Diadromous Species Research Articles

Genetic stock identification of sea trout (Salmo trutta L.) along the British North Sea Coast shows prevalent long-distance migration

Abstract Knowledge of migration behaviour and spatio-temporal habitat use is important for management and conservation, not least for diadromous species where population dynamics are affected by processes occurring in both freshwater and marine habitats. Sampling from a commercial capture fishery for the ecologically and economically important salmonid Salmo trutta L. we genotyped 189 single nucleotide polymorphisms and carried out genetic assignment of origin for 916 migratory sea trout sampled along ∼500 km of Britain’s North Sea coast. Population contributions to stocks showed strong geographical differences. Trout from rivers in the northern part of this coast (Tweed, Aln, Coquet, and Tyne) contributed the majority of migrants, irrespective of location, but fish from local rivers contributed to catches in most locations. Almost all (99%) trout assigned to native British populations and a large proportion (34%) of the sampled catches originated in rivers more than 100 km from capture sites, suggesting long-distance migration to be a common trait, confirming and enlarging historical trends from tagging data. Repeated sampling across years indicated short-term stability in migration patterns. Findings illustrate the scope for genetic stock identification to identify marine habitat use and monitor population-specific migration behaviours in species where stock assessment has not traditionally incorporated marine phase dynamics.

Transposable Elements and Teleost Migratory Behaviour.

Transposable elements (TEs) represent a considerable fraction of eukaryotic genomes, thereby contributing to genome size, chromosomal rearrangements, and to the generation of new coding genes or regulatory elements. An increasing number of works have reported a link between the genomic abundance of TEs and the adaptation to specific environmental conditions. Diadromy represents a fascinating feature of fish, protagonists of migratory routes between marine and freshwater for reproduction. In this work, we investigated the genomes of 24 fish species, including 15 teleosts with a migratory behaviour. The expected higher relative abundance of DNA transposons in ray-finned fish compared with the other fish groups was not confirmed by the analysis of the dataset considered. The relative contribution of different TE types in migratory ray-finned species did not show clear differences between oceanodromous and potamodromous fish. On the contrary, a remarkable relationship between migratory behaviour and the quantitative difference reported for short interspersed nuclear (retro)elements (SINEs) emerged from the comparison between anadromous and catadromous species, independently from their phylogenetic position. This aspect is likely due to the substantial environmental changes faced by diadromous species during their migratory routes.

Partial migration in diadromous fishes drives the allocation of subsidies across the freshwater-marine ecotone

Abstract Migratory animals can act as cross-boundary subsidies sustaining ecosystem functioning, such as diadromous fishes that migrate between fresh water and seawater and carry nutrients and energy across the freshwater-marine ecotone. Frequency and timing of migration are however highly variable within and among populations. We hypothesized that in catadromous fishes (i.e., diadromous fishes that grow in freshwater and spawn in the sea, such as eels), the import of subsidies by migratory juveniles could outweigh the export of subsidies by adults due to skipped spawning migration. We used the diamond mullet Planiliza ordensis, as a model species, and determined life-history traits using a combination of length-to-age data, acoustic telemetry and otolith (fish ear stone) microchemistry. We used a mass balance approach to model individual mass acquisition and allocation, and extended our model to other life-history strategies. Our results showed high intra-population variation of migratory behaviour in P. ordensis, with few individuals migrating every year to spawn. We estimated that an individual P. ordensis acted as a net 42.6g biomass subsidy in fresh water, representing a retention of more than 50% of the juvenile mass at freshwater entry. Our model predicts that skipped spawning is likely to alter the allocation of subsidies in diadromous species, highlighting the important effects of individual variation in migratory behaviour on fluxes of energy and nutrient at ecosystem scales. We encourage future studies to consider how variation in migratory behaviour is likely to affect the direction and magnitude of biomass fluxes across ecotone boundaries.

