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Shark Development - From Egg Cases to Live Births!

Sharks have evolved to survive in every ocean around the world! Specialised reproductive modes have helped them be successful. These include egg-laying and live-birth.

Adaptations are the key to sharks survival in every ocean; from the Greenland Shark in arctic waters, tiger sharks in tropical waters, and catsharks in our local seas! One crucial way sharks have adapted to best survive in their environment is through their modes of reproduction. 

All elasmobranchs (sharks, skates and rays) use internal reproduction. Male sharks have claspers which are inserted into the females' reproductive tract, where they deliver sperm to fertilize eggs. This is different from bony fish (e.g. mackerel) who predominantly use external spawning, i.e. gametes of both sexes are released into the water column. Once fertilization in elasmobranchs has occurred, their reproduction mode diversifies into one of three methods; viviparous, ovoviviparous, and oviparous.

There has been evidence of shark pups being born without reproduction, known as Parthenogenesis! The mother donates both gametes, creating “clone” offspring. Whilst this is good in areas where males aren’t present or limited, a low genetic diversity between generations will make the sharks more susceptible to disease.

Spiny Dogfish

(c) Peter Verhoog

Viviparity

Viviparity involves the shark embryo developing inside the female shark before leading to a live birth. This is the same mode as mammals (including humans!). A placenta connects the pup to the mother for exchange of gases and nutrients. Gestation periods (developing time for the shark pup) vary between species; however, these are usually about 15 months! However, spurdog is an excellent example of very long gestation periods, lasting up to 22 months. This is twice as long as human pregnancies! Once born, the shark pups independent of their parents, fending for themselves. 

Blonde Ray Eggcase

Oviparity

Oviparity involves the mother producing one or multiple eggs in their oviducts, and once fertilized, laying the egg cases on the seafloor. The leathery outer case is comprised of thick protein strands providing excellent protection against wave action and predation! The egg cases are further adapted to help with settlement. For example, catshark egg cases have long string-like appendages called tendrils that wrap around seaweed or coral to keep the egg case from being washed away. Skate egg cases release a goo-like substrate that attracts small debris. This weighs down the egg case keeping it firmly in the sandy or muddy substrates on the seafloor. Inside the egg case, the developing embryo feeds on egg yolk, acting similarly to the placenta and being high in nutrients to help the shark pup grow! Once fully developed, the shark pup will wriggle out through a small self-made hole. The length of time spent within the egg case varies per species. Thornback ray pups can spend between 5-11 months inside the egg case.

Recording your egg cases finds

Empty egg cases can also be found washed up on beaches, especially after a storm. You might have even seen one before without knowing what it is! Whilst these empty egg cases have no purpose to the shark; they still hold an immense value of information. Based on abundance and location, findings can indicate the diversity of elasmobranch species in our local seas as well as potential spawning grounds. In 2018, a blackmouth catshark nursery was found around 200 miles off the west coast of Ireland. If you find any egg cases on your local beach, don't forget to record them on this website! Each egg case recorded is a valuable piece of data that helps us to understand better and protect our local sharks, skates and rays.

Blackmouth Catshark

Ovoviviparity

Ovoviviparity involves the fertilized eggs being maintained within the oviducts of the mother, where they hatch, before escaping – giving the appearance of a live birth! The eggs are better protected within the mother, compared to when laid on the seafloor; therefore, there is a higher chance of survival for the embryos. However, this mode does restrict the amount of offspring the mother can produce – she can only hold a limited amount of eggs within the oviducts respective to the size! Nutrition comes from unfertilized egg yolks also within the oviduct as well as secretions from special glands on the walls of the oviducts. The egg cases usually hatch after around 3-4 months. Zebra sharks and Bull sharks are great examples of those who reproduce by this method.

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