A. Mating activity
1 . Few people have witnessed the mating activity of sharks.
a. In smaller, more flexible species the male coils around the female.
b. In larger, more rigid species the male orients himself parallel and head-to-head with the female.
c. During mating, males of many species bite females on the pectoral fins or the middle of the back to hold onto them. Females often bear scars or marks. Upon examination, these marks show they have been made by upper jaw teeth. In some elasmobranchs, males have longer, narrower teeth than females. In some female sharks, such as the blue shark (Prionace glauca), the skin on the back and flanks is more than twice as thick as the skin on the males.
2. Shark and batoid eggs are fertilized internally, as opposed to external fertilization in many bony fishes. Internal fertilization is a key adaptation for energy-intensive reproduction.
a. When born or hatched, young sharks are fully formed and physically able to fend for themselves.
b. Because these independent shark pups have a better chance for survival, the number of sharks produced in a litter is rarely over 100. The majority of the species bear far fewer pups.
3. Claspers are modified inner edges of the pelvic fins of male sharks and rays. During copulation, the erectile claspers are bent forward. The male inserts one cartilaginous clasper at a time into the female. In some species, claspers contain hooks and spurs that dig into the walls of the female oviduct, anchoring the clasper. Muscles force seminal fluid down a groove in the clasper and into the female oviduct.
B. Embryonic development
There are three types of embryonic development: oviparous, ovoviviparous, and viviparous.
1 . In oviparous sharks, a gland secretes a shell, or case, around the egg as it passes through the oviduct, protecting the shark until it hatches. The mother deposits the egg cases in the sea.
a. When an egg case is first laid, it is soft and pale; the case hardens and darkens in a few hours.
b. The developing embryo receives nutrients from a yolk formed prior to fertilization.
c. Oviparous sharks include horn sharks and swell sharks (Cephaloscyllium ventriosum).
A swell shark (Cephaloscyiiium ventriosum) develops within a tough, leathery egg case.
2. In ovoviviparous sharks, the shell is often just a thin membrane. Sometimes there is more than one egg in a membrane; this group of eggs is called a candle. The mother retains the egg, and the embryo soon sheds the membrane and develops in the mother's uterus.
a. Theoretically, all the embryo's nutrients come from the yolk. In some species, however, the lining of the uterus probably secretes nutritive fluids that are absorbed by embryos.
b. In other species, embryos continue to obtain nutrients after their yolk is absorbed by swallowing eggs and smaller embryos in the uterus. This is termed "intrauterine cannibalism" or ovophagy. In these sharks, usually only one embryo survives in each uterus. (Females have two uteri.)
c. Ovoviviparous sharks include mako sharks (Isurus spp.) and sandtiger sharks (family Odontaspididae).
In viviparous sharks, the developing embryo receives its nutrients from its mother via a yolk sac placenta.
3. In viviparous sharks, the yolk stalk that connects the embryo to the yolk grows long in the uterus. Where the small yolk sac comes in contact with the mother's uterus, it changes into a yolk sac placenta.
a. The embryo receives all its nutrients from the mother in one of two ways:
(1) Tissues of the embryo and the mother are in intimate contact and nutrients are passed directly from the tissues of the mother to the tissues of the developing embryo.
(2) The uterine lining secretes "uterine milk," which bathes the developing embryo. The branched yolk stalk absorbs this fluid.
b. Viviparous sharks include hammerhead sharks.
Gestation periods vary among species and between individuals within a species. Since sharks and batoids are cold-blooded, there is no precise gestation time. The rate at which the embryo develops depends on the water temperature. In general, most embryos develop somewhere in the range of two months (for some rays) to two years (for some spiny dogfish).
An epaulette shark pup emerges from its egg.
Sharks generally bear their young or lay their eggs in coastal nurseries where other large sharks are usually absent.
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