Nudibranchs reproduce sexually. They line up side by side to mate, and then lay eggs in a ribbon-like mass. Some nudibranch species lay eggs as a flat ribbon. Others, the Spanish Dancer included, attach the edge their egg mass ribbons to solid surfaces in a spiral pattern. The Spanish Dancer's egg mass is easy to recognize. It is distinctive in that the ribbons are comparatively wide, and once set down in a spiral, they look like a reddish ruffled rosette. My dive buddy thought it was some tangled piece of clothing!