Estonian Journal of Earth Sciences, 2017, 66, 1, 21–46 doi: https//doi.org/10.3176/earth.2017.01 21 Gill rakers and teeth of three pleuronectiform species (Teleostei) of the Baltic Sea: a microichthyological approach Tiiu Märss a , Mark V. H. Wilson b , Toomas Saat a and Heli Špilev a a Estonian Marine Institute, University of Tartu, Mäealuse St. 14, 12618 Tallinn, Estonia; Tiiu.Marss@ut.ee, Toomas.Saat@ut.ee, Heli.Spilev@ut.ee b Department of Biological Sciences and Laboratory for Vertebrate Paleontology, University of Alberta, Edmonton, Alberta T6G 2E9, Canada, and Department of Biology, Loyola University Chicago, Chicago, Illinois, USA; Mark.Wilson@ualberta.ca Received 16 September 2016, accepted 14 November 2016 Abstract. In this microichthyological study the teeth and bony cores of gill rakers of three pleuronectiform species [European plaice Pleuronectes platessa Linnaeus, 1758 and European flounder Platichthys flesus trachurus (Duncer, 1892), both in the Pleuronectidae, and turbot Scophthalmus maximus (Linnaeus, 1758) in the Scophthalmidae] of the Baltic Sea are SEM imaged, described and compared for the first time. The shape and number of teeth in jaws and on pharyngeal tooth plates as well as the shape, size and number of the bony cores of gill rakers in these taxa differ. The European plaice and European flounder carry incisiform teeth anteriorly in their jaws and smoothly rounded, molariform teeth on pharyngeal tooth plates; the teeth of the plaice are more robust. The gill rakers have similar gross morphology, occurring as separate conical thornlets on gill arches. The bony cores of these thornlets (rakers) consist of vertical ribs with connective segments between them. The cores of gill rakers of the plaice and flounder reveal some differences in details. The plaice has cores with one peak, simple vertical ribs, and nodules on their lower thicker parts, while the flounder has cores with a side-branch and fine vertical ribs, which have parallel ribbing and tend to twist around the lower part of cores. The teeth of the jaws and pharyngeal tooth plates and the raker cores of the turbot are completely different from those of the plaice and flounder. In the turbot two main types of complex gill rakers are attached to the gill arches: one type has ‘sail’-shaped, high elements with one to two rows of fine conical teeth set in sockets; the other type has low tubercles with the same type of teeth. The differences among the species can be useful for studies of taxonomy and phylogeny, as well as for understanding their feeding habits. Key words: Pleuronectiformes, teeth, gill rakers, Baltic Sea, microichthyology, SEM study. INTRODUCTION Methods and approaches developed and used successfully for the comparative, taxonomic and biostratigraphic study of microscopic bony structures (ossicles) of Palaeozoic marine fishes (e.g., Märss et al. 2006, 2007, 2014) have great potential also for similar studies of fishes from younger geological strata, provided that baseline comparative data are available from related taxa of extant fishes. A previous investigation of the flatfish species [European plaice Pleuronectes platessa Linnaeus, 1758 and European flounder Platichthys flesus trachurus (Duncer, 1892), both in the Pleuronectidae, and turbot Scophthalmus maximus (Linnaeus, 1758) in the Scophthalmidae] (Pleuronectiformes) of the Baltic Sea (Märss et al. 2015) revealed a variety of their ossicles such as scales, lateral-line scales, tubercles and swivel- joint platelets. Herein we extend our study of the same pleuronectiform taxa by presenting the results of our examination of the gill rakers, jaw teeth and the teeth of the pharyngeal tooth plates, aspects previously not studied but potentially useful for the investigation of taxonomy, phylogeny and diet. The gill rakers of fishes, varying greatly in number, height, form and arrangement, are known as an important source of systematic characters for the identification and classifications of fishes (e.g., Eastman 1977; Hughes 1984). It has also been long recognized that the develop- ment of the branchial apparatus during fish ontogeny leads to increase in the number of gill rakers and decrease in the space between them, while the height of the rakers increases and the shape can become more complex due to the addition of side processes (e.g., Scofield 1934, pp. 26–29). It is also known that the branchial apparatus reflects the environmental conditions of the fish and that gill rakers may undergo morphological changes as a result of pollution (Kashulin 1997). Given the large amount of variation, including ontogenetic changes and responses to environment, there remains the question of to what extent gill rakers and teeth can © 2017 Authors. This is an Open Access article distributed under the terms and conditions of the Creative Commons Attribution 4.0 International Licence (http://creativecommons.org/licenses/by/4.0).