- Sea urchins have a body that is almost entirely head with a complex nervous system
- Their nervous system shows genetic similarities to vertebrate brains including humans
- Echinoderms shift from bilateral larvae to radially symmetrical adults with unique body plans
Sea urchins, the spiny marine animals, may appear simple, but new research shows their bodies are almost entirely made of "head" and contain a highly complex nervous system that works like an "all-body brain." An international team of researchers discovered that this system has a genetic organization similar to the brains of vertebrates, including humans.
Jack Ullrich-Luter, biologist at the Natural History Museum in Berlin, said that their results show animals without a conventional central nervous system can still develop a brain like organisation.
He added that this fundamentally changes how we think about the evolution of complex nervous systems. Marble-like, spiny marine creatures belong to the phylum Echinodermata, which includes species such as starfish, sea cucumbers, and brittle stars. A distinctive feature of these creatures is that their body plan changes throughout their development. Humans and most animals have bilateral symmetry, meaning their bodies can be divided into two roughly equal halves. Approximately 99 percent of animals, or about 1 million species, share this characteristic and are grouped in the large group known as Bilateria.
However, all bilateral creatures have some asymmetry, such as the human heart being located slightly to the left and the liver being located predominantly to the right.
In contrast, echinoderms initially live as bilaterally symmetrical, free-swimming larvae and adopt radial, pentamerous symmetry as adults. Ulrich-Luter and his team were trying to understand how the same genome can produce two completely different body structures and which cells play a role in this transformation.
The researchers studied young purple sea urchins, Paracentrotus lividus, immediately after metamorphosis. They found that adult sea urchins have a completely "head-like" body and lack a true trunk region. The genes that form the central body structure in other species are active only in internal organs such as the intestine and the water vascular system, which these creatures use for locomotion, respiration, and transporting food and waste.
The study revealed the diversity of neuronal cells. Hundreds of individual neurons expressed both echinoderm-specific "head" genes and ancient genes found in vertebrate central nervous systems. This suggests that sea urchins possess more than just a network of neurons and ganglia, but a brain-like system spread throughout their bodies.
The team also observed that sea urchins possess light-sensitive cells, similar to structures resembling the human retina. Large parts of their nervous system appear to be light-sensitive, raising the possibility that their functions may be influenced by light. One cell type was also found to contain two distinct light receptors, demonstrating that these creatures possess a complex ability to detect and process light, a previously underappreciated ability.
