Sharks and Rays: Course Preview

Reproductive Methods

This discussion was selected from Week 5 of the AMNH online course Sharks and Rays, part of Seminars on Science, a program of online graduate-level professional development courses for K-12 educators. This is an excerpt from an actual course discussion, but learner names have been changed. Explore more sample resources...


Most sharks and rays place a lot of investment in producing only a few young during each breeding cycle. Depending on the species, it may take 12-20 years for large pelagic sharks to reach sexual maturity. Gestation can last between 1-2 years and a litter may contain between 4-6 pups. Smelts, on the other hand, can mature after about 2 years, producing between 8,000 and 50,000 eggs per year. What are the advantages and disadvantages of these two reproductive strategies (producing a lot of young in a short period of time compared to producing few young over a long period of time). How are these reproductive methods related to the size and stability of the populations? What leads you to the conclusions you are drawing?


Henry 26 Sep 6:32 PM

When studying Darwin's Theory of Natural Selection, Overproduction is the first requirement. There must be more offspring produced than could survive to ensure at least one offspring survives. However, there are usually more than one that survives therefore competition happens which will allow for adaptations to reign etc.

It seems that reproduction styles are very importation to species survival. One of my classmates mentioned Pandas. Because their reproduction style is weak (basically timing is everything). Also throw in a single food source, the are doomed.

Also most organisms that produces many offspring tend to have little maternal guidance. They are on their own. Conversely, organisms that produce a few or one offspring tend to have maternal guidance for a period of time, sometimes years.

Gail 29 Sep 11:59 AM

Henry, I agree that the theory of natural selection depends on more offspring than the environment can hold, but this doesn't hold more true for bony fish than sharks. Replacement is two shark offspring, but many have litters of 4-6 pups which does create competition among the offspring.

Jackie 29 Sep 1:35 PM

That part is amazing... manatees, which we have a lot of here in Florida, keep their young around for 5-6 years and some longer... I do think parental care has something to do with it.

Instructor Alex 29 Sep 6:44 PM

Good points. And yet - many of the shark species are functioning against those comments... What say you?
Alex

Jez 31 Jul 2:07 PM

I agree with the comments that extensive parental care has a lot to do with stability of a species....when thinking about Alex's comment that many sharks are going against this statement this is what I was thinking....Sharks, whether are viviparous or oviparous, take some time and energy into making sure that their pups are successful. However, it ends at the "nursery areas" where the mother then leaves and goes off to the open ocean to feed. I think most of their care goes into prenatal care because once the pups are born they are mini versions of their parents and can defend themselves and feed on their own whereas other species need the mother to be around to protect and feed them.

Scientist Alison 1 Aug 6:12 AM

The lack of any type of parental care is probably an indication that none is needed. Indeed the neonates are perfect little sharks, with fully formed teeth and plenty of strength in their jaws. They are probably born very hungry, and since all the systems are in place at this stage feeding is not an issue. Some hornshark neonates will congregate though, and will have a diet slightly distinct from adults. But there may be one interesting case of "parental care": in at least species of freshwater stingray from Brazil (in the Rio Negro), neonates have been observed to remain on their mother's disc for a few days. They apparently just sit there, and no one knows if there is any specific purpose to this. This may be the case in other species of Potamotrygonidae (the family of Neotropical freshwater stingrays), where even specimens collected had neonates on their backs. It is hard to imagine if this is for feeding purposes, because the mother's mouth is underneath, but it may just be for protection. These stingrays have faster gestation periods (some 5 months for some species, I believe), and maybe that is a factor.

Becky 3 Aug 12:37 PM

Alex,
While sharks invest energy when their offspring are developing internally, the care all ends when they are born. So, there is an advantage to one part and a disadvantage to the other. They may be born being able to fend for themselves, but a little help from mom couldn't hurt.
Beck

Instructor Alex 3 Aug 5:23 PM

A little help from either parent.
I am always in awe of the crocodile parents that pick up hatchlings in their massive jaws and gently deposit them in the water for their first swim/bath.