Seahorses are sexually dimorphic, with differing structural characteristics. The most obvious of these is the presence of a brood pouch at the base of the abdomen in males. Males also have a proportionally longer tail than females (Lourie et al. 2004). The minimum recorded size for sexually mature individuals is 5.6 cm (Baum et al. 2003), and males have been reported to develop brood pouches at 5 to 7 months of age (Scarratt 1995). Most seahorse species are sexually and socially monogamous, mating with a single partner for an entire season or lifetime (Baum et al. 2003). Before sexual reproduction, mated pairs undergo a complex courtship process lasting a few days. Both partners may display color changes, becoming pale to whitish during the process (Lin et al. 2008, Martinez et al. 2005). The male inflates his pouch and begins to pursue the female to signal that he is ready to mate. A series of movements follows, including head pointing and the entwining of tails (Lin et al. 2008). Mating behavior culminates in copulation, as the female transfers her eggs to the brood pouch of the male. The male then seals the pouch and fertilizes the eggs. After the male gives birth, courtship may resume immediately.Embryology & Development: Clutch sizes in females may exceed 1000 (Teixeira & Musik 2001), and the reported brood size in males ranges from 97 to 1,552 eggs. The average diameter of eggs is 1.5 mm (Vincent 1990), approximately 2-33% of which were found to be sterile (Teixeira & Musik 2001). The brood pouch of the male acts as a marsupium, protecting the developing embryos and providing them with oxygen through a capillary network. The pouch also serves as an adaptation chamber, altering sodium and calcium concentrations as development progresses until they are similar to the surrounding seawater prior to birth (Linton & Soloff 1964). The average gestation period for H. erectus is 20-21 days (Herald & Rakowicz 1951), and the male gives birth to fry approximately 11 mm in length (eg. Herald & Rakowicz 1951) over the course of about 3 days (eg. Lin et al. 2008). Breeding spans from May to October for populations in the Chesapeake Bay (Teixeira & Musik 2001), with the largest densities of individuals occurring in July for south Florida populations (Powell et al. 2007).
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