The finless porpoise exhibits a pronounced phylogeographic division, characterised by distinct species occupying separate marine regions alongside a single obligate freshwater population (Fig. 1). The Indo-Pacific finless porpoise, N. phocaenoides, occurs throughout the Indian Ocean and the South China Sea (Wang et al. 2008). Its congener, the East Asian finless porpoise N. sunameri, inhabits the East China Sea, the Yellow Sea, the Bohai Sea and the coastal waters off Korea and Japan (Rice 1998; Wang et al. 2008). A zone of overlapping distribution for these two marine subspecies is identified in the Taiwan Strait and its surrounding waters (Fig. 1) (Wang et al. 2008). In marked contrast to these marine populations, the Yangtze finless porpoise N. asiaeorientalis constitutes the sole freshwater-obligate lineage within the genus. Its distribution is confined to the middle and lower reaches of the Yangtze River, encompassing approximately 1,800 km of the main river channel and extends into the interconnected Poyang and Dongting Lake systems (Fig. 1). This spatially restricted and ecologically specialised distribution underscores the distinctiveness of a lineage now recognised as a critically endangered evolutionarily significant unit (Wang et al. 2013).

The body colouration of finless porpoises demonstrates considerable geographical and ontogenetic variation, a well-documented phenotypic plasticity believed to be influenced by environmental gradients and life history stages (Table 1) (Pilleri and Chen 1979; Reeves et al. 1997; Kasuya 1999; Jefferson and Hung 2004; Zhou 2004). In N. phocaenoides, the integument darkens with maturation, progressing to a dark grey or nearly black hue in adults (Zhou 2004; Jefferson and Wang 2011). Conversely, a contrasting developmental pattern is observed in N. sunameri and N. asiaeorientalis, where neonates are born with a darker pigmentation that may undergo subsequent modification. A striking chromatic aberration is found in certain adult N. sunameri individuals from Japanese and northern Chinese coastal waters, which exhibit an almost creamy-white colouration (Fig. 2A) (Zhou 2004; Jefferson and Wang 2011). Furthermore, the Yangtze finless porpoise presents a unique post-mortem change, typically turning black after death (https://news.hsw.cn/system/2014/1128/185103_2.shtml). This collective variation in colour morphs across populations suggests complex interactions between genetic adaptation, environmental factors and potentially, phylogenetic constraint.

The Yangtze finless porpoise exhibits a greater body girth and adiposity than its marine counterparts (Gao and Zhou 1995a; Jefferson and Wang 2011). In terms of body length, adult East Asian finless porpoises are the largest amongst the three, typically reaching approximately 2.00 m, with a maximum recorded length of 2.27 m. In contrast, adult Yangtze finless porpoises and Indo-Pacific finless porpoises are smaller, measuring about 1.70 m in length (Table 1) (Zhou 2004). Finless porpoises lack a dorsal fin, possessing instead a dorsal ridge and a tuberculated area on the dorsal surface (Gao and Zhou 1995a; Jefferson and Hung 2004; Jefferson and Wang 2011). The length and height of the dorsal ridge and the width of tubercule showed significant difference in finless porpoises (Fig. 2B–D). In the Indo-Pacific finless porpoise, the ridge originates posterior to about mid-back of the body and usually accounts for no more than 30% of the body length (Wang 1999), while in the East Asian finless porpoise and the Yangtze finless porpoise, the length exceeds half of the body length and even can reach up to 55% in N. sunameri (Wang 1999). The height of the ridge in N. sunameri usually exceeds 16 mm, even in new-born calves and can reach up to 50 mm or more (Table 1) (Gao and Zhou 1995a; Jefferson and Wang 2011). In contrast, the ridge height of N. asiaeorientalis is generally less than 15 mm (Gao and Zhou 1995a).

