qetzal
Posts: 311
Joined: Feb. 2006
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All these questions about spines and chemicals are easily answered if one simply reads the paper that C.J.O'Brien linked just yesterday.
The chemicals are released when fish digest Daphnia that they have eaten. In other words, the chemicals are digested Daphnia. Those chemicals get released into the water where the fish and the Daphnia live. The Daphnia can detect these chemicals and 'recognize' them (in a non-anthropomorphic way) as a signal that there are Daphnia-eating fish around.
It's rather like when you eat asparagus. One of the digestion products has a distinct odor. You release it into the environment when you pee. If someone is nearby at the time, they can smell it and recognize that you've been eating asparagus. You didn't purposely release the chemical in your urine. It's just an unavoidable consequence of eating and digesting asparagus.
Anyway, the chemicals trigger developmental changes in the Daphnia that result in spines (or bigger spines). I think it's clear that appearance of the spines is not based on a contemporaneous mutational event. The Daphnia already have the ability to respond to the chemical signals by growing the spines.
One could ask why Daphnia don't have spines all the time. My guess is that there's an energetic cost to growing and maintaining the spines, and that it's advantageous not to grow them if they aren't needed for defense. That's admittedly speculative on my part, although it's testable (and may have been tested, for all I know).
SS seems to think that, just because some morphological changes occur in response to environmental cues, therefore all morphological changes occur in response to environmental cues. Why he thinks that's a logical conclusion is beyond me.
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