How Genetics can help rescue endangered species – Case of the whale sharks

Whale sharks are the largest fish in the ocean and are about eighteen to twenty meters long. They weigh around twenty tons and can live up to a hundred years. Though normally pelagic animals, whale sharks are seen in large feeding aggregations in certain parts of the tropical and warm temperate zones around the world, like the Gulf of Mexico and the Indian Ocean. They are known to consume twenty kilograms of food per day that mainly includes plankton, small fish, crustaceans, and coral spawn1.

Source: Wikiemedia Commons
Credit: Courtesy of Wikimedia Commons

The decreasing number and size of whale sharks at some aggregations suggests the species may be declining2. While they are protected in many parts of their range, whale sharks are fished legally and illegally for food (especially for their fins in the Asian markets) and medicinal purposes. Their slow growth and late maturation (when they are 25-30 years old) prevents them from being recovered from over-fishing or habitat disruption.

Whale Shark Habitat Around the Globe. Source: Wikimedia Commons
Whale Shark Habitat Around the Globe. Credit: Courtesy of Wikimedia Commons

Genetic analysis can tell us about whale shark migratory behaviour, social structure and breeding habits. The genetic drift between their populations can help us study the gene flow and interbreeding.  A key question for whale shark conservation is whether the fish aggregations represent isolated populations, or whether whale sharks are migratory and comprise a single global population.

The highly polymorphic microsatellite DNA sequences undergo frequent mutations over time and aid in understanding the population genetics of the species. These mutations are passed on to the offspring and thus provide great resolution for studying the variation and differentiation between populations. The degree of relatedness between different whale shark groups tells us about their migration and breeding patterns (closely related species have similar genetic sequences). For their research, Dr. Schmidt and her colleagues at UIC tagged the satellite sequences to pool the whale shark samples geographically into Pacific Ocean animals, Atlantic Ocean animals and Indian Ocean animals for analysis3. Polymerase Chain Reaction (PCR) was used to amplify and analyse DNA sequences and Principle Components Analysis (PCA) was done to examine the population differentiation. Through DNA analysis of 68 whale sharks from 11 locations, it was shown that the species found in different locations were in fact, quite similar. The little genetic variation among populations confirmed the migratory behaviour and interbreeding among populations. These results further calls for more stringent conservation plans to protect the big fish of the ocean.

As to this day, whale sharks mating has never been observed. The location of their mating is not known either. None of them have been observed giving birth and very young whale sharks are rarely seen. These facts pose a lot of questions regarding the whale shark reproduction. In 1995, a litter of more than 300 whale shark embryos were found inside of a captured female animal at Taiwan5 (the only pregnant female whale shark ever studied scientifically) which suggested that whale sharks are ovoviviparous animals i.e., they give birth to live young that hatch from eggs within the uterus. Paternity analysis through DNA fingerprinting suggested that a single male fathered all the embryos4. Since multiple paternities is common in many shark species, genetics could be used to determine whether the embryos were sired by a single male or multiple males.

The ability to use genetic analysis to study the whale shark behaviour, population and breeding patterns can help us design appropriate tools for their conservation across the oceans. Conservation genetics is an important tool and should lead the way for rescuing many other endangered species around the globe.

Sources:

  1. World Wildlife Org –  http://worldwildlife.org/species/whale-shark
  2. Whale Shark (Rhincodon typus) Issues Paper by The Department of the Environment and Heritage, May 2005, Australian Government – http://www.environment.gov.au/node/15909
  3. Schmidt J.V., Schmidt C.L., Ozer F., Ernst R.E., Feldheim K.A., Ashley M.V., Levine M. (2009) Low genetic differentiation across three major ocean populations of the whale shark, Rhincodon typus. PLoS One, 4:e4988.
  4. Schmidt J.V., Chien C.C., Sheikh S.I., Meekan M.G., Norman B.M. and Joung S.J. (2010) Paternity analysis in a litter of whale shark embryos. Endangered Species Research, Vol 12: 117-124.
  5. Over 300 embryos found in pregnant whale shark – http://www.theepochtimes.com/n2/science/over-300-embryos-found-in-pregnant-whale-shark-41759.html
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Understanding sexual parasitism and cannibalism

The mating ritual of the deep-sea anglerfish is one of the most bizarre in the animal kingdom. The female anglerfish are larger, reaching a length of around 10 centimeters, and the male fish are just a fraction of this and can be more than ten times smaller. These peculiar (read ugly) looking creatures live at a depth of 1 to 3 kilometers in the ocean.  In the darkest depths of the waters where food is scarce, finding a mate is problematic. It is estimated that 80% of the females never encounter a male in their lifetime (which is around 30 years). These fish have adopted a way as to resolve the issue of nutrient and mate acquisition through evolution.

Female deep-sea anglerfish with attached male. Photo: Dr. Theodore W. Pietsch, University of Washington
Female deep-sea anglerfish with fused male (circle). Photo: Dr. Theodore W. Pietsch, University of Washington

The larger female anglerfish release chemical factors known as pheromones into the water for the males to find them. Upon finding her, the male bites onto her skin (the ultimate love bite in the animal kingdom, if you ask me) and gets fused to her permanently! His internal organs degenerates – except for the gonads which are used to impregnate the female – and he now survives solely on the female’s blood vessels to acquire nutrients. He never has to hunt for food in his life. The female can fuse with multiple males (around 6 to 8) ensuring a fresh supply of sperm throughout her life! The males are reduced to a mere small lump of tissue on the female’s skin.

While the anglerfish’s mating ritual serves as a classic case of sexual parasitism, another interesting and even bizarre reproductive behavior is seen in Australian redback spiders, in which the males are consumed (yes, eaten!) by the females during copulation. A male redback spider performs a somersault behavior during sperm transfer and positions it’s abdomen on top of the female’s jaws. The female starts to feed on the male during the duration of copulation. One may ask, why would the male spider want to risk his life and sacrifice himself? The answer is simple: survival of the species through increased reproductive success.

A female Australian redback spider. Photo: Toby Hudson on Wiki Commons
A female Australian redback spider. Photo: Toby Hudson on Wiki Commons

Darwin in 1871, wrote The Descent of Man, and Selection in Relation to Sex, where he proposed sexual selection to explain how some traits evolve to give an advantage in the struggle for reproduction, but reduce the probability of survival. This is exactly what happens in Australian redback spiders. Sexual cannibalism is favored by selection because the cannibalized male spiders receive two paternity advantages. Through a series of behavioral experiments, Maydianne C. B. Andrade found that cannibalized males copulated for longer periods of time during which more eggs are fertilized (advantage #1). After consuming their first male, females reject subsequent males for copulation. This increases the paternity of the first male since the female only produces his off-springs and reduces the probability of her mating with a second male (advantage #2). This explains why sexual cannibalism is in fact, an adaptive strategy for male redback spiders since it increases their reproductive value.

These wild mating rituals of certain creatures may seem strange and even outrageous at first. However, one can appreciate these reproductive strategies when they’re understood in the light of evolution.