An investigation of genetic connectivity shines a light on the relative roles of isolation by distance and oceanic currents in three diadromous fish species

Understanding connectivity is crucial for the effective conservation and management of biota. However, measuring connectivity directly is challenging and it is often inferred based on assumptions surrounding dispersal potential, such as environmental history and species life history traits. Genetic tools are often underutilised, yet can infer connectivity reliably. Here, we characterise and compare the genetic connectivity and genetic diversity of three diadromous Australian fish species: common galaxias (Galaxias maculatus), tupong (Pseudaphritis urvillii) and Australian grayling (Prototroctes maraena). For each species, we investigate the extent of genetic connectivity across a study region in south-eastern Australia (~700 km). We further determine the potential roles of contemporary ocean currents in shaping the patterns of genetic connectivity observed. Individuals across multiple rivers were sampled and >3000 single nucleotide polymorphisms were genotyped for each species. We found differences in genetic connectivity for the three species: common galaxias were highly connected, and Australian grayling and tupong exhibited patterns of isolation by distance. The degree of genetic connectivity for tupong and Australian grayling appeared unrelated to oceanic currents. This study indicates that the degree of connectivity for different diadromous species can vary greatly despite broadly similar life history strategies, highlighting the potential value of genetic tools for informing species-specific management plans.

Environmental flows stimulate the upstream movement of juvenile diadromous fishes

River regulation has degraded aquatic biodiversity globally, and the effects can be pronounced for diadromous species, whose life history processes can depend on flow conditions, such as cues for adult migration, spawning, attracting recruits into coastal rivers and promoting upstream dispersal. Environmental flows are being used to mitigate the effects of river regulation, and understanding their effectiveness is required to improve management practices. This study examined the effects of targeted environmental flows on the upstream dispersal of three temperate catadromous fish species, namely common galaxias Galaxias maculatus, tupong Pseudaphritis urvillii and short-finned eel Anguilla australis. Fyke netting was used to capture fish moving upstream before and during environmental flows in summer and autumn in two coastal rivers. We found significant increases in the catch of young-of-the-year (YOY) common galaxias (6-fold higher) and juvenile short-finned eel (26-fold higher), relative to control sites, during environmental flow pulses compared with stable, regulated base-flow conditions. A significant response was not detected for YOY tupong, despite a 39% increase in the catch. These results demonstrate that environmental flows enhance the upstream movement of juvenile diadromous fishes, a critical process governing population persistence or recovery. The findings provide managers with confidence in the use of environmental flows to support populations.

Taxonomic significance of vertebral column and caudal skeleton of clupeid fishes (Teleostei: Clupeiformes) of Iran

AbstractClupeiformes (generally known as herrings, shads and sardines) with a synapomorphy of recessus lateralis are globally distributed marine, freshwater and diadromous species classified in seven families including the family Clupeidae. In this study, a comparative analysis of the vertebral column and caudal skeleton of twelve clupeid species was conducted on the basis of X‐ray imaging and usual staining method of bones and cartilages. Several osteological features including number of principal branched caudal rays bearing with hypurals on both lobes of the caudal‐fin; number, length, arrangement and shape of epural bones; shape and length of haemal and neural spines of the penultimate vertebrae 2, 3 and 4; length and shape of parhypural; length and shape of neural arch of the urostyle; distribution of the dorsal and procurrent rays, and fusion of parhypural with preural centrum 1 (PU1) were investigated and a detailed account of these structures were provided. The results reveal that the neural spine of the 2nd, 3rd and 4th penultimate vertebrae (NSPU2, NSPU3 and NSPU4) was generally narrow and broad in lower and pointed at upper part with some variation in the size and position related to the dorsal edge of the body in different taxa. The neural arch of the urostyle (Na) has been observed as spindle, lance‐like or knife‐like. It is usually long (≥1/2 length of the NSPU2) with some modification in the studied species. Two or three curved, straight, cone‐shape or pin‐shaped epurals were found in all species with the same number in congeneric species. Opisthural cartilage (O) lays between HYP6 and PLS anteriorly, located at the tip of PLS or traversed horizontally. The presence of this cartilage made a gap between first (longest) and second unbranched caudal rays in upper part of caudal‐fin. It showed six varied shapes (e.g. fusiform, golf club‐like, horn shape, trapezoid shape, crescent and triangle) with different size in comparison with HYP6. This comparative study revealed that some osteological characters such as shape and position of opisthural cartilage, shape of neural arch of the urostyle, and number of epurals could be used as suitable features for the identification of the studied clupeids at the species and genus levels. The improved knowledge of the skeletal structures provided here, can also facilitate the correct identification of clupeid fossils.

Investigating Diadromy in Fishes and Its Loss in an -Omics Era.

