Pietsch has described 72 species and 14 genera of fishes and published numerous scientific papers focusing on the relationships, evolutionary history, and functional morphology of teleosts, particularly deep-sea taxa.

Perhaps his most intriguing work has focused on the evolution of sexual parasitism in deep-sea anglerfishes, a reproductive strategy in which a tiny dwarf male attaches and fuses to a much larger female.

[5] T. W. Pietsch is the author of over 250 scientific and popular articles,[6] including 20 books, that focus primarily on marine ichthyology, especially the biosystematics, zoogeography, reproductive biology, and behavior of deep-sea fishes.

Trees of Life: A Visual History of Evolution, Johns Hopkins University Press, was published in 2012; annotated, illustrated, English translations of the first three volumes of Cuvier’s five-volume Histoire des Sciences Naturelles, depuis leur Origine jusqu’a nos Jours, Publications Scientifiques du Muséum and Bibliothèque Centrale, Muséum national d’Histoire naturelle, Paris, were published in 2012, 2015, and 2018, respectively; and Frogfishes: Biodiversity, Zoogeography, and Behavioral Ecology (with Rachel J. Arnold) and Cuvier’s Historical Portrait of the Progress of Ichthyology, from Its Origins to Our Own Time (second edition) in 2020.

[“How did Peter Artedi Die?”], translated from the English by Hans Aili, Ekström & Garay, Lund, Sweden; and Ichthyopedia: A Biographical Dictionary of Ichthyologists, American Philosophical Society, were published in 2023.