| Summary: | In Europe, the genus Pelseneeria Koehler & Vaney, 1908 includes five species, all ectoparasites of regular sea urchins. Three are living in deep waters (Azores) and two are living in more shallow waters. Information in the literature concerning Pelseneeria stylifera (Turton, 1825) from the North Atlantic area and Pelseneeria minor Koehler & Vaney, 1908 from the Western African Atlantic area and from the Mediterranean Sea is extensively reviewed. A population of Pelseneeria distributed on both sides of the Strait of Gibraltar (Algarve in Portugal and Province of Malaga in Spain) was the subject of a special study. This population includes specimens of a quite atypical shape that parasitize Psammechinus microtuberculatus (Blainville, 1825) (around 700 sea urchins collected), Psammechinus miliaris (Gmelin, 1778) (one sea urchin) and Paracentrotus lividus (Lamarck, 1816) (one sea urchin). The large number of sea urchins observed makes it possible to formulate some considerations concerning the magnitude of the parasitism of Pelseneeria on Psammechinus microtuberculatus. This rate of parasitism is quite variable and probably related to local specific conditions. In Sagres (Algarve - Portugal), it reached 70% in a sample of 439 sea urchins and only 10% in the Mediterranean at Marbella (Spain) in a sample of 235 sea urchins. The gastropods are mainly located on the aboral and or lateral side of the sea urchin, rarely on the oral one. The specimens are found associated with their spawning glued to the surface of the test of the sea urchin. The biggest specimens of Pelseneeria from this area reach 6 mm high. The combination of distinctive criteria, size, profile of the teleoconch and diameter of the larval shell, makes it closer to North-European Pelseneeria stylifera than to Mediterranean and West-African Pelseneeria minor.
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