Learning about human reproductivity can be quite a shock to many kids. One minute we’re parcel cradled swingingly by the beaks of gentle storks delivered to our anticipating parents; the next… well, we learn the gruesome true about how close we once were with our mothers. Naturally we might assume it’s the same for all creatures. Then we incubate that pet egg in kindergarden that crumbles to reveal a cutie patotie chickling, subsequently calling our parents terrible people for making us eat scrambled eggs in the morning (which leads to another corrective conversation, naturally). And we continue to learn more about the diverse nature of well-known land creatures, specifically ones that give birth to live offspring and others which lay eggs. It’s that simple.
Not quite. You might have heard your archaeology professor or biology teacher tell you that all fish lay soft eggs. Since you know and understand that sharks are fish too, you’d think this were truth. But not at all! Sharks are so diverse and so complex that as a whole they present multiple methods of reproduction, including live births and egg-laying! In fact, there are three known methods of how shark babies are made: one when eggs (called yolk sacs) stay inside the mama shark until they’re ready to give birth, another when sharks lay embryo sharks in rigid egg cases along the ocean floor that hatch the little fellas when they’re ready, and one other weird and less common one when the yolk sacs turn into placentas with umbilical cords before birth. These are called yolk-sac viviparity, oviparity, and placental viviparity, respectively.
This is a really common method among sharks, such as the orders of some favorites like Six-gilled sharks, Sawsharks and Anglesharks, to name a few. These sharks carry their baby sharks, or pups, in soft, water-balloon-like eggs full-term. The pups are dependent on these eggs, or yolk-sacs, for growth until they are ready to be born and swim away. Just like all of us primates, some (not all) of these pups also get nutrition from their mothers; but some do not. It’s just another way sharks are so diverse!
This type of reproductive service has a few different variations which we’ll go into later (hint: false catsharks! They’re weird!)
This type of shark development involves a whole new set of diversities. Most oviparous sharks lay one rigid, enclosed eggcase at a time (colloquially Mermaids purses) that incubate little shark embryos until they are ready to hatch and swim away. Usually they incubate on their lonesome, but there are cases of embryos incubating in pairs or groups! Check out California Big Skate or Bajaraja binoculata. They grow and hang out in sets of two and more in single eggcases!
In a few cases, some sharks are known to hold the eggcases for most of the developmental stage before depositing them on the rocky substrate of the ocean floor where they hatch to release the newborn sharks. For example the blackmouth catshark can keep up to 13 eggcases inside of her at one time!
Placental Viviparity (not shown)
This is a tricky one. While production starts off like yolk-sac viviparity, in this case the yolk sac transforms to create a placenta and umbilical cord for the gestating sharks when the nutrients from the yolk sac are depleted. The placenta is still kind of “yolky,” unlike mammalian ones. All but one in the family of Carcharhinid or requiem sharks are placental viviparous. This includes blacktip sharks, whitetip sharks, bullsharks and lemon sharks. The odd one out is the tiger shark, which is yolk-sac viviparous. The sharks in this family can give birth to a few or even a dozen offspring. For example, the Lemon shark can have a litter of up to 17 pups! And they carry them for nearly a year. Imagine that.