SummaryDiadromy, the predictable movements of individuals between marine and freshwater environments, is biogeographically and phylogenetically widespread across fishes. Thus, despite the high energetic and potential fitness costs involved in moving between distinct environments, diadromy appears to be an effective life history strategy. Yet, the origin and molecular mechanisms that underpin this migratory behavior are not fully understood. In this review, we aim first to summarize what is known about diadromy in fishes; this includes the phylogenetic relationship among diadromous species, a description of the main hypotheses regarding its origin, and a discussion of the presence of non-migratory populations within diadromous species. Second, we discuss how recent research based on -omics approaches (chiefly genomics, transcriptomics, and epigenomics) is beginning to provide answers to questions on the genetic bases and origin(s) of diadromy. Finally, we suggest future directions for -omics research that can help tackle questions on the evolution of diadromy.

Understanding the early ontogenetic stages of Mugil liza (Mugilidae): Morphological traits and digestive/metabolic profile of pre-juveniles after recruitment.

The family Mugilidae consists mainly of diadromous species, whose reproduction occurs in offshore waters. Pre-juveniles shift their diet in the surf zone (zooplanktophagous to iliophagous). Later, during their recruitment into estuaries, huge changes take place in their digestive system. However, digestive and metabolic characteristics and some morphological traits at recruitment are unknown for Mugilidae. We performed comparative studies on early and late pre-juveniles of Mugil liza recruited in Mar Chiquita Coastal Lagoon (37°32'-37°45'S, 57°19'-57°26'W, Argentina). We determined digestive enzyme activities (intestine), energy reserves (liver/muscle), total/standard length, total weight, intestinal coefficient, hepatosomatic index and retroperitoneal fat. Pre-juveniles exhibited amylase, maltase, sucrase, lipase, trypsin and aminopeptidase-N (APN) activities, which were maintained over a wide range of pH and temperature, and exhibited Michaelis-Menten kinetics. In late pre-juveniles, amylase (422 ± 131 μmol maltose min-1 mgprot-1 ), sucrase (86 ± 14 mg glucose min-1 mgprot-1 ), trypsin (84 ± 9 μmoles min-1 mgprot-1 ) and APN (0.58 ± 0.08 μmoles min-1 mgprot-1 ) activities were higher (42%, 28%, 35% and 28%, respectively) than in the early stage. Also, the intestinal coefficient was higher in late (3.04) compared to early (2.06) pre-juveniles. Moreover, the liver appeared to be a main site of glycogen and triglyceride storage in late pre-juveniles, muscle being the site of storage in early pre-juveniles, exhibiting higher glycogen, free glucose and protein concentrations (92%, 82%, 32%, respectively). The results suggest that pre-juveniles of M. liza exhibit an adequate digestive battery to perform complete hydrolysis of various dietary substrates, availability of energy reserves and morphological characteristics to support their feeding habit and growth after recruitment. Our results represent an important contribution to knowledge of the ecology and digestive physiology of pre-juveniles of Mugilidae in the wild.

Genomic basis of the loss of diadromy in Galaxias maculatus: Insights from reciprocal transplant experiments

Diadromy is known for having major effects on the distribution and richness of aquatic species, and so does its loss. The loss of diadromy has led to the diversification of many species, yet research focusing on understanding its molecular basis and consequences are limited. This is particularly true for amphidromous species despite being the most abundant group of diadromous species. Galaxias maculatus, an amphidromous species and one of the most widely distributed fishes in the Southern Hemisphere, exhibits many instances of nonmigratory or resident populations. The existence of naturally replicated resident populations in Patagonia can serve as an ideal system for the study of the mechanisms that lead to the loss of the diadromy and its ecological and evolutionary consequences. Here, we studied two adjacent river systems in which resident populations are genetically differentiated yet derived from the same diadromous population. By combining a reciprocal transplant experiment with genomic data, we showed that the two resident populations followed different evolutionary pathways by exhibiting a differential response in their capacity to survive in salt water. While one resident population was able to survive salt water, the other was not. Genomic analyses provided insights into the genes that distinguished (a) migratory from nonmigratory populations; (b) populations that can vs those that cannot survive a saltwater environment; and (c) between these resident populations. This study demonstrates that the loss of diadromy can be achieved by different pathways and that environmental (selection) and random (genetic drift) forces shape this dynamic evolutionary process.