The tuberculated area further distinguishes the three species. In N. phocaenoides, its width ranges from 48 to 120 mm and the number of longitudinal rows of tubercles typically exceeds 10 (Table 1) (Gao and Zhou 1995a). These values are substantially higher than those observed in N. sunameri, where the tuberculated area is 2 to 12 mm wide and the number of rows ranges from 1 to 9, usually 3 to 8 (Table 1) (Gao and Zhou 1995a; Jefferson and Wang 2011). The Yangtze finless porpoise shows the most reduced condition, with a width of only 2–8 mm and 1–5 rows of tubercles (Table 1) (Gao and Zhou 1995a).

Early classification and population division of finless porpoises were based on certain measurements of the skull as main evidence (Pilleri 1972; Pilleri and Gihr 1975; Wang 1992; Gao and Zhou 1995b). Geographical variation in rostrum length and width was first identified in the 1970s (Pilleri 1972; Pilleri and Gihr 1975). Amongst the three taxa, N. phocaenoides exhibits the smallest values for rostrum length-related parameters, whereas rostrum width indices are generally largest in this species, followed by N. asiaeorientalis, with N. sunameri showing the smallest (Table 1). In addition, the Yangtze finless porpoise exhibit reduced tooth counts, elongated temporal fossae, lengthened tympanoperiotic bones and widened occipital condyles relative to marine congeners (Gao and Zhou 1995b). In discriminant analysis, significant differences in parameters such as width of pterygoid, width of internal nares, length of orbital and width of rostrum at 1/2 length were found between N. phocaenoides and other finless porpoises (Gao and Zhou 1995b). Specifically, N. phocaenoides has significantly wider pterygoid and internal nares, as well as shorter length of orbital. Furthermore, specimens from the Indian Ocean exhibited rostrums that were proportionally shorter and broader relative to their counterparts in other regions, whereas individuals from Japan displayed comparatively narrower zygomatic widths (Amano et al. 1992). It is especially noteworthy that N. asiaeorientalis and N. sunameri can be distinguished by the brain box length, middle width of the premaxillae, maximum width of orbital and length of temporal fossa (Gao and Zhou 1995b).

The small kidney structures also showed significant variation (Ni and Zhou 1988) in finless porpoises, which is helpful for finless porpoise to adapt to different water osmotic environment. Especially, the freshwater-inhabiting finless porpoise N. asiaeorientalis showed smaller values across several renal structural indices compared with its marine counterpart N. sunameri (Table 1), including percentage of medullary thickness (PMT), percentage of medullary area (PMA), relative medullary thickness (RMT), relative medullary area (RMA), percentage of papillary thickness (PPT) and percentage of papillary area (PPA) (Ni and Zhou 1988). Consistent with these morphological differences, the Yangtze finless porpoise also exhibits a comparatively weaker urine concentrating capacity.

Comparable differences have also been documented in the visual system (Gao and Zhou 1986). Compared to coastal populations in the Yellow Sea, the Yangtze finless porpoise exhibits significantly shorter mean distances of centre of eye to tip of snout, an adaptation to its turbid freshwater habitat. Morphometric analyses further reveal reduced mean high of eye slit, shorter relative horizontal diameters of ball eye and elongated axial dimensions of eye ball in the Yangtze population (Table 1) (Gao and Zhou 1986). Similar patterns hold for other ocular structures and the Yangtze finless porpoise has smaller eye slits and a substantially reduced lens (Pilleri and Chen 1979; Chen et al. 1980; Pilleri 1980; Gao and Zhou 1986). Furthermore, the number of optic nerves and eye muscle nerves in the Yangtze finless porpoise was also less than those in the Yellow Sea population (Chen et al. 1980; Jefferson and Hung 2004). These may be the adaptation of the Yangtze finless porpoise to the relatively lower light environment in the freshwater (Chen et al. 1980; Gao and Zhou 1986; Jefferson and Hung 2004).

