Question & Answer Session
Question 4
Helen, age ?, from ? asked "hey I have a question about sharks do they lay eggs and what kind of family do they belong to, like fish or something else please write back thanks"
Well the short answer is that Sharks are fish, all sharks are fish, but not all fish are sharks. Sharks are a special kind of fish, Sharks and Rays are cousins, they belong to a group called "Elasmobranchia", thats why shark fans often call themselves "Elasmophiles".
Some species of sharks do lay eggs, others have eggs that grow and hatch inside of them
and yet other species of sharks give birth to live young and have an umbilical cord and placenta, just like mammals.
Sharks that lay eggs are said to be "OVIPAROUS", meaning they lay eggs.
Sharks that have eggs that develop inside them where the baby shark develops in an egg with yolk to provide it with food then hatch and leave the mother are called "OVOVIVIPAROUS"
Sharks that give birth to live young, feeding the baby with an umbilical cord and placenta so nutrients go direct from the mother's blood to the baby are called "VIVIPAROUS".
In more detail :
OVIPAROUS - Sharks that lay eggs - You might have seen "murmaid's purses" on the beach, these are dried empty egg cases of sharks like catsharks (called dogfish in the UK) or hornsharks. The mother lays the egg in a crevice in the rock or around the base of a piece of kelp. The hornshark's eggcase is screw shaped to wedge in a crevice when its pushed in its soft, then it hardens so it won't move with storms, catsharks' egg cases have sticky tendrils that wrap around the kelp. These sharks take between 6months and 2 years to develop in the egg.
Here's a picture / diagram with a cutaway look inside one of these eggcases attatched to a piece of kelp, there's another one with the top still on it. Be careful not to switch the light on or talk to loud with the lid off we don't want to wake the baby. I drew this one specially to answer this question...man I'm too good to you guys!!
The yolk sack provides nourishment to the baby shark as it grows. The egg case feels like leathery plastic, they feel very much like seaweed themselves. When we find them washed up on the beach they're normally dried out and empty, the shark having hatched.
Many sharks are oviparous such as catsharks, hornsharks and swellsharks. The mother looks for a safe, secluded place to lay the eggs, but a place that still has a good flow of water to ensure the baby gets enough oxygen. When they are born they've used up all their yolk and are fully developped and ready to hunt.
OVOVIVIPAROUS - Sharks that have the eggs & yolk sacks develop inside them and then give birth to live young. About 70% of sharks are ovoviviparous. Essentially the mother acts as the safe place for the baby to grow, it feeds off its yolk sack, then its born fully developped and ready to hunt. Whale sharks, Tiger sharks, Bull Sharks are all ovoviviparous. The Bull shark (and its alleged the Tiger Shark) engage in what's called interuterine cannabalism where some of the embryos feed on the less developped embryos while inside the mother.
VIVIPAROUS - Sharks that have a placenta almost exactly like mammals where nutrients can be exchanged between the mother and baby. Interestingly some of these sharks have even more in common with human reproduction than internal fertilization and a placenta, they even have an almost identical gestation period and the same reproductive hormones (chemicals that send signals in the body).
At a later date I'll take more time to describe the second two in more detail
and with some more diagrams.
All sharks produce only a few babies at a time are laid but they are very hardy eggs that are well protected with a lot of energy put into the production / growth of the baby (either inside or outside the mother)
