Where are the Head and Tail in a Sea Star?

Do starfish have five heads or just one head and a tail? Which one is the head?

Thank you Jennifer Haley Grossman for creating the script and illustrating all the images!

#STEMvee #STEMinASL #Starfish #SeaStar #Head #Tail #Symmetry #Echinoderms

ID: Alicia is wearing a black shirt and standing in front of a grey wall. “When we look at ourselves, or even a frog or a fly, we can clearly tell where the head and tail end are located. Like frogs and flies, we have a bilateral symmetry, meaning we have a head end and a tail end, and if you were to draw a line down the midline of our body, we would have a symmetrical half on either side. Most animal groups on earth are bilaterally symmetric and fall under the animal grouping called Bilaterians. For most of these animals, we can confidently rely on their external anatomy to reveal where these fundamental parts of their body plan reside. However, some Bilaterians don’t fit this textbook definition and their external anatomy is useless for the identification of head and tail regions. For example, sea stars are Bilaterians but they look very different, yet they are actually very closely related to humans! Sea stars are a part of a group of animals called Echinoderms. This group also includes sea urchins, sand dollars, sea cucumbers, sea lilies, and brittle stars. Echinoderm development is incredibly unique. As a larva, they are truly bilaterally symmetric, forming mirroring halves on either side of their midline. However, the larva then metamorphosizes into an adult that possesses a five-part radial symmetry, called pentaradial symmetry. Because of this pentaradial symmetry, when we look at the body of a sea star, or any other Echinoderms for that matter, there are no physical features that allow us to tell where the head or tail resides. However, Echinoderms evolved from a bilateral ancestor which had a head and a tail. So, what happened to their head and tail? Before getting into the answer, let’s review some critical animal evolution and biology knowledge. All Bilaterians evolved from a common ancestor which possessed a specific set of architect genes that organized its body from head to tail. Those architect genes have been passed down to all following Bilaterians throughout generations, and they still possess those same body-organizing genes today. These architect genes are expressed only in the specific parts of the body that they are building. We are able to visualize the expression of genes during the development of an organism using molecular biology tools and special microscopes. Using such tools and our knowledge of how and what architect genes are expressed during the development of Bilaterians like ourselves, we can explore the sea star’s hidden “molecular anatomy” to find out where these head and tail genes may be expressed throughout a pentaradial body without an apparent head or tail. Scientists from the Lowe Lab at Stanford were able to tag those architect genes with fluorescent molecules in small sea star juveniles. They then were able to put the sea stars under special lasers of different wavelengths that cause these fluorescent molecules to glow. These regions that were originally invisible, were now literally illuminated and visible to the eye. The most shocking part is that only the head architect genes glowed, meaning there is no tail end in a sea star. They found that the genes that normally build a stereotypical head in other Bilaterians actually build the body of a sea star. So, during their evolution, they lost their tail end and their head and brain have spread out throughout their body and arms. One could say they are basically a walking head.” Throughout the video, illustrations are shown. First is “bilateral symmetry” with a frog and a fly. “Echinoderms” show sea star, sea cucumber, sand dollar, sea urchin, sea lily, and brittle star. “Echinoderm Development” shows sea star larva going through metaorphosis to an adult, showing pentaradial symmetry. “Bilaterian Tree” shows the head and tail and architect genes as a common ancestor for many different types of animals, including echinoderms. “Architect genes are expressed only in body parts they are building” shows a fly and different genes. “Tagging architect genes with fluorescent molecules” shows a scientist looking at a computer and blue florescent light for head genes, orange fluorescent light for tail genes. “Results – they are a walking head!” shows various seastars with only blue florescent light.



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