Nudibranchs are hermaphroditic. These two, the larger about 60 mm long, are mating, right shoulder to right shoulder, exchanging sperm. Later they each will lay an egg ribbon. Depth 12 m.
These tiny snails, about 2 mm, are virtually unknown as few recognize them hidden among grains of sand or on green algae as here. When one of its predators, the six-armed seastar Leptasterias pusilla approaches, the little snails flee while raising their beautiful protective mantle as this one is starting to do. We arranged this encounter in our lab.