These Animals Only Have a Head, No Body
The unorthodox anatomy of Sea Stars (Starfish)
Recent studies on sea stars, those little leggy looking things you sometimes see at low tide in rock pools, has suggested that they, in fact, are simply all head parts, and no body.
How come? How can that be?
The conventional understanding of body axes in animals often relies on bilateral symmetry, where left and right sides mirror each other.
However, echinoderms, particularly sea stars, challenge this norm with their mysterious pentaradial symmetry. Pentaradial symmetry is a type of radial symmetry, which is a characteristic of echinoderms, in which body parts are arranged along five rays of symmetry. It means the organism is in five parts around a central axis.
In a groundbreaking study, Formery et al. employed advanced molecular tools to demystify the perplexing anatomy of sea stars, leading to unexpected revelations about their evolutionary journey and the absence of a trunk.
Decoding unconventional symmetry
Spotting the anterior and posterior in common pets like birds or cats is intuitive, thanks to bilateral symmetry. However, sea stars, belonging to the echinoderm family, introduce a unique challenge with their pentaradial symmetry. The five arms of a sea star are not easily categorized into traditional front and back ends, sparking curiosity among zoologists for over a century.
The enigma of sea star embryology
Embryology provides a potential key to understanding the sea star’s enigmatic anatomy.
The emergence of a bilateral larva in echinoderms during development hints at the location of the anterior and posterior in the adult sea star. Formery and collaborators look at this intricate transformation using molecular techniques, shedding light on the sea star’s evolutionary journey.
Molecular tools unveil the sea star’s secrets
With the aid of molecular tools and RNA tomography, Formery and colleagues investigate the genetic underpinnings of sea star development, and contrary to prior hypotheses, the anterior-to-posterior axis in sea stars is revealed to extend from the median of the arms toward the lateral parts.
This unexpected finding challenges conventional wisdom and reshapes our understanding of echinoderm anatomy.
The sacrifice of the trunk
While uncovering the location of the ‘anterior’ in sea stars, Formery et al.’s study delivers a surprising revelation — the absence of genes regulating trunk development. Unlike typical bilaterians, sea stars lack a trunk, indicating that the evolutionary shift to their type of symmetry led to the sacrifice of this crucial body part.
The entire structure of a sea star aligns with the bilaterian head, setting a precedent for other echinoderms.
Head-centric development
In many animals, the development of the head and trunk involves distinct pathways and sets of transcription factors. Sea stars, and by extension, other echinoderms, exemplify an extreme case where the adult organism halts trunk development, resulting in a tailless form.
This peculiar evolutionary adaptation challenges conventional notions and highlights the diversity of developmental pathways in the animal kingdom.
Redefining our perception of sea stars
Formery et al.’s groundbreaking research not only pinpoints the elusive front and back ends in sea stars but also unravels a broader evolutionary story. The sacrifice of the trunk in favour of pentaradial symmetry marks echinoderms as a unique clade characterized solely by the presence of a head.
This study invites us to re-evaluate our understanding of animal anatomy and underscores the importance of molecular tools in deciphering nature’s intricate designs.
Sources:
A sea star is only a head: Trends in Genetics (cell.com)
Open AccessPublished:January 31, 2024DOI:https://doi.org/10.1016/j.tig.2024.01.008
