The Morphology of the Urogenital Region of the Bluehead Wrasse, Thalassoma bifasciatum: Female Egg Release via Epidermal Rupture
Females of the diandric, protogynous species Thalassoma bifasciatum release a single all-or-nothing explosive burst of eggs in a once-a-day spawning event. The mechanism controlling egg release is unknown but must provide proper spatiotemporal distribution of eggs, i.e., for successful fertilization, one that conforms sufficiently to the corresponding externally released sperm cloud geometry. This is particularly important in pair spawns, where one female releases eggs approximately simultaneously with the sperm release of a single male in a rapid, highly synchronized spawning rush. In order to determine how egg release is controlled, we examined the surface and internal morphology of the urogenital region of mature females using in vivo light microscopy, histology, and scanning electron microscopy. In anesthetized females, manual pressure to the abdomen distended the epidermis between the anus and urinary papilla until it ruptured and eggs burst outward en masse. Females captured before or after natural spawning, and with or without manual pressure applied under anesthetic, consistently exhibited a path for egg release from the oviduct(s) that was a caudoventrally descending irregular sinus within a loose connective tissue mass, which opened with the exterior where the epidermis ruptured; there was no distal duct, surface gonopore, or sphincter. This pathway was immediately rostral to, and separate from, the urinary papilla, its urinary duct, and urinary pore, which had no communication with the oviduct(s). We suggest that the egg load expands the sinus and distends the epidermis until it ruptures due to increased internal pressure, rapidly releasing hundreds of eggs. These results are consistent with a burst-release mechanism that occurs during the pelagic spawning event, perhaps optimizing spatiotemporal mixing of gametes.
Urogenital region of female T. bifasciatum and egg release via rupture of epidermis. (A–D) External changes in the urogenital region (between the anus and the caudal end of urinary papilla) of three anesthetized females; for all, rostral at left. (A) The urogenital region epidermis was distended, uninterrupted, and exhibited a dark semi-circular band rostral to urinary papilla in a gravid, not naturally spawned distended (NNSD) female (TH35) before any manually applied pressure (MP). (B) With slight, incremental MP, the epidermis was maximally distended, giving the appearance of a semi-sphere that filled the field of view (and pushed the papilla dorsally, out of focal plane); this very large distension was only seen in the three NNSD females (female TH37 shown) that had not spawned before capture and were distended before MP (A, above), but all 22 mature females exhibited some distension of the rostral urogenital region with MP. (C) In the same female (and other NNSD females), a small incremental increase in MP caused the epidermis to rupture, releasing a burst of many eggs; the rupture occurred at the dark semi-circular band that had been salient in the initially distended state (A); note white circle delineating one hydrated egg among many. (D) After epidermal rupture and burst release of eggs, the epidermis was non-distended and exhibited a transverse folded edge that overlay/obscured the more dorsal epidermal rupture (or a salient transverse edge [not folded; not shown]); indicated here between two short arrows and the post-egg-release epidermis labeled as “egg flap”; indeterminant female shown (SEM24, IND) before MP. Nineteen of the mature females exhibited this appearance before MP: ten captured before spawning (not naturally spawned [NNS], and perhaps not likely to spawn until the next day) were initially non-distended; three that had naturally spawned before capture (NS); and six whose condition was indeterminant (IND). All 19 showed limited distension and release of very low numbers of eggs with MP: the ten NNS females required slightly more pressure than NNSD females to cause distension that revealed the dark band, caused epidermal rupture, and slower release of smaller numbers of eggs (not shown); the three NS and the six IND females exhibited very little distension, no dark semi-circular band at the epidermis, and there was no rupture/burst event; when present, the very few eggs appeared to exit slowly dorsal to the caudal edge of the egg flap. A–D at same magnification (scale bar in A = 0.5 mm).
