The hydrocoral and the worm

 

Purple hydrocoral (Stylantheca porphyra)

This beautiful encrusting sheet is a colonial animal growing in the low intertidal zone on wave-exposed rocky shores.  It looks like a coral, but is more closely related to hydroids, hence the name "hydrocoral."

Here's a close-up of a smaller sheet (~4-5 cm across).

 

Stylantheca porphyra is found from British Columbia to central California, but is encountered infrequently on Bodega Head.  This striking hydrocoral grows on vertical walls and is often associated with other colorful low zone groups (tunicates, sponges, hydroids, anemones, etc.) — resulting in a brilliant mosaic (see below).

 

 

Each star-shaped pit within the calcareous skeleton contains from 1 to 12 feeding polyps.  Below is a view through a microscope.

 

Interestingly, Stylantheca is a brooder.  Medusae are retained below the surface of the colony where they produce gametes.  Female colonies brood larvae (called planulae) until they are fully developed and can swim or crawl away and settle on a rock.

When researching Stylantheca, I came across an article that mentioned a commensal worm (Polydora alloporis) that lives within hydrocoral skeletons.  The paper contained this picture, showing the worm's tentacles and the paired circular holes used by the worm.

 

From Light, W.J.  1970.  Polydora alloporis, new species, a commensal spionid (Annelida, Polychaeta) from a hydrocoral off Central California. Proceedings of the California Academy of Sciences 37: 459-472.

After reading this article, I reviewed previous pictures of Stylantheca and discovered that many of them appear to have these paired worm holes scattered throughout the colony!  See if you can find some in the photo below.

 

I don't know if this is the same species of worm, but it's likely to be related to Polydora alloporis.