Fresh Water

Fujisawas Sake said:
Another point; osmoregulation. Cephs don't handle freshwater. They are by their nature isosmotic with the seawater and freshwater would stun and most likely kill a ceph quickly. Their metanephridia are no where near as hardy as our kidneys, which are pretty fragile. Fish are better at taking the extremes of freshwater by highly developed kidneys. Very few molluscs are freshwater, and those that are show signs of having returned to the water from a land-based life (pulmonate gastropods).

wasn't the ocean salinity a lot lower when molluscs first arose, though? It seems more plausible that there is genetic leftover that could be reverted to as a pretty simple mutation that could allow them to live in freshwater environments...

- M
snafflehound@work said:
Zebra mussels returned to the water from a land based life! ! ?? !

Sorry, force of habit from staring at a tank full of periwinkles while I'm typing.


Given the amount of specialized adaptation of the cephalopod metanephridia, as well as the amount of necessary ionic concentration in their cells, I doubt a simple mutation would suffice here, though I would say its far from impossible.

Good point about early ocean salinity. I would definitely like to find out more about that.

To slightly digress, and to split an infinitive in the process ... are there any *giant* freshwater invertebrates?

Define "giant"... :wink:

I don't think that there are any "giant" freshwater inverts per se, thought some leeches can get pretty big, and some freshwater crustaceans can get nearly a foot long.

I'm wondering why that is as well... Maybe its an ecological phsyiological constraint, or another example to outcompetition by fishes?

even if the oceans did have far less salt in them then they do now, and the cephs of that day could possibly make an evoultionary switch to freshwater; the cephs of today would have a much harder time evolutionwise to do something such as this, partly because of how much more has been built into their genic code that relies on having a saltwater enviorment and using a "hemocylin" pigmiant. now the only possible canidates are those tidepool octo's. a couple million years of evolution could see them turn fresh or if they develope a taste for insects, terestrial.
I'm really not sure about cephs turning terrestrial, like. I ain't wholly convinced on a few fronts. First is a problem maintaining structure. Not having any supportive skeleton (OK some cephs have shells etc), would mean that movement would be ponderous, so an animal that is agile in the water column, and perfectly evolved over millions of years to achieve this, would really have no benefit from switching their habitat. Second is a desiccation problem. As far as I'm aware, cephs don't have a watertight cuticle (they don't need one). Nautiluses have a calcified shell, but is it watertight? And could you see a terrestrial nautilus dragging that thing about everywhere? Third is the fact that I can't really see any niches open to a ceph on land. It wouldn't be able to catch any prey if it's going to be crawling everywhere. Fourth is reproduction, but that's really not as big a problem as the others, since octopuses at least don't broadcast-reproduce (I don't think). Has anyone ever done work on cephalopod skin? I wonder if there was something about it that couldn't facilitate efficient osmoregulation? Usually the steps go "Marine-->Fresh-->Terrestrial", but not always. I also wondered if maybe marine water has a higher density, what with having more ions etc in it?

Graeme- who knows... maybe they just like it in the sea
In response to queries regarding fresh water gigantism: Margaritifera margaritifera grows to 15 cm, which is a nice size for a mussel, but not for a giant clam... Most of the biggest osteichthyes do live in fresh water: beluga sturgeon, european catfish, arapaima, mekong catfish, but good ole' Mola mola still takes the biscuit when the scales are read. Because Fish are so well adapted to fresh water, there's little niches for molluscs and arthropods left, other than the hard to get to nooks and crannies. The sea is a vast world, compared to the confines of lakes and rivers. Food is the main limiting factor here; you'll see Irrawaddi dolphins, but not Irrawaddi blue whales :biggrin2:

PS: Graeme, Nautilus would likely lose a lot of it's shell once on terra firma, as buoyancy becomes irrelevant, snails prove the effectiveness of hauling around your own little cabin for protection... Snails, being all that's required for land based molluscdom, sort of defeat the argument for the higher organizational level of Nautilus to come over and start competing.
The thing about nautilus is that it doesn't use its limbs for locomotion really, they are extensile tentacles, so how would it scuttle about? Would it compromise its hunting gear in order to migrate to land? If anything, my bed would be on octopuses migrating first, if a cephalopod was to become terrestrial, since they use their arms for way more than just simple prey capture, the most important of these being locommotion! Starting to wonder if the definition of tentacle relates to its extendability as much as its specialist use in prey capture only...

by the way Ob, I love your sig :lol: took a couple of seconds, but I'm digging it!

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