Killifish Heterandria Formosa Research Articles

Response to Selection for Increased Heat Tolerance in a Small Fish Species, With the Response Decreased by a Population Bottleneck

Many natural populations are likely to be impacted by increasing temperatures resulting from climate change. The selection pressures exerted by these impacts may allow some populations to adapt to the elevated temperatures – which could reduce climate change’s impact. At present, there is limited evidence for such “evolutionary rescue”. Moreover, the potential for adaptation may be reduced if populations have experienced population bottlenecks as an initial consequence of climate change or due to other human impacts. This study looked at the response to selection in laboratory populations subjected to selection for heat tolerance, in both normal and bottlenecked laboratory populations of the least killifish Heterandria formosa. The bottlenecked populations had undergone a single bottleneck and were found to have fewer microsatellite alleles and reduced genetic diversity. After four generations of selection, heat tolerance had increased – albeit by only 0.1 °C. The increased heat tolerance was more pronounced for normal populations than it was for those that had undergone a population bottleneck. These results show that some populations, under specific conditions, may be able to adapt to elevated water temperatures. However, this response may be too slow for species with a long generation time and further limited in populations that have undergone drastic reductions in population size. In addition, the observed potential for adaptation to physiological heat stress does not equate to evolutionary rescue from other stresses associated with climate change.

A low level of dietary selenium has both beneficial and toxic effects and is protective against Cd-toxicity in the least killifish Heterandria formosa

As an essential element, selenium (Se) is beneficial at low levels yet toxic at high levels. The present study assessed the effects of dietary exposure to Se in the least killifish Heterandria formosa, and investigated how this exposure influences the effects of a subsequent exposure to cadmium (Cd). The fish were pre-exposed to an environmentally relevant concentration (2 μg g−1 dry wt) of dietary selenite (Se4+) or seleno-l-methionine (Se-Met) for 10 d. The same fish were then exposed to 0.5 mg L−1 of Cd for 5 d. Both Se(IV) and Se-Met rapidly accumulated in H. formosa. Results for the two Se species were generally similar in this study. Fish exposed to Se had lower levels of lipid peroxidation (measured as levels of thiobarbituric acid reactive substances or TBARS) and a higher catalase (CAT) activity. In contrast, their Na+/K+-ATPase activity was reduced. The Cd exposure resulted in an increase in lipid peroxidation and decreases in the activities of catalase and Na+/K+-ATPase. The Cd-exposed H. formosa that were pre-exposed to Se had lower Cd body burdens, less lipid peroxidation, and higher catalase activity, than did fish not pre-exposed to Se. The Se exposure did not have a protective effect on the Cd-induced reduction in Na+/K+-ATPase activity. These results clearly demonstrate that a Se-enriched diet reduces some (but not all) forms of Cd-toxicity and that Se can simultaneously have beneficial and detrimental effects, making it difficult to predict the net outcome of changes in dietary Se levels for fish.

Effects of Water Chemistry on the Life History of the Least KillifishHeterandria formosaand the Absence of Evidence for Local Adaptation

This study explored whether persistent differences in life history traits between populations of Least Killifish Heterandria formosa could be explained by differences in habitat-associated water type, reflecting either local adaptation or phenotypic plasticity. Fish native to a pond and a spring site were reared in the lab in either pond or spring water. Fish reared in pond water reached maturity faster and at the same size as fish reared in spring water, regardless of source population. There was no difference in daily mortality rate between fish raised in the different water types, but the longer time to maturity in spring water produced a lower cumulative survival to maturity. Water type did not affect fecundity, but fish from the spring population had larger offspring, contrary to patterns found in both the field and previously in the lab. Persistent differences among natural populations in life history traits have previously been attributed to differences in population density and predation pressure....

Evolution in Population Parameters: Density-Dependent Selection or Density-Dependent Fitness?

