Oncorhynchus
The genus Oncorhynchus includes five species of Pacific salmon and seven species of Pacific trout (Quinn 2005; Penaluna et al. 2016). Pacific salmon and trout (PST) are iconic and important species culturally, economically, and recreationally (Lichatowich 1999; Behnke 2002; Penaluna et al. 2016). Research on PST has been important for informing biology and fisheries management (Groot and Margolis 1991; Behnke 2002; Penaluna et al. 2016), and ecology and evolutionary processes (Stearns and Hendry 2004; Hendry and Stearns 2004; Quinn 2005; Waples and Hendry 2008).
Modern PST are approximately 6 – 20 million years old, and further speciation and intra-specific diversification has been occurring ever since (Stearley and Smith 1993; Montgomery 2000; Waples et al. 2008; Penaluna et al. 2016). Significant geologic activity, including tectonic action, volcanism, and cycles of glaciation and de-glaciation occurred and thus has been implicated in influencing the speciation of PST (Montgomery 2000; Penaluna et al. 2016). This geologic activity would have also resulted in creation of river drainages and thus geographical isolation that influenced PST speciation (Montgomery 2000). Pacific salmon and trout exhibit a general pattern of isolation-by-distance, with populations near each other being more closely related than those further away (apart from sockeye salmon O. nerka ; Waples et al. 2008; Wood et al. 2008). Pacific salmon and trout home to their natal streams and lakes, and this results in structured populations that are locally adapted to particular environments (Waples et al., 2001, 2008; Brannon et al. 2004; Hendry et al. 2004a; Quinn 2005). Pacific salmon and trout have been described as “…different populations [that] represent ecological types referred to as spring-, summer-, fall and winter-run segments, as well as stream- and ocean-type, or stream- and ocean-maturing life history forms” (Brannon et al. 2004).
Important ISD in PST occurs below the species level (Behnke 2002). Traits of PST that diverge at the intra-specific level include run timing (Groot and Margolis 1991; Brannon et al. 2004), anadromy / freshwater residency (Hendry et al. 2004b; Quinn and Myers 2004), ocean residency, fecundity, territoriality, iteroparity / semelparity, and precocity versus larger and older spawning types (Table 2; see also Fleming and Reynolds 2004; Quinn and Myers 2004; Quinn 2005). This ISD is a continuum determined by a suite of traits that are influenced along temporal clines. One key temporal cline is water temperature, which affects larval development, juvenile residence, and spawn timing (Waples et al. 2001; Brannon et al. 2004; Quinn and Myers 2004). The diversity in life histories and genetics within PST exhibits a direct and strong correlation (Waples et al. 2001). In addition, life history traits in PST are directly related to evolutionary fitness and thus are subjected to strong and consistent selection (Hutchings 2004; Carlson and Seamons 2008). Nevertheless, many questions remain about the extent to which the ISD in PST is a result of phenotypic plasticity versus genetic adaptation (Waples et al. 2001; Hendry et al. 2004b; Waples and Hendry 2008).
Several terms have been used to describe ISD in PST, including “morphotypes”, phenotypes, populations, stocks, “life history forms”, “life history types”, “ecological types”, “races”, “phenotype”, “forms”, “types”, and “subspecies” (Healey 1991; Waples et al. 2001; Behnke 2002; Brannon et al. 2004; Penaluna et al. 2016) — and this list is not exhaustive. The prevailing use of the term “life history” can be found in key tomes (e.g., Groot and Margolis 1991; Behnke 2002). Some authors combine use of terms such as “life history ecotypes” (Wood et al. 2008). In addition, some PST populations have received the designation of “Evolutionary Significant Units” (ESUs; Table 1). This designation enables tracking of demographic characteristics relative to population status.
Lampreys
Lampreys are a basal vertebrate (Janvier 2008; Docker et al. 2015) that first appeared in the fossil record 360 million years ago (Gess et al. 2006) — long before bony fishes (teleosts) like PST and sticklebacks appeared. Forty-two to 45 species of lampreys exist (Maitland et al., 2015; Potter et al., 2015; Riva-Rossi et al. 2020), including 23 – 26 species that are freshwater resident “brook” lampreys without a parasitic life stage, nine freshwater resident parasites, and 10 anadromous and parasitic species (Maitland et al., 2015; Riva-Rossi et al., 2020).
Phenotypic diversity in lampreys has been characterized by the feeding (parasitic vs. non-feeding) and migratory behavior (anadromous or resident; Vladykov and Kott 1979; Salewski 2003). This includes freshwater resident species that do not feed after transformation into the adult life stage (i.e., the so-called “brook lampreys”), and closely related anadromous species that feed parasitically as adults (Docker 2009; Docker and Potter 2019). The brook lampreys are relatively small in body size and females exhibit low fecundity, whereas the anadromous lampreys are relatively large and exhibit correspondingly higher fecundities (Salewski 2003; Docker 2009; Docker and Potter 2019). The closely related pairs or groups of brook and anadromous lampreys have been termed “paired species” or “species pairs”, “satellite species” (more than one species), “life histories” (Vladykov and Kott 1979; Salewski 2003; Docker 2009; Docker and Potter 2019) and recently “ecotypes” (Rougemont et al. 2017; Docker and Potter 2019). A review of studies on parasitic and non-parasitic species pairs of lampreys identified a continuum of genetic and phenotypic divergence within species pairs, with the term “ecotype” being used to indicate different phenotypic expression and partial or full reproductive isolation, whereas life history was used to indicate trade-offs in body size and fecundity associated with feeding type (parasitic or non-feeding) and anadromy versus freshwater residency (Docker and Potter 2019).
The level of genetic relatedness between species pairs depends on the geographic location and circumstances. In some situations, closely related resident and anadromous lampreys can reproduce together; thus they may more aptly be called phenotypes of the same species. In other situations, these phenotypes or paired species exhibit discrete genetic differences. Nevertheless, these phenotypes were originally identified as separate species (Vladykov and Kott 1979; Docker 2009). Resident brook lampreys are expected to display more population structure within a particular river basin than anadromous lampreys, as demonstrated for western brook lamprey (Lampetra richardsoni ; Spice et al. 2019). Anadromous lampreys do not home to their natal streams, and so they display less stock structure (Bergstedt et al. 1995; Bryan et al. 2005; Spice et al. 2014a).
More recently research into Pacific lamprey, Entosphenus tridentatus , has revealed another vector of phenotypic diversity beyond feeding and migratory behavior: bimodal differences in maturation timing. Research into body morphology, gonado-somatic index (GSI), and maturation levels (determined by gonadal histology) revealed phenotypic differences in maturation timing, which were named “stream maturing” and “ocean maturing” Pacific lamprey (Clemens et al. 2013). It was hypothesized that the less-mature life history or phenotype was the commonly recognized stream maturing phenotype that would be expected to spawn one or more years after entering freshwater, whereas the formerly unrecognized ocean maturing form (which is more sexually mature upon entering freshwater) might spawn within the same year of entering freshwater (Clemens et al. 2013). The ocean maturing phenotype was found in the Klamath River estuary (at the river mouth, river kilometer 0). A separate study conducted at this same location used Single Nucleotide Polymorphism markers and GSI verified the existence of the two phenotypes, stream maturing and ocean maturing. This phenotypic diversity was initially referred to as “life histories” (Clemens et al. 2013), and then more recently as “ecotypes” (Parker et al. 2019).
In summary, closely related parasitic and non-parasitic lampreys have been called paired species, species pairs, satellite species, life histories, and ecotypes. And stream maturing and ocean maturing phenotypes of Pacific lamprey have been called life histories and ecotypes.