Morphology of the complete larval development of the symbiotic crab Sestrostoma balssi (Shen, 1932) (Varunidae: Gaeticinae) Author Kornienko, Elena S. Author Korn, Olga M. text Zootaxa 2011 2990 45 58 journal article 10.5281/zenodo.206824 6bd3648e-d667-4b53-b4c2-69461e65b0a5 1175-5326 206824 Sestrostoma balssi ( Shen, 1932 ) Acmaeopleura balssi Shen, 1932 : 155 , figs 98, 100; tab. 6, figs 1, 2 [ type locality: Shangdong Province, northeastern China ]; Itani et al. , 2002 : 46 –47, figs 2, 3. Sestrostoma balssi – Davie & N.K. Ng, 2007 : 265 , figs 7, 8. The development of S. balssi included five zoeae and one megalopa. The minimal time required for larvae to reach the megalopal stage was about 35 days at a temperature of 20–22°C. The body outlines and appendages are illustrated in Figures 1–8 . Only morphological changes are described for the second to fifth zoeal stages. Description of larval stages. ( Figs 1–8 ) First zoea ( Fig. 1 ) Material examined. CL = 0.46±0.02 mm, DRL = 1.20±0.04 mm, N = 10. Minimal duration. 6 days. Carapace ( Fig. 1 А, B). Eyes sessile. Rostral, dorsal, lateral spines present. Rostral, dorsal spines straight, nearly equal carapace length; rostral spine two times longer than antenna; lateral spines longer than 1/2 carapace width. One pair of setae on surface of carapace, at base of dorsal spine. Antennule ( Fig. 1 C). Uniramous, endopod absent; exopod conical, unsegmented, with 2 long, 1 short terminal aesthetascs, 2 short terminal setae. Antenna ( Fig. 1 D). Protopod pointed, with 2 rows of sharp spinules; exopod as a small bud with 1 simple terminal seta. Mandible ( Fig. 1 E). Asymmetrical; without palp. Maxillule ( Fig. 1 F). Coxal endite with 5 plumodenticulate setae; basial endite with 3 plumodenticulate cuspidate, 2 plumodenticulate setae; endopod 2-segmented, proximal segment with 1 simple seta, distal segment with 4 marginal, 1 submarginal plumodenticulate setae. Maxilla ( Fig. 1 G). Coxal endite with 7 plumodenticulate setae; basial endite bilobed, proximal lobe with 4 or 5, distal lobe with 4 plumodenticulate setae; endopod unsegmented, bilobed, with numerous microtrichiae, each lobe with 2 plumodenticulate setae; scaphognathite with 4 marginal densely plumose setae, with long distal process. First maxilliped ( Fig. 1 H). Coxa unarmed; basis with 10 plumodenticulate setae arranged 2, 2, 3, 3; endopod 5- segmented, setal formula progressing distally 2, 2, 1, 2, 1+4, third segment with numerous microtrichiae; exopod unsegmented, with 4 terminal plumose natatory setae. FIGURE 1 . Sestrostoma balssi (Glassell, 1933) , first zoea. A, complete larva, lateral view; B, same, dorsal view; C, antennule; D, antenna; E, mandibles; F, maxillule; G, maxilla; H, first maxilliped; I, second maxilliped; J, pleon, dorsal view. FIGURE 2 . Sestrostoma balssi (Glassell, 1933) , second zoea. A, complete larva, lateral view; B, antennule; C, antenna; D, mandibles; E, maxillule; F, maxilla; G, first maxilliped; H, second maxilliped; I, pleon, dorsal view. Second maxilliped ( Fig. 1 I). Coxa unarmed; basis with 4 plumodenticulate setae arranged 1, 1, 1, 1; endopod 3-segmented, setal formula progressing distally 0, 1, 3+3; exopod unsegmented, with 4 terminal plumose natatory setae. FIGURE 3 . Sestrostoma balssi (Glassell, 1933) , third zoea. A, complete larva, lateral view; B, antennule; C, antenna; D, mandibles; E, maxillule; F, maxilla; G, first