5451 (1), pp. 1-75 : 45-49

Xenosinelobus balanocolus Chim & Tong, 2019

( Figs. 26A–B )

Xenosinelobus balanocolus Chim & Tong, 2019: 415–426 , figs. 1–5, table 1.

Material examined. Station SG74: 1 manca (BNC-002). Station SG75: 2 mancae (BNC-046). Station SG76: 2 non-ovigerous females, 1 male (BNC-070). Station SG77: 2 females (BNC-085, 089). Station SG78: 8 females, 1 male, 9 mancae (BNC-205, 206, 207, 209, 214 and 221).

Remarks. Xenosinelobus is a monotypic genus ( Anderson 2023). It is very similar to its sister genus Sinelobus but can be easily distinguished by the (1) unusually short antenna article-5; (2) presence of a spiniform seta on pereopod-1 coxa and; (3) complete row of dorsotransverse setae on pleonites 1 and 2 ( Chim & Tong 2019). Xenosinelobus balanocolus is a small species with body length barely more than 2 mm. It is the first tanaid species to be recorded from the inside of barnacles ( Chim et al. 2016). Pigmentation on its cephalothorax is similar to that of Zeuxo sp. SG#1 but with honeycomb pattern instead of a band between the eyes. Xenosinelobus balanocolus is known only from the inside of dead Tetraclita barnacles on the rocky shores and seawalls of Singapore Southern Islands (i.e., Lazarus Island and St. John’s Island).

Key to the Tanaidacea of Singapore (adult females unless stated otherwise)

1. Body dorsoventrally flattened. Cephalothorax with prominent rostrum. Ocular lobe with or without spine. Pleonites always subequal; not reduced or fused. Antennule biramous with more than four articles. Antenna with or without squama. Cheliped and pereopod-1 with or without exopod. Pereopods of a wide variety, including digging, swimming, jumping and filter-feeding types. Pereopod-1 with ischium. Pereopods 1–6 with coxa. Uropods always biramous; endopod with 3–26 articles; exopod with 1–14 articles. Spiniform processes absent or present on pereonites, pleonites, cheliped basis and pereopod-1 coxa. Oostegites in four or five pairs..................................................................... Apseudomorpha (2)

– Body cylindrical. Cephalothorax with weak rostrum. Ocular lobe without spine. Some pleonites may be reduced or fused. Antennule uniramous with at most four articles in females.Antenna without squama. Cheliped and pereopod-1 without exopod. Pereopods of only walking, climbing, clinging, and grasping types. Pereopod-1 with or without ischium. Pereopods 4–6 with or without coxa. Uropods uniramous or biramous; endopod with 1–6 articles; exopod with 1–2 articles. Spiniform processes absent on pereonites, pleonites, cheliped basis and pereopod-1 coxa. Oostegites in one or four pairs..... Tanaidomorpha (14)

2. Body coiled, usually inside gastropod shells. Cephalothorax with several plumose lateral setae. Antennule inner flagellum with one article. Antenna with six articles, without squama. Cheliped without exopod. Pereopod-1 of clinging and tearing type, pereopods 2–6 of adhesive type. Pereopod-1 exopod with 14 setae. Pleopods present only on first three pleonites. Uropod endopod and exopod with three and one articles, respectively............................... Pagurapseudes sp. SG#1

– Body not coiled, always free-living. Cephalothorax without plumose lateral setae. Antennule inner flagellum with at least two articles. Antenna with more than six articles, with or without squama. Cheliped with exopod. Pereopods not adhesive type. Pereopod-1 exopod with six or fewer setae if present. Pleopods present on first four or all pleonites. Uropod endopod with more than six articles; exopod with more than one article.......................................................... 3

3. Body sparsely setulose. Pereonites 2–3 with anterolateral notch, pereonites 4–6 with three anterolateral notches. Rostrum with two blunt lobes. Antenna without squama. Cheliped basis subcircular. All pereopods of walking type ................................................................................................. Pagurapseudopsis sp. SG#1

– Body not setulose. Pereonites with or without lateral notches. Rostrum not bilobate.Antenna with or without squama. Cheliped basis not subcircular. Pereopods not all of same type ......................................................... 4

4. Rostrum tip with three tubercles ( Fig. 16B ). Pereonites 3 and 4 with blunt anterolateral apophyses. Antenna with seven articles, without squama. Pereopod-1 much stronger and longer than remaining pereopods..................................................................................................... Poligarida beni Bamber & Marshall, 2013

– Rostrum tip without tubercles. Pereonites with and without lateral apophyses. Antenna with more than seven articles, with squama that arises from peduncle article-2. Pereopod-1 not conspicuously larger than remaining pereopods.............. 5

5. Cheliped with numerous long comb-like plumose ventral setae for filter-feeding. Pereopods 1, 4 and 5 dactylus with a tuft of aesthetascs distally ( Fig. 6B ).............................. Phoxokalliapseudes singaporensis Drumm & Heard, 2011

