cuttlegirl;107258 said:
Many of us have fed our cephalopods fresh water shrimp, so it is possible for them to prey upon fresh water species.
I am wondering if it has something to do with their high metabolism.
I find it interesting that mammals like the Baikal Seal and the freshwater dolphin species have survived in freshwater environments (although they are now threatened by human pollution...).
I like the re-awakening of this thread!
It seems like marine mammals have a rather different approach to osmotic regulation than many other marine animals, in that they tend to keep the land-evolved systems isolated from the water as much as possible, while gill-using animals tend to, by necessity, have a lot of membrane exposure to salt water directly. I'm not sure how marine mammals get hydrated, though.
The fact that so many octopuses live in intertidal zones sure does fly in the face of most of the arguments for why they have never moved into fresh water... another thing that no one has mentioned recently is that cephs are a very ancient group, and I know that the ocean's salinity is believed to have been appreciably less in the Cambrian, so there's some evidence that early shelled cephs existed in lower salinity conditions.
Has anyone else read Peter Ward's
Out of Thin Air book? It's overall about how oxygen levels have been a major and often overlooked driving force for evolution (with many hypotheses that Ward advocates looking at, with appropriate caveats that some may well be wrong... it's a food for thought book.) One of the variants of this story is the hypothesis that cephalopod development of jetting and neutral buoyancy, and hence effective predation, was secondary to development of better respiration: that the mantle inhalation and exhalation through the siphon was to get better circulation of low-oxygen water past the gills, and that had the secondary effect of jet-swimming and higher possible metabolism, which offered an opportunity to prey on the oxygen-starved slower critters in the environment, which nudged the evolution in the direction of free swimming, improved jetting, neutral buoyancy, and tentacles to grab lunch. He also suspects the segmentation of trilobites was more about repeating the body segment that has the gill than anything.
I had already been thinking that the presence of so many cephs in deep, low-oxygen water makes many of the "traditional views" of hemocyanin as inferior to haemoglobin is vertebrate arrogance, particularly in light of Gilly's lab's demonstrations of active, fast, huge humbodts spending a lot of time in the anoxic layer, and reports of nautilus having a good ability to survive low-oxygen and even out-of-water conditions. Ward's book, however, really got me thinking a lot more about the inconsistencies of things I'd read, like "the teleost fishes were able to drive the cephalopods to deeper water because their metabolism was much more effective due to haemoglobin and better oxygen storage in tissues." I'm sure this is true up to a point, but the fact that the cephs are the ones who were more successful in the low-oxygen depths suggests that rather than being metabolically challenged, the cephs outcompete vertebrates more readily at low oxygen levels, so the vertebrate respiratory and metabolic systems may be more tuned to current atmospheric oxygen levels rather than universally "better."
I suspect that cephs' mechanisms for handling low oxygen levels are more in the domain of "less well understood" than vertebrates' systems, and there's a tendency to assume "not understood" is "inferior." I'm not sure how osmotic regulation occurs in any mollusc, or how freshwater bivalves and gastropods differ from their marine counterparts in mechanisms for maintaining homeostasis in freshwater environments, and likewise for terrestrial gastropods. I note that table salt is lethal for slugs, snails, and leeches. Is vastly over-salted water lethal to marine molluscs in a similar way? How about freshwater molluscs in marine salinity levels?
Asking why there are no freshwater cephs seems to also just be a reminder of the huge gaps in our understanding of the history of the modern cephs: the almost complete lack of soft body fossils, and consequently our lack of understanding of the prevalence of non-shelled cephs at various points in their evolutionary history-- which raises a question of whether it's easier, harder, or the same for a shelled ceph to migrate to freshwater (where the pumps in the siphuncle and chamber fluid composition might be impacted) or for a coleoid (where it's harder to be neutrally buoyant in freshwater than denser salt water, although octos, for example, seem happy enough to be negatively buoyant.)
I'm not really buying the "all the niches were occupied" argument as an absolute argument, although it may be sort of a "luck of the draw" argument... It seems like cephs were dominant predators in the ocean before vertebrates had established themselves anywhere, certainly not on land. I'd assume that plants and arthropods, and maybe gastropods, had to be well established on land well before vertebrates... if anything, all the hungry ammonites probably provided pressure for arthropods to escape to dry land. Anyone know if there's a fossil record of when the first terrestrial or freshwater gastropods emerged?
A related question someone asked me that may appeal to the readers of this thread is "how come the queen/drone/worker/hive arthropod model that's so successful for insects on land doesn't seem to occur in marine arthropods?" unless there are examples in marine arthropods that I'm unaware of...
And as long as we're asking such questions, perhaps considering why there are no terrestrial or freshwater echinoderms would be interesting as well... I'd think a starfish or sea urchin would be at least as good a candidate as a snail for crawling about on dry land and eating plants and lichens and such off of rocks.
