How Do Sea Squirts Protect Themselves? (They Just Don’t Taste Good!)

Sea squirts are not the toughest organism in the sea. They’re often soft, immobile creatures, or even blobs floating through the ocean.

But in order to survive, sea squirts have to have some form of defense against predators. So, what do these small, harmless-looking animals do to protect themselves?

Sea squirts protect their soft insides with an outer tunic. It provides physical protection and can also be transparent, which provides camouflage. Many sea squirts also accumulate chemicals that deter fish from eating them. Others can eject part of their digestive system as a distraction and then grow it back later.

Sea Squirts Have Armor

Sea squirts are a part of the Urochordata, or Tunicata, subphylum. The Chordata phylum covers all animals with backbones, including humans.

They need to have a notochord, a kind of flexible backbone, during some point of development to be a chordate.

Sea squirts are technically invertebrates. However, they do have a notochord when they are larvae. This helps them swim around until they find a place to settle.

As they mature, their bodies absorb this backbone-like structure until it disappears.

Another name for sea squirts is tunicate. This comes from their outer layer, or tunic, that protects the rest of their body.

Now, this tunic is not always a strong defense against active attacks. In some salps, it’s a simple sheath that contains their soft internal tissue.

In certain cases, though, it can work as camouflage. Transparent sheaths can make salps difficult to see for visual predators.

But in other species, like the Atriolum robustum, the sheath is a much firmer barrier. It can offer some protection against small fish and parasites. Some even have spicules, or spines, to deter predators.

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The tunic also protects against pathogens.

Sea Squirts Taste Bad

A sea squirt’s main line of defense is how unpalatable they are to most fish. A common way of passive resistance to predators is in the form of bad tastes or smells.

Some sea squirts produce chemicals that give them an undesirable taste. Tunicates in the Aplidium genus produce high levels of indole alkaloids. Research shows that these alkaloids deter predatory sea stars.

Other species accumulate heavy metals such as vanadium. Experts believe that acidic vanadium makes sea squirts unpalatable to predator fish.

Over time, the fish learn that sea squirts aren’t very good food and will leave them alone.

This only qualifies for animals such as fish, though. For humans, it’s a different story.

In the Mediterranean, Asia, and South America, people often catch and eat sea squirts. This includes the tunic, which people pickle to eat later.

They Eviscerate Themselves As Distractions

Sea cucumbers are well-known for eviscerating themselves. This is the process of ejecting internal organs, often to distract predators.

But there is also a species of sea squirt that can do this as well.

The Polycarpa mytiligera is a tropical sea squirt that attaches to coral reefs. Triggerfish and pufferfish often hunt in these areas, which can be dangerous.

While rarely a food target, the occasional fish will bite at the sea squirt, sometimes even by accident.

Polycarpa mytiligera will respond by ejecting its gut, which has two possible uses. One, it can distract the fish from eating the rest of the sea squirt. Or two, its unsavory nature will cause the fish to lose interest.

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Evisceration sounds like an extreme measure against predators at first. Besides losing part of their digestive system, ejection tears their branchial sac. They use this sac for filter feeding and respiration.

The key to evisceration’s success is the sea squirt’s ability to regenerate. P. mytiligera can regrow and repair their digestive system and branchial sac. It’s also a fast process; they can heal in less than three weeks.

So, eviscerating themselves is not really that harmful. It saves sea squirts from the occasional predator and doesn’t cause lasting damage.

Now, tunicates do get their “sea squirt” name from another ejecting property. If startled or taken out of the water, they’ll “squirt” out any water in their bodies.

However, there isn’t much proof that this is a defense of any kind. The water is just that – water. It doesn’t contain poison or a distracting cloud of ink like squids.

Sea squirts eat by filtering water for microorganisms. Therefore, their bodies are full of water by nature. Startling or squeezing one contracts its muscles as a reflex, causing the water to shoot out.

Why Is Protection Important For Sea Squirts?

Many sea squirts are sessile, which means they attach themselves to hard surfaces. As larva, they have the ability to swim around, but only to find a good place to attach.

Once they find a suitable place, such as on coral or even boat hulls, they suction themselves to the surface. Then, they undergo a metamorphosis. They absorb the notochord that lets them swim, and also gain their tunic.

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Afterwards, they spend most of their lives stuck to the hard surface and act as ocean purifiers, like whale sharks and clams. They filter bacteria from the water as food.

Water goes through their oral siphons, and then filters out of their atrial siphon.

As these sea squirts are immobile, they have no chance to run away from potential predators. They have to have a good defensive system since they can’t outright attack other animals.

Even vagile tunicates (ones that can move around) still need a good defense. This is because of their soft internal bodies and sheaths.

Salps and pyrosomes look like free-floating jellyfish and can’t attack other animals. Their best defense is their transparency and their undesirable taste.

To End

Without defensive mechanisms, no animal would survive long enough to reproduce. Even organisms that look harmless must have some way to either protect themselves.

Sea squirts are very soft creatures without the ability to attack. They either attach to hard surfaces or float through the ocean. They don’t have claws or teeth with which to defend against predators. Oftentimes they’re completely immobile and can’t even swim away from danger.

So, sea squirts have developed outer layers, or tunics, to protect their soft insides. These work either through camouflage or as a barrier against predators and parasites.

Some can also eject parts of their bodies as a distraction. They sacrifice one part to keep the predators from eating the rest of them.

They also accumulate chemicals and heavy metals that make them unpalatable to fish. In short, sea squirts just don’t taste good enough to warrant eating.

James Ball

James has had a lifelong passion for animals and nature, tracing back to his childhood where he first began fostering intimate knowledge and connection with pet frogs and snakes. He has since honed this interest into a career as a trained Wildlife Biologist, specializing in Biogeography, sustainability and conservation. In addition to his professional pursuits, James maintains an active lifestyle, regularly indulging in outdoor activities such as hiking, and musical pursuits like playing piano and swimming.

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