A new study shows that live-bearing shark mothers provide substantial post-partum investment to their young, giving them enlarged ‘super-livers’ that they feed off during their first few months of life.
A Bangor University-led international team of researchers examined sharks captured incidentally by beach protection nets around KwaZulu-Natal, South Africa. Their report is the first to demonstrate that live-bearing carcharhinidae sharks — the largest order of sharks that include common types such as blue sharks, catsharks, swellsharks and sandbar sharks often called Requiem sharks — provision their young with a maternal head-start in the form of energy reserves stored in the pup’s liver. These reserves help the pups through the dangerous first few weeks of life, when prey are difficult to catch and predators most threatening.
This study provides the first evidence of a decline in liver mass of newborn sharks, from 20% of body weight at birth to 6% when they start to feed themselves. The research shows that, during the critical period after birth, shark pups lose weight by consuming their liver reserves and that this weight loss is not necessarily an indication that the sharks are in a poor nutritional state, as has been previously thought.
Lead researcher Nigel Hussey from the School of Ocean Sciences at the University of Bangor said:
“It is likely that the liver reserves enable the newborn sharks to acclimatize themselves to their environment and to develop their foraging skills, We know that large sharks use their livers as an energy store, but we had no idea that the mother provisions her young with additional liver reserves to enhance their survival.”
While sharks have swum the world’s oceans for nearly 400 million years, their reproductive habits appear to be far from primitive. The study found a dramatic increase in the size of pups born later in the year, when the risk of predation is lowest. This suggests mothers have some flexibility in when they give birth, thereby helping to maximize each pup’s chances of survival.
The widely-reported global decline of many shark species calls for conservation actions. The identification of a peak in reproductive output could mean targeting conservation efforts by directing fishing effort towards capturing smaller or larger fish, while protecting the pups most likely to survive.
“We have much work to do to improve our basic understanding of shark biology if we are to implement effective management plans” said Hussey.
The team comprised researchers from the Bangor University, the KwaZulu-Natal Sharks Board, the University of KwaZulu-Natal, and the Australian Institute of Marine Science and their findings were published in the “Maternal Investment and size-specific reproductive output in carcharhinid sharks” article in the Journal of Animal Ecology on October 21, 2009.
The work was supported by grants from the Natural Environment Research Council of the UK and operating funds from the Australian Institute of Marine Science.
Copyright © 2009 by Marine Science Today, a publication of OceanLines LLC