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The biology of seahorses:

The male seahorse, rather than the female, becomes pregnant. This unusual mode of reproduction is the most extreme form of male parental care yet discovered, although it arises from a general bias towards paternal care among fishes. Eighty-nine of 422 families of bony fish exhibit parental care, with almost half of these (36) being cases of paternal care. (See Blumer, L. S. 1979. Male parental care in the bony fishes. Quarterly Review of Biology. 54: 149-161.)

Sexual maturity in males is usually determined by the presence of a brood pouch. Male seahorses are able to become pregnant any time during the breeding season, which varies with species, and is most likely dependant on temperature of the water. Other factors that may affect the timing of the breeding season are monsoon patterns and the lunar cycle.

Male and female Hippocampus guttulatus entwine their tails     PHOTO COURTESY J.A. RODRIGUEZ
Most species of seahorses are monogamous, forming pair bonds that last the entire breeding season (and perhaps even last over several breeding seasons), although some species may not be pair-bonded. Pair bonds are reinforced by daily greetings in which the female and male change colour and promenade and pirouette together. This dance lasts several minutes, and then they separate for the rest of the day. The greetings occur throughout the male pregnancy, and are even thought to ensure that the male and female are ready to re-mate at the same time. Once the male has given birth and it is time to re-mate, sometimes only hours later, this greeting is extended into a courtship that, for one species, lasts up to nine hours.

The female inserts her ovipositor into the male’s brood pouch, where she deposits her eggs, which the male fertilizes. The fertilized eggs then embed in the pouch wall and become enveloped with tissues. The pouch acts like the womb of a female mammal, complete with a placental fluid that bathes the eggs, and provides nutrients and oxygen to the developing embryos while removing waste products. The pouch fluid is altered during pregnancy from being similar to body fluids to being more like the surrounding seawater. This helps reduce the stress of the offspring at birth.

Pregnancy lasts between two and four weeks, the length decreasing with increasing temperature. At the end of gestation the male goes into labour (usually at night), pumping and thrusting for hours to release his brood. Young are miniature adult seahorses, independent from birth, and receive no further parental care. Newborns of most species measure 7-12 mm. The number of young released averages about 100-200 for most species, but can be a low as five for the smaller species, or as high as 1,500.

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