– Cheliped without numerous long comb-like ventral setae. Pereopods dactylus simple, without a tuft of aesthetascs........ 6

6. Pereopod-1 coxa with large spine ( Fig. 4C )...................................................... Apseudidae (7)

– Pereopod-1 coxa without large spine...................................................................... 8

7. Adults about 2–4 mm in length. Body slender, about 11 times longer than wide. Rostrum large, wide, subtriangular, slightly curved, and ventrally directed ( Fig. 4C ). Ocular and epistomal spines absent. Pereonites without notches but pereonites 2–6 with curved and anteriorly directed anterolateral spines ( Fig. 4D ). Pleonites and pleotelson each with irregularly shaped and three-pointed projections on lateral margins. Pleotelson without lateral budges and posterior spine. Antennule peduncle article- 1 inner margin without spines and denticles; inner flagellum with four articles; outer flagellum with two articles. Cheliped carpus robust, about 2 times longer than wide. Chela fixed finger greatly reduced in males ( Fig. 4G ). Pereopod-1 without exopod; coxa spine without setae. Uropod endopod with six articles; exopod three six articles............................................................................................. Mendamanus ailurostoma Bamber, 1999

– Adults about 4–6 mm in length. Body stout, about 6 times longer than wide. Rostrum short, slender and spiniform. Ocular and epistomal spines present. Pereonites without spines but pereonites 3–6 with lateral notches. Pleonites with curved and posteriorly directed lateral spines. Pleotelson with two lateral budges and a small posterior spine. Antennule peduncle article-1 inner margin with large medial spine and many denticles; inner flagellum with nine articles; outer flagellum with three articles. Cheliped carpus slender, about 7 times longer than wide. Chela fixed finger not reduced in males. Pereopod-1 with exopod; coxa spine with four short and one long simple setae. Uropod endopod with 27 articles; exopod with seven articles............................................................................................... Siegius sp. SG#1

8. Pleon long, at least 40% of body length. Pleotelson as long as all pleonites combined. Cheliped carpus with a row of setae. Pereopod-1 merus and carpus with dense row of long comb-like ventral setae for filter-feeding. Pleopods present only on first four pleonites....................................................................... Numbakulla sp. SG#1

– Pleon short, about 20–30% of body length. Pleotelson as long as or shorter than all pleonites combined. Cheliped carpus without a row of setae. Pereopod-1 without dense row of setae. Pleopods present on all five pleonites.................. 9

9. Rostrum minute. Pleonites with lateral spines. Antennule peduncle (excluding common article) with three articles; inner and outer flagella with two and 5–6 articles, respectively. Antenna flagellum with four articles. Cheliped carpus shorter than propodus. Male cheliped fingers without large teeth. Pereopods 2–6 of clinging, tearing, walking, and climbing types. Uropod endopod with more than 3–4 articles; exopod with 9–11 articles................................ Calozodion sp. SG#1

– Rostrum conspicuous. Pleonites with or without lateral spines. Antennule peduncle (excluding common article) with three or four articles; inner and outer flagellum with at least 10 and 12 articles, respectively. Antenna flagellum with at least eight articles. Cheliped carpus longer or shorter than propodus. Male cheliped fingers with or without large teeth. Pereopods 2–6 of jumping type. Uropod endopod with more than four articles; exopod with at least 11 articles............ Parapseudidae (10)

10. Body slender, about 12 times longer than wide. Rostrum semi-circular with a short, curved and ventrally directed spine. Ocular lobe with spine. Epistomal spine long, curved and dorsally directed. Antennule peduncle article-1 inner margin without spiniform setae. Antenna peduncle article-2 with two-prong apophysis. Cheliped basis with spiniform seta. Male chela fingers without large tooth…................................................................................. 11

– Body stout, about 5–6 times longer than wide. Rostrum not curved. Ocular and epistomal spines absent. Antennule peduncle article-1 inner margin with 1–2 spiniform setae. Antenna peduncle article-2 without two-prong apophysis. Cheliped basis without spiniform setae. Male chela fingers with large tooth.................................................. 12

11. Pereonites with anterolateral spines ( Fig. 11B ) and hyposphenia. Pleonites short, each about half as long as pleotelson.Antennule inner flagellum with 11 articles and outer flagellum with 13 articles. Antenna with 14 articles. Cheliped basis without spines or spiniform setae. Pereopod-1 exopod with five setae; coxa with 2–3 spines; claw bifid and with two ventral spines................................................................................................ Actenos sp. SG#1

