We measured nine P. volitans larvae from 2.5 – 12.9 mm (Fig. 2 a – g) and examined others as supplemental material. Smallest preflexion larva characterized by precocious pectoral fins, deep base, elongate rays. Gut with single loop or “ twist ” (Fig. 2 a), and terminates slightly beyond mid-body; hindgut becomes rugose in early postflexion larvae. Flexion larvae with tips of longest developing pectoral rays to or beyond posterior margin of dorsal-fin base; by early postflexion, these rays extend to about terminal margin of caudal peduncle (CP) or beyond, if unbroken. Flexion larvae with pronounced “ hump ” in snout dorsal profile created by elevation of rostral cartilage (i. e., ascending process of premaxilla). Head deep, angle of lower jaw pronounced; snout typically 10 – 15 % shorter than OD; CP shortens, deepens as notochord flexes. Upper jaw terminates near mid-orbit; maxilla deepens near terminal margin, its maximum depth> 55 % OD. CP depth nearly equal to its length in postflexion and transforming larvae (Table 2). Seven BR. Pigmentation Pigmentation sparse in all larvae examined (Fig. 2 a – e). Internally, head with melanophore between lobes of midbrain in smallest larva, and on dorsoposterior surface of hindbrain and brainstem by 3.0 mm. Flexion larvae add pigment to ventrolateral surface of each lobe of hindbrain (otic region), which together with midbrain pigment forms “ triangular ” pattern when viewed downward from top of head (Kitchens et al. 2017: fig. 3). Early postflexion larvae (~ 5.0 mm) with series of subsurface melanophores across dorsoposterior margin of midbrain, and dorsal and lateral surfaces of hindbrain and brainstem; this pigment gradually obscured by cranial ossification and thickening musculature in early transformers (~ 10.0 mm). Behind head, pigment on dorsoposterior margin of hindgut near anus in smallest larva (Fig. 2 a) becomes more conspicuous in flexion larvae. Dorsal surface of visceral mass along body wall lightly pigmented at all sizes; dorsal surface of gas bladder unpigmented until early postflexion. All internal pigment gradually obscured by thickening abdominal musculature in early transformers. Externally, pigment concentrated along body midlines. Pigmentation in preflexion larvae varies from opposing dorsal and ventral blotches to narrow longitudinal swath located along posterior third of body margins; lateral midline with one to three “ dash-like ” melanophores (Fig. 2 a). Flexion larvae with pigment typically restricted to narrow longitudinal swath about 4 – 5 epimeres wide near posterior margin of developing dorsal-fin base; another swath about 2 – 3 hypomeres wide near posterior margin of developing anal-fin base; and mid-lateral series of 3 – 5 “ dash-like ” pigments along CP. One of two flexion larvae with small pair of melanophores embedded in ventral midline of CP anterior to developing hypural elements; a single melanophore embedded between upper and lower hypural plates, and pigment scattered over lower plate (Kitchens et al. 2017: fig. 3); pigment lacking at these locations in postflexion larvae. Swath of pigment along body margin and posterior half of soft dorsal-fin base, and above posterior third of anal-fin base, extends onto adjacent 1 – 2 myomeres of CP in postflexion larvae. By 6.0 mm, series of 3 – 4 melanophores added near mid-base of developing spinous dorsal fin, these pigments on pterygiophores 9 through 11 in 8.5 mm larva (Fig. 2 c). Pigmentation unchanged to slightly reduced along dorsal, lateral, and ventral body thereafter until early transformation. During that period, pigment typically restricted to narrow “ dash-like ” series of melanophores below soft dorsal-fin base and along margin of adjacent epimeres, 0 – 3 melanophores along lateral midline of CP, and along last 3 – 4 pterygiophores of anal-fin base. Early transformers add pair of enlarged melanophores to dorsal surface of head between parietal spines (Fig. 2 f); single melanophore to small cluster along dorsal rim of