The reproductive system of the seahorse

By Charley Malligan

Habitat of the seahorse

Seahorses are typically located in the shallow tropical waters that are quite warm. They can be found living along the coral, the sea grass and mangroves that provide protection and are close to the shore. Many of the larger species live in the Mediterranean Sea. The seahorse lives in areas that have still water or slow moving water. This is because they can’t swim very well and are slow moving. They have to anchor to various forms of coral or grass to be able to rest. The coral where the seahorse lives offers them a place to rest. Approximately 25% of seahorses will spend their entire life in the coral. Destruction has caused very low numbers of seahorses in some areas. Water pollution is one of the main reasons why the seahorse habitat has continued to be in trouble. Approximately 50% of the population of seahorses in the world declined from 1990 to 1995. This was a combination of natural factors as well as the efforts of humans.


Seahorse reproduction is unusual because the male is the one that becomes pregnant. Seahorses are monogamous which means that they only mate with one partner. Monogamy is particularly rare for fish so this is yet another trait that makes the seahorse unique. Female seahorses generally have vibrant colouring and are larger in size in order to attract a mate. Male seahorses often have more bland colouring and are smaller in size.

Males are equipped with a special birthing pouch. The male becomes pregnant when a female deposits her eggs into the pouch which takes about 5-10 seconds. The female can transfer a range from 10-1000 eggs into the male. The average diameter of the egg is 1.5 mm. Seahorse fertilisation is internal but strangely, the males excrete their sperm into the sea water where they must find their way into the pouch before the egg is transferred as then the pouch closes and is sealed. Male seahorses only produce a small amount of sperm compared to humans but still give birth to 100's of babies.

The mating pair entwine their tails and the female connects a tube called an ovipositor to the male's pouch. She does this several times for short intervals to avoid becoming exhausted. In between, the female rests and the male contorts himself  to get the eggs in place in his pouch. In the pouch, each egg attaches itself to the pouch wall which contains a fluid that insures that the eggs receive oxygen and nutrients. After the egg has been fertilised, the male moves away and attaches himself by his tail to a nearby plant so that he can rest. The eggs remain in the pouch until they hatch, then tiny baby seahorses emerge from the pouch.

The fact that the males are pregnant in this process is an advantage in reproduction because the load of reproducing is shared between two partners. While the male is bearing the young, the female can prepare more eggs to implant into the male once he has given birth. Some seahorses can give birth in the morning and be pregnant again by evening.


The embryo of a seahorse develops in the pouch of the male, this means that development is internal. The pouch protects the developing embryos and provides them with oxygen through tiny tubes. Seahorses are ovoviviparity which means that they lay eggs that hatch within the body and they then give birth to live young. This strategy works to the advantage of the habitat that they live in because if they were to lay there eggs externally, they would would not be protected and may float away in the water because they are so small. Having the eggs develop internally, allows the eggs to be protected from the threats that this habitat presents. The gestation period for seahorses is 2-5 weeks depending on the temperature of the water (the higher the temperature, the shorter the gestation period). During gestation the male will become more and more unsociable, and his pouch will grow fuller (as seen in the image below).

Parental Care

Seahorses do not nurse their young, as soon as the male gives birth to them they are left to fend for themselves. The infant mortality rate for seahorses is extremely high with very few living long enough to become mature and mate on their own. This is most likely due to the small size of the offspring and having to fend for themselves and find food to survive. Seahorses are in the r-selection category because they reproduce quite quickly and in large numbers so that at least some offspring survive. I think that there is no parental care because of the large number of offspring, even without parental care, at least some offspring will survive and so the parents can spend that time producing more offspring. A disadvantage is that not a lot of the babies will survive and it is unlikely that a lot of them will not make it to adulthood.   


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