Joined: April 2005
hmm... fox, fox, your name sounds familar now that you mention it.
wouldn't be related to andrew fox by any chance?
in any case...
First off, I'd like to say that even tho i worked with many species of sharks for many years, I still don't consider myself an "expert" in this field. there are many researches who specialize in shark research and behavior, even in CA, that i would quickly refer to as more expert than myself.
My primary specialty was in the study of pomacentrids (damslefishes), and shark research was an interesting sideline i participated in for several years when living in the Monterey Bay area in CA.
Henry Molet at the Montery Bay Aquarium has done a nice job of summarizing the various research groups working on elasmobranchs, which you can see here:
second, this will likely be a rambling missive, so tune out whenever you get bored
However, I'm happy to answer what questions i can, and will readily admit when I'm out of my depth on any specific issue.
Whatever JAD was really trying to get at, as usual, is likely beyond human comprehension. I'll take a stab at it tho.
There are in fact many species of sharks that have live birth (viviparous). Elasmobrachs as a group exhibit the entire range of gestation type, from completely oviparous (egg laying), to ovoviviparous (meaning they retain their eggs internally until they hatch), as well as what would be considered viviparity.
In viviparous species, again, there is a range of species that exhibit more or less "true placenta", meaning that they provide nourishment through a blood supply from the mother directly to the offspring.
interestingly, viviparity is not a feature shared by even all lamnid sharks.
White sharks, for example, are ovoviparous, while lemon sharks are viviparous.
there have been a few studies to look at the selective advantages of each type of gestation method, and i seem to recall one paper that looked at ovoviparity vs. vivaparity to compare offspring growth rates in sand tigers vs. lemon sharks.
sand tigers are the poster child for oophagy - that is a form of cannibalism where the young feed on the eggs that the mother produces, or even their fellow embryos (embryophagy).
comparing the growth rates of embryos in sand tigers to lemon sharks, one can see tremendous advantages to oophagy over straight viviparity, which may be one reason why ovoviparity is maintained in so many species of sharks.
see here for an overview (and a decent shark bio site)
I'm not a paleontologist, so I'm not as familiar with when the various modes of gestation are typically considered to have evolved in the various species and families of sharks.
However, I do know that sharks as a group have essentially remained unchanged in morphology and physiology for tens of millions of years, so I would suspect that both the evolution of heat-exchange systems and viviparity appeared quite some time ago.
the best person i can think of to elucidate the current school of though on this would be Dr. Barbara Block, who was working at Hopkins Marine Station in Monterey last i checked.
here's a link to her stuff:
she published a paper on the evolution of heat exchange systems in many species of fishes about a decade ago that would cover this issue quite nicely:
Trying to address JAD "pant-loading" hypothesis however, I suspect JAD remembered that sharks in general have an ancient lineage (the elasmobranchs have existed as a group for over 350-400 million years), so perhaps he is thinking that since all these gestation types exist in modern sharks, that makes them great examples of "frontloading".
however, there have been numerous studies indicating the evolution of viviparity independently in several families of sharks like this one:
EDIT: hmm. this link doesn't seem to work correctly, so here is the abstract:
|Proceedings: Biological Sciences |
ISSN: 0962-8452 (Paper) 1471-2954 (Online)
Issue: Volume 264, Number 1386 / September 22, 1997
Pages: 1309 - 1315
URL: Linking Options
Evolutionary transitions among egg-laying, live-bearing and maternal inputs in sharks and rays
N. K. Dulvy, J. D. Reynolds
Sharks and rays are thought to have a large number of independent origins of live-bearing. We examined evolutionary transitions to live-bearing and maternal input to embryos in this subclass by optimizing reproductive characters onto a composite phylogeny. Egg-laying (40 per cent of all species) is the likely ancestral reproductive mode for this clade, and there is evidence that live-bearing has evolved independently 9–10 times and maternal input 4–5 times. Most transitions (12–15) have been toward live-bearing with provisioning limited to yolk. These have occurred from egg-laying ancestors or live-bearing taxa that provide maternal input to embryos. Only 2–3 transitions have occurred in the other direction, i.e. away from yolk-only bearing. Egg-laying has evolved from live-bearing ancestors in skates, Rajidae (25 per cent of all species) and possibly in the zebra shark, Stegostoma fasciata. Thus, although there has been an overall trend toward the evolution of live-bearing in elesmobranchs, the evolution of additional maternal input has been extremely labile.
which of course completely refute the idea of Pant-loading in sharks.
similarly, we can find far more references of indpendent evolution of quite a few traits in most other groups.
sharks as a whole are still an understudied lot, mostly because they aren't as common as fruit flies, for example, and are notoriously difficult to study in the field (and in the lab, for that matter).
even so, there is plenty of research on the evolution of both gestation strategies and heat exchange systems in sharks.
one thing i would like to point out since we are talking about sharks, is that regardless of the method of gestation used, sharks as a group have VERY slow rates of reproduction.
this has caused them to be highly susceptible to commercial fisheries, and a great many species are in trouble right now.
please encourage anyone thinking about using products made from shark cartilage not to do so.
these products have NO proven medical benefit whatsoever, and in combination with the shark fin fishery (for asian shark-fin soup), are decimating shark populations worldwide.
here's a nice little summary:
ok. i'm gonna end this "chapter" here.
let me know if there is something specific, or you wanted to know what it's like to work with sharks, or see pictures etc.