Many divers have an insatiable thirst for information about sharks, and many amazing facts are already commonly known, such as a shark’s ability to detect one part of blood in a million parts of water, or the fact that their skin is made up of myriad tiny teeth. Here we’ll look at a few lesser known shark facts, offering further proof that sharks are indeed the most fascinating fish in the sea.
A Light In The Darkness
Many deep-sea fish use bioluminescence as a defense tactic, to attract prey, or to communicate with each other in the darkness of the lightless depths. In most cases, either the nervous system controls these lights or they are the product of symbiotic bacteria living on the host animal. The exception to this rule is the velvet belly lanternshark, a small shark that grows no bigger than 24 inches/60 centimeters and inhabits Atlantic waters to depths of up to 8,170 feet/2,490 meters. This amazing animal boasts a series of light-emitting photophores across its eponymous velvet belly, along its lateral line and around its fins, all of which appear to be controlled either by hormones or neurotransmitters, giving the shark the ability to change the brightness of its lights at will. It is believed that different light strengths are used for different purposes, some for warding off potential predators, and others for facilitating social interactions.
Embryonic Defense Mechanisms
Research conducted by shark biologist Ryan Kempster in 2013 at the University of Western Australia in 2013 revealed that embryonic bamboo sharks are capable of responding to a predator threat while still in their egg cases. Normally, the embryos move constantly within their eggs to enable the circulation of fresh, oxygenated seawater. By doing so, they emit small signals that could alert a nearby predator to their presence. These signals include smell, carried on the water currents generated by their movement, and the electric signals given off by the beating of their gills. However, when Kempster introduced an electronic signal into to their tank to simulate a potential predator, the embryonic bamboo sharks reacted by freezing, holding their breath and curling their tails protectively around themselves. In the wild, their ability to play dead in a similar situation would most likely result in their remaining undetected, and therefore surviving until birth.
Puget Sound, off the coast of Seattle, Washington, is home to a significant population of bluntnose sixgill sharks, which scientists from NOAA, the Washington Department of Fish and Wildlife, and the Seattle Aquarium recently discovered are mostly brothers and sisters. Through the sampling of genetic tissue biopsies taken from the sharks, it became apparent that the majority of the population surveyed were members of the same litter, meaning that once their mother gave birth and departed, the resulting offspring chose to stay together in the Sound. The number of sharks in the Sound differs from one year to the next as groups of siblings come and go, but the phenomenon of the pups choosing to travel and stay together seems to occur again and again. The sharks are observed via a research station that doubles as the Sixgill Shark exhibit of the Seattle Aquarium, in the hope of shedding more light on this unusual discovery. Sixgill sharks are ovoviviparous, meaning that their young are born live after hatching from eggs in the womb, in litters sometimes numbering over 100 pups.
Natural Sun Protection
When scientists collected juvenile scalloped hammerhead sharks from the murky waters of Kaneohe Bay, Hawaii, and kept them in a shallow, clear water tank for observation, the increased sun exposure caused the baby sharks to tan significantly. That the color change from light brown to almost black was caused by the sun was proven by applying opaque patches to the baby sharks, beneath which they remained untanned after 21 days in the clear-water tank. Over the course of three weeks, the melanin content in the sharks’ skin increased by 14 percent, confirming that the hammerheads represented the first documented case of tanning in a non-mammalian species. This ability to adapt to sun exposure may explain why sharks appear to have a resistance to skin melanoma while other fish species have been proven to suffer from skin cancers. However, while some scientists hope that this discovery might help inform advancements in human sun protection measures, it should be noted that sharks do suffer from some forms of cancer despite what popular media may claim.
Evolved For Effective Reproduction
The relatively violent way in which sharks reproduce is well documented, from the twin claspers of the male to the double-thickness skin that some female sharks have developed to protect themselves from the amorous biting of their partners. Shark reproduction is remarkably effective too, perhaps thanks to the males’ siphon sacs, two muscular organs on the underside of the shark’s body that help propel the sperm from the claspers into the female. Just before reproduction occurs, the sacs fill with seawater, which is then forcibly ejected as they contract, carrying sperm along with it into the claspers, and then into the female reproductive tract. The siphon sacs and their stream of seawater perform a double function — not only do they increase the effectiveness of the male’s sperm, but they also serve to flush away the sperm of competing males that may have copulated with the female immediately beforehand.