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Invertebrate of the Week #3 – Marrus orthocanna

We’re sticking with the ocean theme this week and highlighting the spectacular physonect siphonophore Marrus orthocanna.

Marrus orthocanna. Author: Kevin Raskoff (

Siphonophores are a member of the same taxonomic Class as jellies (Hydrozoa) and are an intriguing group of animals. Though they function as a single entity, they are actually comprised of several specialized individuals called zooids which combine to produce a single super-organism.

Detail of the pneumatophore of Marrus orthocanna.
Detail of the pneumatophore of M. orthocanna. Image: Kevin Raskoff

Regardless of their form or specialty, each zooid within the siphonophore is derived from single fertilized egg which develops into a structure known as a protozooid.

In physonects like Marrus orthocanna, the protozooid elongates, forming a pneumatophore and a mouth at opposite ends.  (You can see the pneumatophore of Marrus orthocanna quite well; it’s the organge bubble structure at one of the colony.)

The mouth of M. orthocanna.
Detail of photo taken Kevin Raskoff ( which shows the mouth end of M. orthocanna.

From there, two growth zones develop which will give rise to the individual zooids that will eventually come to serve specialized roles within the colony such as propulsion, digestion, etc.  In fact, the zooids are so specialized and integrated into the colony that they are unable to survive on their own and must remain together.

With such intimate integration, it’s easy to make comparisons between the sub-specialization of siphonophore zooids and the similarly specialized tissues that make up other organisms.  Perhaps the siphonophore body plan helped give rise to the cell-based architecture of other phyla.  Of course, more research needs to be done before we start drawing those kind of conclusions but it’s a fascinating thought nonetheless.

Further Reading
  • Mapstone and Arai. Siphonophora (Cnidaria: Hydrozoa) of Canadian Pacific Waters. NRC Research Press, 2009

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