• SharkieSophie

What Came First, The Shark or The Egg?

Updated: Nov 12

Something that has long-baffled scientists about sharks, is how incredibly diverse their reproductive strategies are. Some sharks lay eggs... others give birth to live young... some do something in between. Some sharks have huge litters, some only one pup and others are literally pregnant all of their adult life! Even close relatives can have completely different ways of breeding. And at the centre is an on-going debate about what came first.. Did the first sharks lay eggs or have live births? Which evolved earliest? And why is each shark so different?


Female lemon shark giving birth to live offspring, with placental cord still visible (Image Source: www.youtube.com)

There are a myriad of different ways that sharks reproduce and for many species and there are not really two species that breed exactly the same way. This has allowed sharks to evolve into many different evolutionary niches and to breed successfully in many different situations. Rather than different categories, there is more of a continuum of different ways that sharks reproduce and every species is unique. Yet there are some broad terms we can use to describe the different methods:

Oviparity

ōvum (Latin) "egg" + perə (Proto-European) "to bring forth"


Oviparity in the swell shark (Ebert et al, 2021)

Oviparity involves a female shark laying fertile eggs after she has mated with a male. The eggs then develop, before young sharks hatch out. Around 40% of sharks are oviparous. They are mostly small, "benthic" species (that live on or near to the ocean floor), like catsharks, skates, all bullhead sharks and some carpet sharks (Abel & Grubbs, 2020, Ebert et al, 2021).


The benefits are that this type of reproduction is not too costly to the mother - she does not need to invest a lot of time or energy (Abel & Grubbs, 2020, Ebert et al, 2021).


Heterodontus shark egg case (Image Source: BMCL / Shutterstock

On the other hand, this means that she also cannot protect the young from predators and mortality rates can be very high. This is especially true in sharks that breed by "immediate oviparity"; where the female only gestates eggs for a short period of time before laying them. Slightly less so in sharks with "delayed oviparity"; where gestation periods are longer and female investment is higher (Abel & Grubbs, 2020, Ebert et al, 2021).


Many scientists believe that egg-laying was the ancestral reproductive method for sharks; that is to say it evolved first and then other methods appeared later in evolutionary history (Musick & Ellis 2005, Abel & Grubbs, 2020).


Shark eggs (Image Credit: Simon Infnager @photosimon, Source: https://unsplash.com)

Ovoviviparity

Also known as "aplacental viviparity" or "yolk sac viviparity"

ōvum "egg" + vividus "to live" (Latin) + perə (Proto-European) "to bring forth"


Between egg-laying and live young, there are sharks which do a bit of both; hatching eggs internally to then give birth to live young. Around 40% of sharks are ovoviviparous, including dogfish, angel, frilled, gulper, sleeper and cow sharks, and also torpedo rays, guitarfish and wedgefish (Abel & Grubbs, 2020, Ebert et al, 2021).


Ovovivparity of the piked dogfish (Ebert et al, 2021)

In oviviviparous reproduction, the egg hatches internally and the young sharks are nourished by a yolk sac whilst inside the mother. This is known as "lecithotrophy" (Greek - lecithin "the fat found in egg yolk" + trophos "feeder"). This means there is limited input from the mother after she has produced the eggs. (Abel & Grubbs, 2020, Ebert et al, 2021).


The benefit of ovovivipary is that the pups are protected inside the female for a longer period compared to ovipary and the cost to the mother is still relatively low compared to vivipary (Ebert et al, 2021).


Sarawak swellshark embryo (Nakaya et al, 2020).

Viviparity

Also known as "placental viviparity"

vividus (Latin) "to live" + perə (Proto-European) "to bring forth"


Finally, around 18% of sharks give birth to live young, which must be nourished by the mother after thay have completely absorbed their egg case (Ebert et al, 2021).


Viviparity in the bonnethead (Ebert et al, 2021)

There are a few different forms of viviparity, where the developing young are nourished in different ways. Some species, like the great white shark (Carcharodon carcharias) and their relatives, keep producing eggs throughout their whole pregnancy, which the developing embryos eat! This is known as "oophagy" (Greek - oon "egg" + phagos "to eat"). It requires energy from the mother to feed the young in this way (Abel & Grubbs, 2020, Ebert et al, 2021).