Diadromy in a large tropical river, the Mekong: more common than assumed, with greater implications for management

Diadromous fishes, those that migrate between fresh and marine waters, are among the most vulnerable species to river infrastructural development. These fish need to move between fresh water and the sea, so any obstruction to migration can block access to critical habitat areas. The Lower Mekong Basin (LMB) is experiencing an unprecedented boom in river development, with many dams and irrigation schemes being installed. Although general patterns of fish migration are known in the LMB, there is relatively little information on diadromous fish migration, so the overall impacts of river development on their population status is likely underestimated for many species. In this study, we reviewed the available information for over one thousand Mekong fish species to identify evidence for diadromy. Our results found that diadromy is likely a more common life history trait in the LMB than previously assumed. For instance, 61 of the 1136 Mekong fish species assessed exhibit diadromous traits (44 amphidromous species, 9 anadromous species, and 8 catadromous species). Many of these species are known to migrate relatively long distances, connecting with the Mekong estuary to some extent to complete their life cycles. Based on routine fish catch monitoring data, these suspected diadromous fishes contributed around 3% to the total catch across the basin and included a number of endemic species of significant economic value. Riverine development must therefore proceed in a balanced manner to protect diadromous species diversity and associated fisheries resources, whilst also meeting food and energy demands in the region.

An innovative bivariate approach to detect joint temporal trends in environmental conditions: Application to large French rivers and diadromous fish

Most key life-events of organisms are synchronized by complex interactions of several environmental cues to ensure optimal survival and growth of individuals and their offspring. However, global change is known to affect multiple components of ecosystems and cues at the same time. Therefore, detecting joint trends in covariate time series is a crucial challenge in global change ecology that has rarely been addressed so far. In this context, we designed an innovative combination of kernel density estimations and Mann-Kendall trend tests to detect joint temporal trends in a pair of environmental variables. This methodological framework was tested on >30 years (1976–2019) of water temperature and discharge data for 6 large French rivers (the Garonne, Dordogne, Rhône, Rhine, Loire and Vienne rivers). The implications of such trends in both temperature and discharge for diadromous species key life-cycle processes were then explored by checking if significant bivariate environmental changes occurred during seasons of upstream and downstream migration, and reproductive activities. Results were contrasted between rivers and seasons: many rivers displayed an increase in the number of days with high water temperature and low river discharge, but local discharge regulation measures could have mitigated the trend in discharge. Our findings showed that species migrating or spawning in spring were likely to be strongly impacted by the new environmental conditions in the Garonne, Loire and Rhône rivers, given the marked changes in water temperature and discharge associations detected by our new method. Conditions experienced by fall-running and spawning species have been strongly affected in all the rivers studied. This innovative methodology was implemented in a new R package, ChocR, for application to other environments and ecosystems.

Environmental influences on the juvenile migration of the threatened amphidromous Australian grayling (Prototroctes maraena)

Many diadromous species are threatened globally. Management actions to conserve these species need to be based on a quantitative understanding of their life history requirements. The transition between marine and freshwater habitats is a critical component of the life cycle of diadromous fishes, but is poorly understood for many species. We investigated recruitment patterns of juvenile Australian grayling (Prototroctes maraena), a nationally threatened amphidromous fish, entering freshwater environments in four streams in south-east Australia across 3years (2016–18). Migration occurred from September to December, with peak abundances between late October and early November (mid-austral spring). Catches were also related to mean daily discharge in the preceding 7days, with highest catches at intermediate flows (~100–150MLday–1). Our results can inform management actions, such as environmental flows, for attracting and facilitating the movement of juvenile Australian grayling into freshwater from marine nursery areas.

Modeling diadromous fish loss from historical data: Identification of anthropogenic drivers and testing of mitigation scenarios.

Diadromous fishes have drastically declined over the last century, especially in Europe. Several authors have highlighted the role of large dams in this decline, but in fact, its causes are potentially multiple and cumulative, including degradation of local environmental conditions and widespread fragmentation of hydrographic networks associated with the pervasive establishment of smaller barriers. Consequently, there is a need to improve the identification and prioritization of the drivers of diadromous species loss in order to identify and apply the most appropriate conservation and restoration measures. In this study, we used both historical sources (from mid-18th to early 20th century) and current data to quantify the long-term loss of diadromous taxa over 555 sites throughout the French river network. Then, we modeled the effects of several anthropogenic pressures (e.g. barriers, water quality, hydrological and river morphological alterations) on diadromous taxon loss. Lastly, we assessed the potential consequences of four different scenarios of anthropogenic pressure reduction. Due to uncertainties in historical sources, some species were grouped into taxa leading to a potential underestimation of actual species extinctions. Despite this limitation, our results showed that the decline in diadromous assemblages is widespread but with contrasting magnitudes depending on site locations. The maximum height and density of barriers appeared as the major factors of taxon loss. Over the scenarios tested, we observed that exclusively improving local conditions have much more limited effects than restoring river continuity. Focusing actions on large dam removal did not show the strongest responses compared to removing medium and small-sized barriers. For effective and sustainable restoration of diadromous fish assemblage, (1) historical occurrences of diadromous fishes should be used as an indicator for assessing recovery, and (2) undertaken measures must be adapted to each basin to target and limit the number of barriers to remove while allowing diadromous fish recovery.

