A great variety of reproductive strategies occur in this order, including some of the most elaborate known for any group of fishes. In a few families (Centriscidae, Macroram-phosidae, Pegasidae, and perhaps Aulorhynchidae), the eggs and larvae are pelagic, and spawning probably is accomplished (at least in some of these families) in a style similar to broadcast spawning, in which eggs and sperm are released directly into the water column. In pegasids, spawning involves a courtship ritual whereby a female and a male swim vent to
vent, releasing eggs and sperm simultaneously. In other families, reproduction is a far more complex process in which the male carries the eggs, sometimes in a special pouch, within which the eggs may be fertilized. In Solenostomus, the females carry the fertilized eggs in pouches made of their greatly extended pelvic fins, repeatedly opening and closing them to fan the eggs. As a preliminary behavior to spawning, many species employ complex courtship rituals, in which males compete for the female by dancing, inflating their pouches, or performing in some other way. A male stickleback lures the female into his nest and fertilizes the eggs there, sometimes attracting several females in succession. Some species are reproductively active throughout the year, whereas others are seasonal. Many species of syngnathids form monogamous pairs.
Reproduction has been studied in detail for many syng-nathids. In seahorses, the females insert their eggs into the pouch of the males, using an abdominal projection known as the ovipositor (an everted egg duct); the eggs are fertilized by the sperm located in the pouches. During this process the male and female face each other. Inside the pouch the eggs are enveloped in tissue that supplies oxygen to the eggs (through diffusion from capillaries) as well as hormones by bathing the small portion of the egg that protrudes from the surrounding tissue. The pouch remains sealed. The male carries the eggs to term (incubation may last from 10 days to six weeks, depending on the species and surrounding temperature), during which time he appears very pregnant. The male actively expels the young (by forcefully moving back and forth) over a period of a few hours through an opening at the top of the pouch. Usually, about 100 young are born in this manner, but some species produce up to 400 young, whereas others produce between 10 and 50 (measuring close to 0.4 in, or 1 cm); pairs may have several broods in one year. The young do not receive any further parental care and are on their own immediately after birth. Young may congregate, and their sexes can be distinguished after a few months, when the pouches of the males become apparent (i.e., when they become sexually mature). Young resemble adults shortly after birth. As many as 1,000 young are produced per year by each couple in this manner, although the actual number varies between species, as reproduction may occur continuously.
Many pipefish males carry the eggs in the anterior tail region (just underneath the dorsal fin) without enclosing them in a pouch, fertilizing the eggs when they are deposited. The eggs are clearly visible in these cases. Courtship has been observed in detail in the species Corythoichthys isigakius from Japan. In this species, after a couple has consented to mate, the male and female repeatedly circle each other with their heads raised, exposing their undersides while remaining in an almost vertical position. When the male is ready to incubate the brood and the female is full of eggs, they practice egg transfer for several days until the process is actually enacted. The male's tail is flattened laterally as an indication that he is able to receive the eggs. The female places her small, greenish eggs on his tail, pushing them into place so that they remain attached. The eggs hatch after a few weeks. Hatching may be helped by the male, who shakes vigorously until all the eggs have separated. The young then are ready to begin their short pelagic life.
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