The sandtiger shark (Carcharias taurus) goes one step further and after the young have chomped their way through the other eggs, they move on to eat their live brothers and sisters for nourishment! This is known as "intrauterine cannibalism" (to learn more, you can check out Shark Tartare) (Abel & Grubbs, 2020, Ebert et al, 2021).



In other species the cost to the mother is even higher - known as "matrotrophy" (Latin/Greek - mater "mother" + trophos "feeder"). For example, some sharks nourish their young by secreting protein- and fat-rich milks from the wall of their uterus. This is known as "histotrophy" (Greek - histo "body tissue" + phagos "to eat"). In other species, the developing embryos attach themselves to the uterus wall via a cord. This is not like the ambolical cord in humans, as it does not connect to the mother's blood stream. Instead, the pups are nourished through the placenta, hence the name "placental viviparity" (Ebert et al, 2021).


Viviparity is very costly to the mother because she has to invest a lot of time and her own energy into nourishing her young as they grow. Some sharks can be pregnant for years and require a long recovery period before they can breed again. On the other hand, the pups are born big and strong, so they have a very high chance of survival, which makes the investment worth it (Ebert et al, 2021).



So, which came first, live sharks or eggs?

Scientists always thought that viviparity arose recently in the evolutionary history of sharks. However, some scientists have argued that, in fact, having live young was the ancestral reproductive method for sharks, and egg-laying arose later on. They have suggested that egg-laying evolved recently, as it allowed small-bodied species to produce more young more rapidly, but that subsequently, some larger sharks then reverted back to the ancestral form of live-birth (Abel & Grubbs, 2020, Ebert et al, 2021).


Different reproductive methods of closely related shark species (Musick & Ellis, 2005)

If we look at the shark family tree (known as a "phylogenetic tree"), we can see that giving birth to live young (via ovoviviparity or true viviparity) is the most dominant method, with 60% of sharks breeding this way (Musick & Ellis 2005, Abel & Grubbs, 2020).

If we are to believe that egg-laying came first, we would expect to see this type of reproduction in the oldest species of sharks. However, this is not the case! the "Hexanchiformes" (six-gills and cow sharks) are thought to be one of the oldest evolutionary lineages of sharks, around for as long as 190 million years, yet they are viviparous. The same pattern can also be found in the phylogenetic tress for rays (aka "Batoids"). Whatsmore, if we consider fossils of ancient species of extinct sharks, we find carpet sharks ("Orectolobiformes") that gave birth to live young! So, this reproductive method has been around for a long time! This suggests that the ancestral form of reproduction was live birth and egg laying came later (Musick & Ellis 2005, Abel & Grubbs, 2020).

On the other hand, there are many scientists that argue that egg-laying came first and birthing live young evolved more recently. They argue that because some of the most ancient known species of sharks, like Cladoselache, were egg-layers, that this is proof that at one time, all sharks and their ancestors were oviparous. Also, as the modern bullhead sharks (Order "Heterodontiformes") are most closely related to the ancient "Hybodont" sharks, which lived around 240 million years ago, and they all lay eggs, many think it is just logical that egg-laying came first (Abel & Grubbs, 2020).


We may never know the answer for sure... But it's a pretty exciting mystery to try to unravel! But one thing is for certain, the myriad of different ways that sharks reproduce is truly amazing!



To learn more about ancient sharks you can check out The Good, The Bad and The Ugly and I Will Survive!



References

Abel DC & Grubbs RD (2020). Shark Biology and Conservation: Essentials for Educators, Students, and Enthusiasts. Johns Hopkins University Press, Canada. IBAN: 9781421438368.


Ebert DA, Dando M& Fowler S (2021). Sharks of the World: A Complete Guide, Second Edition. Princeton University Press: UK. IBAN: 978-0-691-20599-1.


Musick JA & Ellis JK (2005). Reproductive evolution of chondrichthyans. In: Hamlett W (Ed.), Reproductive Biology and Phylogeny of Chondrichthyes: Sharks, Batoids, and Chimaeras, 45–79. Science Publishers, Plymouth, UK. Access online.


Nakaya K, White W & Ho H (2020). Discovery of a new mode of oviparous reproduction in sharks and its evolutionary implications. Scientific Reports, 10:1. Access online.


By Sophie A. Maycock for SharkSpeak.

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