premaxilla; and single to short series of melanophores along dorsal midline near mid-CP. Pigment along anal-fin base typically reduced to one or two melanophores, often large, on lateral surface of penultimate and antepenultimate pterygiophores. Largest transformer (12.9 mm) adds enlarged melanophore or blotch to surface of head (midbrain region), and small cluster to snout between anteroventral margin of orbit and upper lip. Fin development and pigmentation Pectoral fins precocious, base deep, typically 53 – 56 % BDc (Table 2); all rays present in early postflexion larvae (Table 3). Pigment lightly scattered over mid- to outer-margin of blade and developing rays in flexion larvae, and beginning to consolidate into irregularly shaped patches in postflexion larvae. By 7.0 – 8.0 mm, scattered pigment at several locations along inner third to mid-shaft of fins, on webbing between rays, on inner third of shaft only of rays 4 – 6, and near mid-shaft of rays 13 – 14; early transformers add pigment along outer third of shaft of most rays. Tips of longest unbroken rays extend to or slightly beyond terminal margin of CP through late postflexion; pectoral fins become relatively shorter during transformation. Early postflexion larvae with three to four rays in pelvic fin, and all elements by 7.0 mm (Table 3). By 8.5 mm, pigment on outer third of webbing between pelvic spine and outermost ray (Fig. 2 d); tips of middle three rays extend to or slightly beyond first pterygiophore of anal-fin base. By 11.0 mm, longest pelvic rays extend to or beyond mid-anal-fin base, with part of innermost ray attached by short membrane to abdominal wall; tip of pelvic spine when pressed against body extends to or slightly beyond first pterygiophore of anal-fin base. Bases of soft dorsal and anal fins thickening outward from mid-base in flexion larvae. Early postflexion larvae with all dorsal- and anal-fin pterygiophores, and most elements developing; full complement of elements present by 8.0 mm (Table 3). First and second spines of anal fin formed by 7.0 mm; tips of last 3 – 4 soft dorsal rays, last two rays of anal fin now extend to or slightly beyond terminal margin of hypural plate. Gap between first and second dorsal spines narrower than between subsequent spines, length of first spine about one-half that of second spine; length of first anal spine <65 % that of second (Table 2). Pterygiophore of first dorsal spine typically inserted over operculum. If 13 dorsal spines, first anal spine inserted below 12 th to 13 th spine; if 12 spines, first anal spine inserted below 12 th spine to first ray of soft dorsal fin. First “ true ” ray of anal fin (fourth element) inserted below first to second ray of soft dorsal fin; terminal pterygiophore of anal fin inserted below penultimate to antepenultimate pterygiophore of soft dorsal fin. Caudal fin with 7 + 7 primary rays by 8.0 mm; early transformers with 22 total caudal-fin elements (Table 3). Largest transformer with aberrant number of dorsal-fin elements (21 total, typical count 23 – 24; Table 3), lacks first three pterygiophores and associated structures, fourth pterygiophore underdeveloped. Origin of spinous dorsal fin displaced posteriorly, anteriormost pterygiophore (fourth) located over dorsal splay of sixth epimere (Fig. 2 g); third element of anal fin untransformed; dorsal, anal, and caudal fins unpigmented in largest transformer. Cephalic and opercular spination; supraocular and nasal cirri; and sensory pore development Number, relative length, and orientation of PPO spines change as larvae develop. Smallest larva with small PPO- 2 along upper margin, and PPO- 3 along lower margin. Flexion larvae add two small spines along anterior shelf of preopercle (APO), APO- 2 along upper margin behind lower rim of orbit, and APO- 4 along lower margin near angle of lower jaw (Fig. 3 a). Early postflexion larvae add small APO- 1 and PPO- 1 along upper margin, and add APO- 3 and PPO- 4 along lower margin by 7.0 mm. APO spines 1 through 3 generally