Rocinela dumerilii (Lucas, 1849) is obtained on the horse mackerel, Trachurus trachurus (Linnaeus, 1758) from the Sea of Marmara, in Turkey. This species is a free-living organism from the Aegean Sea of Turkey, the Mediterranean Sea and Atlantic. Its micropredator behaviour is discovered for the first time in this study. Elthusa sinuata (Koelbel, 1879) is also collected on the gill cavity of Fries’s goby, Lesueurigobius friesii (Malm, 1874) in this study. This host is a new record for Elthusa sinuata. The morphological characters of Rocinela dumerilii species are given by drawings.
