I'm with you on food, might add salinity, but I'm not sure I buy the respiration thing: many cephs live in deep, low oxygen conditions, and do just fine....
Boy, this is one of those questions that really fascinates me... why haven't cephs gone to freshwater (along with why is semelparity adaptive in octos...)
I have to disagree with the not enough food part. While freshwater does have lower bimass overall that marine environments, there are some that have quite high primary production and very high biomass, such as the Amazon river and its tributaries, the Mekong river etc. Even in North America native freshwater mussels can be very abundant, (Margaritifera falcata reaches densities of 39 individuals per square meter , with average individual size about 8cm). These areas are comparable with many tropical marine areas (not coral reefs), which are relatively barren and have low primary productivity. Cephalopods seem to do fine in the deep sea as well, which has a much lower biomass density than many freshwater.
I really don't buy much into the respiration argument either. Nearly all surface waters (down to a 50 m or so), be it freshwater or marine, are oxygen saturated. Saturation level is dictated by water temperature. So really this isn't a problem. Rainbow trout have similar respiratory levels and thrive in freshwater. And it isn't a problem with extraction efficiency, as octopuses can extract up to 76% of the oxygen from their ventilatory stream in normoxic conditions.
I would tend to agree with Monty that it is likely partly a osmoregulation problem. But seriously, there are some cephs that frequent intertidal areas, like O. rubescens
, that seem to be fairly resilient to reasonable salinity changes. Additionally, gastropods, the mollusks most closely related to cephs, haven't seemed to have had trouble invading freshwater or terrestrial environments.
I would also say variability in temperature would be another problem in freshwater for cephs, but then again we have cephs that frequent intertidal areas that seem to be fairly resilient to this as well. I will use O. rubescens
as an example again, I have seen them go from 11 C to 0 C in the space of three hours, stay down there for about four hours and brought back up to 11 C again over the course of another three hours, and went on to live a long (for an octopus) life. That was as accute change, but O. rubescens
occurs from Alaska to the Sea of Cortez, so that suggests that they physiologically can handle a large range of temperatures.
Anyhow, thats my
. Sorry if that was a bit long, But I find this all really interesting. I have a hunch if some research was to dedicate a descent about of time on the question, they would uncover some really interesting stuff about ceph physiology.