External fertilization is a wasteful process but still enables survival of many aquatic species. For external fertilization to be successful certain prerequisites are needed: high population numbers, aggregation of these large groups and synchronous release of gametes. Animals are great resource maximizers. Once they discover a high value resource they will exploit it as efficiently as possible. Gametes (sperms and eggs) released during external fertilization of large groups of fish such as the Jack are extremely nutritious for filter feeders such as whale sharks. Being there at the right moment requires time-mapping and spatial memory so that those recurring events are not missed.
Whale sharks (Rhincodon typus) mostly feed on carbon rich plankton limited in phosphorus but they can also feed on small fish if the opportunity strikes. The nutrient content of plankton averaged over worlds oceans is known as Redfield Ratio with Carbon/Nitrogen/Phosphorus (C/N/P) being 106:16:1. As you can see phosphorus is the limiting nutrient. Sperms and eggs contain high fraction of nitrogen and phosphorus in the form of protein and nucleic acids (DNA) far exceeding these proportions. Whale sharks do not miss this opportunity.
Each year, during the full moons of spring, whale sharks gather in a remote reef located 30 miles off the coast of Belize called Gladden Spit. More than 35 species of fish aggregate in this particular location to spawn. Whale sharks are attracted by the huge cloud of gametes released into the water column. Not so long ago the site was endangered due to uncontrolled fishing and overwhelming numbers of SCUBA dive operations. A local non-governmental conservation organization, Southern Environmental Association (S.E.A) works to manage the area and protect it. One of their initiatives is the promotion of sustainable tourism based on whale sharks.
Befriending Giants – Whale Sharks of Oslob from Blue Sphere Media on Vimeo.
These ocean voyaging gentle giants are threatened worldwide. We have very little understanding of their reproductive biology. Georgia Aquarium in Atlanta has started an ambitious program in 2007 to understand these fishes in captivity. The aquarium has four whale sharks inside the world largest (24,000 m3) tank designed specifically for them. As you can see in the video above these fishes can be “trained” to follow a feeding boat. This behavior makes it possible to raise them in captivity.
Natural history of whale sharks are a curious research area for marine biologists. What is their movement patterns? How long do they live? When and where do they breed? Even a poo sample can excite biologists. Neither mating nor birthing of whale sharks has ever been observed. A pregnant female captured in July 1996 with 300 pups provided evidence that whale sharks are ovoviviparous. Continuous monitoring of four captive individuals may provide some valuable information on elusive reproductive biology of these fishes. There are a number of organizations employing citizen science projects for monitoring whale sharks in the wild.
Whale Shark Dive – Georgia Aquarium from Uzay Sezen on Vimeo.
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