Recent observations on the epidermal ultrastructural characteristics of marine and freshwater finless porpoises have also revealed significant divergence, which may be correlated with adaptations to distinct osmotic environments (Zhou et al. 2025). Specifically, the epidermis of the Yangtze finless porpoise exhibited a higher abundance of stratum basale cells. At the outermost surface, the lipid-rich stratum corneum comprised fewer cell layers and featured enlarged neutral lipid droplets, an adaptation that curbs passive water influx under hypotonic conditions. Conversely, in the East Asian finless porpoise, the stratum basale displayed a more orderly cellular arrangement, correlating with densely packed keratin filaments and structurally reinforced desmosomes across all epidermal strata at the ultrastructural level. Such intimate intercellular packing likely curtails transepidermal water loss in response to elevated external osmotic pressure (Zhou et al. 2025).

The age at sexual maturity and reproductive longevity in finless porpoise exhibit marked geographical variation, reflecting adaptive life-history strategies to distinct ecological conditions (Table 1). Comparative analyses indicate that populations in the South China Sea and the southern East China Sea reach sexual maturity the earliest, with males maturing at approximately three years and females at four years of age and have a maximum recorded longevity of 21 years (Zhou 2004). In contrast, species inhabiting the Yellow Sea demonstrate a delayed onset of maturity, with both sexes attaining sexual maturity at around four years, coupled with an extended potential breeding lifespan, as evidenced by a maximum observed age of 24 years (Zhou 2004). The most pronounced divergence in life-history parameters is observed in the critically endangered Yangtze finless porpoises, which exhibit the most protracted developmental trajectory. This freshwater species reaches sexual maturity later, at approximately five years and has a shorter maximum observed longevity of 17 years (Zhou 2004). These demographic parameters are critical for assessing population viability and formulating conservation strategies, particularly for the imperilled Yangtze River population.

In the past decades, various molecular markers have been applied to reveal the genetic variability and significant differentiation in the finless porpoise (Yang and Zhou 1997; Yang and Zhou 2000; Yang et al. 2002; Zheng et al. 2005; Yang et al. 2008; Chen et al. 2010; Li et al. 2011; Li et al. 2013; Lin et al. 2014; Lin et al. 2017; Lee et al. 2019; Kim et al. 2025). Obvious population genetic structure was revealed amongst different populations (Yang et al. 2002, 2003, 2008; Zheng et al. 2005; Xu et al. 2010) by using mtDNA control region, microsatellite, nuclear SNPs, MHC, AFLP etc. For example, based on mtDNA control region, very significant overall population structure in finless porpoises in Sino-Japanese waters was found (Φ_ST = 0.61, P < 0.001; F_ST = 0.52, P < 0.001) (Yang et al. 2008). Although the differentiation between the Yangtze finless porpoise and either the Yellow-Bohai Sea finless porpoise or the South China Sea finless porpoise was relatively smaller, they all have reached statistical significance (Yangtze River vs. Yellow/Bohai Sea: Φ_ST = 0.16, F_ST = 0.10, Yangtze River vs. South China Sea: Φ_ST = 0.59, F_ST = 0.55). Even if there is overlap in the distribution areas between different finless porpoise populations, very limited (or even no) gene flow was detected (Chen et al. 2010; Li et al. 2013; Lee et al. 2019).

