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  Topic: Shark evolution, Warm-bloodedness in the Great White< Next Oldest | Next Newest >  
Alan Fox



Posts: 1402
Joined: Aug. 2005

(Permalink) Posted: Jan. 14 2006,10:36   

I don't know where I came across the news (to me) that Great White Sharks maintained their body temperature higher than that of the surrounding water, and thus are able to be active predators on colder water than other species. But it shows to me that evolution is still happening.

I do recall John A Davison mentioning more than once that some species of shark (presumably those that bear live young, such as the Great White, have a "true" placenta. I'm not sure what John meant by true in this context, possibly to do with his front loading hypothesis.

Luckily for me we have a resident shark expert on PT who may expand on the subject.

Over to you, Sir T.

  
sir_toejam



Posts: 846
Joined: April 2005

(Permalink) Posted: Jan. 14 2006,11:56   

hmm... fox, fox,  your name sounds familar now that you mention it.

wouldn't be related to andrew fox by any chance?

http://www.sharkfoundation.com/

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:

http://homepage.mac.com/mollet/Links.html#Elasmobranch%20Research

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.

that said...


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)

http://www.elasmo-research.org/educati....ism.htm


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:

http://www.tunaresearch.org/reprints/reprints.html

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:

http://www.springerlink.com/(4jlzgo....02877,1

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:

http://www.journals.royalsoc.ac.uk/(sf2cju....02024,1

EDIT:  hmm. this link doesn't seem to work correctly, so here is the abstract:

Quote
Proceedings: Biological Sciences
ISSN: 0962-8452 (Paper) 1471-2954 (Online)
Issue: Volume 264, Number 1386 / September 22, 1997

Pages: 1309 - 1315
DOI: 10.1098/rspb.1997.0181
URL: Linking Options  
Evolutionary transitions among egg-laying, live-bearing and maternal inputs in sharks and rays


N. K. Dulvy, J. D. Reynolds

Abstract:


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:

http://www.healthwatcher.net/Quackerywatch/Shark-cartilage/

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.

cheers

  
sir_toejam



Posts: 846
Joined: April 2005

(Permalink) Posted: Jan. 14 2006,12:04   

did you want to know more about how the heat exchange system works in sharks (same as in bony fishes that also exhibit this, like tunas, btw).

hot stuff (heh)

also of note is that this is the same construction that allows bony fishes to adjust the gas volume in their swimbladders.

  
Alan Fox



Posts: 1402
Joined: Aug. 2005

(Permalink) Posted: Jan. 14 2006,12:15   

Bloody ####, Sir T

That was more than I bargained for, and it's past my bedtime.
Much appreciated though, and I will have a look at links and get back with any further thoughts. Blog time will be a bit restricted from tomorrow as have to go back to work

  
Dean Morrison



Posts: 216
Joined: Dec. 2005

(Permalink) Posted: Jan. 14 2006,12:35   

Aren't Leatherback turtles 'warm blooded' also? They occasionally stray into cool British waters....

  
sir_toejam



Posts: 846
Joined: April 2005

(Permalink) Posted: Jan. 14 2006,12:57   

turtles warmblooded you say?

hmm. news to me.

you should ask Lenny.

he's more of an expert on reptiles than I.

however, a quick search shows that by jove, you are correct (well, at least that they share a similar heat exchange system).

wasn't hard to find, and yes, they use a similar counter-current exchange system using capillaries.

http://www.bio.davidson.edu/people....low.htm

note that's DAVIDSON.edu, not davison :)

hmm. that would make the rete-mirabila system of heat exchange an even better example of convergent evolution than i had anticipated.

It appears to be quite a ubiquitous trait among ectothermic pelagic swimmers, to be sure.

hmm.  which brings up the question of whether ALL sea turtles exhibit this, or only ones that have a more pelagic existence, or spend a lot of time migrating in the open ocean.

note however, that we still consider ectotherms, even with heat exchangers, to still be ecototherms.

the system for regulating body temperature is entirely different from a true endotherm, like a mammal.

  
sir_toejam



Posts: 846
Joined: April 2005

(Permalink) Posted: Jan. 14 2006,13:58   

oh, and continuing on the thread title;

the evolution of shark species in general, and white sharks in particular, is an active and ongoing area with several heated debates between various factions.

you can kinda get a picture of this from this recent article:

http://www.sciencedaily.com/releases/2005/05/050502144430.htm

to put it simply, there is still much debate about the exact lineage of Carcharodon carcharias, with the older synthesis having them coming from the famed "Megalodon", while more recent theories have them as being derivative of more modern lamnids.

I'm sure this picture gets even more interesting as the molecular biologists get involved.

  
Alan Fox



Posts: 1402
Joined: Aug. 2005

(Permalink) Posted: Jan. 21 2006,04:05   

Is this paper on shark evolution, the one you intended to link to, Sir T?

  
sir_toejam



Posts: 846
Joined: April 2005

(Permalink) Posted: Jan. 21 2006,08:46   

lol.  

I don't know because your link goes to the same place!

gotta register to get to where it was supposed to go, and I never bothered.

  
Schmitt.



Posts: 3
Joined: Dec. 2005

(Permalink) Posted: Jan. 24 2006,02:23   

Quote (Dean Morrison @ Jan. 14 2006,18:35)
Aren't Leatherback turtles 'warm blooded' also? They occasionally stray into cool British waters....

Leatherback (and other sea) turtles are poikilothermic, but due to their large volume and low surface area, thick layers of fat and counter-current blood vessels in their forelimbs, are able to maintain a fairly high body temperature/metabolic rate.

    
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