Sea squirts, also called tunicates, are marine filter feeders that might surprise you with their hidden connection to humans. Unlike their mobile relatives, adult sea squirts typically live a sessile (attached) lifestyle, often forming colonies on rocks, piers, or other underwater structures.
Their sac-like bodies and vibrant colors might resemble sponges, but they possess a more complex internal anatomy that places them closer to vertebrates on the evolutionary tree. With over 3,000 species ranging in various sizes, shapes, and colors, they have a lot more to them than they first appear.
They play a crucial role in the larger ecosystem, are surprisingly evolutionarily advanced, and possess a sophisticated reproductive cycle that is well worth exploring.
Appearance
Adult sea squirts have a fairly basic appearance and typically display a tube-like, bell-like, or urn-like shape with two open ends known as siphons. Water and oxygen draws in through the incurrent siphon and move out through the excurrent siphon.
These marine animals reach a maximum size of approximately 10cm, although they can be found much smaller.
As there are thousands of species, there is a large array of colors and patterns that ascidian can be found in. Among these are white, pink, orange, yellow, brown, and blue, or a mixture of various colors in random-seeming patterns. Some are also known to exhibit bioluminescent hues.
Did you know…
They belong to the phylum Chordata, which includes all animals with a spinal cord, a supporting notochord (backbone), and gill slits at one point in their life cycle. Tunicates have all these primitive vertebrate features as larvae when they resemble tadpoles. Usually within 24 hours of planktonic life, the tunicate larvae settle down on a hard surface and attach themselves with adhesive organs. The tunicate then transforms, rearranging its organs (absorbing its notochord, nerve cord, and tail) and becoming a full-grown sea squirt.
Habitat
Sea squirts are found in saltwater environments at a variety of depths and water temperatures.
As there are thousands of species, it can be expected that each has adapted to thrive in its specific environment.
The marine creatures can be found in both shallow and deep water, ranging from the intertidal zone to the seabed.
If you are on the hunt for some ascidians, you will find their adult forms attached to rocks and other hard substrate on the ocean floor, on hulls of ships, or attached to mussels, crabs, and oysters.
It is through attaching themselves to moving objects such as crabs and ships, that have been able to move larger distances and colonize over an extensive habitat.
Interestingly, their attachment to shellfish can have a negative economic effect, as it takes time and money for them to be removed before sale.
Diet
Adult sea squirts are sessile creatures, meaning they attach themselves to a surface and remain in place.
This, as with everything in the animal kingdom, has its exceptions, though, as some species move extremely slowly, detaching one part of their bodies from the substrate and reattaching another.
This, of course, happens at an extremely slow rate, with faster colonies moving at approximately 1.5cm per day. As they remain mostly in place, sea squirts don’t rely on hunting prey but rather on a filter-feeding system.
Water is pulled in through their inhalant siphon, where it passes through the pharynx.
In the pharynx, plankton is trapped by a layer of mucus. The plankton, food particles, and debris then pass into the stomach for digestion and then through the intestines before being expelled through the exhalant siphon.
As you may notice, this is very similar to the digestive system of humans. This is because although adult sea squirts have no backbone, their juveniles do have a spine and, like humans, are part of the phylum Chordata.
Reproduction
Most sea squirts are hermaphroditic, they produce both male and female gametes. The eggs remain safely inside and are fertilized by sperm that is pulled in through the inhalant siphon.
Fertilized eggs are released into the water column, where free-swimming larvae develop, closely representing the appearance of tadpoles.
These tadpole-like larvae will swim freely for a short while before attaching themselves to a suitable substance, where they will go through a metamorphosis and develop into their adult forms.
Although solitary sea squirts have both male and female reproductive organs, an individual is unlikely to fertilize itself, further allowing greater genetic diversity and evolutionary changes.
Colonial species reproduce slightly differently, with some species exhibiting budding, where new individuals grow off established adults, and eventually break off as zooids, creating a new sea squirt with the same DNA. This eliminates the juvenile stage but also removes the opportunity for genetic diversity.
Threats
Due to their sessile nature, sea squirts are easy prey for a variety of predators such as larger fish, crustations, eels, and sea snails.
As they attach themselves early in life, they need to develop unique defense mechanisms. These include a combination of their rubbery textured armor known as the tunicate, and an unpalatable taste due to the presence of chemicals such as indole alkaloids (known to deter sea stars), while other species absorb heavy metals that are toxic to most fish life.
5 More Interesting Facts about Sea Squirts
- One known species (Polycarpa mytiligera) will expel its gut in a process known as evisceration.
- Some species live individually; others live in groups or colonies.
- The body has an outer protective covering, the tunic, which contains a cellulose-like substance.
- Some species can self-fertilize.
- Sharks, skates, and other bottom-dwelling fish eat sea squirts.
FAQs
How do sea quirts get their name?
Sea squirts get their name from their habit of contracting their bodies when disturbed and sending out a jet of water.
Are sea squirts eaten by humans?
While sea squirts are not commonly consumed, certain species are considered a delicacy in some parts of the world. In Korea and Japan, for example, sea squirts known as “meongge” or “hoya” are used in local dishes.
How long do sea squirts live?
The lifespan of sea squirts varies among species, typically ranging from a few months to a few years. Species, environmental conditions, and reproductive strategies all influence their lifespan.
Do sea squirts have any economic importance?
Some sea squirts can negatively impact aquaculture by fouling structures and occupying cultivation space. That said, compounds extracted from certain sea squirt species are being researched for potential medical applications.
