Darwin's 'Survival of the Fittest' is the mechanism which underlies every single stage of the evolutionary process. Over many millions of years, animals are subjected to pressures which cause them to evolve - those individuals which are well-adapted to their environment will survive to have many offspring, whereas those which are poorly adapted die young or fail to reproduce, wiping their inadequacies out of the gene-pool. Survival of the fittest is at work in every aspect of animals' lives; when they feed the strongest will eat the best, when they mate the most attractive individuals will get the girls and when they are pursued by predators the fastest or smartest will survive. And survival of the fittest can also take action before an animal is even born! In sharks... this is through cannibalism!
Sandtiger Sharks are Truly Unique
Sandtigers (Carcharius taurus) are medium-sized sharks from the order Lamniformes. Reaching a maximum of 3 metres in total length (TL), mature females are considerably larger than the males and have a characteristically bulky body. With a very wide-ranging distribution, these sharks can be found in the coastal regions and offshore waters of every continent (excluding Antarctica). Despite their fearsome looking teeth, sandtiger sharks are very docile around humans and feed primarily on small bony fish, squid and crabs, that live near to the sea floor (Compagno, 2001).
Sandtigers reproduce every other year by a process known as "ovoviviparity" - this means that eggs hatch internally, to then be born as live young. This is not uncommon in sharks and can be found in many different evolutionary lineages (Gilmore et al, 1983; Gilmore et al, 2005).
But what makes sandtigers so unique is that their reproductive method is actually not at all like any other species... During gestation the offspring undergo several different phases, where they gain their energy from different sources... including cannibalism! (Gilmore et al, 1983; Gilmore et al, 2005).
Phase 1: Fertilisation
Sandtiger sharks form large aggregations during the mating season, whereby one female will mate with multiple different males. She is capable of becoming pregnant (the scientific term is "gravid") by several different males at once, but she will not necessarily give birth to babies from each of these guys (known as "multiple paternity" or "polyandry"). In fact, she will only birth to two offspring per pregnancy - one from each of her uteri (Gilmore et al, 1983; Gilmore et al, 2005).
Phase 2: Nourishment from the Egg Sac
After mating, up to 50 fertilised eggs may be inside the female, spread between her two uteri. These become enclosed in groups of 16 - 23. At this point the offspring absorb nutrients from the egg case they are in and possibly also from the uterine fluid. When they grow to a critical size, around 5.5cm TL, the embryos will hatch from their egg capsules. By the time they reach 10cm TL these hatchlings will have completely absorbed all the energy from their yolk sac and the will need another source of energy... (Gilmore et al, 1983; Compagno, 2001; Gilmore et al, 2005; Chapman et al, 2013).
Phase 3: Intrauterine Cannibalism
When the hatchings are around 10cm long, they posses sharp, functional teeth... At this point they begin to use those teeth to eat all the siblings in their group! This is known as "intrauterine cannibalism" or embryophagy" or "adelphophagy". During this phase there can be an enormous size difference between the offspring within the litter - with the hatchlings which were fertilised first or those which grow fastest, hatching first and eating all his/her smaller siblings. Only one hatchling per uterus will survive this phase, after having ingested all the others! (Gilmore et al, 1983; Gilmore et al, 2005; Chapman et al, 2013).
Phase 4: Oophagy
Approximately 100 days after fertilisation, the two remaining sandtiger offspring will have run out of live brothers and sisters to eat. As the mother continues to produce eggs throughout her 9 - 12 months pregnancy, at this point they turn their attention to these unfertilised eggs for energy. This is known as "oophagy" (egg-eating). The growing hatchlings will sustain themselves this way until "parturition" (aka birth), at which point they will have reached 1 metre TL! They will be completely independent after birth and require no parental care from their mother. They also have few predators due to their already formidable size (Gilmore et al, 1983; Compagno, 2001; Gilmore et al, 2005).
It's like something out of science fiction, right!?
Survival of the Fittest In Utero
So why on Earth has this reproductive strategy developed? Scientists believe that this unusual mode of reproduction has evolved because it ensures that only the strongest offspring are born - the embryo which can grow larger fastest and is quickest off the mark to start eating his/her siblings will survive. This ensures that, even if the female has mated with multiple males, she will only invest her valuable reproductive resources into those with the best genetics, because all the others will die. Not only do the males have to compete for a female to mate with, their offspring must be able to fight for survival from the moment they hatch! This is survival of the fittest taking place very, very early in life... while the sharks are still in the womb!
What a dog eat dog (or shark eat shark) world!
References
Chapman DD, Wintner SP, Abercrombie DL, Ashe J, Bernard AM, Shivji MS & Feldheim KA (2013). The behavioural and genetic mating system of the sand tiger shark, Carcharias taurus, an intrauterine cannibal. Biology Letters, 9: 20130003. Access online.
Compagno LJV (2001). Sharks of the World: An annotated and Illustrated Catalogue of Shark Species Known to Date. Volume 2: Bullhead, Mackerel and Carpet Sharks (Heterodontiformes, Lamniformes and Orectolobiformes). FAO Species Catalogue for Fishery Purposes, 1:2. FAO:Rome. Access online.
Darwin C (1859). On the Origin of Species by means of natural selection, or, The preservation of favoured races in the struggle for life. J. Murray: London.
Gilmore RG, Dodrill JW & Linley PA (1983). Reproduction and embryonic development of the sand tiger shark, Odontaspis taurus. Fishery Bulletin, 81:1. Access online.
Gilmore RG Jr., Putz O & Dodrill JW (2005). Oophagy, intrauterine cannibalism and reproductive strategy in Lamnoid sharks. In: Hamlett WC (Ed.). Reproductive Biology and Phylogeny of Chondrichthyes: Sharks, Batoids and Chimaeras, Volume 3. Science Publishers Ltd.: Enfield, USA. Access online.
By Sophie A. Maycock for SharkSpeak
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