Let’s Talk About (Shark) Sex, Baby

The cartilaginous fishes, or “Chondrichthyes”, including the sharks, skates, rays and chimaeras are an extremely broad group of over 1,050 species. There are over 500 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...

Pregnant tiger shark (Image credit: Todd Bretl, Source: www.toddbretl.com)

All sharks reproduce sexually (meaning 2 individuals of different sexes produce offspring together) through “internal fertilisation”, but there are several very different methods that the sharks employ. These can be broadly categorised into three contrasting strategies: “oviparity”, “ovoviviparity” and “viviparity” (Parsons et al, 2008).

Oviparity

Producing eggs that mature and hatch after being expelled from the body, aka egg-laying sharks.

Oviparous sharks deposit eggs in a safe spot on the ocean floor. At this point neither parent protects or sustains the eggs. The eggs themselves nourish the growing embryos, through the yolk, and the young will hatch later if the eggs are not taken by predators.

Oviparity is the most primitive mode of reproduction, meaning it evolved earliest.

Oviparity can be found throughout different “taxonomic” groups of sharks, including:

• the “Orectolobiformes” order (zebra sharks (Stegostoma tigrinum (formerly Stegostoma fasciatum), carpet sharks (family “Parascylliidae”) and some bamboo sharks (family “Hemiscylliidae”))

• the “Heterodontiformes” order (horn shark (Heterodontus francisci)).

• the “Carcharhiniformes” order (catsharks (family “Scyliorhinidae”))

Horn shark lodging its egg into rocks (Image credit Jen Thompson, Source www.naroomanewsonline.com.au)

Shark eggs are commonly called “Mermaid’s purses”. They are all covered in a tough, leathery protective layer, but in terms of shape and size, they are extremely diversified. Some egg cases are capsule-like, whereas other are corkscrew shaped, and some have tendrils which are used to attach the egg to the “substrate" for security (Wourms & Demski, 1993).

If you would like to learn more about different egg cases, you can check out Don't Put All Your Eggs in one Basket and there are also excellent resources available at the SharkTrust website.

Ovoviviparity

Producing eggs which hatch inside the body of the mother, where the young further develop and are born alive, aka aplacental viviparity. This strategy differs from “viviparity”, as the pups are not nourished by a “placenta” whilst developing inside the mother.

Ovoviviparity is the next step in evolution between oviparity and viviparity.

This reproductive method is also found across different taxonomic groups of sharks, including:

• the “Lamniformes” order (the great white shark (Carcharodon carcharias), the pelagic thresher shark (Alopias pelagicus) and mako sharks (genus “Isurus”)),

• the sawsharks (order “Pristiophoriformes”),

• the “Triakidae” family (such as the gummy shark (Mustelus antarcticus) and the tope (Galeorhinus galeus)),

• the “Squaliformes” order (the Greenland shark (Somniosus microcephalus))

• the “Squatiniformes” order (angel sharks).

Within this strategy there are also species, such as the, shortfin mako shark (Isurus oxyrinchus), which are “oophagous” (aka “intrauterine cannibalism”), meaning that when the young are hatched, they will gain nourishment by eating other egg cases which are still unhatched inside the mother.

Viviparity

Internal eggs develop into pups, which are nourished through the placenta (via an umbilical cord attaching from mother to pup between the “pectoral fins”), aka live-bearing sharks. The sharks are born as perfect miniatures of their adult form.

Viviparity is the most recently-evolved mode of reproduction in sharks and is now the dominant strategy in “extant” shark species.This reproductive method is prevalent across in the Carcharhiniformes order, but can also be found in the order of Orectolobiformes (whale shark (Rhinocodon typus)).

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 species show they are very similar and yet, they will have entirely different modes of reproduction. For example, the tiger shark (Galeocerdo cuvier), is the only species within the Carcharhinidae family which is ovoviviparous (Swift et al, 2016). This suggests that the different strategies evolved independently and repeatedly, and the changes were gained and lost again in the different shark lineages (we call this “convergent evolution”). 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 shark evolution of reproductive strategies is complex, dynamic and ever-adapting.

References

Parsons GR, Hoffmayer ER, Hendon JM & Bet-Sayad WV (2008). In: Fish Reproduction, (Eds) Maria J, Rocha MJ, Arukwe A & Kapoor BG. CRC Press. 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.

Tomita T, Kiyomi Murakumo K, Ueda K, Ashida H, Furuyama R (2019). Locomotion is not a privilege after birth: Ultrasound images of viviparous shark embryos swimming from one uterus to the other. Behavioural Note, 125, 122–126. 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, 7-21. Access online.

By Sophie A. Maycock for SharkSpeak.