maxilliped; H, second maxilliped; I, pleon, dorsal view. Third maxilliped. Absent. Pereiopods. Absent. Pleon ( Fig. 1 J). Five somites; somite 2, 3 each with a pair of dorsolateral processes; somites 2–5 each with a pair of posterodorsal simple setae; pleopods absent. Telson ( Fig.1 J). Bifurcated, furcal rami longer than proximal part of telson, without lateral spines; distal part slightly curved upwards, with 2 rows of spinules; posterior margin with 3+3 plumodenticulate setae, with fairly deep median notch. Second zoea ( Fig. 2 ) Material examined. CL = 0.57±0.02 mm, DRL = 1.57±0.06 mm, N = 10. Minimal duration . 5 days. Carapace ( Fig. 2 А). Eyes stalked. Four pairs of setae on surface of carapace; each ventral margin with 2 setae. Antennule ( Fig. 2 B). Exopod with 4 long terminal aesthetascs. Antenna ( Fig. 2 C). Unchanged. Mandible ( Fig. 2 D). Unchanged. Maxillule ( Fig. 2 F). Basial endite with 5 plumodenticulate cuspidate, 2 plumodenticulate setae; exopod seta present. Maxilla ( Fig. 2 F). Proximal part of scaphognathite with 5, distal part with 3 densely plumose setae. First maxilliped ( Fig. 2 G). Exopod with 6 terminal plumose natatory setae. Second maxilliped ( Fig. 2 H). Exopod with 6 terminal plumose natatory setae. Third maxilliped. Presents as a small bud. Pereiopods. Absent. Pleon. Somite 1 with 1 dorsomedial seta. Telson. Unchanged. Third zoea ( Fig. 3 ) Material examined. CL = 0.63±0.04 mm, DRL = 1.96±0.04 mm, N = 7. Minimal duration . 6 days. Carapace ( Fig. 3 A). One pair of setae on dorsal spine; each ventral margin with 4 setae. Antennule ( Fig. 3 B). Exopod with 3 terminal aesthetascs. Antenna ( Fig. 3 C). Unchanged. Mandible ( Fig. 3 D). Unchanged. Maxillule ( Fig. 3 E). Basial endite with 1 lateral seta; epipod seta present. Maxilla ( Fig. 3 F). Coxal endite incompletely bilobed; proximal, distal lobes of basial endite each with 5 setae; proximal part of scaphognathite with 8 or 9, distal part with 5 densely plumose setae. First maxilliped ( Fig. 3 G). Coxa with 1 plumose seta; third segment of endopod with addition sparsely plumose seta; exopod with 8 terminal plumose natatory setae. Second maxilliped ( Fig. 3 H). Exopod with 8 terminal plumose natatory setae. Third maxilliped. Increased in size to a little degree. Pereiopods. Present as buds. Pleon ( Fig. 3 I). The sixth pleonal somite differentiated. Telson ( Fig. 3 I). Posterior margin with 4+4 plumodenticulate setae. Fourth zoea ( Fig. 4 ) Material examined. CL = 0.88±0.05 mm, DRL = 2.77±0.16 mm, N = 10. Minimal duration. 7 days. Carapace ( Fig. 4 A). Two pairs of setae on dorsal spine; each ventral margin with 7 or 8 setae. Antennule ( Fig. 4 B). Exopod with 4 terminal, 1 subterminal aesthetascs. Antenna ( Fig. 4 C). Biramous, endopod about of 1/3 of protopod. Mandible ( Fig. 4 D). Unchanged. Maxillule ( Fig. 4 E). Coxal endite with 7 plumodenticulate setae; basial endite with 12 or 13 plumodenticulate setae (5 cuspidate, 2 lateral). Maxilla ( Fig. 4 F). Coxal endite bilobed; proximal lobe of coxal endite with 4 or 5, distal lobe with 3 plumodenticulate setae; proximal lobe of basial endite with 7, distal lobe with 6 plumodenticulate setae; scaphognathite with 19–21 plumose setae. First maxilliped ( Fig. 4 G). Endopod setal formula progressing distally 2, 3, 2, 2, 2+4; exopod with 10 terminal plumose natatory setae. Second maxilliped ( Fig. 4 H). Exopod with 