A review on the activity of Na+/K+-ATPase in branchial ionocytes and its role in salinity adaptation among diadromous species

Freshwater and saltwater environments have opposite osmotic gradients. In freshwater, organisms tend to uptake ions from the surrounding, while in saltwater, they need to excrete the excess ions. Branchial ionocytes play a significant role in euryhaline teleost species that migrate to different salinity gradients as part of their life cycle. Ionocytes are highly dependent on the activity of Na+/K+-ATPase (NKA) in salinity adaptation. Thus, this review article determines the factors affecting NKA activity in ionocytes and its role in salinity adaptation among diadromous species. Several findings suggest that NKA activity is affected by various factors such as hormones, nutrition, and gene expression. Ionocytes enriched with NKA have many functions in diadromous depending on their types, location, size, and number in branchial necessary for these organisms to carry out their biological function in ionic-regulatory processes.

A review on the activity of Na+/K+-ATPase in branchial ionocytes and its role in salinity adaptation among diadromous species

A review on the activity of Na+/K+-ATPase in branchial ionocytes and its role in salinity adaptation among diadromous species

Anguillid eels as a surrogate species for conservation of freshwater biodiversity in Japan

To monitor and manage biodiversity, surrogate species (i.e., indicator, umbrella and flagship species) have been proposed where conservation resources are focused on a limited number of focal organisms. Using data obtained from 78 sites across six rivers in the mainland Japan and the Amami-Oshima Island, we demonstrate that two anguillids – the Japanese eel (Anguilla japonica) and the giant mottled eel (A. marmorata) – can act as surrogate species for conservation of freshwater biodiversity. Anguillid eels were the widest topographically-distributed species ranging from near the mouth to the upper reaches of rivers. Moreover, stable isotopic analyses indicated that eels are likely one of the highest-order predators in freshwater ecosystems. A significant positive relationship was found between the density of eels and the number of other diadromous species collected. However, the optimal models revealed that both the density of eels and the number of other diadromous species were significantly negatively correlated with distance from the river mouth and cumulative height of trans-river structures from the river mouth to each site. This suggests the positive relationship between eel density and number of other diadromous species was indirect and related to river-ocean connectivity. Given their catadromous life-cycle, and global commercial and cultural importance, as a taxa, anguillid eels can act as indicator, umbrella and flagship species, and a comprehensive surrogate for conservation of freshwater biodiversity.

Preference for saline water of an amphidromous goby maintained during migration to upstream freshwater areas.

This study aimed to reveal the salinity preference of juveniles of an amphidromous goby, Sumi-ukigori Gymnogobius petschiliensis, while migrating to a freshwater area. Salinity choice experiments revealed that juveniles of this species significantly prefer brackish water (salinity 20) to freshwater (salinity 0) when acclimated to a salinity of 20 in advance. Additional experiments revealed no preference between brackish water and seawater (salinity 35). Since body size was not correlated with the strength of preference for brackish water, and adults of this species are also known to prefer brackish water at a salinity of 20 to freshwater, the preference for saline water may be consistent after migration to a freshwater area. Considering that juvenile G. petschiliensis would often migrate to freshwater areas just after entering streams, the migration should be against its salinity preference. This directly contrasts with other diadromous species, which prefer the salinity of destination areas during and after migration. Adult and juvenile G. petschiliensis may take advantage of high euryhalinity to choose habitats where such ecological costs, such as high predation risk and interspecific competition, are low (i.e., freshwater areas).

