All Bottlenose Dolphins around the world were previously recognized as Tursiops truncatus, but currently, the genus is considered to be composed of two species: T. truncatus (Common Bottlenose Dolphin) and T. aduncus (the smaller Indo-Pacific Bottlenose Dolphin) (Wang et al. 1999, 2000a,b). The Common Bottlenose Dolphin has a worldwide distribution in tropical and temperate latitudes but exhibits a strong ability to adapt to local conditions, a complex social structure and strong site fidelity which results in considerable habitat partitioning throughout its range. This, in turn, has created strong population differentiation accompanied, in some cases, by marked morphological differentiation. The taxonomy of Bottlenose Dolphins is confused due to this geographical variation, and it is very possible that additional species will be recognized in the future. Three subspecies are currently recognized by the Society for Marine Mammalogy's Committee on Taxonomy (2017) and a recent re-assessment of Tursiops taxonomy worldwide conducted by the International Whaling Commission confirmed their validity (IWC 2018). These subspecies are the Black Sea Bottlenose Dolphin (T. t. ponticus Barabash-Nikiforov, 1940) which differs morphologically and genetically from other forms including those in the nearby Mediterranean Sea (Barabash-Nikiforov 1960, Geptner et al. 1976, Natoli et al. 2005, Viaud-Martinez et al. 2008). The Lahille Bottlenose Dolphin (T. t. gephyreus) (Lahille, 1908), a larger form in the coastal waters of the western South Atlantic Ocean, is morphologically and genetically different from the offshore population in eastern South America (Costa et al. 2015, 2016; Fruet et al. 2011, 2015). The third subspecies is the nominate subspecies that includes the remaining Common Bottlenose Dolphins worldwide (T. t. truncatus (Montagu, 1821)). In the western North Atlantic, two forms, offshore and coastal, are distinguishable on the basis of morphology and ecological markers (Mead and Potter 1995) and have fixed genetic differences (Le Duc and Curry 1997, Hoelzel et al. 1998, Kingston et al. 2009, Rosel et al. 2009, Van Waerebeek et al. 2017a) and according to the recent IWC review the coastal population should be recognized as at least a different subspecies.

Meester et al. (1986) recognised three subspecies, including the nominate subspecies (Taphozous perforatus perforatus) from Egypt (where this species was initially described), T. p. sudani Thomas, 1915 from Sudan, and T. p. haedinus Thomas, 1915 from Kenya. However, it remains uncertain how the geographically isolated southern African population (named T. p. rhodesiae, and described from the Shashi-Limpopo confluence; Harrison 1964) is related to these subspecies (Monadjem et al. 2010). T. p. rhodesiae was originally recognised as a subspecies of T. sudani (T. sudani australis), which was previously identified as a distinct species from T. perforatus (ACR 2015).

No subspecies of the Rough-toothed Dolphin (Steno bredanensis) are currently recognized (Committee on Taxonomy 2017). However, based on genetics there is evidence of population structuring, both within and between ocean basins, and even within specific archipelagos (Oremus et al. 2012, Da Silva et al. 2015, Albertson et al. 2016). Analyses of photo-identification and satellite tag data indicate population structuring within the Hawaiian archipelago (Baird et al. 2008, Baird 2016), and similar population structuring has been identified in the Society Islands, French Polynesia, based on photo-identification data (Oremus et al. 2012). It is likely that subpopulation structure exists throughout the species' range. Many current publications erroneously attribute the name Steno (Delphinus) bredanensis to G. Cuvier in Lesson (1828), rather than to Lesson (1828) (Smeenk 2018), and this latter attribution is now followed by the Committee on Taxonomy (2020).

Recent genetic work indicates that the genus Stenella is paraphyletic and it is likely that the Delphininae will be restructured in the coming years (LeDuc et al. 1999, Perrin et al. 2013). Four subspecies of Spinner Dolphins are currently recognized: S. l. longirostris (Gray's Spinner Dolphin), S. l. orientalis (Eastern Spinner Dolphin), S. l. centroamericana (Central American Spinner Dolphin) and S. l. roseiventris (Dwarf Spinner Dolphin) (Perrin 1990, Perrin et al. 1999). There is a zone of hydridisation between Gray's Spinner and the Eastern Spinner where an intermediate form called the White-bellied Spinner occurs (Andrews et al. 2013). Smaller individuals in Arabian waters (Red Sea and Arabian Gulf) (Van Waerebeek et al. 1999) and morphologically different animals in West Africa may represent as yet undescribed subspecies (Cadenat 1959, Jefferson et al. 1997). Several studies have demonstrated significant meta-population genetic structure, especially where relatively insular dolphin communities are strongly associated with island resting habitat such as in the Hawaiian Archipelago (Andrews et al. 2013) and in the South Pacific (Oremus et al. 2007).