10 terminal plumose natatory setae. FIGURE 4 . Sestrostoma balssi (Glassell, 1933) , fourth zoea. A, complete larva, lateral view; B, antennule; C, antenna; D, mandibles; E, maxillule; F, maxilla; G, first maxilliped; H, second maxilliped; I, third maxilliped; J, pereiopods; K, pleon, ventral view. FIGURE 5 . Sestrostoma balssi (Glassell, 1933) , fifth zoea. A, complete larva, lateral view; B, antennule; C, antenna; D, mandibles; E, maxillule; F, maxilla; G, first maxilliped; H, second maxilliped; I, third maxilliped; J, pereiopods; K, pleon, ventral view. FIGURE 6 . Sestrostoma balssi (Glassell, 1933) , megalopa. A, complete larva, dorsal view; B, antennule; C, antenna; D, mandibles; E, maxillule; F, maxilla (arrows indicate tubercles; setules on scaphognathite omitted). Third maxilliped ( Fig. 4 I). Biramous. Pereiopods ( Fig. 4 J). Pereiopod I chelated. Pleon ( Fig. 4 K). Somite 1 with 3 dorsomedial setae; somites 2–5 with uniramous pleopod buds. Telson ( Fig. 4 K). Unchanged. Fifth zoea ( Fig. 5 ) Material examined. CL = 1.05±0.06 mm, DRL = 3.20±0.12 mm, N = 8. Minimal duration . 11 days. Carapace ( Fig. 5 A). Three pairs of setae on dorsal spine; each ventral margin with 8–12 setae; posterodorsal arch with 2 setae. Antennule ( Fig. 5 B). Antennular base broadened, with 2 short setae; endopod bud present; exopod with 10 aesthetascs in tree tiers (3+2+5), 1 short simple terminal seta. Antenna ( Fig. 5 C). Endopod slightly shorter than protopod, incompletely segmented. Mandible ( Fig. 5 D). Palp bud present. Maxillule ( Fig. 5 E). Coxal endite with 9–11 setae; basial endite with 15–17 setae of different size (6 or 7 cuspidate, 3 lateral); two epipod setae present. Maxilla ( Fig. 5 F). Proximal lobe of coxal endite with 6 or 8, distal lobe with 3 setae; proximal lobe of basial endite with 8 or 9, distal lobe with 8 setae; scaphognathite with 27–32 plumose setae. First maxilliped ( Fig. 5 G). Coxa with 3 plumodenticulate setae (2 long, 1 short); exopod 2-segmented, with 12 terminal plumose natatory setae; small unarmed triangular epipod presents. Second maxilliped ( Fig. 5 H). Exopod incompletely 2-segmented, with 12 terminal plumose natatory setae. Third maxilliped ( Fig. 5 I). Increased in size, incompletely segmented. Pereiopods ( Fig. 5 J). Increased in size, incompletely segmented. Pleon ( Fig. 5 K). Somite 1 with 5 dorsomedial setae; somite 2–5 with biramous pleopods, somite 6 with uniramous uropods. Telson ( Fig. 5 K). Posterior margin with 5+5 plumodenticulate setae. Megalopa ( Figs 6–8 ) Material examined. CL = 1.45±0.09 mm, СW = 1.22±0.05 mm, N = 4. Minimal duration . Undetermined. Carapace ( Fig. 6 A). Pear-shaped, with small paired tubercles behind eyes, one posterodorsal tubercle. Carapace surface covered with scattered short simple setae; each ventral margin with 30–40 plumose setae. Rostrum short, directed ventrally. Ocular peduncle with 3 short setae on frontal margin; cornea not wider than ocular peduncle. Antennule ( Fig. 6 B). Biramous; peduncle 3-segmented, setal formula 8, 2, 1; endopod unsegmented, with 1+3 setae; exopod 4-segmented, setal formula 4 or 5 aesthetascs, 5 or 6 aesthetascs, 5 or 6 aesthetascs+2 sparsely plumose setae, 5 aesthetascs (4 long, 1 short)+1 terminal sparsely plumose seta. Antenna ( Fig. 6 C). Consists of 11 segments; peduncle 3-segmented, with 1, 1 or 2, 1 or 2 setae; flagellum 8- segmented, setal formula 0, 