Observations on Hopkinsia sp.

Dear David,
Yes I'm well and had a good Christmas break. Thanks for the interesting pictures. Yes the bryozoan is being overgrown by hydroids. This bryozoan often has hydroids growing on it, but usually just a few as in the photo at the top of the page. All sessile organisms (ones that are fixed and can't move away) in the sea have a problem with "fouling organisms" which are plants and animals which settle on and smother sessile organisms. Fouling organisms get their name because they are a major problem when they foul the bottom of boats. They often have very short lifecycles, growing rapidly to maturity. they settle on all bare surfaces.

Many sessile plants and animals have evolved mechanisms to keep fouling organisms at bay, through chemicals and special structures. This bryozoan clearly hasn't succeeded. Nudibranchs, even those that feed on cnidarians, are usually not immune to hydroid nematocysts, so it unlikely that Hopkinsia would be much use in clearing an area already overgrown with hydroids. Quite possibly the hydroids have affected the Hopkinsia population by preventing larval nudibranchs from settling out of the plankton, or by eating the larvae.

I am not sure that your two animals are mating. They have their reproductive apparatus on the right side of the body so during mating they need to be aligned so their right sides are touching. Your animals may be thinking about it but I don't think their right sides are together.

The pink bryozoan's name, Pleurotoichus clathratus, is at the top of the page. Thanks for this information. I don't think I have seen the egg mass of this species before.
Best wishes,
Bill Rudman.