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Let’s Talk About (Shark) Sex, Baby

The cartilaginous fishes, or “Chondrichthyans", including the sharks, skates, rays and chimaeras are a broad group of over 1,000 species - there are over 540 species of sharks alone! As well as differences in morphology, this diversity also takes the form of different diets, habitats, behaviours, distributions; also known as different “life-history strategies”. One area where the sharks are especially diverse is in their reproductive methods. So how do sharks breed? Why are there different methods? And are there any similarities across all the species?



A Beautiful and Unique Snowflake

There are a myriad of different ways that sharks reproduce and there are not really two species that breed exactly the same way. If you compare this to other animals it becomes very apparent how astounding this is (Abel & Grubbs, 2020; Ebert et al, 2021).

There are several different reproductive methods employed by sharks and rays, with each popping up repeatedly in different lineages (Katona et al, 2023)

For instance, if you look at boney fishes, a huge majority of species reproduce by "spawning" - with females laying eggs and males coming in to externally fertilise them. The embryos develop within these eggs, thanks to nutritional resources stored in the yolk and then hatch out into the world as larva, that eventually morph into their adult shape. This is a strategy that is conserved quite broadly throughout the fish world. Consider too, the mammals. Barring only a few monotremes, like the platypus, and marsupials like kangaroos and opossums, mammals tend to all breed in pretty similar ways. A male and a female mate, there is internal fertilisation when egg meets sperm and then the mother carries the baby as it develops, to eventually give birth to live offspring (Abel & Grubbs, 2020; Ebert et al, 2021).


Shark reproductive methods are much, much more diverse. Rather than separate categories, there is more of a continuum of different ways that sharks reproduce and every species is unique. In order to simplify everything a bit, we can describe shark reproduction in two broad groups: those that lay eggs (known as "oviparity") and those that give birth to live young (known as "viviparity") (Abel & Grubbs, 2020; Ebert et al, 2021).


However, within these groups each species will carry their young for a different length of time, nourish the developing pups in a unique way and deliver them into the world in various ways. In some sharks the eggs hatch internally (known as "ovoviviparity"), in others they are laid. Some developing pups only rely on yolk (known as "yolk only viviparity"), others eat unfertilised eggs (known as "oophagy"), some suck up interuterine milk, where others are connected to the mother (known as "placental viviparity"). It's enough to make your head spin! (Musick et al, 2005; Abel & Grubbs, 2020; Ebert et al, 2021; Katona at al, 2023).



A Mixed Bag

It is interesting to note, that reproductive strategies are all found across different taxonomic groups, meaning that even very closely related species may have developed different modes of reproduction. Genetic comparison of closely related sharks have shown they can be very similar species and yet, they will have entirely different modes of reproduction (Abel & Grubbs, 2020; Ebert et al, 2021).

This pregnant female bull shark will give birth to live young, rather than laying eggs (Image Credit: Steve Barze / Shutterstock)

For example, the tiger shark (Galeocerdo cuvier), is the only species within the Carcharhinidae family which is ovoviviparous (also known as "aplacental viviparous") - where the female's eggs hatch internally and then continue to develop inside the mother before being born (Musick et al, 2005; Swift et al, 2016; Abel & Grubbs, 2020; Ebert et al, 2021; Katona at al, 2023).


Within the Chondrichthyans group as a whole, it is estimated that viviparity evolved as many as 18 different times across the different lineages (Wourms & Demski,1993). This shows us that the different strategies evolved independently, and the changes were gained and lost again in the different shark lineages (we call this “convergent evolution”). Which method came first is still up for debate (Musick et al, 2005; Abel & Grubbs, 2020; Katona at al, 2023). To learn more check out What Came First, The Shark or the Egg?



Let's Get Together

The most notable feature which is conserved across all sharks, is that they all breed by "sexual reproduction" (Just to be awkward there are a small number of chondrichthyans that are also capable of asexual reproduction. To learn more, check out Like A Virgin and Don't Need No Man!). This means that a male and female combine their genetic material, with half of the chromosomes (aka "chromatids") coming from the father and half from the mother. Males produce sperm cells (called "spermatozoa") and females produce eggs ("ova"). These "gametes" have half the normal amount of genetic material as regular body cells, so when they combine, they create a viable cell with the right amount of DNA (Abel & Grubbs, 2020).


Tiger sharks, unlike all their closest relatives, are ovoviviparous (Image Credit: Daniel Lamborn / Shutterstock)

In contrast, there are many, many species (mostly microbes) that breed "asexually". In these cases there are not two parents. Instead the single-celled organism duplicates its genetic material and then divides itself in half, donating a full set of chromosomes to each daughter cell. This method is quick, but as all of the new generation are clones of the parent, these organisms must swap and share genetic material amongst themselves in other ways, in order to shake up their diversity. Otherwise, they run the risk of their genetic similarities leaving the population vulnerable to threats (Abel & Grubbs, 2020; Ebert et al, 2021).


The mixing of genetic heritage means that all offspring are physically and genetically dissimilar from both of their parents, so every single generation of sharks is slightly different from the last. The genetic variation which arises between individuals over the generations, is what has allowed the sharks and rays to adapt to so many different ecological niches, in various habitats all around the world, and to survive for over 400 million years (Abel & Grubbs, 2020; Ebert et al, 2021).



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.


Katona G, Szabó F, Végvári Z, Székely Jr T, Liker A, Freckleton RP, Vági B & Székely T (2023). Evolution of reproductive modes in sharks and rays. Journal of Evolutionary Biology. Access online.


Musick JA, Ellis JK & Hamlett W (2005). Reproductive evolution of chondrichthyans. Reproductive biology and phylogeny of chondrichthyes: sharks, batoids and chimaeras, 3. Access online.


Swift DG, Dunning LT, Igea J, Brooks EJ, Jones CS, Noble LR, Ciezarek A, Humble E & Savolainen V (2016). Evidence of positive selection associated with placental loss in tiger sharks. BMC Evolutionary Biology, 16, 126. Access online.


Wourms JP & Demski LS (1993). The reproduction and development of sharks, skates, rays and ratfishes: introduction, history, overview, and future prospects. Environmental Biology of Fishes, 38. Access online.



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