The Schmeilsche lobus in Diaptomidae (Crustacea: Calanoida): comparative morphology and diagnostic potential

The Schmeil’s (Schmeilsche lobus) organ is a specific cuticular outgrowth on the dorsal surface of the second pair of swimming legs. Using a scanning electron microscope, its shape was studied and described in 40 representatives of the Diaptomidae family. The Schmeil’s organ has a characteristic shape, similar in females and males of the same species, resembling a triangle or a fold. A triangle, unlike a fold, has a more elongated shape tapering to the top, it can be with a sharp, rounded or curved angle. The Schmeil’s organ may look like a longitudinal or transverse fold, more or less deep. The presence of a Schmeilsche lobus was first established in a representative of the Paradiaptominae subfamily Metadiaptomus asiaticus (Uljanin, 1875), as well as in 10 species of Diaptominae. The species differentiation based on the Schmeil’s organ structure is useful at the case of the great similarity of the key diagnostic structures, for example, it is differ between the genera Leptodiaptomus and Acanthodiaptomus, which have the similar morphology of the fifth pair of legs in females, as well as in subgenera within the genus Arctodiaptomus and species of the genus Eudiaptomus, with great similarity in males’s fifth pair of legs structure. The considered examples confirm that the presence and shape of the Schmeil’s organ can be used as an additional diagnostic feature.