THINK: HICKIES BUT WITH SHARP TEETH, LEADING TO CERTAIN PREGNANCY. Written by Safina Center Fellow, Dr. Demian Chapman
Every August we become acutely aware of the possibility of a shark attack. This doesn’t happen because it is late summer and we are all headed to the beach. It happens because about 80% of Discovery Channel’s Shark Week programming and advertising consists of some poor actor splashing about in a sea of red coloring dye, trying to make it look as if a rubber shark is tearing him or her apart. This image always reminds me that there is one group of animals that does endure shark attacks on a regular basis. They have evolved to survive the injuries inflicted upon them by sharks. This group of animals actually needs to be attacked by a shark for survival. That group of animals is, of course, all female sharks. That’s right, female sharks have to endure a shark attack just to become pregnant.
There are not many absolutes in shark reproduction other than this: all sharks and their relatives, the rays and skates, practice internal fertilization. Males possess modified pelvic fins called “claspers”, which they are able to insert into the female during copulation and transfer sperm into her reproductive tract. Males actually have a pair of sacs running underneath their skin that they fill with seawater before they mate. These water-filled sacs are used as a hydraulic system to propel the sperm as far up the female’s reproductive tract as possible. Females of some species can actually store this sperm in specialized glands for long periods (a year or more) before using the sperm to fertilize their ova. For others, fertilization happens right after they mate.
So how do male sharks actually secure a female for mating? Physical purchase is necessary to get the needed leverage to insert the clasper. But sharks don’t have hands to hold the female still. What they do have is a mouth–with teeth. So this is where the shark attack comes in. Male sharks have to bite the head, gill or pectoral fin region of the female in order to mate. Not only that, they also have to hold on for several minutes. Shark claspers are outfitted with large spurs. When a clasper is inserted into the female’s reproductive tract it splays open like a scroll and these spurs anchor it in place, just as the male’s jaws and teeth hold the front end of the mating pair together. The pair stops swimming and sinks, eventually coming to rest on the seafloor if the water is shallow enough. For many species, both male and female are unable to breathe efficiently as they mate. When the pair finally disengages, both male and female are physically exhausted and have to rest on the bottom, rapidly ventilating their gills to repay the oxygen debt they have incurred.
Female sharks often suffer what look to be horrific bite wounds after mating. The teeth of some sharks, such as tigers, bulls and whites, are adapted to removing large chunks of tissue from big prey, including other sharks. So how do females withstand bites from male sharks? These bites can kill other animals. First of all, male sharks bite the female when mating but do not attempt to remove tissue. The bites look bad but they are nothing compared to what the shark is capable of. (In fact, many cases of shark bites on humans are like this as well…investigatory but not an all out attempt to feed. That’s not to say that shark bites on humans are related to mating, just that there are motivations other than feeding for a shark to bite.) Female sharks have also evolved a defense mechanism. If you look at a cross section of a female shark’s skin you will see that it is noticeably thicker than that of a similar-sized male. And sharks have remarkable healing abilities. Ecotourism operators that dive with bull sharks in Fiji on an almost daily basis have observed that the females show up with extensive mating wounds during a certain period of the year but these wounds heal up within weeks.
Despite their thick skin and ability to heal, mating is a rough experience for female sharks. I suppose I shouldn’t have been surprised to find several years ago that female sharks sometimes reproduce without mating. That’s right, sharks are capable of occasional, asexual reproduction. I used DNA fingerprinting techniques (just like you might see on the Maury Povitch show to prove that one of Maury’s guests is the father of another guest’s baby) to prove that a certain baby hammerhead shark had no father at all. (Virgin Birth in a Hammerhead Shark.) To date researchers have found that females of at least four shark species are capable of this type of reproduction.
Nevertheless, asexual reproduction is rare in sharks and seems to produce fewer young per event then having a sexually-produced litter. So for now at least, male sharks still have a role to play in reproduction even if it does involve all of that biting.
Photo #1 caption - Healing wounds on the body of a mature female oceanic whitetip recently bitten by another shark, probably during mating.
All Photos by Brenda Anderson, University of North Florida.