0, 2 or 3, 2–4, 1, 4, 2 or 3, 3; four distal segments with long serrate setae, rest segments with short simple (rarely sparsely plumose) setae. Mandible ( Fig. 6 D). Palp 2-segmented, distal segment with 8 setae. Maxillule ( Fig. 6 E). Coxal endite with 18–21 plumodenticulate setae; basial endite with 22 or 23 marginal (9 plumodenticulate, 12 or 13 plumodenticulate cuspidate), 3 or 4 lateral sparsely plumose or simple setae; endopod reduced, unsegmented, with 6 setae; at base of maxillule 4 plumodenticulate setae. Maxilla ( Fig. 6 F). Coxal endite bilobed, proximal lobe with 11–14, distal lobe with 5 setae; basial endite bilobed, proximal lobe with 12–14, distal lobe with 10–14 setae; endopod reduced, 2 or 3 setae similar to scaphognathite setae in proximal part, 1 or 2 plumodenticulate or simple setae in distal part; scaphognathite with 52–60 densely plumose marginal setae, with 4 or 5 simple setae on surface. First maxilliped ( Fig. 7 A). Epipod triangular, with 3 or 4 setae in proximal part, with 7–9 plumodenticulate setae in distal part; coxa with 12–18 plumodenticulate setae; basis with 14–20 setae; endopod reduced, incompletely segmented, with 2 or 3 lateral simple setae, 3 or 4 terminal plumodenticulate setae; exopod 2-segmented, proximal segment with 2 distal short plumose setae, distal segment with 4 or 5 terminal plumose setae. Second maxilliped ( Fig. 7 B). Epipod with 1 plumose seta in proximal part, 3–5 setae in distal part; coxa, basis fused, with 3 or 4 setae; endopod 4-segmented, setal formula 2, 0, 6 or 7, 8–10; exopod 2-segmented, proximal segment with 1–3 lateral short simple setae, distal segment with 5 or 6 terminal plumose setae. Third maxilliped ( Fig. 7 C). Epipod with 11 or 12 plumodenticulate setae in proximal part, 19–23 setae in distal part; coxa, basis fused, with 8–12 setae; endopod 5-segmented, setal formula 11 or 12, 5–9, 4 or 5, 10–12, 9; exopod 2-segmented, proximal segment with 2 or 3 simple setae, distal segment with 6 terminal plumose setae. FIGURE 7 . Sestrostoma balssi (Glassell, 1933) , megalopa. A, first maxilliped; B, second maxilliped; C, third maxilliped (setules on plumodenticulate setae omitted). Pereiopods ( Fig. 8 A–E). Covered by rare short simple setae. Cheliped short, not longer than antenna, palm nearly equal dactylus, dactylus and fixed finger crossed. Dactyli of pereiopods 3–4 each with 1 appreciable spine in centre of inner margin; dactylus of pereiopod 5 with 2 terminal, 1 subterminal long curved serrate setae, small claw. Propodi of pereiopods 3–4 each with strong serrate seta in distal part. Sternum ( Fig. 8 F). With 5 pairs of setae in anterior part. Pleon ( Fig. 8 G, H). Consists of 6 somites, covered by simple setae (somite 1 with 8 setae, somite 2 with 14 setae, somite 3 with 12 setae, somite 4 with 12 setae, somite 5 with 10 setae, somite 6 with 6 setae). Somites 2–4 with small posterolateral processes; somites 2–5 with biramous pleopods, somite 6 with uniramous uropods. Pleopods ( Fig. 8 I, J). Exopods of pleopods 1–3 each with 14–16 plumose natatory setae, exopod of pleopod 4 with 12 or 13 plumose natatory setae; endopods of pleopods 1–4 each with 3 cincinnuli; uropod with 8 or 9 plumose setae on exopod, 1 seta on basal segment. Telson ( Fig. 8 H). Nearly rectangular; width somewhat greater than length; posterior margin with 1 pair of small tubercles; 4 pairs of setae on dorsal surface.