The Thaliacea comprise a class of marine animals within the subphylum Tunicata. Unlike their
benthic
The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning "t ...
relatives the ascidians, thaliaceans are free-floating (
pelagic
The pelagic zone consists of the water column of the open ocean, and can be further divided into regions by depth (as illustrated on the right). The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or wa ...
) for their entire lifespan. The group includes species with complex life cycles, with both solitary and colonial forms.
Anatomy
The three orders of thaliaceans are
filter feeder
Filter feeders are a sub-group of suspension feeding animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feedin ...
s.
Pyrosome
Pyrosomes, genus ''Pyrosoma'', are free-floating colonial tunicates that usually live in the upper layers of the open ocean in warm seas, although some may be found at greater depths. Pyrosomes are cylindrical or cone-shaped colonies up to long, ...
s are colonial animals, with multiple tiny ascidian-like
zooid
A zooid or zoöid is a single animal that is part of a colonial animal. This lifestyle has been adopted by animals from separate unrelated taxa. Zooids are multicellular; their structure is similar to that of other solitary animals. The zooi ...
s arranged in a cylinder closed at one end. All of the atrial siphons point inwards, emptying into a single, common
cloaca
In animal anatomy, a cloaca ( ), plural cloacae ( or ), is the posterior orifice that serves as the only opening for the digestive, reproductive, and urinary tracts (if present) of many vertebrate animals. All amphibians, reptiles and birds, a ...
in the centre of the cylinder. As the water exhaled by the zooids exits through a common opening, the water movement slowly propels the pyrosome through the sea. Salps and doliolids have a transparent barrel-shaped body through which they pump water, propelling them through the sea, and from which they extract food. The bulk of the body consists of the large
pharynx
The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its struc ...
. Water enters the pharynx through the large buccal siphon at the front end of the animal, and is forced through a number of slits in the pharyngeal wall into an atrium lying just behind it. From here, the water is expelled through an atrial siphon at the posterior end. The pharynx is both a respiratory organ and a digestive one, filtering food from the water with the aid of a net of
mucus
Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It is ...
slowly pulled across the slits by
cilia
The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike projecti ...
salp
A salp (plural salps, also known colloquially as “sea grape”) or salpa (plural salpae or salpas) is a barrel-shaped, planktic tunicate. It moves by contracting, thereby pumping water through its gelatinous body, one of the most efficient ...
s alternate between asexual and sexual life stages. Salp colonies can be several meters in length. Doliolids and salps rely on muscular action to propel themselves through surrounding seawater.
Thaliaceans have complex lifecycles. Doliolid eggs hatch into swimming tadpole larvae, which are the common larval stage for other
urochordates
A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata (). It is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one time ...
. Pyrosomes are
ovoviviparous
Ovoviviparity, ovovivipary, ovivipary, or aplacental viviparity is a term used as a "bridging" form of reproduction between egg-laying oviparous and live-bearing viviparous reproduction. Ovoviviparous animals possess embryos that develop insi ...
, meaning the eggs develop inside the "mother" without the tadpole stage. Salps are
viviparous
Among animals, viviparity is development of the embryo inside the body of the parent. This is opposed to oviparity which is a reproductive mode in which females lay developing eggs that complete their development and hatch externally from the m ...
, meaning the embryos are linked to the "mother" by a placenta. This then develops into an oozoid, which reproduces asexually by budding to produce a number of blastozoids, which form long chains (see image). The individual blastozoids then reproduce sexually to produce the eggs and the next generation of oozoids.
The dorsal, hollow nerve cord and
notochord
In anatomy, the notochord is a flexible rod which is similar in structure to the stiffer cartilage. If a species has a notochord at any stage of its life cycle (along with 4 other features), it is, by definition, a chordate. The notochord consis ...
found in
Chordata
A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These fiv ...
has been lost, except for a rudimentary one in some doliolid larvae.
The jell pump and the carbon cycle
Thaliaceans play an important role in the ecology of the sea. Their dense faecal pellets sink to the bottom of the oceans, and this may be a major part of the worldwide
carbon cycle
The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and Earth's atmosphere, atmosphere of the Earth. Carbon is the main component of biological compounds as well as ...
.
