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.