MBL Assistant Scientist Zak Swartz, who is developing the starfish as a model system, provides comment in this article.

Pop quiz: How many arms do sea stars have?

The answer seems obvious: one, two, three, four, five. Five arms. Anyone who has stumbled across one splayed on the seashore knows this.

But if that is your answer, bad news: You’re wrong. In new research published Wednesday, scientists detail how sea stars evolved into their distinct star shape, and it may have just upended everything commonly known about the animal’s anatomy. The answer is stranger than anything most scientists expected.

Simply put, the sea star, also known as starfish, appears to be mostly just a head. Molecular analysis of its genes suggests its ancestors evolved to lose their trunk. Read rest of the article here.

Source: How Many Arms Do Starfish Have? If You Said ‘Five,’ You’re Wrong | The Washington Post