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The ' trailing ' behaviour is found in all species of Risbecia, where pairs of animals are often found following each other in single file, the second animal in the line usually touching the ' tail ' of the one in front( Sea Slug Forum).
Hypselodoris pulchella is a long slender species with a body size up to 80 mm or even 110 mm. The mantle edges are well extended along the body, with a thin bluish border. The mantle and the foot are creamy white, covered with numerous small rounded yellowish-orange spots of different sizes. There is always a diffuse pattern of pale violet-brown on the back; this is absent in the similar species Hypselodoris ghardaqana. The sensory tentacles( the antennae like rhinophores) are bluish. The main respiratory organs( gills) vary from bluish to orange. These sea slugs usually raise and lower rhythmically their head, vibrating their gills. They often show a characteristic ' trailing ' behaviour, with two individuals following closely each other. This species feeds on sponges( Wikipedia).
Habitat This benthic species can be found at a depth of 3 – 30 meters( Wikipedia).
The Yellowish-Orange Spotted Chromodorid Nudibranch( Risbecia pulchella [ Hypselodoris pulchella ] Rüppell & Leuckart, 1830) from the Sea of Fujairah, Gulf of Oman, Arab Sea, Indian Ocean. Photo by Divemaster Ola Mostafa Khalaf. 31.12.2016. http:// marine-life-uae. webs. com /
Gazelle: The Palestinian Biological Bulletin- Number 154 – October 2017