by Matthew Cobb
Last year we discussed one of the questions I routinely ask my students – why are there no insects in the sea? Today’s poser is a question a student, Xaali, recently asked me: Why are there no freshwater cephalopods?
Now I don’t know much about molluscs (arthropods are more my bag), but the main reasons might be:
• Salinity (but other molluscs seem to cope quite well, and what about low salinity areas in estuaries? Is it all to do with the cephalopod ‘kidney’?)
• Oxygen levels (but is there really a difference between freshwater and marine O2 levels?)
• Food availability or some other ecological factor (but there are some pretty big bodies of freshwater out there, so this seems unlikely)
Whatever factor is involved (and there may be more than one), it would presumably have to be pretty substantial. Although the idea of cephalopods migrating up rivers to lakes might seem unlikely, at least some freshwater lakes will have been formed from inlets where cephalopods were presumably trapped, and then died out as the water turned less saline.
So, WEIT readers, what are your hypotheses? And above all how could you test those hypotheses?
For those who’d like an introduction to the issue, there is this thread over at tonmo.com which has stretched over a couple of years… Try not to repeat those folk!
Oh, and I expect PZ to weigh in here!