Sunday, March 08, 2009

Is the name "Sea Star" more accurate than "Starfish"?

It has become common in recent popularizations to suggest that echinoderms of the family Asteroidea should be given the common name "sea star" rather than the traditional "starfish". The complaint is understandable -- starfish aren't fish. In fact, humans are more closely related to fish, as they both are vertebrates while starfish are not. But is "sea star" actually a better supported name? A taxonomic analysis suggests not.

Let's consider stars, the sea, fish, and starfish.  How can we compare such diverse entities? Obviously not by molecular means. But we can return to the traditional means of cladistic characters. In this analysis I used:  1) Living 2) Primarily water 3) Non-trivial concentration of dissolved NaCl, and 4) presence of backbone.

These characters can be encoded in a PHYLIP matrix in the following manner.
    4    4
Star 0000
Fish 1111
Sea 0110
Starfish 1110
By standard parsimony this yields the following midpoint-rooted tree






So, while "starfish" is inaccurate, "sea star" is even worse!

3 comments:

Joe Dunckley said...

surely you'd want (4) to be "absence of a backbone", not presence?

Jonathan Badger said...

Well, I suppose it would be following proper convention to do what you suggest, as absence is the ancestral state, but as far as the program itself is concerned, it doesn't matter -- you get the same tree either way.

Emmanuel said...

Hey Jonathan,

I stumbled upon your blog, and this is a very interesting exercise, and also quite fun ~:0) ! So to play the devil's advocate here, I think your analysis is really biased against the star, because all the features you are analyzing have a 0 value for the star. So what if you were considering a feature called "shaped like a star", which would be the precise reason why a starfish would be renamed "sea star"?
Secondly, I disagree with the 0 value for "Non-trivial concentration of dissolved NaCl". Stars do have NaCl (see: http://tiny.cc/0fVcZ and also http://tiny.cc/IvPCV): it might not be dissolved in a liquid medium, but NaCl is NaCl.
So now, rebuilding your tree with "Star 00101" (the last column being "shaped like a star") would probably give you an entirely different result :0)

-Emmanuel