The Digital Fish Library
Featured Fish
By Kara Yopak, Ph.D.
Videos
surface (Video by K. Yopak) Swimming with Whale Sharks
(Video by K. Yopak)
Taxonomy
A sketch of the from Compagno (1984). 
Two whale sharks off the coast of Cancun, Mexico. (Photo by: K. Yopak) The whale shark, Rhincodon typus, is a cartilaginous fish that was identified in 18281 and received its name for its whale-like proportions and filter feeding mechanisms. The cartilaginous fishes (sharks, skates, rays, and holocephalans) are comprised of approximately 1100 extant species2, 10 of which grow up to lengths greater than
4m3
. The largest fish in the world’s ocean4, the whale shark is a migratory5-7, epipelagic filter-feeding member of the Orectolobiformes order.
The Orectolobiformes order is comprised of 33 species across 7 families (Brachaeluridae, Ginglymostomatidae, Hemiscyllidae, Orectolobidae, Parascyllidae, Rhincodontidae, and Stegostomatidae), with Rhincodon typus as the sole species of the family Rhincodontidae8,9. This family is characterized by large external gill slits and sieve-like filter screens present on the internal gills, with a wide, flat head. They have a nearly terminal mouth, 300-350 rows of miniscule teeth, and prominent keels on the caudal peduncle8. The ‘checkerboard’ coloration pattern of this species is quite unique, with a light underside and a dark dorsal surface, patterned with light-colored spots and vertical stripes.
Biology and Ecology
The whale shark has circumglobal distribution in tropical and warm-temperate seas, found in both oceanic and coastal waters. It is found from New York south to Central Brazil in the West Atlantic and from Senegal to the Gulf of Guinea in the East. In the Western Pacific, the whale shark is distributed from Japan to Australia and in the Eastern Pacific from Southern California to Chile, as well as throughout the Indian Ocean4,10. Rhincodon typus is a highly migratory species, likely related to plankton blooms or changes in water temperature4. They have been sighted singly but are also reported in predictable, annual aggregations of hundreds of individuals occurring throughout their circumglobal distribution4,10-14.
Recent satellite tagging studies have documented the whale shark's long distance migrations and vertical migrations to depths of over 1 mile6,15,16. Individuals are found in shore in shallow water areas as well as far offshore10, often seen close to the surface feeding. The whale shark consumes planktontic and nektonic organisms, including fish eggs, small crustaceans, teleosts (sardines, anchovies, mackerel, small tunas), and squid4,8. Gut content analysis data reported on the proportions of the primary components of the whale shark diet, which consisted of 38.2% bony fishes, 14.6% cephalopods, 31.4% zooplankton (mainly euphausids, or “krill”), and 15.8% marine plants and algae17.
R. typus grows up to total lengths of at least 12m4,16,18, though it has been reported up to 18-21.4m4,5, and maximum size is as yet uncertain. The lifespan of the whale shark is still unknown, though a captive whale shark, maintained at the Okinawa Churaumi Aquarium in Japan, has been living at least 12 years. This shark reproduces via ovoviviparity4,19,, though the whale shark was thought to be oviparous for many years, e.g. 20,21. An ovoviviparous lifestyle describes a reproductive mode by which embryos develop within egg cases in utero and feed solely on yolk until they hatch or are about to hatch22. There have been documented cases of between 200-300 egg cases with developing embryos at various stages of development in a female whale shark’s uteri19,23. Whale sharks are pupped and are free swimming at between 55-64cm in length4. The gestation period for R. typus is unknown, although it has been suggested that they may reproduce biennially, similar to one of their closest phylogenetic relatives, the nurse shark (Ginglymostoma cirratum)24. Captive studies on the whale shark hope to provide more insight into the life history of these unique species.
Unique Feeding Ability
A juvenile whale shark engaging in vertical suspension feeding off the coast of Cancun, Mexico (Photo by: K. Yopak)
A whale shark feeding at the water’s surface (Photo by: K. Yopak)Whale sharks, along with the megamouth sharks, Megachasma pelagios25 and basking sharks, Cetorhinus maximus26, represent an enormous lifestyle change in chondrichthyan evolution, with extremely large body sizes and a unique feeding mechanisms for a planktivorous lifestyle. Some batoids (rays, skates), such as the mobulid rays, employ planktivorous filter feeding as well, consuming prey both at the surface and in the midwater. The whale shark employs suction filter feeding mechanisms27, as opposed to relying on forward motion to opportunistically capture prey, such as the dynamic filtering mechanism of the basking shark4. However, Rhincodon can also use continuous ram filter feeding and vertical suspension feeding. During vertical suspension feeding, this species will hang at a 45-degree angle in the water column and suck prey into its mouth15,28. Whale sharks feed at or close to the surface4, and it has been suggested that they can target and approach high plankton concentrations27 to maximize prey consumption.