Density-dependent selection is one of earliest topics of joint interest to both ecologists and evolutionary biologists and thus occupies an important position in the histories of these disciplines. This joint interest is driven by the fact that density-dependent selection is the simplest form of feedback between an ecological effect of an organism's own making (crowding due to sustained population growth) and the selective response to the resulting conditions. This makes density-dependent selection perhaps the simplest process through which we see the full reciprocity between ecology and evolution. In this article, we begin by tracing the history of studying the reciprocity between ecology and evolution, which we see as combining the questions of evolutionary ecology with the assumptions and approaches of ecological genetics. In particular, density-dependent fitness and density-dependent selection were critical concepts underlying ideas about adaptation to biotic selection pressures and the coadaptation of interacting species. However, theory points to a critical distinction between density-dependent fitness and density-dependent selection in their influences on complex evolutionary and ecological interactions among coexisting species. Although density-dependent fitness is manifestly evident in empirical studies, evidence of density-dependent selection is much less common. This leads to the larger question of how prevalent and important density-dependent selection might really be. Life-history variation in the least killifish Heterandria formosa appears to reflect the action of density-dependent selection, and yet compelling evidence is elusive, even in this well-studied system, which suggests some important challenges for understanding density-driven feedbacks between ecology and evolution.

Occurrence of copper acclimation in the least killifish Heterandria formosa, and associated biochemical and physiological mechanisms

We investigated the occurrence of copper acclimation in the least killifish, Heterandria formosa using both lethal and sublethal endpoints. We also investigated potential mechanisms underlying the observed acclimation. To assess the occurrence of acclimation, fish were exposed to either a background Cu level or to 15μg/L Cu for seven days and subsequently exposed to a lethal Cu level (150μg/L Cu). During the latter exposure, fish were monitored for survival till all fish had died, and (during the first 8h of this exposure) for changes in whole-body Na levels and lipid peroxidation (LPO). During the high-level Cu exposure, fish pre-exposed to copper had a significantly longer time-to-death than did the control fish. Similarly, neither whole-body Na nor LPO changed in the Cu-pre-exposed fish during the 8h of the exposure to 150μg/L Cu – while both decreased significantly in the control fish. Thus, acclimation was evident for both time-to-death and the sublethal endpoints. These results also indicate that Cu toxicity may involve both Na loss and LPO, and that Cu-acclimation may be brought about by prevention of these effects. Our follow-up study on potential mechanisms underlying this copper acclimation used a similar pre-exposure/exposure design. Fish were subsampled at the end of the 7-day acclimation period – just before the commencement of high-level Cu exposure (T0), after 4h of this Cu exposure (T4), and again after 8h of this Cu exposure (T8). Whole-body Cu accumulation, Na+/K+-ATPase activity, metallothionein levels, and catalase activity were quantified for these time points. While Cu levels were higher in the Cu-pre-exposed fish than in the control fish at T0, net Cu accumulation was faster in the control fish than in the Cu-pre-exposed fish during the subsequent high-level Cu exposure. Consequently, changes in Cu accumulation dynamics may play a role in the resistance. Metallothionein induction may also play a role in the observed acclimation, as Cu-acclimated fish had a significantly higher metallothionein concentration compared to the control fish. There was no evidence of involvement of Na+/K+-ATPase in the acclimation, as the activity of this enzyme remained lower in the pre-exposed fish than in the control fish throughout both Cu exposure periods. There was limited evidence that a reduced loss of catalase activity plays a role in the acclimation; catalase activity did not differ after the pre-exposure period but was significantly higher in Cu-acclimated fish than in the control fish at T8.

Seasonal nekton assemblages in a flooded coastal freshwater marsh, Southwest Louisiana, USA

Marsh flooding and drying may be key factors affecting seasonal nekton distribution and density because habitat connectivity and water depth can impact nekton accessibility to the marsh surface. Recent studies have characterized freshwater nekton assemblages in marsh ponds; however, a paucity of information exists on the nekton assemblages in freshwater emergent marshes. The principal objectives of this study are to characterize the seasonal nekton assemblage in a freshwater emergent marsh and compare nekton species composition, density, and biomass to that of freshwater marsh ponds. We hypothesize that 1) freshwater emergent marsh has lower taxa richness than freshwater marsh ponds; and 2) freshwater emergent marsh has a lower seasonal density and biomass than freshwater marsh ponds. Mosquitofish Gambusia affinis and least killifish Heterandria formosa were abundant species in both habitats while some abundant species (e.g., banded pygmy sunfish Elassoma zonatum) in freshwater ponds were absent in freshwater emergent marsh. Our data did not support our first and second hypotheses because taxa richness, seasonal density and biomass between freshwater emergent marsh and ponds did not statistically differ. However, freshwater emergent marsh was dry during the summer months and thus supports no fish species during this period. Additional long-term research on the effects of flow regime in the freshwater marsh on nekton assemblages would potentially improve our understanding of nekton habitat requirements.