Recent genetic work suggests that the genus Stenella is paraphyletic, and it is likely that the Delphininae will be restructured in coming years (LeDuc et al. 1999, Perrin et al. 2013). Morphological and genetic studies strongly suggest that the Mediterranean and eastern North Atlantic Striped Dolphin populations are isolated from each other, with little or no gene flow across the Strait of Gibraltar (Calzada and Aguilar 1995, García-Martínez et al. 1995, Archer 1997, Gaspari 2004). No subspecies are recognized.

The genus Stenella is paraphyletic, and it is likely that the Delphininae will be restructured in the coming years (LeDuc et al. 1999, Perrin et al. 2013). Two subspecies are recognized: S. attenuata attenuata in oceanic tropical waters worldwide, and S. a. graffmani in the coastal waters of the eastern tropical Pacific (ETP) (Jefferson et al. 2015, Perrin 2018).

No subspecies of False Killer Whale (Pseudorca crassidens) are currently recognized. However, there is evidence of geographic variability in skull morphology and life history (Kitchener et al. 1990, Ferreira et al. 2014). Based on genetics there is substantial population structure, both between and within-ocean basins, and even within a single archipelago (Martien et al. 2014). This species has been intensively studied only in Hawaii (Baird 2016) and more recently in New Zealand (Zaeschmar 2014). In Hawaii, three subpopulations have been identified based on a combination of genetic studies (Martien et al. 2014), satellite tagging (Baird et al. 2010, 2012, 2014, Bradford et al. 2014a), and analyses of photo-identification data (Baird et al. 2008, Baird 2016). These include two partially overlapping insular populations, one around the main Hawaiian Islands, and one in the northwestern Hawaiian Islands, as well as a partially overlapping pelagic population that ranges throughout and beyond the U.S. Exclusive Economic Zone (EEZ) around the Hawaiian Archipelago. In New Zealand, based on high resighting rates of photo-identified individuals and a relatively small number of individuals documented, it is likely that this represents a distinct subpopulation (Zaeschmar 2014). While the number of subpopulations world-wide is unknown, this is due primarily to incomplete sampling in most of the range. Based on the evidence from Hawaii, subpopulation structure likely exists throughout their range.

This taxonomic unit is treated as a single species even though there is extensive and growing evidence that it is, in fact, a complex of multiple forms with morphological, genetic, ecological, and behavioural differences that merit subspecies if not also species designations. At the time of writing (June 2017), the Committee on Taxonomy of the Society for Marine Mammalogy (<a href=""https://www.marinemammalscience.org/species-information/list-marine-mammal-species-subspecies/"">www.marinemammalscience.org/species-information/list-marine-mammal-species-subspecies</a>), which is generally regarded as the authority for marine mammal taxonomy, recognized a single killer whale species, Orcinus orca (Linnaeus, 1758), and two unnamed subspecies in the eastern North Pacific, the ENP resident killer whale (O. o. un-named subsp.) and the ENP transient killer whale (O. o. un-named subsp.) also known as Bigg's killer whale. The Committee noted, however, Other forms of killer whales in the North Pacific, North Atlantic and Antarctic [Southern] Ocean may warrant recognition as separate subspecies or even species, but the taxonomy has not yet been fully clarified or agreed (Morin et al. 2010; Foote et al. 2009, 2013). One population (a distinct population segment) of the ENP resident subspecies (the southern resident population) was listed as Endangered under the Canadian Species at Risk Act in 2003 and the U.S. Endangered Species Act in 2005.

The population of the western part of the distributional range (NW Africa, Europe, Caucasus and Levantine regions) is considered as being the nominotypic subspecies (M. emarginatus emarginatus); in the Asian part of the range, one or two subspecies are differentiated, namely M. e. sogdianus and M. e. desertorum.