A Seafloor Dwelling Head: The Astonishing Body of a Starfish
New scientific breakthroughs have finally unraveled the mysterious body structure of sea stars, revealing their unique and enigmatic biology like never before
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The heads of various creatures are typically distinguishable, yet the identification of sea stars has perplexed scientists. This ambiguity, however, has recently been resolved.
Sea stars have five arms, each with tube feet used for movement on the seafloor. This has led to speculation among naturalists about whether sea stars have defined front and back ends, and if they possess heads at all.
However, recent genetic research proposes a contrasting viewpoint. It suggests that sea stars are primarily heads, lacking torsos or tails, and may have lost these characteristics through the course of evolution. The researchers explain that the peculiar fossils of sea star ancestors, which seemed to have a type of torso, now align more logically with the new findings when viewed from an evolutionary perspective.
The research results were published in the journal Nature on Wednesday.
According to lead study author Laurent Formery, a postdoctoral scholar at Stanford University and the University of California, Berkeley, "It appears that the sea star lacks a trunk entirely and can be best described as a head that crawls along the seafloor. This contradicts the previous assumptions made by scientists regarding these creatures."
of echinoderms, which includes starfish, sea urchins, and sea cucumbers, has long perplexed scientists. However, recent advancements in genetic sequencing have presented exciting opportunities to gain deeper insights into their evolutionary history and relationships with other species, even those seemingly unrelated to them. By unlocking these revelations, scientists hope to shed light on the fascinating and enigmatic world of echinoderms.
Sea stars, along with sea urchins, sand dollars, and sea cucumbers, are part of a group known as echinoderms. These remarkable creatures possess distinct body structures that consist of five sections, setting them apart from bilateral animals that exhibit symmetrical head-to-tail body plans with corresponding left and right sides.
Laura Brown/Courtesy Northwestern Medicine
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Sea stars originate as fertilized eggs which later hatch and develop into larvae that drift in the ocean, resembling plankton, for a duration of several weeks to several months before eventually settling on the seabed. Once settled, they undergo a remarkable process that involves the transformation of their body shape from bilateral symmetry to a pentaradial form.
According to Christopher Lowe, a senior study coauthor and a marine and developmental biologist at Stanford University, this perplexing phenomenon has intrigued zoologists for centuries. Lowe states, "The transition from a bilateral body plan to a pentaradial plan and the comparison between any aspect of the starfish and our own body plan has remained a longstanding enigma in the field of zoology."
Researchers are striving to solve the puzzling connection between bilateral animals, such as humans and insects, and echinoderms, which share a common ancestor. This intriguing discovery earned the Nobel Prize in Physiology or Medicine in 1995.
"How the different body parts of echinoderms correlate with those observed in other animal groups has perplexed scientists throughout their study," stated Dr. Jeff Thompson, a lecturer at the University of Southampton and coauthor of the research. "While bilateral relatives possess a segmented body structure consisting of a head, trunk, and tail, comprehending the connection between these regions and the body of a starfish, for instance, remains a considerable challenge."
Unraveling the enigma of echinoderms
Researchers utilized micro computed tomography scanning to obtain an unprecedented three-dimensional visualization of the shape and structure of sea stars. Subsequently, advanced analytical techniques were employed by team members to identify the expression sites of genes in the tissue and to precisely locate specific RNA sequences within the cells. Gene expression refers to the activation of gene information, rendering it functional.
Specific molecular markers serve as blueprints for the body plan, guiding each cell to its proper location within the body.
Lowe stated that when the skin is removed from an animal and the genes related to distinguishing a head from a tail are examined, these same genes are responsible for determining body regions in all animal groups. "Therefore, we disregarded the anatomy and questioned whether there is a concealed molecular axis underlying the unusual anatomy, and what role it plays in the development of a starfish forming a pentaradial body plan," Lowe explained.
The nervous system of a starfish is shown here during an analysis.
Laurent Formery and his team used the data to generate a 3D map, showing the expression of genes as sea stars underwent growth and development. This allowed them to identify the specific genes responsible for regulating the development of the starfish's ectoderm, which encompasses its skin and nervous system.
Genetic markers associated with head development were detected throughout the sea stars, with a particular concentration in the central region of the star and each limb. However, the gene expression for the torso and tail sections was largely absent, indicating that sea stars exhibit the most remarkable instance of head and trunk decoupling known to date, according to Formery, a researcher at the nonprofit research organization Chan Zuckerberg BioHub in San Francisco.
The research received funding from the Chan Zuckerberg BioHub, established in 2021 by Dr. Priscilla Chan and Mark Zuckerberg, as well as NASA, the National Science Foundation, and the Leverhulme Trust.
Researchers stained the genetic material of starfish with fluorescent labels, enabling the scientists to map the behavior of the animals' genes.
According to Laurent Formery, the absence of certain gene expressions in starfish compared to other animals, such as vertebrates, suggests a significant component of their body structure is absent. The genes responsible for shaping the trunk of the animal were not expressed in the outer layer. This implies that the overall body plan of echinoderms is equivalent to the head in other animal groups.
The echinoderm body plan is believed to have evolved in a more complex manner than previously believed, according to our research. Sea stars and other echinoderms developed their unique body structures after their ancestors lost their trunk region, enabling them to move and feed in ways distinct from other animals. Thompson expressed excitement over these discoveries, stating that as a researcher of echinoderms for the past decade, these findings have fundamentally shifted their perspective on this group of fascinating creatures.
Unlocking new perspectives
While animal research predominantly focuses on species that resemble humans, exploring groups such as echinoderms could offer profound insights into the intricate tapestry of life's evolution on our planet.
"Many animals lack impressive nervous systems and live in burrows in the ocean while actively hunting for prey. These modest creatures often go unnoticed by people, but they likely hold the key to understanding the origins of life," stated Lowe.
By studying the development of creatures like sea stars, we may gain valuable insights into the diverse strategies employed by different species to maintain their well-being.
According to Daniel Rokhsar, a coauthor of the study and a professor at the University of California, Berkeley, along with being a researcher at the Chan Zuckerberg BioHub, working with less researched organisms presents a greater challenge. However, he believes that studying these unique animals and their unconventional behaviors can expand our understanding of biology. In turn, this expanded knowledge can contribute to finding solutions for ecological and biomedical issues.
