Don't Put All Your Eggs in One Basket
Updated: Apr 27, 2021
One of the things that is most fascinating about sharks, is how remarkably varied their reproductive methods are. Shark reproduction is far more diverse compared to other animal groups. Despite being closely related, some sharks give birth to live young (known as "viviparity"), whilst others lay eggs ("oviparity"), where others hatch eggs internally, to then give birth to live young ("ovoviviparity"). Even when we talk about only oviparous sharks, there is still an incredible range of contrasting egg cases and reproductive strategies, employed by different sharks.
It is thought that egg-laying sharks came first in evolutionary history and live-bearing sharks evolved more recently. Many members of the order Carcharhiniformes (the ground sharks), which are the youngest in evolutionary history, are viviparous. Yet, there are still many species of egg-laying sharks around today, in many different lineages. All members of the order of bullhead sharks (Heterodontiformes) are oviparous and egg-laying can also be found in the carpet sharks (Orectolobiformes) and the ground sharks (Nakaya et al, 2020).
But even oviparous sharks all have massive differences in their reproductive methods. If you take a glance at the egg cases of different sharks, you can see immediately how different they are - varying in size, shape and colour.
Whatsmore, the way that the eggs develop within the mother can vary from species to species. Some oviparous sharks reproduce by "single oviparity". This means that the female has a single egg case in each of her two "oviducts", which are only retained for a very short time before being laid. This means a pregnant female will only carry two embryos at any one time, but she can repeat this process, in order to have dozens of offspring within one spawning season. The egg cases of these sharks are commonly thick and strong, and have long tendrils which can be used to attach the egg case onto the seabed (Nakaya et al, 2020).
Comparatively, sharks which reproduce by "multiple oviparity" will have numerous egg cases during each pregnancy. These eggs are retained within the female for much longer and so they develop more extensively before they are laid. The tendrils of these egg cases tend to be shorter and finer (Nakaya et al, 2020).
However, recently a group of scientists in Malaysia discovered that shark reproduction might be even more diverse than we previously thought! They discovered that the Sarawak swellshark (Cephaloscyllium sarawakensis), did not fit into either the single oviparity or the multiple oviparity category. Instead, these sharks had a single egg case in each oviduct (like single oviparity), but also retained the eggs for an extended period of time, to allow the embryo to develop more fully (like multiple oviparity) (Nakaya et al, 2020).
The eggs of these sharks had a tough, thick shell, very long, strong tendrils for attachment and a smooth surface which was completely transparent, so the scientists could clearly see the yolk and the developing embryo inside (Nakaya et al, 2020).
The researchers concluded that this shark has a different method of reproduction than it's close relatives, so they coined a term to describe it; "sustained single oviparity". They hypothesised that retaining the eggs within the mother for an extended period is advantageous for this species, because it offers the offspring added protection from predators. Upon depositing the eggs, the transparent egg case is perfectly adapted, as the patterned, pigmented skin of the small sharks camouflages them against the seabed. They also noted that, whilst this reproductive strategy limits how many offspring each mother can have during one spawning season (known as "low fecundity"), it allows the offspring to be born at a relatively large size (around 12cm long). As this species must only reach around 35 cm length to be sexually mature, they are able to maintain a relatively high "lifetime fecundity", by rapidly growing to maturity and reproducing many times throughout the course of their lives (Nakaya et al, 2020).
With babies this weirdly cute... who wouldn't want to have lots of them!