Hi I'm Stefano and I will start this module by introducing the group of Tunicates.
I will give you their general characteristics and synapomorphies
and then together with Sébastien and Jean-Philippe in the
next chapters we will go more in depth in their biology. Let's start!
The subphylum Tunicata encompasses a large group of invertebrates, they are
exclusively marine and they belong to our own phylum: the chordates.
In fact, they share their sister group with the vertebrates and they share
a common ancestor. In the next chapters, we will tell you, despite their
diversity, why they belong to our own phylum.
So the body plan of tunicates will be described more in detail later.
For now, what you need to know is that they're all filter feeders. They have two
siphons, one inhalant and one exhalant, where the water flows through.
They have a pharynx. They have a central and a peripheral nervous system.
They have muscles. They have a heart - a pumping heart - and then they have
cavities where mesenchymal cells are flowing through.
Also called urochordates, the tunicates take their name from the tunic.
The tunic is an extracellular matrix, it's cellulose-based, and it surrounds the body
of these animals.
The tunic is normally used for defense. In fact some species can have spicules
embedded inside it. But it's also used to attach to the substrate
or to recognise other animals.
The tunicates are the only animals that have cellulose, and the cellulose is synthetized
by one enzyme that is called cellulose synthase, and it's probably a horizontal
gene transfer from some photosynthetic algae or bacteria.
There are a little more than 3,000 species of tunicates that have been described,
and probably there are as many that haven't been described yet.
Tunicates definitely lack a lot of charisma in comparison to the other
marine invertebrates but I tell you they are spread all over the world in all latitudes.
We can find them in the first 100m from the surface and they can occupy artificial
and natural habitats. They can also be invasive and biofouling problems
in many aquaculture facilities, as you can see from this picture.
Tunicates are all hermaphorites and they all reproduce sexually.
This is true for some species and for some of them this is the only way to reproduce
and we call them solitary species. Then there are other species that
on top of the sexual reproduction they also reproduce asexually and they form
colonies of genetically identical individuals in this case we talk about colonial tunicates.
Depending on the class, the different life cycles can become really complex
and we won't go in detail right now, but in the next chapters of this module
we will look in detail at their sexual reproduction, their metamorphosis,
and also their asexual reproduction by budding.
Tunicates have been historically divided in three classes: there are the appendicularians
also called are larvaceans, the thaliaceans, and the ascidiaceans.
The first two are strictly pelagic while in the ascidians or the ascidiacians
we have only the larval phase that is pelagic, and after
metamorphosis we have benthic animals.
Most of the described species of tunicates belong to the class ascidians.
Ascidians have a biphasic life cycle. The embryos develop into a swimming larvae
and the larvae retain synapomorphies of chordates such as a notochord, a metameric
musculature and a dorsal, centralized central nervous system.
The larva swim for a while and then it finds a suitable substrate and metamorphoses
into a sessile form that becomes a huge part of the benthic macrofauna.
Ascidians are the most studied animals also in the laboratory and they will be
the focus basically in the next chapter of this module.
The class of appendicularians or larvaceans they have lost their metamorphic and post-
metamorphic phase. In some ways they are neotenic and they retain the larval form.
They're all planktonic, they reproduce only sexually and they don't form colonies.
There are only 84 species of appendicularians that have been described and none of them
are bigger than 1cm. Appendicularians modified their tunic by producing a
complicated arrangement of filtres that allow the food in the surrounding water
to be brought in and concentrated. This bubble-like tunic structure is called the house.
The high efficiency of this method allows the larvaceans to feed on much
smaller plankton than most other filter feeders.
As the animal grows and the filter gets clogged, the house is regularly replaced.
In some species, a house is kept for no more than a few hours before being replaced.
Larvaceans are the only tunicates that abandon their tunic, and the discarded
houses contribute extensively to plankton biomass and account for a significant part
of organic material sinking in the ocean depths.
The class of thaliaceans is exclusively pelagic, and it has very complex life cycles
which normally alternate sexual and asexual reproduction.
In this class, there are three orders: pyrosomes, doliolids, and salps.
The pyrosomes are all colonial.
The barrel-shaped zooids form swimming colonies that are tubular and they can
reach even several metres of length. The atrial siphons are all inwards and
they expel water and propel the colony through the water.
The pyrosomes are all ovoviviparous, and they develop their embryos inside the zooids.
Salps and doliolids are also transparent and are barrel-shaped. They also propel
themselves thanks to very strong circular muscles. They have complex life cycles and
alternate sexual and asexual reproductions, and they form floating huge colonies.
So I hope you have learned something about these amazing marine creatures
and I invite you to test yourself before moving to the next chapter using the
quiz of Chapter 1.