Urogenital region morphology of females before and after egg release. (A) Top: ventral view of NNSD female before egg release, rostral to left; note distension of rostral urogenital region between anus and urinary papilla, whose epidermis was continuous with that of urinary papilla, with dark semi-circular band showing before egg release. Bottom: lateral view of female before egg release, rostral to left, ventral to bottom; hydrated eggs are shown within sinus, representing how they might contribute to distension extending beneath ventral body scales (dashed line). (B) Top: ventral view of female after egg release; note post-egg-release egg flap, with no distension and no semi-circular dark band, with a visible transverse edge where the epidermis was discontinuous/separate from the more caudal urinary papilla. Dimensions of characters measured in vivo are shown between vertical arrows (egg flap width) and arrowheads (papilla width). Bottom: lateral view of female after egg release; note egg exit location where the empty sinus extends to the epidermal rupture, and egg flap receded dorsal to ventral body scales (dashed curve). Dimensions of characters measured in vivo are shown between vertical lines and horizontal arrows (papilla length [shorter] above urogenital length [longer]).
Surface features of female egg flap and urinary papilla after egg release. (A) Low-power (70X) scanning electron microscopy (SEM) photomicrograph of the urogenital region of a representative female (SEM24, IND+MP) with ventral body scales and rostral wall of the anus removed to expose the post-egg-release egg flap and urinary papilla; rostral to left; rectangle indicates region of higher magnification. (B) High-power (275X) SEM photomicrograph showing the single, small urinary pore and surface of the papilla; rostral to left. It could not be determined if this female was captured/examined before or after natural spawning and manual pressure was applied during in vivo examination before SEM fixation (i.e., categorized as indeterminant, IND+MP). Scale bars at 0.1 mm.
Sagittal sections showing the descending egg path, epidermal rupture, and associated features. (A) Midsagittal section of an adult female captured during spawning period, and with no manual pressure applied (TH50, IND without MP); note the sinus descending ventrocaudally from the OD to the urogenital region epidermis overlayed by the more ventral egg flap (showing midline epidermis not ruptured, but thinnest at point nearest sinus); dashed line indicates approximate plane of transverse section through the urinary duct (UD) and urinary papilla, immediately caudal to egg flap region (Fig. 5C); rostral to left, ventral to bottom. (B) Parasagittal section of same female showing continuous egg path from OD (with egg) into sinus, continuing to epidermal rupture through fissure in LCT. Ligament muscle (LM) shown where it approaches anal-fin spine, but still lateral to it; rostrally the LMs were lateral to sinus and thus not in section; rectangle indicates region of higher magnification in C, and dashed lines, left and right, indicate approximate planes of transverse sections in Figure 5A and B, respectively. (C) Higher magnification of sinus, fissure within LCT (indicated with arrowheads), separated ends of ruptured epidermis (arrows), and egg exit (between arrows). LCT = loose connective tissue, LM = ligament muscle, OD = oviduct, UB = urinary bladder, UD = urinary duct. Scale bars at 0.5 mm.
Transverse sections showing the descending egg path, epidermal rupture, and urinary papilla. (A) Transverse section through rostral urogenital region (approximated by leftmost dashed line in Figure 4B (female TH50); note that Figure 4B is parasagittal section not including anatomy at midline, whereas Figure 5A–C are centered on midsagittal plane, ventral to bottom), from female TH33 (NS+MP), showing sinus continuous with OD, separating LCT; note merged, common OD. (B) Transverse section through caudal egg flap of rostral urogenital region (approximated by rightmost dashed line in Fig. 4B), from female TH35 (NNSD+MP); arrows indicate two ends of ruptured egg flap epidermis (epidermal rupture and egg exit is space between arrows at ventral extent of sinus; spanned with extracellular secretions); the epidermis on right of figure is curved dorsally in “C” shape partially separate from LCT, suggesting larger opening for egg exit. (C) Transverse section through urinary papilla caudal to egg flap and rupture (approximated by dashed line in Fig. 4A), from female TH33 (NS+MP), showing papilla with urinary duct only. LCT = loose connective tissue, LM = ligament muscle, OD = oviduct, UD = urinary duct. Scale bars at 0.5 mm.
Contributor Notes
Associate Editor: M. T. Craig.