122 — Feathered hydroid, Pennaria disticha

A colony of Pennaria disticha, underneath
Ngapipi Bridge, Tamaki Drive,
Auckland, 18/5/18.

Under Ngapipi Bridge, Tamaki Drive, 18/5/18.

Under Ngapipi Bridge, Tamaki Drive, 18/5/18.
This could be a different species.

Under Ngapipi Bridge, Tamaki Drive, 9/4/16.

I think it would be a safe to say that most people don't notice hydroids when they visit the sea shore, and if they do they assume that they're some kind of seaweed. However, although they look like plants, hydroids are actually carnivorous animals. They belong in the phylum Cnidaria, in which you'll also find jellies, sea anemones, and corals.

The feathered hydroid, Pennaria disticha*, is what's known as an athecate hydroid. Athecate essentially means without a cup, and this refers to the flowery red bit of the hydroid (called a hydranth). In athecate hydroids, the hydranth sit out by itself and is not protected by a sheath or cup. The hydranths shown here are ~3–5mm in size. Colonies can form clumps and get to around 20x20x20cm.

Feathered hydroids are fouling organisms and have been transported all over the world attached to the bottom of ships. It was thought that they originated in the Atlantic and spread from there. However, a recent genetic study found that instead of one cosmopolitan species, Pennaria distcha is probably several genetically distinct cryptic species^1.

If you happen across a colony like the one pictured here it's best not to touch it, as the colonies are venomous. You can be stung and apparently it's quite painful. Their venomous nature means that these hydroid colonies are considered to be a bit of a nuisance when they grow on aquaculture facilities ^{2, 3.}

*(Goldfuss, 1820)




More info:

^1. Miglietta, M. P., Odegard, D. Faure, B., Faucci, A. (2015). Barcoding techniques help tracking the evolutionary history of the introduced species Pennaria disticha (Hydrozoa, Cnidaria). PlosOne, 10(12), e0144762. doi:10.1371/journal.pone.0144762

^2. Bosch-Belmarab, M., Azurro, E., Pulis, K., Milisenda, G., Fuentes, V., Ons, K-D., Micallef, A., Deidun, A., Piraino, S. (2017). Jellyfish blooms perception in Mediterranean finfish aquaculture. Marine Policy, 76, 1–7. doi.org/10.1016/j.marpol.2016.11.005

^3. Tezcan, Ö. D., Sarp, S. (2013). An unusual marine envenomation following a rope contact: A report on nine cases of dermatitis caused by Pennaria disticha. Toxicon, 61, 125–128. http://dx.doi.org/10.1016/j.toxicon.2012.10.019