[Coral-List] Thread submission
mpnolan at lbl.gov
Thu Dec 13 23:21:16 UTC 2018
googling the fascinating question
Why aren't there any freshwater cephalopods?
from my computer at the top
ABC Science ask an expert:
But while cephalopods are clearly adept at exploiting extreme
environments, they are not found in freshwater.
"While we can't be 100 per cent certain, it's unlikely that there have
ever been freshwater cephalopods," says cephalopod expert and Head of
Science at Museum Victoria, Dr Mark Norman.
It's all to do with osmosis.
"It is probable that they never developed a sodium pump that would
help them cope with osmotic change in freshwater," Norman explains.
Freshwater dwellers have salty blood relative to the water around
them. Without a mechanism in place to control it, osmosis would
equalise salt concentrations between the animal and the water
surrounding it, pumping salt out of the body and flooding it with
freshwater. A sodium pump, like that found in freshwater fish species,
uses chloride cells on the gill surface to actively absorb sodium and
potassium ions from the environment. Any excess water taken in at the
same time is excreted as urine.
Marine dwellers have the opposite problem, and need to conserve fresh
water while expelling salt. Cephalopods pump seawater through their
gills and use their kidneys to filter out fresh water from the ocean.
Salts and waste water are channelled through the funnel.
"Dr Mark Norman is the Head of Science at Museum Victoria, Dr Norman
spoke to Rachel Sullivan."
On Thu, Dec 13, 2018 at 8:07 AM Douglas Fenner via Coral-List
<coral-list at coral.aoml.noaa.gov> wrote:
> That is a good question, I don't know that there is any answer at this
> point. Certainly there are fresh water gastropods and bivalves, but to my
> knowledge there aren't any fresh water chitons (polyplacophora),
> aplacopherans, scaphopods (tusk shells), or monoplacopherans. See
> https://en.wikipedia.org/wiki/Freshwater_mollusc So at the class level,
> there are more Mollusc classes that are restricted to salt water than there
> are that have entered fresh water. The main challenge probably is
> osmoregulation, regulating salt content and water content of the body and
> cells. The Wikipedia article on freshwater molluscs says that they have
> characteristically low tissue salinities and some of the freshwater clams
> have the lowest tissue salinities known. So they are tough and tolerant of
> low salts, instead of being good at regulating salts. Of course gastropods
> (snails and slugs) make it onto land, there are lots (most are pulmonates,
> they have lungs instead of gills), but none of the other classes make it
> onto land. The Wikipedia page on Cephalopods
> https://en.wikipedia.org/wiki/Cephalopod says that "None of them can
> tolerate freshwater <https://en.wikipedia.org/wiki/Freshwater>, but the
> brief squid, *Lolliguncula brevis
> <https://en.wikipedia.org/wiki/Lolliguncula_brevis>*, found in Chesapeake
> Bay <https://en.wikipedia.org/wiki/Chesapeake_Bay>, is a notable partial
> exception in that it tolerates brackish water
> <https://en.wikipedia.org/wiki/Cephalopod#cite_note-4>" and "Cephalopods
> are thought to be unable to live in freshwater due to multiple biochemical
> constraints, and in their +400 million year existence have never ventured
> into fully freshwater habitats.
> <https://en.wikipedia.org/wiki/Cephalopod#cite_note-5>" The groups that
> have not made it into freshwater have also not made it onto land.
> Cheers, Doug
> On Wed, Dec 12, 2018 at 4:17 AM Nadia Jogee via Coral-List <
> coral-list at coral.aoml.noaa.gov> wrote:
> > Hi all,
> > I’ve been following the ‘why is an octopus smart’ thread and has made me
> > pitch to you helpful lot a question I’ve often pondered- Why aren’t there
> > any freshwater cephalopods?
> > Every other group of mollusc have evolved both salty and fresh forms, yet
> > not the cephalopod. This question isn’t for any other reason other than
> > curiosity and wondered if anyone has any ideas!
> > Nadia
> > Sent from my iPhone
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> Douglas Fenner
> Ocean Associates, Inc. Contractor
> NOAA Fisheries Service
> Pacific Islands Regional Office
> PO Box 7390
> Pago Pago, American Samoa 96799 USA
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