– Pereonites without spines and hyposphenia. Pleonites long, each as long as pleotelson.Antennule inner flagellum with 20 articles and outer flagellum with 19 articles. Antenna with 21 articles. Cheliped basis with ventromedial spiniform seta. Pereopod-1 exopod with three setae; coxa without spines; claw not bifid and with five spiniform ventral setae............................................................................... Biropalostoma spiniferum Guţu & Angsupanich, 2004

12. Rostrum with conspicuous spine. Pleonites without curved and posteriorly directed lateral spines. Pleonite-1 with complete dorsotransversal row of plumose setae ( Fig. 15A ). Pleotelson as long as wide. Antennule peduncle article-1 inner margin with two spiniform setae. Pereopod-1 of swimming type ....................... Longiflagrum koyonense Angsupanich, 2004

– Rostrum with minute spine. Pleonites 4–5 with curved and posteriorly directed lateral spines. Pleonite-1 without complete dorsotransversal row of plumose setae. Pleotelson longer than wide. Antennule peduncle article-1 inner margin with spiniform seta. Pereopod-1 of digging type ........................................................................ 13

13. Pereonites 2–3 posterolateral corners with curved spiniform setae ( Fig. 13 ). Pleonite-1 with incomplete dorsotransverse row of plumose setae. Pleonites 2–3 and pleonites 4–5 with plumose and simple lateral setae, respectively. Male cheliped basis without dorsal spine................................................................. Platylicoa angela Bamber, 2013

– Pereonites 2–3 posterolateral corners without curved spiniform setae. All pleonites with simple lateral setae. Male cheliped basis with dorsal spine................................................................ Brachylicoa sp. SG#1

14. Five free pleonites. Pereopod-1 ischium present. Pereopods 4–6 of climbing or clinging types. Pleopods present on all pleonites. Uropods biramous...................................................................... Paratanaoidea (15)

– Three free pleonites and two reduced or fused pleonites. Pereopod-1 ischium absent. Pereopods 4–6 of grasping type. Pleopods present only on pleonites 1–3. Uropods uniramous............................................... Tanaidoidea (20)

15. Pereonite-1 reduced. Chela forcipate. Pereopods 1–6 with coxa. Oostegites in one pair (on pereopod-4)...................................................................................... Akanthinotanais cf. gerlachi Sieg, 1977

– Pereonite-1 not reduced. Chela not forcipate. Pereopods 1–3 with coxa. Oostegites in four pairs...................... 16

16. Uropod longer than pleotelson; endopod with three or more articles; exopod with two articles.......... Leptocheliidae (17)

– Uropod not longer than pleotelson; endopod with two articles; exopod with one or two articles....................... 19

17. Antennule with three very stout articles ( Fig. 17A ). Male chela fixed finger reduced ( Fig. 17D ). …......................................................................................... Konarus crassicornis ( Stebbing, 1905)

– Antennule with four articles. Male chela fixed finger not reduced................................ Leptocheliinae (18)

18. Antenna peduncle article-2 with large distal spine ventrally. Male cheliped greatly enlarged and robust. Uropod exopod as long as endopod article-1.............................................................. … Chondrochelia sp. SG#1

– Antenna peduncle without large spines. Male cheliped greatly elongated and slender ( Fig. 19 ). Uropod exopod twice as long as endopod article-1 ( Fig. 19 )................................................. Leptochelia cf. splendida Guţu, 2016

19. Pleonites without plumose setae. Antennule with three articles in females. Male cheliped with large flange ( Fig. 20C ) on outer margin and ridges on inner margin. Uropod as long as pleotelson; endopod article-2 as long as article-1; exopod with two articles................................................... Nesotanais rugula Bamber, Bird & Angsupanich, 2003

– Pleonites 1–4 each with large basally swollen plumose seta on lateral margins. Antennule with four articles in females. Male cheliped without flanges or ridges. Uropod shorter than pleotelson; endopod article-2 about 1.5 times longer than article-1; exopod with one article............................................................... Aparatanais sp. SG#1

20. Cephalothorax with or without honeycomb pattern across centre. Eyes with dotted or banded pigmentation in between. Pleonites without dorsotransversal row of setae. Pleonites 4 and 5 reduced but not fused. Uropod with three or four articles. Zeuxo (21)

– Cephalothorax with honeycomb pattern across centre. Eyes with blotched pigmentation in between. Pleonites 1 and 2 each with a dorsotransversal row of setae. Pleonite-5 fused to pleotelson. Uropod with three articles............... Sinelobinae (22)

21. Cephalothorax and rest of body densely covered with polka dot pigmentation. Pleonites 1 and 2 with some dorsolateral plumose setae. Antennule with pigmentation. Uropod endopod with three articles................. Zeuxo cf. coralensis Sieg, 1980

– Cephalothorax with distinct dark coloured band between eyes and honeycomb pattern across the centre. Pereonites with pigmented band. Pleonites with somewhat irregular pigmentation; pleonites 1 and 2 without dorsolateral plumose setae. Antennule without pigmentation. Uropod endopod with four articles................................. Zeuxo sp. SG#1