Life history variation along a salinity gradient in coastal marshes

Coastal habitats are susceptible to changes in the environment associated with alter- ations in salinity. A field study was conducted on natural populations of the sailfin molly Poecilia latipinna, the western mosquitofish Gambusia affinis and the least killifish Heterandria formosa col- lected from coastal marsh sites along a salinity gradient, to investigate the influence of salinity on body condition and reproductive life history traits. In brackish marsh sites male P. latipinna had the best body conditions, while females had similar body conditions across all 3 marsh types. Female P. latipinna had greater reproductive allotment and fecundity in brackish marshes, where this fish was most abundant. Specimens of G. affinis collected from fresh marsh sites had less favorable body condition, and females had lower reproductive allotment and fecundity than those collected from higher salinities. While G. affinis was more abundant in freshwater marshes than in higher salinity marshes, this higher abundance did not correspond with a better body condition or higher reproduc- tive effort, suggesting that G. affinis may be stressed in freshwater. There was no difference in the conditions between (both male and female) H. formosa from fresh and intermediate marshes. Female H. formosa from fresh marshes had a similar reproductive allotment and lower fecundity than those from intermediate marshes. The 3 species exhibited different life history patterns along the salinity gradient, and some of these patterns conflicted with expectations based on species abundances. When assessing habitat quality along an environmental gradient, measures of abundance should be accompanied by more sensitive indicators of environmental stress.

Laboratory culturing and selection for increased resistance to cadmium reduce genetic variation in the least killifish, Heterandria formosa

Populations exposed to environmental contaminants can undergo intense selection pressures, which in turn can lead to a loss of genetic variation. We assessed this loss of genetic variation in the least killifish Heterandria formosa for laboratory populations that had undergone eight generations of selection for an increased resistance to cadmium. Using microsatellite markers, we compared genetic variation between three selection and three control laboratory populations and between these laboratory populations and the source population. Heterozygosity was lower in each selection population than it was in its paired control population, with this difference being statistically significant in two of the three comparisons. This is evidence that adaptation to environmental contaminants can result in an overall loss of genetic variation. Furthermore, the laboratory populations had much lower heterozygosity than did the source population. The latter loss of genetic variation, probably a result of random drift, did not prevent the laboratory populations from showing a strong response to the selection for cadmium resistance. The loss of genetic variation resulting from maintaining populations in the laboratory demonstrates that it is important to maintain a large population size for such populations and that the potential for loss of genetic variation in laboratory populations is taken into consideration in ecotoxicology when extrapolating from laboratory to natural populations.

Patterns of Habitat Use among Vegetation‐Dwelling Littoral Fishes in the Atchafalaya River Basin, Louisiana

AbstractInformation regarding the relative value of different aquatic macrophytes as fish habitat is lacking for large floodplain river systems, such as the Atchafalaya River basin in south‐central Louisiana. We made seasonal comparisons of fish (total length, <100 mm) density, diversity, and biomass and physicochemistry among submersed hydrilla Hydrilla verticillata, emergent bulltongue Sagittaria lancifolia, and floating water hyacinth Eichhornia crassipes. In addition, we examined the influence of habitat structure and physicochemistry on fish assemblage composition and density. Although there were no seasonal differences in fish biomass among plant types, hydrilla consistently supported the greatest fish density and diversity across seasons. While hydrilla and water hyacinth fish density changed little among seasons, density in bulltongue was higher during low water. Of the 12 most common fish species, the pirate perch Aphredoderus sayanus and yellow bullhead Ameiurus natalis showed no consistent pattern of habitat use. However, the densities of golden topminnow Fundulus chrysotus, bluegill Lepomis macrochirus, warmouth L. gulosus, redear sunfish L. microlophus, and pugnose minnow Opsopoeodus emiliae were highest in hydrilla, whereas the density of bantam sunfish L. symmetricus was highest in water hyacinth. A seasonally variable pattern was found for western mosquitofish Gambusia affinis, redspotted sunfish L. miniatus, sailfin molly Poecilia latipinna (higher in bulltongue), and least killifish Heterandria formosa (lower in water hyacinth). Overall, littoral fish distributions reflected responses to several spatially and temporally variable environmental factors (e.g., water quality, plant complexity, and seasonal depth changes). We suggest that the consistently high fish density and diversity in hydrilla across seasons were related to an optimization of favorable water quality (particularly dissolved oxygen [DO]), architectural complexity, and biovolume. However, during low water, as hydrilla and water hyacinth canopies became very dense and water hyacinth DO levels remained low, bulltongue was more heavily used by several species, presumably because of a more favorable combination of DO, complexity, and particularly depth.

