Thaliacea
Thaliacea is a class of marine animals within the subphylum Tunicata, which includes the salps, pyrosomes, and doliolids. In contrast to their benthic relatives, the ascidians, thaliaceans spend their entire lifespan free-floating (pelagic). This group encompasses species with intricate life cycles, featuring both solitary and colonial forms.
The three orders of thaliaceans are filter feeders. Pyrosomes are colonial animals, consisting of multiple tiny ascidian-like zooids arranged in a closed cylinder. All of the atrial siphons are directed inwards, leading to a single, shared cloaca at the center of the cylinder. As the zooids exhale water through a common opening, the resulting water movement gradually propels the pyrosome through the sea. Salps and doliolids have transparent barrel-shaped bodies that pump water, propelling them through the sea and providing a source of food. The majority of their body is occupied by a large pharynx. Water enters the pharynx through a large buccal siphon at the front end of the animal and is then forced through slits in the pharyngeal wall into an atrium located just behind it. Finally, the water is expelled through an atrial siphon at the posterior end. The pharynx serves as both a respiratory and digestive organ, as it filters food from the water using a net of mucus that is slowly pulled across the slits by cilia.
Doliolids and salps undergo alternating asexual and sexual life stages. Salp colonies can reach lengths of several meters. Doliolids and salps rely on muscular action to propel themselves through the surrounding seawater.
Thaliaceans have intricate lifecycles. Doliolid eggs hatch into swimming tadpole larvae, which are the typical larval stage for other urochordates. Pyrosomes are ovoviviparous, indicating that the eggs develop inside the "mother" without going through a tadpole stage. Salps are viviparous, as the embryos remain connected to the "mother" by a placenta. Eventually, the embryos develop into oozoids, which reproduce asexually by budding to generate numerous blastozoids that form long chains. Subsequently, the individual blastozoids reproduce sexually, producing eggs and the subsequent generation of oozoids.
It is worth noting that in Thaliacea, the dorsal, hollow nerve cord and notochord characteristic of Chordata have regressed, except for a rudimentary form observed in some doliolid larvae.