In contrast to bony fish, sharks always practice internal fertilisation. For this, they have reproductive organs similar to those of the mammals.
Male and female sharks are easy to distinguish from birth, due to their external reproductive organs. Male sharks have paired reproductive organs, called claspers, which developed from the inner rim of the pelvic fins. A tube runs through either clasper; its front opening is called apopyle, the rear one hypopyle. In front of the actual clasper base, in the front area of the pelvic fins, there are two muscular, ampoule-like siphon sacs.
Together with the claspers, they are important for insemination. A small tube leads from the siphon right to the apopyle, the “entry” of the clasper tube.
In sharks, the siphon is filled with sea water that is then ejected under pressure as an injection. This sea water is used to wash the semen, which is already present in the tube during copulation, into the female cloaca. Only one of the two claspers is used for copulation. The time of sexual maturity of male sharks can be determined by the hardness (hardening from calcium deposits) of the claspers. In young sharks, the claspers are soft and flexible, but in sexually mature animals, calcium deposits have made them very hard and inflexible.
In some shark species, such as the blue sharks, sperms can be enclosed in small packages, so-called spermatophores, and stored for a little while. The shark’s testicles are deep inside the shark’s body – below the spine at about the height of the first dorsal fin. Spermatogenesis (formation of sperm) happens in the testicles, from where the sperm is brought to the two claspers through the spermatic duct.
Female sharks only have a cloaca, i.e. an opening that is used for reproduction as well as for excretion of dung and urine.
Copulation behaviour in sharks was only observed in a very few species and is, in general, not well researched yet.
However, it is assumed that it is very similar for most species. One unsolved question regarding certain solitary species is the one of how animals willing to copulate will find each other in the vast seas, in particular if they are migratory
rather than living only in a small area.
Where a male and a female shark of the same species meet and are willing to copulate, they first need to indicate this willingness to the partner. The male then tries to turn the male into the right position
(usually belly to belly), which is not always possible without using violence and may cause bite marks and abrasions in both partners. Then the male inserts a clasper, which may be equipped with a hook to attach inside the cloaca.
Even before mating, the male aligns his claspers forwards using his pelvic fin muscles.
When sharks mate, the sperm is washed into the female’s cloaca. The sperms fertilise the eggs either at once or can be stored for a while – in the case of small-spotted catshark (scyliorhinus canicula) even up to two years.
The oocytes are formed in the two ovars. Depending on type, either both ovars are functional (oviparous species), or only the right (e.g. carcharhinides and hammerhead sharks) or only the left one (most rays).
Mature, unfertilised oocytes are brought to the infundibulum, where the eggs are fertilised, by the oviducts. In the middle section of the oviducts, the shell gland, the eggs are covered in a protective shell.
The section behind this stores the eggs until they are laid. Viviparous species, i.e. livebearers, produce no egg shell.
Instead, the young develop in the uterus (again, depending on species, one or both sides are functional) and are nourished through a placenta-like organ.
Types of reproduction
In the course of evolution, sharks developed different reproductive mechanisms.
|Only about 30 % of all shark species are oviparous, i.e. lay eggs - like, e.g., turtles – and leave the young, which will hatch some time later, to themselves.
In the laid egg of the catshark, the tiny animal and the comparatively huge yolk are clearly visibly.
Shortly before the shark hatches, the yolk supply has declined and the animal has grown a lot.
|Live-bearing sharks can be categorised by two different courses of pregnancy.
Placental pregnancy in sharks is very similar to pregnancy in mammals. This means that a placenta develops with the fertilised eggs embedded in it. The young develop within the mother’s body and hatch through the cloaca’s birth channel when they have reached a certain size.
The embryo grows in the female’s uterus without a yolk-sac and is nurtured by the mother’s metabolism. Right after birth, the young sharks are independent from their mother and start looking for food in their own. Live-bearing species are, e.g., blue sharks.
This term is used to describe a mixed form of oviparous and viviparous reproduction. In this case, eggs are not laid but remain within the uterus.
Here, the sharks grow in their egg hulls and are then also ejected through the birth channel.
In sand tigers, the young will first eat their weaker siblings or unhatched eggs after hatching before they are ejected from the uterus.
For a shark, birth is never without danger, since the animals are only able to move and defend themselves within limits in this period. Therefore, female sharks usually find protected places away from other members of their species or larger predators. Protected environments such as lagoons, sea grass fields, etc. usually also are where the young sharks spend their first years.
Since sharks do not take care of their brood, the young sharks are on their own from the very first and may even have to avoid their own mother, who might consider them food.