How brainless sea sponges use sunset as a cue to prepare for metamorphosis

Well before I start collecting coats, keys or shoes, my dog always knows when we’re about to leave. By picking up on subtle cues, he anticipates our next movements, reminding me he’s smarter than often shows.

But anticipation isn’t always about the brains. Our new research reveals even an animal with no brain at all can anticipate what’s coming next.

The metamorphosis of the sea sponge

Sea sponges are one of our most distant animal relatives, lacking all the familiar sensory apparatus. They have no brain, no nerves and no neurons. Being not much more than well-organised collections of specialised cells, they’re pretty much as simple as animal life gets.

And yet, sea sponges carry out a wildly complicated process much like a backwards version of a caterpillar’s transformation, from squidgy grub to free-flying bug. Starting out as free-swimming larvae, about half the size of a chia seed, they transfigure themselves into shapeless, spongy lumps, cemented in place.

My colleagues and I have researched this miraculous metamorphosis for years. We have discovered that environmental cues play a very important role.

For instance, swimming larvae are all looking for a suitable alga – the perfect spot to settle on and make their forever home. But in 2020, our group found that if we prevent larvae from experiencing sunset, they never settle, even if in the vicinity of some 5-star algae.

How could missing sunset disrupt such an important event in a young sea sponge’s life?

What sunset does to sea sponge DNA

In a new study, our team shows that sunset is a critical signal that pre-emptively tells sea sponge larvae to “get set”.

While the larvae are still freely swimming, long before settlement is on the cards, sunset triggers modifications to parts of their DNA. Sponge larvae anticipate their big moment by pre-organising parts of their DNA to be primed and ready for “go” time.

By measuring changes in the animals’ DNA (which carries their genetic material) and RNA (which shows which genes are switched on), our team captured genetic freeze-frames from before and during the transformation.

Measuring RNA is like recording all the notes being played at any one time by a musical ensemble. But capturing DNA, using an approach known as chromatin accessibility profiling, tells us something different.

The technique maps open or “accessible” regions of an animal’s genome, versus regions considered “inaccessible” or closed. If a region is accessible, previously dormant genes can be “read” and spring into action. You can imagine this as like selectively highlighting lines of a musical score to indicate which sections the musicians should play.

Our new results reveal that regions of DNA encoding genes related to settlement and metamorphosis get primed (made “accessible”) long before the genes themselves get switched on. In other words, lines of the musical score get marked up well before the musicians begin playing the notes.

The mystery of a rapid transformation

The larvae’s anticipatory genetic action means that, when the right time comes, they can initiate metamorphosis at a jaw-dropping speed. In around 30 minuets, the time it takes glue to dry, they become firmly attached and begin disassembling their larval body.

Sea sponges aren’t the only marine invertebrates to change form. Corals, sea urchins and oysters metamorphose too. And they all do it out in the open, without any protective structure like a caterpillar’s cocoon. Being exposed to dangers while transforming, they need to be fast.

But how they achieve this speed has been a longstanding mystery. We think the anticipatory approach found in sea sponges may be broadly used by diverse species to initiate rapid metamorphosis.

Familiar genes

To coordinate such a complex biological transformation with the daily timing of sunset is an extraordinary feat for an animal without a brain. As it turns out, many of the sponge genes controlling this are also used within our own bodies. For instance, a gene called CLOCK, which is involved in sponge metamorphosis, also sets our circadian rhythms.

Despite appearances, there’s a lot we can learn from sponges about genes like these, which are fundamental to animal life.

After all, with an ancestry dating back some 700 million years, sea sponges are one of the oldest animal groups still alive today. Research into these unusual creatures opens a valuable window onto early animal evolution — including how the first animals came to use the tempo of the planet to anticipate what’s coming next.

This article is republished from The Conversation under a Creative Commons license. Read the original article.