Sea Star Regeneration of Arms
Date: Spring 2013
Why do sea stars sometimes regrow 2 arms instead of just 1?
Yes, sea stars can regrow a few arms at once if needed!
In regeneration, there are a series of "signaling" that takes place that begins the process of growing a new arm. Simplified greatly, the remaining living cells have always had living cells boarding it and if these cells are missing or dead, the starfish DNA in the remaining cells start the series of replacment cell divisons leading to a new arm. (This process is an entire course in graduate school so this is not in detail).
Sometimes this missing arm signaling begins the process twice resulting in two arms. It is automatic and once started goes through completion.
Thanks for the question. Seastars may regrow two arms instead of one. Here's how it could happen: Normally, one cell develops and begins growing an appendage which later becomes the arm. However, if that one cell divides (by mitosis) when it is not supposed to, there will now be two cells and each can regrow an arm. Thus, the seastar will regrow two arms instead of one.
I hope this helps. Please let me know if you have more questions.
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Update: November 2011