Abstract
The scientific literature includes a plethora of terms to denote
within-species phenotypic diversity (e.g., morphotypes; ecotypes;
ecomorphotypes; ecophenotypes; polymorphisms; and life histories). Here
we discuss a particular situation in which different terms (ecotypesand life histories) have been used to describe the same
within-species diversity of adult Pacific lamprey Entosphenus
tridentatus , note an important challenge in using “ecotypes”, and
conclude with a recommendation to use “life histories” to describe the
ocean- and stream-maturing forms of Pacific lamprey.
The scientific literature includes a plethora of terms to denote
within-species phenotypic diversity. These include morphotypes (Lessios
and Weinberg 1994; Chavarie et al. 2013; Collyer et al. 2015); ecotypes
(Gregor 1944; Arostegui et al. 2018; Cruz-Font 2019); ecomorphotypes
(Baker et al. 1998; Segura-Trujillo et al. 2018; Kloh et al. 2019);
ecophenotypes (Schönborn and Peschke 1988; Sorensen and Lindberg 1991;
Proćków et al. 2018); polymorphisms (Skúlason and Smith 1995; Skúlason
et al. 2019; Jamie and Meier 2020); and life histories (Stearns 1989;
Winemiller and Rose 1992). Several of these terms have common roots, and
are often used inter-changeably or in combination (e.g., Baker et al.
1998; Chavarie et al. 2013; Brannon et al. 2004; Wood et al. 2008;
Palacios et al. 2012). The use of these terms may suggest perceived or
actual novelty, a unique take on biological phenomena, or an attempt to
follow precedents. Here we discuss a particular situation in which
different terms (ecotypes and life histories) have been used to
describe the same within-species diversity of adult Pacific lampreyEntosphenus tridentatus , note an important challenge in using
“ecotypes”, and conclude with a recommendation to use “life
histories” to describe the ocean- and stream-maturing forms of Pacific
lamprey.
The Pacific lamprey is a jawless and boneless anadromous fish from a
primitive taxon (Petromyzontiformes; Docker et al. 2015; Potter et al.
2015). Parker et al. (2019) genotyped Pacific lamprey at adaptive and
neutral single nucleotide polymorphisms and measured the proportional
egg mass of females to identify a genetic basis for phenotypic variation
in ocean- and stream-maturing lamprey. Their research offers an
important contribution to the understanding of evolution and ecology in
general, and of Pacific lamprey in particular. The results of Parker et
al. support the conclusions of Clemens et al. (2013), who initially
identified this same diversity from comprehensive phenotypic
characteristics that included morphology and physiology of proportional
body and organ sizes and dimensions, and use of histology to identify
stages of gonadal development. Rather than “ecotypes”, Clemens et al.
described the ocean- and stream-maturing forms as “life histories”.
We offer three inter-related reasons to argue that, given the present
scientific knowledge regarding the Pacific lamprey, “life histories”
is a more appropriate (i.e. conservative) term than “ecotypes” to
describe within-species variation of maturation schedules:
- Current information is insufficient to conclude that the ocean- and
stream-maturing forms are genetically distinguishable phenotypes
occupying different habitats, rather than life histories that display
life history trade-offs. Ecotype has been defined as “the product
arising as a result of the genotypical response of an ecospecies to a
particular habitat” (Turesson 1922, cited in Gregor 1944), and has
been used to describe the use of different niches by different forms
of the same species (e.g., benthic and limnetic forms of the
threespine stickleback Gasterosteus aculeatus ; Baker et al.
2005, and the forms of marine snail Littorina saxatilis that
have adapted to the action of waves or predation by crabs; Westram et
al. 2014). In lampreys, the term ecotypes has been synonymized with
closely-related species pairs, where one species is parasitic as a
juvenile, whereas the other species does not have a juvenile feeding
stage (Rougemont et al. 2017). Docker and Potter (2019) furthered this
argument with the description of two situations where lamprey ecotypes
could either be partially or fully reproductively isolated. It is not
known whether ocean- and stream-maturing Pacific lamprey occupy
different niches at any point in their life cycle, and they co-occur
during the freshwater spawning migration. Further, assessment of the
neutral alleles of ocean- and stream-maturing Pacific lamprey suggests
one panmictic population with no ecotypic differentiation (i.e., no
reproductive isolation; Parker et al. 2019). We appreciate that Parker
et al. raise the concept of ecotypes when referring to the
reproductive variation of ocean- and stream-maturing Pacific lamprey
because this has important evolutionary and management implications.
However, to conclude that environmental selection was responsible for
the evolutionary divergence of ecotypes from a common ancestor would
require evidence that these ecotypes encountered some degree of
reproductive isolation. Knowledge of the different habitats occupied
by each phenotype could greatly enhance monitoring and conservation
practices for Pacific lamprey.
- The names of the phenotypes were chosen by Clemens et al. (2013) to
place understanding of them within the template of what is known about
other, more well-studied species. The terms ocean- and
stream-maturing, and life histories were used by Clemens et al. to
place the morphological and physiological information on Pacific
lamprey from their study into the context of the extensive
evolutionary and ecological literature on Pacific salmonOncorhynchus spp. (Healey 1991; Hendry and Stearns 2004; Quinn
2005), particularly steelhead O. mykiss (Busby 1996; Brannon et
al. 2004; Quinn 2005), as a template to advance understanding of
Pacific lamprey. Pacific salmon are sympatric with Pacific lamprey,
and steelhead (like Pacific lamprey) spawn in the spring (Quinn 2005;
Clemens et al. 2013).
- “Life history” connotes demographic diversity of phenotypes, which
pertains to correlations and trade-offs among organismal
characteristics of mortality, growth, fecundity, age-at-maturation,
and body size (Stearns 1989; Winemiller and Rose 1992; Ricklefs and
Wikelski 2002). Clemens et al. used the term “life history” to place
the within-species diversity they identified for Pacific lamprey into
the well-established life history paradigm of Winemiller and Rose
(1992). Clemens et al. furthermore used the scheme of Winemiller and
Rose to conclude that Pacific lamprey, as a species, is a “periodic”
life history strategist.
It will be important to identify the habitat use and extent of
distribution of ocean- and stream-maturing life histories of Pacific
lamprey. If ocean-maturing lamprey spawn in the same year that they
enter freshwater, then one would predict life history differences
relative to stream-maturing Pacific lamprey. For example, less time
holding in fresh water prior to spawning could yield higher adult
survival and fecundity for ocean-maturing lamprey relative to
stream-maturing lamprey. This is because less time holding in freshwater
before spawning could translate to less exposure to predation or other
mortality sources, and more energy that could be used for generating a
higher fecundity rather than being used to prolong life prior to
spawning. We conclude that “ecotype” suggests a fundamental difference
in geographic or habitat distribution between ocean- and stream-maturing
Pacific lamprey that has not been demonstrated. However, “life
histories” more accurately describes the diversity of schedule over
which conspecifics mature and reproduce.