Taxonomy
The class is a relatively small one, and is divided into three orders:
Class Thaliacea World Register of Marine Species. Retrieved 2014-02-13.
* Order
Pyrosomida
Pyrosomes, genus ''Pyrosoma'', are free-floating colonial tunicates that usually live in the upper layers of the open ocean in warm seas, although some may be found at greater depths. Pyrosomes are cylindrical or cone-shaped colonies up to long, ...
Jones 1848
** Family
Pyrosomatidae
Pyrosomes, genus ''Pyrosoma'', are free-floating colonial tunicates that usually live in the upper layers of the open ocean in warm seas, although some may be found at greater depths. Pyrosomes are cylindrical or cone-shaped colonies up to long, ...
Pyrosoma
Pyrosomes, genus ''Pyrosoma'', are free-floating colonial tunicates that usually live in the upper layers of the open ocean in warm seas, although some may be found at greater depths. Pyrosomes are cylindrical or cone-shaped colonies up to long, ...
Salpinae
A salp (plural salps, also known colloquially as “sea grape”) or salpa (plural salpae or salpas) is a barrel-shaped, planktic tunicate. It moves by contracting, thereby pumping water through its gelatinous body, one of the most efficient ...
Lahille 1888
**** Genus '' Brooksia'' Metcalf 1918
**** Genus ''
Iasis In Greek mythology, Iasis (Ancient Greek: Ἴασις means 'healing, remedy') was one of the Ionides Nymph, nymphs whose spring waters were believed to cure diseases. She was an Ancient Elis, Elean naiad-daughter of the Potamoi, river-god Cytherus ...
Ihlea
''Ihlea'' is a genus of tunicates
A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata (). It is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including verteb ...
Pegea
''Pegea'' is a genus of tunicates
A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata (). It is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including verteb ...
Salp
A salp (plural salps, also known colloquially as “sea grape”) or salpa (plural salpae or salpas) is a barrel-shaped, planktic tunicate. It moves by contracting, thereby pumping water through its gelatinous body, one of the most efficient ...
Soestia
''Soestia'' is a monotypic genus of tunicates
A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata (). It is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (incl ...
Thetys
Thetis (; grc-gre, Θέτις ), is a figure from Greek mythology with varying mythological roles. She mainly appears as a sea nymph, a goddess of water, or one of the 50 Nereids, daughters of the ancient sea god Nereus.
When described as ...
Thalia
Thalia, Thalía, Thaleia or Thalian may refer to:
People
* Thalia (given name), including a list of people with the name
* Thalía (born 1971), Mexican singer and actress
Mythological and fictional characters
* Thalia (Grace), one of the three ...
Doliolida
The Doliolida are an order of small marine animals of the subphylum Tunicata. They are in the class Thaliacea, which also includes the salps and pyrosomes. The doliolid body is small, typically 1–2 mm long, and barrel-shaped; it featu ...
Dolioletta
''Dolioletta'' is a genus of tunicates in the family Doliolidae.
Characteristics
Members of the genus ''Dolioletta'' are transparent, gelatinous barrel-shaped animals, usually less than one centimetre long. They move in jerks by contracting the ...
'' Borgert 1894
**** Genus '' Doliolina'' Garstang 1933
**** Genus '' Dolioloides'' Garstang 1933
**** Genus ''
Doliolum
''Doliolum'' is a genus of tunicates, the members of which move via jet propulsion. A detailed description can be found at .
Species
The World Register of Marine Species lists the following species:Doliopsoididae Godeaux 1996
**** Genus '' Doliopsoides'' Krüger 1939
**Suborder Doliopsidina
*** Family
Doliolunidae
Doliolunidae is a family of tunicates belonging to the order Doliolida
The Doliolida are an order of small marine animals of the subphylum Tunicata. They are in the class Thaliacea, which also includes the salps and pyrosomes. The doliolid ...
Robison, Raskoff & Sherlock 2005
**** Genus '' Pseudusa'' Robison, Raskoff & Sherlock 2005
*** Family Doliopsidae Godeaux 1996
**** Genus '' Doliolula'' Robison, Raskoff & Sherlock 2005
**** Genus '' Doliopsis'' Vogt 1854
*** Family Paradoliopsidae Godeaux 1996
**** Genus '' Paradoliopsis'' Godeaux 1996