Starfishes are just one big head and no limbs, say scientists

Imagine a Halloween scene like no other — a human head skittering about on hundreds of tiny feet amid all the other eerie decor. Now, I know what you're thinking: why delve into macabre fantasies when the holiday has already come and gone? The answer lies in the peculiar revelations surrounding starfish, which have stirred our imaginations and compelled us to explore the unnerving aspects of these aquatic creatures.

While the starfish may appear to boast numerous "arms" extending from its central body, recent research has uncovered a startling truth. These creatures, along with other echinoderms such as sea urchins, sea cucumbers, and sand dollars, are essentially just heads. Yes, you heard it right - they're all head and no body, unlike most creatures who possess a head, torso, limbs, and a backside.

What has made these creatures infinitely more puzzling is the fact that their evolutionary records suggest they evolved from ancestors with bilateral symmetry, much like humans, where the body consists of two mirrored halves.

Recently, scientists embarked on a quest to understand how these echinoderms, characterised by their symmetrical bodies divided into five equal sections, defy this norm. They constructed a three-dimensional map of gene expression within the sea star. This allowed them to pinpoint where specific genes were active during the creature's development, focusing on genes responsible for the growth of the ectoderm, which includes the skin and nervous system.

Their investigation yielded intriguing results. While the gene signatures linked with head development were scattered throughout juvenile sea stars, something critical was amiss. When the researchers compared the expression of genes responsible for coding the starfish's torso and tail sections with those in other animal groups, they discovered that these essential components of the body plan were notably absent. Even markers typically associated with the front of an animal's head were strangely located near the middle of each of the sea star's five arms.

One of the study's co-authors, Laurent Formery, an evolutionary biologist at Stanford University, succinctly summed up this revelation, suggesting that the sea star was completely missing a trunk, and could be best described as just a head crawling along the seafloor.

These groundbreaking findings lead researchers to speculate that sea stars and their echinoderm counterparts likely evolved their unique five-section body plan by shedding the trunk region present in their bilateral ancestors. This transformation probably provided them with distinct advantages in terms of mobility and feeding, setting them apart from animals with symmetrical body plans.

As for the question of why these creatures evolved in this peculiar manner, consider this: the oldest known starfish fossils date back over 200 million years, predating even the earliest known dinosaurs. So, while they may appear as odd, head-crawling entities, they've clearly thrived for an astonishingly long time, suggesting they've found a successful formula for survival.

The comprehensive findings of this study have been documented in the journal Nature, and can be accessed here.