High-spined Commensal Hydroid

Image: Seascapeza
Hydractinia altispina

The hydroid that makes a monkey orangutan of a mollusc!

It may be difficult to believe, but if you can look past this orange suit of fuzz and tentacles, there's actually a snail under there! It's called the Scaly Dogwhelk (Nucella squamosa), it reaches 5 cm (2 in) long and it's found on the west coast of South Africa.

This individual just happens to be going through some personal hygiene issues at the moment...

Image: H. Zell

If you gave it a good scrub, you'd be able to see the lumpy, bumpy shell beneath. Apparently these lumps and bumps are just perfect for a crusty, orange covering of High-spined Commensal Hydroids!

Hydroids are related to jellyfish and sea anemones. Like so many others, this High-spined variety is colonial; each individual is connected to one another by tubes through which they can share food. Some polyps collect food with their enthusiastically outstretched tentacles and stinging cells, just like tiny sea anemones. Others, the little orange balls, are completely devoted to reproduction.

Many hydroid colonies produce jellyfish who drift away before reproducing. For most hydroids, that's the only way they can be sure their young will fly the nest and end up more than an inch away from the parent colony. But the High-spined Commensal Hydroid has a ride! They're racing through the ocean on a snail mount, so they can just release their larvae straight away. This larvae is a planula, a minute blob covered in cilia to let them swim and crawl.

Image: Peter Southwood

Aside from the feeding polyps and orange sex-balls, the High-spined Commensal Hydroid also has numerous defensive spines which work against at least some of the Scaly Dogwhelk's own predators. It is a fuzzy suit of armour after all!

That's why it's called "commensal" rather than "parasitic" or simply "nasty". It's a friendly relationship wherein the hydroid gets to journey around and the snail gets some extra defence points.

So next time you see a man in a monkey-suit talking to himself, don't assume he's crazy...