What is a sea squirt?

Inside the animal that eats its own brain

(Image credit: Nick Hobgood/Wikimedia Commons)

Sea squirts might look like plants of the ocean, but they are actually rather advanced animals. When fully grown, these potato-shaped organisms sit brainless and stationary at the bottom of the ocean. However, before this stage they had a much more active life.

Born looking similar to tadpoles with a backbone and muscular tail, the sea squirt is free to explore the ocean. The young sea squirt is tasked with finding a suitable place to live the rest of its life. It does this using its sucker located on its head. Once attached to the surface, whether that is the sea bed, bottom of a ship or back of a crab, the sea squirt no longer needs its brain. Fed automatically by the sea’s nutrients as water streams through its gills, all the animal needs to do is release reproductive cells for the continuation of the species. In fact, shortly after finding the surface to spend the rest of its life on, the sea squirt eats its own brain and its tail disappears.

Protecting themselves might sound difficult without a brain, but this comes as an automatic response. They react to touch by squirting water and waste products, deterring predators.

Squirt anatomy

(Illustration by Nicholas Forder)

1. Huge filter

This area filters out waste products and excess water, while nutrients and some water remain to be digested.

2. Opening siphon

This siphon brings water into the sea squirt. Hair-like structures inside the tube form a current to draw it in.

3. Nerve cells

With no brain, a group of nerve cells allows the body to sense touch. This is crucial in triggering its squirt response.

4. Release siphon

Giving them their name, sea squirts tend to squirt water when they are under attack or need to release waste. Contracting their bodies forces water and waste out of this tube. It is possible for them to squirt out their entire digestive tract and then grow it back again.

5. Slither heart

The elongated heart contracts at one end, which spreads to the other side of the heart and then changes direction. This pumping system circulates blood around their bodies.

6. Reproductive organs

Though they have both male and female reproductive organs, they can not reproduce by themselves. The eggs stay in the body waiting to be fertilised by another sea squirt.

7. Stomach

Like in our stomachs, the sea squirt uses enzymes to break down the filtered food. 

8. Waste products

The products that haven’t been used in the body are released into a large cavity. This cavity also holds the water and waste from the first filtering process.

9. Protective tunic

The animal has a thick outer layer to protect internal organs. It has evolved to need this layer as it spends the majority of life immobile and with limited defence.



For more science and technology articles, pick up the latest copy of How It Works from all good retailers or from our website now. If you have a tablet or smartphone, you can also download the digital version onto your iOS or Android device. To make sure you never miss an issue of How It Works magazine, subscribe today!