An inspection-based assessment of obstacles to salmon, trout, eel and lamprey migration and river channel connectivity in Ireland

Knowledge of the location, physical attributes and impacts of obstacles on river connectivity is a requirement for any mitigating action aimed at restoring the connectivity of a river system. Here, we present a study that recorded the numbers and physical diversity of obstacles in 10 river catchments in Ireland, together with the impact these structures had on overall river connectivity. A total of 372 obstacles were recorded, 3 of these were dams, and the remainder were low-head weirs/sluices, obstacles associated with road or rail crossings of rivers and natural structures. The degree of fragmentation was estimated in each catchment by calculating obstacle density and the Dendritic Connectivity Index (DCI). DCI scores were calculated for 4 native Irish fish species with different life-histories, namely diadromous (Atlantic salmon, sea trout, European eel, sea lamprey) and potamodromous (brown trout). Obstacle density ranged between 1.2 and 0.02 obstacles/km of river. Six of the 10 catchments had at least one obstacle located on the mainstem river at least 5 km from its mouth/confluence. These 6 catchments typically had the lowest connectivity scores for diadromous species and ranged between 0.6 and 44.1 (a fully connected river would receive a maximum score of 100). While there was no significant correlation between obstacle density and the DCI score for diadromous fish, a significant negative correlation was detected between obstacle density and the DCI score for potamodromous brown trout. Here, we highlight the merit of these obstacle assessments and associated challenges for decision-making relating to prioritisation of obstacles for removal or modification.

Impacts of current and future large dams on the geographic range connectivity of freshwater fish worldwide.

Dams contribute to water security, energy supply, and flood protection but also fragment habitats of freshwater species. Yet, a global species-level assessment of dam-induced fragmentation is lacking. Here, we assessed the degree of fragmentation of the occurrence ranges of ∼10,000 lotic fish species worldwide due to ∼40,000 existing large dams and ∼3,700 additional future large hydropower dams. Per river basin, we quantified a connectivity index (CI) for each fish species by combining its occurrence range with a high-resolution hydrography and the locations of the dams. Ranges of nondiadromous fish species were more fragmented (less connected) (CI = 73 ± 28%; mean ± SD) than ranges of diadromous species (CI = 86 ± 19%). Current levels of fragmentation were highest in the United States, Europe, South Africa, India, and China. Increases in fragmentation due to future dams were especially high in the tropics, with declines in CI of ∼20 to 40 percentage points on average across the species in the Amazon, Niger, Congo, Salween, and Mekong basins. Our assessment can guide river management at multiple scales and in various domains, including strategic hydropower planning, identification of species and basins at risk, and prioritization of restoration measures, such as dam removal and construction of fish bypasses.

Evolutionary patterns of diadromy in fishes: more than a transitional state between marine and freshwater

BackgroundAcross the tree of life there are numerous evolutionary transitions between different habitats (i.e., aquatic and terrestrial or marine and freshwater). Many of these dramatic evolutionary shifts parallel developmental shifts that require physiological, anatomical and behavioral changes for survival and reproduction. Diadromy (scheduled movement between marine and freshwater) has been characterized as a behavior that acts as an evolutionary intermediate state between marine and freshwater environments, implying that diadromous lineages are evolutionarily transient. This hypothesis comes with assumptions regarding the rates of evolutionary transitions in and out of diadromy as well as rates of speciation and extinction in diadromous fishes.ResultsBased on a published phylogeny of 7822 species of ray-finned fishes, state speciation and extinction models of evolutionary transition between marine, freshwater, and diadromous species suggest transition rates out of diadromy are 5–100 times higher that transition between marine and freshwater or into diadromy. Additionally, high speciation and low extinction rates separate diadromous fishes from marine and freshwater species. As a result, net diversification (net diversification = speciation – extinction) is about 7–40 times higher in diadromous fishes compared to freshwater and marine respectively. Together the transition, speciation, and extinction rates suggest diadromy is the least stable of the three states.ConclusionEvolutionary transitions to diadromy are rare in fishes. However, once established, diversification rates in diadromous lineages are high compared to both marine and freshwater species. Diadromous lineages tend to be more transient than marine or freshwater lineages and are found to give rise to marine and freshwater specialists in addition to diadromous descendants. Although diadromy is not a necessary evolutionary intermediate between marine and freshwater, these results support the interpretation of diadromy as an important, occasionally intermediate state, that contributes to biodiversity in fishes in all environments. This evolutionary instability of diadromous lineages is counteracted by their relatively high diversification rates. These findings highlight the importance of integrating the dynamics of diversification and major evolutionary transitions for understanding macroevolutionary patterns.