Posted by: Rob Viens | June 8, 2012

The Immortal Hydrozoa

Today, June 8th, Darwin was back wading in the bay.  It really is sort of interesting how he seems to focus on different ecosystems or animal taxa each day.  It makes for interesting blogging.  Today he simply writes:

“Collected some Corallines on the rocks, which surround part of Botofogo Bay.” (June 8)

Curious as to what he might have found a looked in his Zoological Notebook and found reference to a specimen he collect in June 1832 – almost certainly the coralline specimen from today (marked Sample #265).  Of it he writes:

“Cells oval, attached by one end in irregular scattered groups on irregular cylindrical jointed hollow transparent much branched stems. Polypus tubular, conical, lengthened with 8 long tapering arms.— Growing in large tufts at low-water mark. Polypi hanging out.— Stems irregularly divided, interwoven, membrano-gelatinous.” (Zoological Notebook)

A footnote in the notebook (from the 20th century) notes that this was identified as “Sertularia Lamarck, a term formerly covering both bryozoans and hydrozoans”. For the moment let’s assume that these were not bryozoans (another story, for sure). Therefore, that leaves with the fact that these Sertularia, although related to corals (they are both in the Phylum Cnidaria), must be part of an entirely different class – the Hydrozoa

Some readers may be familiar with the most well-known of the Hydrozoa – the hydra.  Hydra have a tube-shaped body with a mouth at on end. (As Jim pointed out in an earlier comment, this mouth opening in Cnidaria is also the anus. The mouth/anus is surrounded by several tentacles that carry food into the opening. (Remember that the Hydra from Greek mythology was a serpent or “dragon” with many heads.) One other cool thing about hydra, while we are on the subject – they have amazing regenerative capabilities and (even cooler) they never seem to age.  That means, short of being preyed upon or suffering a tragic accident, they are effectively immortal.  Not bad for a little tiny sea polyp.

Hydra under the microscope (from Wikipedia Commons):


Each hydra is an individual organism, but many Hydrozoans live in colonies (much like corals do). And the Sertularia appears to be one of these colonial varieties.  The most common members of this genus (because we humans like to use them for decorations or in our aquariums) is the Sertularia argentea – also called the “Air fern”.

Air ferns are like “condominiums” for hydrozoa.  They are made of chitin secreted by Hyrozoans that effectively form a “skeleton” for the colony. Each individual Sertularia polyp lives in a cup-like depression on the skeletal structure. (This is a lot like corals, which form this skeletal framework out of calcium carbonate.) What you end up with is a branching, fern-like animal colony that is often mistaken for a plant. An example is show below (from the Encyclopedia of Life (

 Sertularia argentea

Not surprisingly, people often actually mistake this animal colony for a plant. (RJV)


  1. One cool thing about the Cnidarians is that they were probably around, and involved in, the origin of multicellularity…

    This phylum exhibits radial or biradial symmetry, a clear advantage for sedentary animals like the hydra in that they can respond to stimuli that come from all directions. They are also probably the first (along with the phylum Ctenophora) animals to get to the tissue-level of organization, being diploblastic. And as Rob mentioned, they have a gastrovascular cavity, with food, digestive wastes, and reproductive stages entering and leaving through one opening (the mouth).

    Probably the coolest “innovation” regarding the Cnidarians is that they have the very beginnings of a nervous system, a structure commonly referred to as a nerve net. This might have been the very, very beginning of the evolution of animal nervous systems, ultimately culminating in the “big-brained Homo sapiens.

  2. Thanks Jim! I was not aware of the significance of their nervous system, but I certainly love to explore the significant steps in the evolution of life. I’ve always been a big fan of the early steps toward multicellularity and particularly of the Cnidarians and Porifera (sponges).

  3. […] (as Darwin describes) is the anus. (Notice that this is not exactly the same as the cnidarians (see The Immortal Hydrozoa).  In the corals, the mouth and anus are the same opening – in the echinoderms the digestive […]

  4. […] are from the Phylum Cnidaria which includes corals, jellies, anemones and hydra (see The Immortal Hydrazoa).  Cnidaria have two basic body forms – the free-swimming medusa (with a mouth opening on the […]

  5. […] – Obelia geniculata (from See The Immortal Hydrozoa for more on […]

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