Marine unicorns: Pyrosomida

This week, it has been spread through Social Media a video of a marine animal that is hardly seen. A group of divers, while they was diving in the Philippines, had the opportunity to observe a marine unicorn or Pyrosomida. Or maybe not! Some professionals said that this is a Thysanoteuthis squid egg case. In fact, it’s easy to distinguish a Thysanoteuthis squid egg case from a Pyrosomida: while in the egg case the balls (eggs) are distributed forming a spiral, in the Pyrosomida is an homogeneous mesh of organisms.  In this article we will talk about Pyrosomida. 

INTRODUCTION

Pyrosomida are an order of marine animals included in the Chordata (a group that also includes vertebrate animals). In concrete, they are inside the Thaliacea class in the Urochordata or Tunicata group. There are 2,000 species widely distributed in all oceans, from close to shore to big depths. The reason of their name, Tunicata, is that they present a tough tunic that covers and protects the animal and that contains cellulose. Urochordata or Tunicata can be classified into three classes: Ascidians (or sea squirts), Larvacea and Thaliacea.

Urochordata classes. (A) Ascidian (Picture: Gronk, Creative Commons), (B) Larvacea (Picture: Rocco Mussat Sartor, Università degli Studi di Torino); and (C) Thaliacea (Picture: Mingorance Rodríguez, Creative Commons)

THALIACEA

The Thaliacea are pelagic organisms similar to a lemon or a barrel, with a transparent and gelatinous body; and for this reason is difficult to see them when they are in the sea surface. Each individual is constituted for a belt of circular musculature and an inhalant and exhalent siphon in opposed poles. They can live in two ways: while some of them are solitaries, that is that each individual live independently one from another; other form colonies that can measure some metres.

They get around with body contractions, so they bomb water through the body and propel themselves with a water stream. This also allow them to breath and to feed on the particles of the water.

Most of them are luminescent and produce bright light during the night.

PYROSOMIDA

As we said, Pyrosomida are a group of marine pelagic animals included in the Thaliacea, which have been explained in the previous section.

The individuals live grouped in colonies, that measure between 20 and 30 cm in the Mediterranean, despite in tropical seas they can be 4 meters long and, in extreme cases, more than 10 meters. Each individual is called blastozooid and measures few millimetres, the body present an oral siphon inside the colony and a cloacal outside. Despite each organism maintains its individuality, they live together under the same tunic. Colonies have an inner cavity, which communicates with the exterior through an opening.

They are filtering animals, so water pass through faringial fissures with feeding particles and, thanks to an organ that produces mucus (called endostyle) and Listers tongues, they form a feeding cord.

Concerning to reproduction, each individual release gametes inside the colony and then these are expeled outside. After fecundation, it is formed the oozooid, called ciatozooid, which produce four individuals by budding, and these constitute the tip of the new colony. These four organisms, by budding, produce the entire colony.

Pyrosomida are the most bioluminescent organisms of the zooplanckton because they produce a blue light that can be seen easily some metres under the water.

Young colony of Pyrosomida. These animal measured about 1 cm long. (Picture: Nick Hobgood, Creative Commons). 
Pyrosoma atlanctium (Picture: Show_ryu, Creative Commons)

REFERENCES

• Notes of the subject Chordata of the Degree in Biology of the University of Barcelona
• Hickman, Roberts, Larson, Anson & Eisenhour (2006). Integreted principles of Zoology. Mc Graw Hill (13 ed).
• Storer, TI; Usinger, RL; Stebbins, RC & Nybakken JW (1975). Zoología general. Ed. Omega (6 ed)
• http://www.sensaciones.org/down/aquanet/aquanet-34.pdf

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