• Phylum –  CHORDATA (Dorsal tubular nerve cord, gill-slits and notochord present)
  • GroupACRANIATA (No head, cranium or brain)
  • SubphylumUROCHORDATA (Marine. Body covered by a thick test. Notochord present only in larval tail)
  • ClassTHALIACEA (Test with circular muscle bands. Free-living, pelagic)
  • OrderPYROSOMIDA (Compact tubular colony. No larva. Muscle bands at end of body)
  • GenusPyrosoma


Pyrosoma is distributed in tropical and sub-tropical regions.


Pyrosoma is pelagic, marine, colonial and bioluminescent urochordate, found at a
depth of 500 meters. A few forms are abyssal.


  • It is a thimble-shaped hypopleustonic colony, 25 cm to 1 metre in length and consists of several individuals, called blastozooids, embedded in a common test having test processes and terminal opening.
  • Colony floats horizontally and is like a balloon cylinder. Zooids are arranged in the wall of the cylinder.
  • Branchial and atrial apertures are at opposite ends. Branchial openings or mouths of individuals or zooids open to the outside, while atria open into a common cloaca with a tenninal outlet, from which a continuous jet emerges.
  • Colony moves by a jet propulsion. Rhythmic contractions of muscular bands in the body wall cause ejection of water through pharynx and atrium with sufficient force to propel the organism.
  • Each zooid consists of a large branchial sac with endostyle and dorsal lamina, neural complex, heart and atrium.
  • Branchial sac contains 50 gill-slits divided by internal longitudinal bars. Endostyle communicates with peripharyngeal band, retropharyngeal band and dorsal lamina. Intestine makes a loop around stomach and ends into atrium. Heart is ventral. Lobed testis is found behind ovary.
  • Close to the mouth of each zooid arises a tongue-like process of the test, called as buccal appendage. Other structures seen are testes, anus, stomach, oesophagus intestine, atrial muscles, stigmata, mass of mesodermal cells, dorsal tubericle, muscle fibers, luminous organs and tentacles. 
  • Hermaphroditic. Reproduction sexual and asexual.
  • A single fertilized egg develops within blastozooid and gives rise to asexual oozooid. It gives stolon that forms 4 ascidiozooids or tetrazooid which degenerates and gives rise to cyathozooid. It is enclosed in a test and by repeated budding forms a colony. 
  • Asexually colony multiplies the number of zooids by direct stolon budding, producing blastozooids.


The most outstanding characteristic of the creature is the shining of powerful light without heat, a phenomenon called as bioluminescence. Animal produces light on stimulation and when all colonies produce light, the entire area is illuminated as much as one can read the book. Light is produced by bioluminescent cells, which contain curved inclusions.

According to some, these bioluminescent cells contain luminescent symbiotic bacteria. However, in any case, light is emitted by the chemical interaction between two compounds, luciferin and luciferase, in the presence of moisture.


Since the animal contains opposite branchial and atrial siphons with thimble-shaped colony and above features, hence it is Pyrosoma.




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