Author: Natasha Hynes
Sharks that give birth to live shark pups (this is called viviparity) do so through three main modes. The first mode is placental viviparity, where the pups grow by feeding on a placenta in the mom (like humans!). The second mode is lecithotrophic viviparity (also called ovoviviparity/aplacental viviparity). This is where the pups feed on a yolk sac but they are still growing inside the mom. The third mode is called oophagy. This is where the pups are growing inside the mom and feeding on extra eggs that the mom has produced. It's thought that oophagous species produce extra eggs to feed a few pups so that they will be big when they’re born. Bigger pups means they could be safer from predators when they're born. But, investing more energy into making the pups big usually means having less pups. Researchers from Canada and the US brought years of information together to compare how each mode affects pup size and litter size.
Scientists use ultrasound to better understand shark reproduction
This image shows a tiger shark pup in the womb. Credit: Sulikowski Lab
The researchers looked for a few key pieces of information in the literature. First, they selected shark species based on what mode of reproduction they use. They tried to select species that were of similar size across all three modes. Once they chose their species, they recorded the following:
1. maximum length of females of this species
2. minimum female length at maturity
3. maximum female weight
4. minimum female weight at maturity
5. minimum and maximum pup length
6. minimum pup weight
7. median litter size
8. maximum litter size
9. how long the female is pregnant
10. what habitat/depth you can find the species in
Armed with all this information, they were ready to make their comparisons.
As expected, the researchers found that oophagous species have fewer pups than the other viviparous species. They also found that oophagous species give birth to pups that are longer than lecithotrophic species (but not always placental viviparous species). Yet, the litters of oophagous species do not weigh significantly more than the litters of lecithotrophic species. This means that even though the pups of oophagous species are bigger, it doesn’t mean that the mom spent more energy growing them compared to other species. This finding was the same for sharks of different families and orders, which means that oophagy developed independently in different species because of the benefit of bigger pups!
When looking at how many pups the species had and how often they had them, the researchers found that species that give birth to less pups at a time might make up for this by having pups more frequently. The example given in the study is the shortfin mako, which can be pregnant for around 2 years (!!!) and have around 12 pups at a time. However, the common thresher has 2-4 pups at a time but is only pregnant for 9-12 months. They also found that litter size and frequency of the same species can change depending on where the population is.
Overall, the study further supported that oophagous species gave birth to fewer (but sometimes larger) pups than placental and lecithotrophic viviparous species, but that having shorter pups doesn’t necessarily mean that the mom spend less energy growing them. Since sharks are known for having very few babies on average, knowing the ins and outs of shark reproduction helps immensely with conservation management measures.
References
Miller, E., Wails, C.N. & Sulikowski, J. It’s a shark-eat-shark world, but does that make for bigger pups? A comparison between oophagous and non-oophagous viviparous sharks. Rev Fish Biol Fisheries (2022). https://doi.org/10.1007/s11160-022-09707-w
NOAA. (2020, November 8). Atlantic Shortfin Mako Shark. NOAA Fisheries. https://www.fisheries.noaa.gov/species/atlantic-shortfin-mako-shark