SERIES: Invertebrate of the Week

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 (
Marrus orthocanna. Author: Kevin Raskoff (

Siphonophores are a member of the same taxonomic class as jellies (Hydrozoa) and are an equally intriguing group of animals. Though siphonophores physiologically and anatomically function as a single animal, they are actually comprised of several specialized individuals called zooids that combine to produce a single super-organism.

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

Regardless of their morphology or specialty, each zooid within the siphonophore traces its origins back to a single fertilized egg which develops into a structure known as a protozooid.

In physonects like Marrus orthocanna, the protozooid elongates to form a pneumatophore and a mouth at opposite ends. You easily can identify the pneumatophore of Marrus orthocanna. It’s the orange bubble structure at one end 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 ultimately give rise to the individual zooids that 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.

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!

Further Reading