For ages naturalists have wondered about the head of a starfish, a creature with five identical arms. When you inspect a worm or a fish, the head is obvious. But with five evenly matched arms, any one of them could lead the starfish along the seafloor. The question of which end is the head and which is the tail remained unsettled for a long time. This unusual body plan led many scientists to think starfish might not have a conventional head or legs.
Today, papers from laboratories at Stanford University and UC Berkeley help settle that mystery. In essence, while the research detected genetic signatures linked to head development across developing starfish, the expression of genes tied to a traditional trunk and tail was largely absent.
Researchers employed a suite of advanced molecular and genomic techniques to map how and when different genes arise during starfish development. A team from Southampton used micro-CT scanning to reveal the animal’s shape and structure in unprecedented detail.
Another finding showed that molecular signatures typically linked to the foremost part of a head appeared toward the middle of each arm, with these signals becoming more posterior and extending toward the arm edges.
Actually it’s all heads
And the study, published on November 1 in Nature, suggests that starfish were not headless cheaters of evolution but rather that bodily trunks have been lost while heads persisted.
“It’s as if the starfish were missing a trunk entirely, and the clearest way to describe it is this: a head crawling on the seabed,” said Laurent Formery, a postdoctoral researcher and lead author of the study. “This is not what scientists previously believed about these animals.”
Two of the study’s three co-authors, marine biologist Christopher Lowe of Stanford and UC Berkeley’s Daniel Rokhsar, a specialist in the molecular evolution of animals, have collaborated for a decade.
Most animals, including humans, are bilaterally symmetrical, meaning they can be divided into two mirrored halves along a head-to-tail axis. In 1995, the Nobel Prize in Physiology or Medicine acknowledged work showing that bilateral body plans arise from a set of encoded molecular switches governing development. The same genetic programming is shared by many species, including vertebrates like humans and fish, as well as numerous invertebrates such as insects and worms.
Researchers have since confirmed that this genetic blueprint is common across most animal groups, including vertebrates and invertebrates alike, underscoring a deep link in how bodies are built.
Fivefold symmetry instead of double
The starfish’s body design has long puzzled scientists. Rather than a simple front and back, adult echinoderms such as starfish, sea urchins, and sea cucumbers show fivefold symmetry with no obvious head or tail. Evaluating how genetic programming could drive this unusual fivefold pattern has been a major challenge.
Some researchers proposed that the head-to-tail axis might stretch from the animal’s armored back to its ventral side covered by tube feet. Others suggested that each of the five arms could correspond to a separate head-tail axis. Yet confirming any of these ideas has been difficult because typical gene-expression methods were developed for model organisms like mice and flies and do not translate easily to developing starfish tissue. For years, Lowe and colleagues sought genetic evidence showing how starfish develop, but a comprehensive view required tools not yet available.
Innovative technology
Lowe found a path forward during a regular Biohub meeting in San Francisco. A colleague suggested reaching out to PacBio, a Silicon Valley company advancing genome sequencing devices. For five years, PacBio has been refining a method to read long stretches of DNA in one go, using tiny chips packed with millions of reactors that can read large DNA segments in a single run.
Unlike traditional sequencing that chops DNA for accuracy, PacBio’s HiFi sequencing preserves long, intact DNA strands, delivering precise data. This approach enabled the team to study starfish genetics from scratch and at a scale not previously possible.
“The sorting that would have taken months can now be done in hours.” “These advances let us start from scratch in ways we could not five years ago,” said David Rank, a former PacBio scientist and co-senior author. The project opened doors to studying an organism rarely examined in the lab and conducting a level of detail that was out of reach a decade ago.
A new genetic analysis system made it possible to see traces of the brain in the body of the star
This technology enabled researchers to sequence starfish genomes and apply spatial transcriptomics to map where genes are active within the animal. They examined three directional axes to detect patterns along the body: center to arm tips, bottom to top, and one region to another across the arms. To visualize key genes more clearly, they labeled them with fluorescent dyes, constructing a detailed distribution map inside the starfish.
Brains in arms
The results challenged common hypotheses about starfish anatomy. In bilaterally symmetric animals, forebrain activity aligns with the midline of the body. In starfish, the gene expression linked to higher brain regions tracked along the midline but extended toward the outer edges of the arms. Genes marking different trunk regions in humans and other animals appeared only at the arm edges, not along a central trunk.
The findings suggest that echinoderms, particularly starfish, clearly show how head and body regions can become separated. This discovery invites many new questions and suggests that some ancient starfish ancestors might have carried a trunk in the fossil record.
The door to new discoveries
The team now plans to explore whether the same genetic pattern appears in sea urchins and sea cucumbers. Formery also aims to learn what starfish can teach about how nervous systems evolve, a topic that remains only partially understood in echinoderms.
Understanding starfish and their relatives could illuminate fundamental questions about animal evolution and inspire innovations in medicine, according to researchers. Starfish move water through thousands of tube feet to walk and extend their stomachs to digest prey, illustrating how unusual life forms can unfold unexpected strategies to stay healthy. Studying these strategies might broaden approaches to human diseases.
“Less studied organisms are harder to work with, but discovering unusual animals broadens our view of biology and can help solve ecological and biomedical challenges,” Rokhsar remarked.
Reference: Nature, 2023. This study adds to a growing body of work on how diverse life forms navigate development and evolution.
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