Laboratory culturing and selection for increased resistance to cadmium both reduce genetic variation in the least killifish Heterandria formosa

Laboratory culturing and selection for increased resistance to cadmium both reduce genetic variation in the least killifish Heterandria formosa

Metallothionein-like protein in the least killifish Heterandria formosa and its role in cadmium resistance.

Metallothioneins (MTs) are a class of low molecular weight, cysteine-rich, metal-binding proteins ubiquitous in animals. They function in metal regulation and detoxification. An MT-like protein was separated by gel-permeation high-performance liquid chromatography (HPLC) in the least killifish Heterandria formosa exposed to 6 mg/L of Cd for 26 h. Western blot analysis showed that this protein cross-reacted with a cod-MT antibody. We quantified MT-like protein levels by the Cd-hemoglobin saturation assay in fish from two Cd-resistant and two control laboratory populations. Fish from the resistant populations had higher MT-like protein levels than those from the control populations; this difference was found for basal levels and for levels after 26 h of exposure to Cd. Our results suggest that the MT-like protein detoxified Cd in the least killifish and accounted for at least part of the increased resistance in the Cd-resistant laboratory populations.

Changes in cadmium accumulation as a mechanism for cadmium resistance in the least killifish Heterandria formosa

Evolution of resistance to heavy metals may involve complex changes at the physiological level. We report here on research into changes in cadmium uptake and accumulation as mechanisms underlying the increased resistance to Cd in laboratory lines of the least killifish, Heterandria formosa. We compared Cd accumulation between three control lines and three lines selected successfully for an increased Cd-resistance. Individuals of the Cd-adapted lines had a lower short-term (2 h) uptake of Cd than those of the control lines. A time-series experiment showed that selection-line fish had a lower rate of Cd accumulation than fish from the control lines during the first 14 h of exposure to 6 mg/l of Cd. At the 14 h time point, Cd accumulation in the control-line fish approached lethal levels, while Cd levels in selection-line fish from this point on increased at a slower rate than in the initial 14 h. Overall, the relatively small changes in uptake and accumulation patterns caused lethal body burdens in the fish from the selection lines to be reached at more than twice the exposure duration at which this occurred in fish from the control lines. Lethal Cd levels appeared to be the same in control- and selection-line fish. Our results suggest that reduced uptake and accumulation of Cd accounted for approximately two-third of the increased resistance in the Cd-adapted lines.

The Suitability of Calcein to Mark Poeciliid Fish and a New Method of Detection

The suitability of calcein as a marker for population studies depends on (1) the assumption that marked individuals have unaltered viability, (2) the fidelity of the calcein label, and (3) the facility with which calcein can be used. We examined the effects of calcein on survival, growth, and the timing and size at sexual maturity of least killifish Heterandria formosa and present a new method for detecting calcein. To test the assumption that marked individuals have unaltered viability, juvenile least killifish were immersed for 24 h in a 250-mg/L solution of calcein. A control group of same-aged juveniles was immersed in the same volume of water for 24 h without calcein. All individuals were then removed and reared individually in separate containers. Upon examination under an epifluorescent microscope, all calcein-treated fish showed fluorescent green marks on their scales and fin rays, whereas controls did not. The calcein treatment had no significant effect on growth and survival through 9 weeks nor on the age and size at maturity. We also designed a portable detector (uses InGaN blue LEDs as a light source) for distinguishing calcein-marked individuals; using either this new detector or a standard epifluorescent microscope, the fluorescent mark was visible on the test fish for up to 5 weeks in the laboratory, although some individuals required remarking (due to fading) at 2–3 weeks postimmersion. The calcein tag was also visible in the vertebrae of ethanol-preserved specimens for up to 6 years, provided specimens were stored in the dark.