overlay their PPO counterparts, with APO- 4 inserted between PPO- 3 and - 4, but closer to PPO- 4. PPO- 1 directed toward upper margin of opercle, PPO- 2 nearly horizontal, PPO- 3 directed toward pelvic-fin base, PPO- 4 directed ventrally (Fig. 3 b). PPO- 2 and - 3 similar in length, longer than - 1 and - 4; mean length of PPO- 2 and - 3 about 50 % OD in flexion and early postflexion larvae (Table 2). PPO- 4, shortest, length less than one-half that of PPO- 2 and - 3; all PPO’s regress in length relative to OD during transformation. By 8.5 mm, cephalosensory canal along outer margin of PPO with three pores: uppermost near PPO- 1, middle pore between - 1 and - 2, lowermost pore between - 2 and - 3 (Fig. 3 c). By 11.0 mm, sensory canal added along ventral margin of infraorbital ridge with five pores. Largest transformer with lateral margins of PPO- 1 through - 3 weakly serrate; opercle, interopercle, and subopercle lack spines at sizes examined. Most cranial ridges and spines indistinct in preflexion larvae, and weakly developed in flexion and early postflexion larvae. Pair of small dorsoposteriorly directed parietal spines present in late preflexion larvae; flexion larvae with pterotic, lower posttemporal, supraocular, and postocular ridges developing; pterotic ridge with short medial spine. Height of pterotic ridge increases toward posterior margin, short spine on terminal margin in early postflexion larvae; terminal margin of lower posttemporal ridge with short spine by 7.0 mm. Pterotic ridge two times longer, higher than lower posttemporal ridge at 7.0 mm, pterotic ridge nearly three times longer by 8.5 mm. Largest transformer, however, with lower posttemporal ridge longer, and two times higher than pterotic ridge. Transformers also add vertical pair of short, parallel, irregularly sculpted sphenotic ridges behind orbit. Nuchal spines short, acute, conjoined to outer margin of parietal base in flexion larvae, parietals longer than nuchals, both elevated and directed upward about 30 ° above longitudinal axis of head. Parietal spines over two times longer than nuchals by 8.0 mm (Fig. 3 a-c), about three times longer by 10.0 mm; both spines gradually regress during transformation. By 8.5 mm, larvae with anterior margin of parietal spines weakly dentate; posterior margin of parietals, and anterior and posterior margins of nuchals smooth in all examined. Supraocular and postocular ridges continuous in early postflexion larvae, supraocular ridge low, postocular ridge rises abruptly along anterior margin to acute peak near mid-orbit, ridge height decreases thereafter along dorsoposterior margin of orbit. Supraocular and postocular ridges project outward above orbits like “ blinders, ” postocular more so than supraocular ridge. By 8.5 mm, supraocular ridge broadly rounded, small spine near terminal margin, ridge height about one-half that of postocular. Early transformers with anterior margin of postocular ridge weakly dentate, spine midway along ridge. Lateral ridge along first infraorbital (IO 1) low, poorly developed; small spine on anterodorsal (upper, U) margin (IO 1 - U 1) by 7.0 mm, and on anteroventral (lower, L) margin (IO 1 - L 1) by 8.0 mm (Fig. 3 b – c). Lateral ridge crosses onto second infraorbital (IO 2) by 8.5 mm, spine on its posteroventral margin (IO 2 - L 1) by 10.0 mm; lateral ridge crosses third infraorbital (IO 3) in early transformers; and terminates on fourth infraorbital (IO 4) below posterior margin of orbit, small spine on its terminal margin by 11.0 mm. Transformers add weak outer interorbital and upper posttemporal ridges. Outer interorbital ridge connects to coronal base, bifurcates terminally into low coronal (inner) and tympanic (outer) ridges; both ridges lack spines and serrations in all examined. Transformers also add short, unpigmented supraocular cirrus, and small, unpigmented nasal cirrus along dorsal rim of lower nostril by 11.0 mm. Largest transformer with small, blunt spine on terminal margin of upper posttemporal ridge.
