- Joined
- Jan 19, 2007
- Messages
- 314
There is an important difference here between calcium carbonate, which is the calcium in shells and cuttlebones, and ionic calcium (calcium chloride most commonly when not free ionic) used in neurons and other cells. Even cephs with no calcified structures need calcium for cellular functions, like all animals.
About the speculation of high Ca2+ in tissue because of the 'rich' innervation - I have some doubts about this. Lots of other peripheries have very, very dense innervation (mammalian skin and body hair innervation, for example, is pretty darned extensive). Yes, cephalopods use their motor neuronal network to show off with chromatophores, and they do have a LOT of peripheral cell bodies in the arms, but I will bet you mammalian tissue rivals it.
Mammals absorb their calcium via their diets, and (presumably) cephalopods do too - I actually don't know for sure.. I guess they could absorb it directly from SW... But animals generally have no trouble getting enough of it from a regular dietary source without extra supplementation. In captivity, where food sources are far more homogenous than in the wild, supplemental calcium might be needed (just like it is in humans with bad diets...). For animals needing to make a shell in captivity (like Nautiluses), calcium supplementation might be necessary.
The original observation that calamari supplies a significant amount of daily calcium - I'm guessing this is common to many marine food sources.
About the marine-freshwater transition - it seems unlikely that calcium needs are a limiting factor. Many molluscs (with highly homologous physiologies and similarly substantially complex nervous systems) have colonised freshwater habitats just fine. I'd love to know why cephalopods never made it to freshwater, too. Any more thoughts on that topic?
About the speculation of high Ca2+ in tissue because of the 'rich' innervation - I have some doubts about this. Lots of other peripheries have very, very dense innervation (mammalian skin and body hair innervation, for example, is pretty darned extensive). Yes, cephalopods use their motor neuronal network to show off with chromatophores, and they do have a LOT of peripheral cell bodies in the arms, but I will bet you mammalian tissue rivals it.
Mammals absorb their calcium via their diets, and (presumably) cephalopods do too - I actually don't know for sure.. I guess they could absorb it directly from SW... But animals generally have no trouble getting enough of it from a regular dietary source without extra supplementation. In captivity, where food sources are far more homogenous than in the wild, supplemental calcium might be needed (just like it is in humans with bad diets...). For animals needing to make a shell in captivity (like Nautiluses), calcium supplementation might be necessary.
The original observation that calamari supplies a significant amount of daily calcium - I'm guessing this is common to many marine food sources.
About the marine-freshwater transition - it seems unlikely that calcium needs are a limiting factor. Many molluscs (with highly homologous physiologies and similarly substantially complex nervous systems) have colonised freshwater habitats just fine. I'd love to know why cephalopods never made it to freshwater, too. Any more thoughts on that topic?