Destroyer of cities

The tide was coming in on the White Rock beach, and we were wading through the shallows, to see what treasures the waves were washing ashore. Mostly, last Sunday, we were dodging Lion's Mane jellyfish (of which more, later), but a long strip of bull kelp tickled my ankles, and I noticed these circles on it.

Membranipora serrilamella, Kelp-encrusting bryozoan

Bryozoans. At least I recognize them now. Tiny colonies, each animal in its own separate calcium "box", in this case, spreading out to form a crust on the kelp. I brought the whole strip home to photograph it at leisure.

Bird's-eye view of the walls.

Where the colonies don't compete with others, they form almost perfect circles. If you look closely, you can see the bump of the tentacles at one end of each "brick".

In the water, the rolled edge of the kelp allows a side view of the tentacles, one tube with a spray of tentacles per case.

As always, I looked up my find in "Marine Life ..." and Kozloff. "Marine Life" had a decent photo and a brief description, but in Kozloff, I found,

Some colonies of Membranipora, after being taken out of the water, have what look like little lumps of jelly of about the same color as the colony. If the colony is submerged in clean sea water and examined with a strong hand lens ... the little blobs may prove to be one of the marine zoologist's delights, Doridella steinbergae.

The accompanying photo showed Doridella and its half-circle egg masses. And I remembered seeing tiny white semi-circles among the bryozoans; I should have been paying attention, but I was looking for unblemished colonies.

The kelp was in my aquarium. I fished it out and spread it full length in shallow water. Oh!

Egg masses of Doridella steinbergi, Cryptic nudibranch.

Another cluster of egg masses.

Where there are eggs, there should be egg layers. But where? With reason, the sea slug is called "Cryptic"; they're a miniature blob of translucent jelly, with white lines to match the walls of the bryozoan. I looked directly at several without seeing them, thinking they were small colonies of Membranipora, until I noticed the pattern of white lines, squiggly rather than rectangular.

Two sea slugs, and the edge of a colony of membranipora.

Another Doridella, with egg masses.

I poked this one with a plastic toothpick, and it woke up and started to crawl over the kelp. It's about as fast as a land-based slug. Here it is, crawling onto a bryozoan colony:

The two white ear-like protrusions on the front end are sense organs, the rhinopores.

Most nudibranches have prominent gills at the back, but as part of this one's camouflage, the gills are hidden underneath a flap of skin. Another nudibranch, the Corambe pacifica, also feeds on the kelp bryozoan; it is almost identical to Doridella, except that its gills show through a small notch at the tail. Photos for comparison: Doridella, (They call it Corambe steinbergae here.) and Corambe pacifica. The C. pacifica lays its eggs in a spiral, which helps with the identification.

In my search for information, I came across a great blog, Biological Tales from the Brine Queen. In her post on this sea slug, she writes,

They feed on Membranipora by creating a suction seal, using their rasping tongue (called a radula) to cut through the outer membrane, then they suck out the insides.

And they leave the bryozoan colony with bare patches and irregular edges. Which is why I missed them on the first photo shoot.

.