Where is the starfish's head? A new answer to the age-old mystery

Where is the head of a starfish? | Tuchong Creative

If you were to put a hat on a sea star, where would you put it? This question may sound a bit nonsensical, but it actually involves a serious question in the fields of zoology and developmental biology: Where is the head of a sea star?

In life, whether we look at a worm or a fish, we can easily tell where the head and tail are. However, the head structure of a starfish, which has five identical "arms" that can be used to propel itself across the seabed, has puzzled naturalists for centuries.

On November 1, 2023, a new study published in the journal Nature provided an unexpected answer: Although starfish do not have a specific head, they have head-like areas distributed in the center of the body and in the center of each of its "arms**."

A map of gene activity in starfish

Starfish, like sea urchins and sea cucumbers, are echinoderms. When they are young, they float in the ocean as plankton. As they grow, they sink to the seafloor and perform a "growth magic" - from the bilateral symmetry (across the midline) of their childhood to the five-pointed star shape of their adult form, known as pentaradial symmetry.

This body configuration is unusual. Most animals we know of are bilaterally symmetrical. In previous studies, scientists have shown that the bilateral symmetry of animals is due to the action of a series of molecular switches, which are encoded by genes and expressed in specific head and trunk regions. The vast majority of animal species, including vertebrates such as humans and fish, and invertebrates such as insects and worms, have the same genetic program.

(Note: Gene expression refers to the process by which information encoded by genes is transcribed and translated to produce functional gene products.)

So, what kind of genetic program drives sea stars to change from bilateral symmetry in their juvenile stage to pentaradial symmetry with no clear head or tail in their adult stage?

Exploring this question has been more difficult than expected, largely because methods for measuring gene expression were developed in a few model organisms, such as mice and fruit flies, and they don’t work well in starfish.

Biologist Laurent Formery , first author of the new paper, began studying the early development of sea urchins as a graduate student. Before they become five-fold symmetrical adults, sea urchins are also bilaterally symmetrical.

The corresponding author of the paper is Professor Christopher Lowe , whose laboratory team has been eager to map the gene activity of developing sea stars for many years. After Formery joined Lowe's research team, Formery's knowledge of echinoderm development was combined with Lowe's expertise in molecular biology techniques, and they successfully created a 3D map of gene expression in sea stars.

Uncovering complex answers

In the new study, the researchers explored this question using techniques such as RNA tomography, which can pinpoint where genes are expressed in a tissue, and in situ hybridization, which can pinpoint specific RNA sequences in cells.

They sliced ​​the starfish's body tissue into thin slices and examined differences in gene expression from three different directions:

1. From the center of the starfish to the tip of the "arm"

2. From the "top" to the "lower abdomen",

3. From one edge of the "arm" to the other edge.

Using RNA tomography, they identified the genes expressed in each section.

To better observe the activity of certain key genes, they labeled the starfish tissue with fluorescent dyes. Using in situ hybridization, they monitored the tissue samples to see where the genes were expressed, creating a detailed 3D map of gene expression in the starfish.

Their results showed that expression of genes corresponding to the forebrain of most bilaterians was located along the midline of the starfish's arms, while expression of genes corresponding to the midbrain was located at the edges of the arms.

Overall, the researchers found that all of the bilaterian genes associated with the head were expressed in the sea star, but only one of the bilaterian genes associated with the trunk was found in the sea star, and this gene was located at the very edge of the sea star's "arm," suggesting that there is a tail-like region along the starfish's circumference.

In this sense, the starfish is not headless, but it is more like it has lost its body, with only a "head" crawling on the seabed.

Diverse Biological

Such results suggest that echinoderms, especially sea stars, are the best examples of head-torso separation that we know of today. The researchers say that most studies typically focus on groups of animals similar to humans, but if you focus too much on these familiar species, it's hard to learn new information.

Our planet is home to 34 different animal phyla, all of which have come up with different solutions to the same basic biological problems over a period of about 600 million years. Among these animals, most are gentle creatures that live in ocean caves. People are not usually fascinated by these animals, but they may represent the beginning of life.

This article is a work supported by Science Popularization China Starry Sky Project

Team: Principle

Reviewer: Ye Sheng, Professor of Beijing University of Aeronautics and Astronautics

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.