Fig. 3

External observations during seahorse brood pouch formation. Ventral views (a–h) and lateral views (i–l) of the tail of a male seahorse, observed 20–30 days (a–f, i, j), five months (g, k), and eight months (h, l) after birth. (a) Before formation of the brood pouch primordia. The order of brood pouch development is depicted in b–f: (b, i) primordium of the brood pouch as parallel linear projections; (c) elongation of the projections; (d) a seam line forming a Y-shape; (e) a seam line forming an I-shape; (f, j) formation of the pore-like pouch entrance; the expanding pouch as development proceeded to five months (g, k) and eight months (h, l) after birth. Arrowheads point to edges of the brood pouch. af, anal fin; an, anus. Scale bars: 1 mm