In recent years, with the rapid development of high-throughput genome sequencing technology and bioinformatics analysis methods and their application in the finless porpoise, more evidence for the population differentiation and adaptive evolution of finless porpoises has been provided (Zhou et al. 2018; Yin et al. 2022; Yin et al. 2024). Zhou et al. (2018) performed de novo sequencing for one Yangtze finless porpoise individual and whole-genome re-sequencing for additional 48 finless porpoise individuals from Chinese waters. Over 13 million high-confidence single nucleotide polymorphism (SNP) sites were identified to conduct in-depth population genomic analyses. A series of evidence to support the genetic differentiation and speciation in finless porpoises was collected: 1) Phylogenetic reconstruction resolved the Yangtze finless porpoise as a distinct clade compared with the Indo-Pacific finless porpoise clade and the narrow-ridged finless porpoise clade (Fig. 3A); 2) Population structure analyses, principal component analysis, identity by descent patterns, linkage disequilibrium decay and demographic history reconstruction collectively supported significant and stable genetic divergence between the Yangtze porpoise and its marine counterparts, demonstrating their establishment as distinct evolutionary lineages (Fig. 3B); 3) Species delimitation analyses, based on Bayesian factors, further support the three-species classification of finless porpoises as statistically superior to the two-species model. The wide-ridge and narrow-ridge porpoise lineages diverged approximately 100,000 years ago, while the Yangtze finless porpoise split from its marine narrow-ridge counterparts 5,000–40,000 years ago. These divergence times suggest that sea-level fluctuations during the Last Glacial Period likely facilitated the establishment of the Yangtze freshwater population and subsequent speciation events; 4) Significant positive selection signals were detected in a suite of genes associated with osmoregulation (ADCY1, DYNC2H1 and SLC14A2) and renin-angiotensin system function (ACE2) between the Yangtze finless porpoise and its counterparts inhabiting high-salinity environments. These findings are consistent with earlier morphological measurements of small kidney structures by Ni and Zhou (1988), collectively indicating that the Yangtze finless porpoise has undergone adaptive evolution in osmoregulatory capacity to maintain water-electrolyte balance in freshwater environments. Furthermore, telomere-to-telomere gap-free genome assemblies of the Yangtze finless porpoise and East Asian finless porpoise revealed: 1) Substantial genomic divergence between the two species; 2) Their phylogenetic divergence is further confirmed; 3) Positively selected genes in the Yangtze finless porpoise are associated with DNA damage repair, while those in the East Asian finless porpoise are linked to high-salinity tolerance (Yin et al. 2024).

A consistent pattern of genetic differentiation between the Yangtze finless porpoise and its marine relatives has emerged from studies of their distribution, morphology, anatomy, physiology, genetics and genomics. Taken together, all of these findings point to a complete genetic, ecological and reproductive separation, with no gene flow or genetic exchange occurring between the Yangtze finless porpoise and the marine finless porpoise. Although not yet internationally recognised as a distinct species, the Yangtze finless porpoise is already acknowledged as such in China, where it has demonstrated substantial flagship conservation value. This value is evident across multiple dimensions. At the national policy level, China established a dedicated Yangtze Finless Porpoise Conservation Day in 2019 (24 October), the first such day for a single wildlife species in the country and the recently released Yangtze Finless Porpoise Conservation Action Plan (2026–2035), jointly issued by eight ministries, led by the Ministry of Agriculture and Rural Affairs, explicitly designates the species as a flagship and umbrella species (Ministry of Agriculture and Rural Affairs 2025). Societally, the porpoise is widely known as the “aquatic giant panda” and the “smiling angel”; its charismatic morphology has made it one of the most prominent ecological symbols in Chinese media and public communication (Hao et al. 2024). The species has been adopted as an urban eco-cultural icon by several Yangtze-side cities and its conservation has catalysed a broad public-participation paradigm built on government guidance, scientific support, engagement of social organisations and media promotion (Hao et al. 2024). Ecologically, the Yangtze finless porpoise is the sole surviving apex predator in the river, occupying the top trophic level of the food web and its population status directly reflects the health of the entire river ecosystem (McDonell 2025). Accordingly, population recovery has become a primary indicator of the effectiveness of key Yangtze conservation policies, including the Yangtze River Protection Law (2021) and the ten-year fishing ban (2021–2031). The population has risen from a low of 1,012 individuals in 2017 to 1,249 in 2022 and further to 1,426 in 2025, representing an increase of more than 40% since 2017, a recovery that has been described as exceptionally rare amongst freshwater cetaceans. In conservation science, a mature three-pillar conservation infrastructure has been established, integrating in-situ protection (eight reserves, Lu et al. (2025)), ex-situ conservation (four naturalised populations comprising over 150 managed individuals and an annual reproductive output exceeding ten calves; Hao et al. (2024)) and captive breeding (five captive-born individuals including three second-generation offspring, Hao et al. (2024)). The successful conservation practices developed for the Yangtze finless porpoise have been formally proposed as a global conservation framework for endangered small cetaceans.