A comparative SEM study of ossicles in the Pleuronectiformes (Teleostei) of the Baltic Sea

The morphologies of ossicles of the European plaice Pleuronectes platessa Linnaeus, the European flounder Platichthys flesus trachurus (Linnaeus) (Pleuronectidae), and the turbot Scophthalmus maximus (Linnaeus) (Scophthalmidae) are described and compared. The examined material of these flatfishes (Pleuronectiformes) originates from Estonian waters, the eastern Baltic Sea. The cycloid and ctenoid scales, tubercles, lateral line scales and segments, and swivel-joint platelets noticeably differ among the three studied species. Because of characteristics of ctenii and development of the ctenial pattern, the scales of the flounder have been separated into a new subtype, the overgrowing ctenoid scales belonging to a general type of ctenoid scales. Two different appearances of tubercles, ctenoid and circuloid, are described for the flounder and turbot, respectively. Although the ossicles of the European plaice and European flounder differ in many details, they are not as highly modified as are the circuloid tubercles of the turbot. In the European plaice, a strong difference also occurs between the scales of males and females. In the European flounder and turbot, nodular ultrasculpture on the external surface of ossicles and structure formed by Mandl’s corpuscles on their internal surface, known to be caused by mineralization processes, are imaged under scanning electron microscope and also described.