Gail (response to Henry) 29 Jul 3:39 PM
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 (response to Henry) 30 Jul 11:38 AM
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 (response to Henry) 31 Jul 7:41 AM
Good points. And yet - many of the shark species are functioning against those comments... What say you?
Alex
Jez (response to Alex) 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 (response to Jez) 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 (response to Alex) 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.
Becky
Instructor: Alex (response to Becky) 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.
