Aquarists from the Shedd Aquarium have described a series of successful captive spawnings by its pair of Zebra sharks, Stegostoma fasciatum, in which the female laid over 300 eggs during a four-year period.
Collections Manager Lise Christopher and Senior Aquarist Heather Thomas reported their findings in the public aquarium journal the Drum and Croaker, and explained that the pair of sharks, which were caught off the north east coast of Australia, started to display spawning behaviour shortly after they were introduced into the aquarium.
Shortly after arrival our pair began to display pre-copulatory behaviour that primarily included the male grasping onto the caudal fin of the female while swimming. After taking hold of her fin, the female turned upside-down in the water column, gradually sinking to the floor of the exhibit, wrote Christopher and Thomas.
With both animals now on the bottom, the male would either hold onto her caudal fin or grasp her pectoral fin. This behaviour occurred frequently throughout the year, continuing for several hours at a time, occasionally concluding with copulation.
When the female was in the mood, she would lie still on the bottom off the aquarium floor and allow the male to insert his clasper. However, when she wasn't in the mood, the male was sometimes seen to force himself upon her, by pushing her against the corals and copulating forcefully.
EggsThe Aquarium saw its first eggs from the sharks in April 2004. A few days before the eggs were laid, the attachment tendrils, which anchor the eggs to seaweed, could be seen protruding from the vent of the female. The eggs were laid early in the morning, so the staff set up cameras to film the sharks depositing their egg sacs.
Christopher and Thomas wrote: Several days before egg-laying occurs, attachment tendrils can be observed at the opening of her vent. Because egg-laying typically takes place during evening and early morning hours, video surveillance provided us an opportunity to observe her behaviour.
Our female would swim in tight circles around the rockwork, cinderblocks or artificial corals she was attempting to lay her eggs on while rubbing her vent on the structure and oscillating her body back and forth. As she began to expel the eggs, the tendrils would attach to the structure pulling subsequent eggs from her oviduct.
Successful programmeThe 313 eggs laid resulted in 78 baby sharks, which were grown-on in separate enclosures away from their parents.
The fast-growing shark pups reached over a kilogram in weight and over 70cm in length in just four months. The offspring have been re-homed at a number of other public aquaria.
For more information see the paper: Christopher, L and H Thomas (2009) " Notes on captive propagation and rearing of Zebra sharks, Stegostoma fasciatum at John G. Shedd Aquarium. Drum and Croaker, 40, 2009. pp 3-8.