Life History and Behavior of sharks
Life History
Most sharks live about 20 - 30 years. There are however shark species that can live over 100 years, like the whale shark.
Sharks practice internal fertilization, as opposed to bony fish.
There are at least two documented cases in which a female shark who had not had contact with a male has conceived a pup on her own through parthenogenesis.
Most sharks are ovoviviparous, which means that the eggs hatch in the oviduct within the mother's body. Still inside their mother, the young keep on being nurished by remnants of the yolk and the oviduct's fluid. Then, they are born alive and fully functional.
Young Sandtiger sharks will feed on their siblings in their mother's womb, ensuring that only the strongest one will survive. This leads to the mother giving birth to two baby sharks; one from each uterus. This cannibalism among siblings is however very unusual in sharks; it is much more common in for example birds of prey.
But also oviparity (eggs are laid in the water; 20 % of species, mostly cat sharks) and viviparity (development within mother with a placental link, like in mammals) are observed.
Now that baby sharks are already fully developed and instinctively guided about what to do, the mother doesn't have to stick around and leaves them after brith.
Sharks produce only a small number of well-developed young. Typical numbers per reproductive cycle are 2 offspring. Some species have up to 100 offspring per reproductive period.
Adolescent sharks grow rather fast. After reaching maturity their growing rate decreases significantly, but they keep growing their whole life.
The young sharks reach maturity relatively late, ranging from 3 - 25 years.
Behavior
Only a few species are solitary hunters, many sharks are cooperative feeders and hunt in packs. Sharks can be highly social , remaining together in large schools of over 100 individuals.
Sharks are relatively intelligent and show signs of short and long term memory and they can learn by observation. In fact, shark have some of the largest brains of all fish. Their brain-to-body ratio is similar to that of birds and mammals.
Usually, sharks swim and cruise at an average speed of 8 km/h. When attacking prey, they can increase their speed considerably. The fastest shark is the Shortfin Mako and it can reach a speed of up to 80 km/h.
Most sharks are carnivorous, but some sharks like the Basking shark or the Whale shark have independently evolved into plankton filter feeders. They swim with their mouths open. 1.5 Million Liters of water pass through the Basking shark's mouth every hour.
The courtship in sharks can have several different forms. Some males bite the females to show their interest. The females sometimes have scars; some species have evolved thicker skin in females for that reason.
The Cookie-cutter shark is one that lives deep down in the ocean. To attract prey in the vast darkness, its whole lower body glows in a ghostly green produced by bioluminescent organs.
Swellsharks that belong to the Cat sharks can swim between rocks and then swell so that predators can't pick them out and eat them.
Most sharks hunt during the night.
Source: http://tywkiwdbi.blogspot.ch/2009/03/shark-egg-case.html
Figure: Shark egg of a cat shark
Source: http://www.quantum-immortal.net/physics/biolum.php
Figure: Swellshark under a rock
Copyright December 2014; Olivia Lucie Meier, Sharkworld.