Physiology of Finfish and Shellfish (2+1)

3.1.21. Modes of reproduction

1. Live bearers/ internal bearers

Most sharks give birth to live young, but some release eggs that hatch later. The gestation period (the time the embryo spends inside the female, developing) can be up to almost two years long (the spiny dogfish shark may have the longest shark gestation period).

Baby sharks (called pups) are born with a full set of teeth and are fully ready to take care of themselves. They quickly swim away, even from their mothers who might eat them. Litter size ranges from one or two pups (for a great white shark) to over 100 (for a large blue shark and the whale shark).

i. Ovi-ovoviviparous

This is considered to be the first evolutionary step towards live bearing. In some sharks and rays after internal fertilization and after a number of weeks on incubation large few embryos are laid each covered with a horny capsule. Among teleosts, they lay their embryos immediately after fertilization.

ii. Ovo-viviparous or Aplacental Viviparity

This is best known from squaliform sharks and from the coelacanth (Latimeria). In these fishes, development is internal. The eggs hatch and the babies develop inside the female's body but there is no placenta to nourish the pups. The large active young are born after the yolk sac has been absorbed or having their young developed inside them without direct maternal nourishment .

The eggs develop in the oviduct of elasmobranchs and the ovary of teleosts. The gestation period can last from a day or 2 in small tropical teleosts to 1 or 2 years in sharks. The pups eat any unfertilized eggs and each other (they are oviphagous). Very few pups in a litter survive until birth due to this form of sibling cannibalism. Eg. for teleosts are Scorpaneidae, Cottidae, Hexagrammidae.

iii. Viviparous

To be precise the female keeps her developing eggs in the final section of the oviduct, called the womb or uterus and the babies are fed by a placenta which transfers nourishment from the mother to the babies (via an umbilical cord which is connected to the baby shark behind the between the pectoral fins). The placenta helps transfer of nutrients and oxygen from the mother's bloodstream and transfers waste products from the baby to the mother for elimination. F ollowing a long gestation period gives birth to fully developed, extremely independent pups, in other words, sharks give birth to living offspring. The long gestation period inside the mother's body effectively protects the young from enemies. The number of pups in a litter ranges from 2-20 or more.

Eg. Hammerhead sharks (Sphyrinidae), carcharhnidae, Embiotocidae, Poecillidae.

2. oviparous, or "egg-laying"

Lay undeveloped eggs, external fertilization (90% of bony fish), internal fertilization (some sharks and rays).

In fishes, oviparity is most common. The adult can produce many offspring, which they broadcast into the plankton column. When the offspring settle out of the plankton, they may be in totally new environments, allowing for a great area in which the young may survive. This mode also comes with its disadvantages; when born, the fish must first go through a larval stage for growth before they transform into the adult stage. In this larval stage, they must fend for themselves in obtaining food and avoiding predation. They may not find a suitable environment when they settle out of the plankton column. The survival of individual eggs is very low, so millions of eggs must be produced in order for the parent to successfully produce offspring. The other modes have their advantages, the eggs are much less prone to predation when carried within the mother, and the young are born fully advanced and ready to deal with the environment as miniature adults. These advantages come with a price-tag also; the adult must supply nutrients to its offspring and can only produce a few eggs at a time. The young are limited to the environment that their parents were in, and if this environment is deteriorating, they are stuck with it.

T he sharks deposit eggs in the ocean which will hatch later if they are not eaten by predators. The eggs are not guarded by either parent. Shark eggs (sometimes called "mermaid's purses") are covered by a tough, leathery membrane. Their shape ranges from pouch-like to screw-shaped (like the California horn shark and the Port Jackson shark). Some eggs (like those of cat sharks) have tendrils that attach the egg to objects on the sea bed. The egg has a yolk that feeds the embryo, very much like a chicken egg. Oviparous sharks include the Zebra shark, the cat sharks, swell shark, the necklace carpet shark, some Epaulette sharks, and the Horn shark.