The Weird World of Shark Eggs

You may have seen a fierce and violent shark attacking humans in movies, but very few of us have encountered a shark in real life. There are over 500 species of sharks, each inhabiting unique environments and displaying diverse shapes and characteristics. However, the mystery which we discuss in this article revolves around shark eggs. Read on to learn and explore new things about shark eggs. 

Do Sharks Lay Eggs? 

A number of species of sharks can lay eggs. But the hatching mechanism can be different from what traditional birds’ hatching process looks like. Similarly, not all sharks lay eggs; in fact the majority of them give birth to intact baby sharks. Scientists have classified shark reproduction methods into three main types. 

Sharks Reproduction Methods

Sharks have three main methods of reproduction: oviparity, viviparity, and ovoviviparity.

Oviparity

Some species of sharks lay eggs. This is known as oviparity, and those laying eggs are called oviparous sharks. Some of the prominent sharks in oviparous category include Zebra shark, Horn shark, Catshark, Port Jackson Shark, Swellshark, Angel Sharks, Tasselled Wobbegong, and Carpet sharks. 

Where are Oviparous Sharks Eggs?

The eggs of oviparous sharks are often encased in a leathery protective case sometimes called a “mermaid’s purse”. These egg cases contain a yolk sac which provides nourishment for the developing embryo. The mother shark will deposit the egg cases in a safe location, like crevices or buried in the sea floor. Once the baby sharks, called pups, are fully developed, they hatch from the egg case and are immediately independent. Notably, mother sharks that use this method do not guard their eggs after laying them.

Viviparity

Sharks that give live birth to pups are viviparous and this method of reproduction is called viviparity. The pups (baby sharks) develop inside the mother and are nourished by a placenta, which delivers oxygen and nutrients from the mother to the pups. Viviparous pups are born live and fully formed.

Ovoviviparity

Many sharks use a combination of the two methods, called ovoviviparity. The pups develop from eggs inside the mother shark but are not connected to her by a placenta. Instead, they feed off the yolk of their egg and possibly other eggs or siblings in the uterus. Hence, the pups born through ovoviviparity are fully formed and need no further nourishment. 

What Does a Shark Egg Look like? 

There are different varieties of shark eggs and they look different from each other, depending on the eggs belonging to which species of shark. However, the following are some examples of what shark eggs look like, depending on the species:

Horn Shark (Heterodontus francisci): Horn sharks lay spiral-shaped eggs, which the female then wedges into crevices to keep them safe from predators.

Port Jackson Shark (Heterodontus portusjacksoni): Like their relatives the horn sharks, Port Jackson sharks also lay distinctive spiral-shaped eggs.

Catsharks (Scyliorhinidae family): Many sharks in this large family, including the small-spotted catshark (Scyliorhinus canicula), lay eggs. These eggs are typically encased in tough capsules with curly tendrils at the corners that help secure them to seaweed or other underwater structures.

Bamboo Sharks (Chiloscyllium): These are small, tropical sharks that lay eggs, which they secure to the sea floor.

Swell Sharks (Cephaloscyllium): These sharks are also oviparous and their eggs are often found washed up on beaches.

How Many Eggs Does a Shark Lay at a Time

The number of eggs a female shark lays at a time can greatly vary between species and even between individuals. Typically, oviparous sharks often lay anywhere from one to two eggs at a time but may do this multiple times a year. For example, the small-spotted catshark can lay two egg cases every couple of days during its breeding season, while the nursehound, another type of catshark, may lay up to 50-100 eggs per year.

Each egg case typically contains one developing shark pup, although in rare instances two pups might share an egg case.

How Many Eggs per Breeding Season?

The average number of eggs laid by sharks per breeding season can greatly vary among different species of sharks. Typically, oviparous (egg-laying) sharks produce fewer offspring at a time than viviparous (live-bearing) sharks.

As an example, the small-spotted catshark can lay around 100 to 200 eggs in a single breeding season. Another species, the Port Jackson shark, might lay only about 10 to 15 eggs in a breeding season. Similarly, depending on the species, Bamboo Sharks can lay between 100 to 150 eggs per year.  

Keep in mind that the survival rate for shark eggs is relatively low due to predation and other environmental factors, so laying a large number of eggs is a strategy to ensure that at least some offspring survive to adulthood.

What is the Survival Rate of Shark Eggs? 

In general, it is estimated that only a small fraction of the eggs laid by any given shark will survive to hatch into pups. The survival rate can be as low as 10% or even less in some cases, underscoring the challenges faced by these eggs. Moreover, it is worth noting that some sharks, including some catsharks, exhibit a unique behavior called “intrauterine cannibalism” or “adelphophagy” where developing embryos consume their unhatched siblings in the womb, reducing the number of viable offspring but potentially increasing their chances of survival. 

Do Sharks Care for Their Eggs? 

Sharks that lay eggs (oviparous sharks) do not hatch them in the sense that they do not actively care for the eggs until they hatch. Instead, they deposit the eggs in a safe location and the pups are left to develop and hatch on their own.

Which Type of Shark is Friendly? 

Not all sharks are aggressive toward humans; for instance, Nurse Sharks are generally more docile and often tolerate human presence near them.

Do All Shark Eggs Survive to Develop into Pups?

No, not all shark eggs will survive to develop into juvenile sharks. Many eggs are eaten by predators, or the embryo inside may not develop fully. This is one reason why sharks lay multiple eggs or have multiple pups.