22. Pleonites 1–2 dorsotransversal row of setae incomplete. Pleotelson without anterolateral budge. Antennule with pigmentation. Antenna article-5 long, almost as long as article-4. Male cheliped with two marginal ventral apophyses on merus and one inner-ventroproximal rounded apophysis on carpus. Pereopod-1 coxa without spiniform seta. Pereopods 2–3 dactylus and unguis together slender and slightly curved. Uropod endopod with three articles.......................... Sinelobus sp. SG#1

– Pleonites 1–2 dorsotransversal row of setae complete. Pleotelson with anterolateral budge. Antennule without pigmentation. Antenna article-5 very short, 0.1 times as long as article 4. Male cheliped merus and carpus without apophyses. Pereopod-1 coxa with spiniform seta. Pereopods 2–3 dactylus and unguis together stout and greatly curved. Uropod endopod with two articles......................................................... Xenosinelobus balanocolus Chim & Tong, 2019

Figures

FIGURE 4. Mendamanus ailurostoma Bamber, 1999 from Singapore: (A) non-ovigerous female, dorsal habitus; (B) ovigerous female, lateral habitus; (C) rostrum and pereopod-1 coxal spines (arrows); (D) lateral spines (arrows) on pereonites 3–6; hyposphenia (arrows) on (E) pereonite-2 and (F) pereonite-6; (G) male cheliped with reduced fixed finger (arrow). A, C–D: CR1117-P02-02-02; B, F: CR1016-P02-02-03; E: CR1114-P02-03-02; G: CR1016-P02-01-01. Scale bar:A–B = 1 mm; C = 500 µm; D–G = 200 µm.

FIGURE 6. Phoxokalliapseudes singaporensis Drumm & Heard, 2011, Singapore: (A, B) adult male, SUB-0490, dorsal and lateral habitus, respectively, with insets showing distal spiniform seta on antennule peduncle article-1, propodus and claw on pereopod-6, and aesthetascs on pereopod-1 dactylus; (C) lateral habitus of ovigerous female (S5G1); (D) large spine on propodus of male cheliped, 5115TB1-041. Scale bar: A–C = 1 mm; D = 500 µm.

FIGURE 11. Actenos sp. SG#1, Singapore: (A) adult male, CR1118-P02-01-01, lateral habitus; (B) female with oostegites, CR0717-P02-01, dorsal habitus, with inset showing anterolateral spines (arrows) on pereonites 2–6. Scale bar = 2 mm.

FIGURE 13. Platylicoa angela Bamber, 2013, non-ovigerous female with detached pereopods, CR0513- P06-02, Singapore, dorsal habitus, with inset showing posterolateral curved spiniform setae (arrows) on pereonites 2 and 3. Scale bar = 2 mm.

FIGURE 15. Longiflagrum koyonense Angsupanich, 2004, JS- 7226, from Singapore: (A) lateral habitus of adult male, with inset showing dorsotransverse row (arrow) of setae on pleonite-1; (B, C) lateral and dorsal habitus, respectively, of non-ovigerous female. Scale bar = 2 mm.

FIGURE 16. Poligarida beni Bamber & Marshall, 2013, CR1209- P08-01, from Singapore: (A, B) dorsal and ventral habitus, respectively, of non-ovigerous female, with inset showing rostrum (arrow) bearing three terminal tubercles. Scale bar = 500 µm.

FIGURE 17. Konarus crassicornis (Stebbing, 1905), Singapore: (A) non-ovigerous female, SEN-PRO-00.0-TAN07, dorsal habitus, with inset showing 'fat' peduncle antennule article-1 (arrow); (B) non-ovigerous female, SEN-PRO-02.5-TAN05, lateral habitus; (C) adult male, SEN-PRO-07.5-TAN08, dorsal habitus; (D) adult male, SEN-PRO-05.0- TAN09, lateral habitus, with inset showing the reduced chela fixed finger (arrow). Scale bar = 1 mm.

FIGURE 19. Leptochelia cf. splendida Guţu, 2016 adult male, Singapore, HAN-EXP-10.0-TAN01, dorsal habitus, with insets showing left cheliped and uropods. Scale bar = 1 mm.

FIGURE 20. Nesotanais rugula Bamber, Bird &Angsupanich, 2003, Singapore, SG30 No. 36784–36809: (A, B) non-ovigerous female, dorsal and lateral habitus, respectively; (C) adult male, lateral habitus, with inset showing flange (arrow) on left chela. Scale bar = 500 µm.

FIGURE 26. Xenosinelobus balanocolus Chim & Tong, 2019, Singapore: (A) female with oostegites, BNC-221-01, dorsal habitus; (B) adult male, BNC-205-03, dorsal habitus. Scale bar = 1 mm.