Absence of countergradient and cogradient variation in an oceanic silverside, the California grunion Leuresthes tenuis 

Abstract

Spatial adaptations that maximize a species' fitness in its local environment, particularly in the form of countergradient variation (CnGV = genotypic influences counteract environmental effects on trait variation) and cogradient variation (CoGV = genotypic influences accentuate environmental plasticity), have been documented in many estuarine and anadromous fishes. However, their prevalence in more oceanic fishes, where extensive gene flow and reduced environmental selection may preclude local adaptation, remains unclear. If oceanic species lack genetic structure, they may be limited in their ability to adapt to global climate change. We examined the potential for adaptation in oceanic fishes by testing whether an oceanic silverside, the California grunion Leuresthes tenuis, displays CnGV in growth capacity and CoGV in vertebral number across a latitudinal gradient as found in estuarine silversides. Common garden experiments revealed that reaction norms for growth in response to temperature differed little among 3 L. tenuis populations from different latitudes. Growth rates of wild adult L. tenuis measured from scale analysis suggested that growth may be slightly faster at more southern latitudes, likely due to environmental variation. Mean vertebral number in wild L. tenuis, determined from radiographs, were nearly identical for all populations. Overall, our results did not provide evidence for spatial adaptations in these traits of L. tenuis, in contrast to the strong patterns of latitudinal trait adaptation in other, more estuarine silversides. This study provides support for the finding that higher gene flow levels and weaker environmental selection pressures of oceanic species limit local adaptation.

Temperature and photoperiod effects on sex determination in a fish

“…Our laboratory trial, however, did not find evidence of sex-dependent growth rates; hence, size differences between sexes in the wild likely result from different birth dates. In addition, the size difference between males and females does not increase after year 1 (Brown et al, 2012).…”

“…Sex determination in L. tenuis likely occurs around 20-25 mm, based on experiments in M. menidia and other studies, which indicated that sex is determined just prior to differentiation (Conover and Fleisher, 1986;Conover and Kynard, 1981). This means that fish spawned in March or April would reach the critical sex determination period in May-June (based on growth rate of 0.34 mm/day at 17°C; Brown et al, 2012), at which point photoperiod would be lengthening. For instance in Malibu, CA (34°N), photoperiod would be increasing from 13.5 light hours: 10.5 dark hours to 14.5 light hours: 9.5 dark hours.…”

“…Female L. tenuis are known to be larger than males by their first spawning (Brown et al, 2012;Clark, 1925), and there is evidence that fecundity in L. tenuis is positively correlated with female size (Thompson, 1919). This suggests that larger females have a higher fitness; hence, the production of females early in the breeding season should be advantageous.…”

“…For instance in Malibu, CA (34°N), photoperiod would be increasing from 13.5 light hours: 10.5 dark hours to 14.5 light hours: 9.5 dark hours. However, fish spawned in July or August would reach the critical sex determination stage in August/September (given a growth rate of 0.50 mm/day at 21°C, Brown et al, 2012) when the photoperiod would be shortening, from 13.5 light hours to 12 light hours at 34°N.…”

“…Upon reaching a mean ± SD total length (TL) of 7.8 ± 1.3 mm about 1 week after hatching, larvae from each population were randomly separated into three temperature treatments (17, 21, and 25°C), which span their thermal range (Brown et al, 2012;Ehrlich and Muszynski, 1982;Reynolds et al, 1977), and were maintained under a long photoperiod regime of 15 L:9 D. Temperature was controlled by heating and cooling apparatuses and was measured daily. For each population/ temperature treatment, there were three replicate containers (20 L) of 40 fish each.…”