Reproduction in sharks

2/14/2017

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In this blog post, we will be discussing the different types of reproduction in Chondrichthyans: oviparity (egg-laying), viviparous with yolk-sac or viviparity (live birth).

Many chondrichthyans use the “K strategy” of reproduction, meaning they have a few but well-equipped offspring (shark babies, called pups, do not need parental care once born). This is, however, a slow method of breeding, as gestation periods average between 9-12 months (and some sharks take even longer). Not all give birth to a few pups- some can give birth to hundreds (see the bigeye thresher or whale sharks)! And like other animals in the animal kingdom, many do not give birth to young every year. In fact, some have a resting period between each litter, lasting from 1-2 years.

The actual process of fertilization of the eggs was discussed in a previous post (HOW SHARKS MATE), so we will be covering the growing of the embryo up to the point where it is ready to be born.

what happens after mating?

Once a male has “done the deed” (aka deposited sperm), the eggs become fertilized. Not all sperm has to be used at once, as some females can actually store sperm for future use (e.g. dusky, blue, Atlantic sharpnose, scalloped hammerhead, chain catshark). In fact, some can mate with multiple males, meaning the pups in the same litter can have different fathers.

Can you tell what part of the inside of this egg (a chicken egg) will develop into the embryo (and eventual chick)? It’s not the yellow part (called the yolk; though that was my first guess too), but a tiny microscopic speck on the surface on the yolk- the little white bit! As time goes on, the embryo will start to develop its main organs and systems- usually the heart is one of the first. Soon other organs will begin to take shape as the embryo begins to grow larger.

There are multiple ways the growing embryo can receive nutrients. Embryos that are nourished solely by the yolk are called lecithotrophic; embryos nourished by both yolk and mother are called matrotrophic. Some Chondrichthyans, such as some stingrays, are nourished by the yolk and later histotroph (“uterine milk”) produced by the mother. The nutrients this milk provides to the pups isn’t well studied. But, I have it on good authority that this milk tastes eerily similar to condensed milk. Another way of nourishment is placental viviparity, in which part of the uterine lining secretes a nutritive substance called embryotrophe, which is ingested by the embryo. This only occurs in the Carcharhiniforme order of sharks. Aplacental viviparity is where the embryo ingests the yolk at the beginning until one side is covered in blood vessels that grow with the uterine wall and create a yolk sac placenta. From here the nutrients in the mother’s blood are shared via that placenta.

As the baby continues to grow, it starts to develop a pattern on its skin that helps it best camouflage to its surrounding environment. From here, we will break up the three forms of reproduction. First we have egg-laying:

oviparity

Oviparity is when mother Chondrichthyans (sharks, skates, rays, and chimaeras) will lay tough egg cases by either burying them under soft substrate (think sand and mud) or anchoring them to a structure (such as a rock cluster, coral, kelp stalk) with tendrils or wedging them securely. Eggs are usually laid in pairs.

These young tend to come out smaller due to their limited nourishment (once they run out of yolk, that’s it). They receive oxygen through the water in their surrounding environment, and excrement (body wastes) seeps out. Inching towards maturity, these pups begin to wriggle and rotate around the small confines of the egg. Once ready to be born, the pups can push themselves out of the egg case and into the open ocean.
There are two distinct methods of live bearing:

viviparity via yolk-sac

Most sharks are viviparous with yolk-sac, where eggs are fertilised and hatch inside the female shark but the embryos lack a placental connection to the oviduct or uterus and so do not feed off the mother. These eggs are not as thick as oviparous shark egg cases, staying flimsy. The pups will live on their yolky food supply and develop like oviparous sharks would. After being born pups may stay in the uterus to take advantage of some extra sustenance: unfertilized eggs. The practice of first born pups eating the unfertilized eggs is called oogeny; the practice of the first born pups to eat their live womb-mates is called intrauterine cannibalism.

vivparity (live birth)

The developing pups in an animal that is viviparous allows for the mother to provide nourishment via the nutrient-exchange we discussed earlier (similar to a placenta). Here, blood vessels from the yolk sac (which are attached to the baby shark’s blood system) lie along the mother’s blood vessels; nourishment and oxygen passes one way while waste travels the other way.

Once the pup is mature and ready to be born, they move into the cloacal chamber and out through the vent slit. Most are born tail first, but some can be born head first.

Next in this series is our article looking at the anatomy of shark egg cases.

did you know this all before? what did you learn that's new?

glossary

Aplacental viviparity: Where the embryo ingests the yolk at the beginning until one side is covered in blood vessels that grow with the uterine wall and create a yolk sac placenta. From here the nutrients in the mother’s blood are shared via that placenta.
Dorsal fin: Fin located on the back.
Embryo: A new organism in the earliest stage of development.
Gestation: The period in which a fetus develops, beginning with fertilization and ends at birth.
Histotroph: Uterine milk.
Intrauterine cannibalism: The practice of the first born pups to eat their live womb-mates.
K strategy: Animals have few but well-equipped offspring.
Lecithotrophic: Embryos that are nourished solely by the yolk.
Matrotrophic: Embryos nourished by both yolk and mother.
Oogeny: Eating unfertilized eggs.

Oviparity/oviparous: Where adult Chondrichthyans lay eggcases (also known as ‘mermaid’s purses) and the baby develops inside the egg before hatching.
Pelvic fins: Fins located behind the gills on either side.
Placental viviparity: Part of the uterine lining secretes a nutritive substance called embryotrophe, which is ingested by the embryo.
Pup: Give birth to baby sharks (pups).
Pups: Baby sharks.

Viviparous: Eggs hatch inside the female's body and the babies are fed by a placenta which transfers nourishment from the mother to the babies (via an umbilical cord which is connected to the baby shark between the pectoral fins).
Viviparous with yolk-sac: Also known as aplacental viviparous and previously known as ovoviviparous/ovoviviparity; it is the production of eggs that are fertilised and hatch inside the female shark but the embryos lack a placental connection to the oviduct or uterus and so do not feed off the mother.
Yolk: The portion of an egg that supplies food for the developing animal embryo.