Neuroanatomy and the use of MRI
Brain of the whale shark, Rhincodon typus, in lateral view, modified from Yopak and Frank30. A. Gross morphology. B. Sagittal slice of MR data. C. Digital segmentation of the major structures of the brain and cranial nerves. Scale bar corresponds to 1cm.
Little is known about the biology, behavior, and biogeography of R. typus, and, until recently, even less was known about their neural development29,30. The study of species with unique behavioral and morphological characteristics is critical when teasing apart evolutionary trends, yet becomes difficult, as often those specimens are rare and/or difficult to acquire. Traditional methods for brain analysis, such as gross dissection and histological sectioning, are highly informative, e.g. 31,32-35 yet also highly invasive and thus impractical for the study of valuable specimens36. Magnetic Resonance Imaging (MRI) offers a solution to these current drawbacks, as it is unique in its ability to non-invasively acquire high-resolution images of soft tissue structures. A recent study on the brain organization of the whale shark employed MRI as an effective tool to quantify these rare brains without destroying them30.
The whale shark had small brain for its body size, similar to other large bodied elasmobranchs such as the great white shark, Carcharodon carcharias34, basking shark, Cetorhinus maximus37, and megamouth shark, Megachasma pelagios38. It has been proposed that the relatively small brains of planktivorous predators were related to their opportunistic and somewhat passive predation strategies39, which might be less cognitively demanding in terms of sensory and/or motor requirements in comparison to more agile hunters30,34,37-39.
When compared to over 60 other shark species, the whale shark possessed a relatively small telencephalon (42% of the total brain) with a slight protrusion of the central nucleus of the dorsal pallium29,30. A large telencephalon has been associated with social and environmental complexity in sharks34,40. The dorsal pallium in particular has been linked to complex “social intelligence”41-44, and its brain characteristics suggest moderate social behaviors. The most notable characteristic of the whale shark brain was a large and highly foliation cerebellum, characteristics previously documented in agile, active predators such as the mako shark, Isurus oxyrinchus, and the bigeye thresher shark, Alopias superciliosus34. Though not considered an agile predator, the cerebellum occupied 29% of the brain of the whale shark. Although the function of the cerebellum is currently an area of debate, these results suggested that foliation of this brain structure might reflect sensory acquisition more than previously thought30. Statistically, this study found that the brain of the whale shark was most similar to another planktivorous species, the basking shark, demonstrating a possible convergent evolution in the brains of these two animals with similar but independently evolved lifestyles30.
Conservation Effort and Ecotourism
Whale shark aggregations off the coast of Cancun, Mexico are prime spots for both ecotourism and scientific research, allowing for up-close encounters with this species (Photo by: Kara Yopak).
Though this is the largest living shark, and indeed the largest fish in the sea, this species is harmless to humans18. The whale shark has been targeted by various fisheries for consumption, leather products, and health supplements in Taiwan, China, India, the Maldives, the Philippes, and Senegal4,16and is listed as vulnerable on the IUCN Red List due to the unregulated fisheries45. In addition to the value of whale shark products, Rhincodon has become a huge commodity in ecotourism operations, due to their docile nature and inshore aggregations in some parts of the world4,15,16. The most popular ecoutouristic sites include whale shark aggregations at Ningaloo Reef, Belize, the Sea of Cortez, the Seychelles, KwaZulu-Natal, Kenya, the Galapagos, Veraval in India, and Mexico46. These operations have raised both public awareness and scientific research on this species, though the behavioral and abundance impacts of human interactions on the whale shark are as yet unknown46. Offshore aggregations have facilitated access to these animals for scientists; several satellite tagging projects, which study the movements and behavior of the whale shark, have documented travel over extensive distances, e.g.5,6, size segregation, and individual re-sightings16,46.
Rhincodon typus has also been successfully maintained in captivity throughout the world, primarily in Asia. At present, whale sharks are housed at the Georgia Aquarium in Atlanta, the Osaka Aquarium Kaiyukan, the Okinawa Churaumi Aquarium in Japan, the Kenting National Museum of Biology and Aquarium in Taiwan, the Polar Ocean World in Qingdao, China, and the Atlantis Hotel in Dubai, though the Atlantis Hotel had received much public pressure to release the shark as of October 2008. The longest surviving whale shark in captivity was at the Okinawa Churaumi Aquarium, which has maintained 16 whale sharks between 1980-1998, one of which had lived 12 years as of 2005. Though housing these animals in captivity has raised ethical questions, aquariums promote public awareness and inspire support for global conservation efforts, in addition to providing researchers with critical data on growth rates, reproduction and fecundity, and lifespan,e.g.47-49.