Increasing size makes coordination harder in Placozoa, an animal without a brain or neurons.

What is it about?

We investigated how the Placozoa, the simplest living animals that moves by the collective force of thousands of cells, is able to coordinate the action of its cells across its body, and how increasing in size makes coordination increasingly difficult. As the animal gets bigger, movement across the animal's body becomes less coordinated, and we show that this is the result of cells communicating less effectively with each other. We use simulations to explore if there were any way to overcome this trade-off between increasing size and decreasing coordination, and find that this is inescapable. This suggests that there is a size limit for such decentralized animals, which may have motivated the evolution of more advanced structures like nervous systems.

Featured Image

Photo by Scott Webb on Unsplash

Photo by Scott Webb on Unsplash

Why is it important?

A question that impacts life at all scales - from cells to cities - is to what extent things should be organized in a centralized hierarchy or as a distributed, leaderless network. We tackle this question from the perspective of the Placozoa species Trichoplax adhaerens, arguably the simplest multicellular animal. This animal lacks a brain, nervous system, head, or any other organs, instead acting as a flat carpet of cells that searches for food by crawling on surfaces through the collective action of thousands of cells. We show how as this animal grows in size and cell count, coordination becomes increasingly difficult, and use simulations to reveal that there is no escaping this trade-off for an animal with such a simple, decentralized anatomy. Our work reveals the limit of coordination in such simple structures or networks, and suggests that as groups grow in size, the complexity of the network also needs to increase through hierarchies and long-range communication channels. Ultimately this work has implications not only for biology and the evolution of nervous systems, but also for how we structure our organizations and societies.

Perspectives