Marine Biology

Reproduction in Seahorses



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Seahorses are fascinating creatures, not simply because they are really strange looking fish but because it is the males that become pregnant not the females. They are also monogamous, which is very rare in fish. This is thought to be because seahorses that stay with one partner they form a team that gets better at producing young. Some species only mate for one season and others don’t have their own mate but do mate only within their own small group. The mating season for seahorses is from May to August.

Distinguishing Sex

To attract mates, female seahorses are usually bigger than the males have more vibrant colours. Males are smaller and have more muted colouring. The most obvious difference in live seahorses is the birthing pouch on the males, just below his chest area, which looks round and very smooth whilst the females have a rough, pointed area in the same place.

Competition for Mates

The male seahorses compete among themselves for a mate when the mating season begins. To attract a female they will fight each other, slapping each other with their snouts repeatedly and wrestling with their tails.

Courtship

Before mating, males and females court for several days. Just before dawn they start rituals that look as if they are dancing. The will also swim side by side at the same speed as if they want to mirror each others movements.

Mating

During mating, the female inserts her oviduct into the male’s brooding pouch whilst entwining their tails. The female lays between 250 to 600 eggs at one go. The female does this a few times for short periods with rests in between so as not to exhaust herself. Whilst the female is resting, the male twists himself around to get the eggs into position in the brood pouch. When the eggs are in the pouch, they are fertilized by the male and the sack is sealed tightly. The eggs are then the total responsibility of the male. After mating has finished the male finds a nearby plant and attaches himself to it with his tail and the female has to wait a few hours for her oviduct to go back.

Pregnancy

The male seahorse has a pregnancy in just the same way as females of other species. The eggs are attached to the male by a capillary network which transfers nutrients to them. The male makes sure that the salinity in the pouch is the same as that of his surrounding so the young are ready to go into the salty seawater. The length of the pregnancy depends on the species of seahorse and can last from about two weeks to about six weeks. In some species, the female will check on the male every day, whilst others leave them completely alone. The males can become very aggressive when they are pregnant.

Birth

Just before the birth, the male will puff up and become very large, probably to put off predators as he has no other defences against them. Birth usually takes place at night but can occur during the day. In the preparation for birth, the pouch of the male seahorse extends and becomes almost spherical in shape. The labour and birth can last for several hours before the young are born. For about ten minutes, the male seahorse suffers from muscular spasms, bending backward and forwards. The babies are born in an explosive action. Baby seahorses are born looking the same as their parents but smaller. When the birth is over, the male’s pouch goes back to normal over the next hour and a few hours later he is ready to mate again. The male seahorse can be pregnant for most of their lives.

The fry immediately go to the surface of the ocean and attach themselves to anything they find, be it a piece of sea weed or each other and start to feed on small shrimps, other marine organisms and plankton. Brood can vary in size from 8 to 200 depending on the species.

As the young seahorses have to fend for themselves immediately after birth, the mortality rate is very high and less than 1% of them survive to maturity. The average lifespan of seahorse is 4 to 6 years.

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