Abstract
Populations of desert pupfish (Cyprinodon macularius;
pupfish), a federally-listed endangered species, inhabit irrigation
drains in the Imperial Valley agricultural area of southern
California. These drains have varying degrees of selenium (Se)
contamination of water, sediment, and aquatic biota. Published
Se toxicity studies suggest that these levels of Se contamination
may pose risk of chronic toxicity to Se-sensitive fish, but
until recently there have been no studies of the chronic toxicity
of Se to desert pupfish.
A life-cycle Se exposure with pupfish was conducted to
estimate dietary and tissue thresholds for toxic effects of Se on
all life stages. The dietary exposure was based on live oligochaete
worms (Lumbriculus variegatus) dosed with Se by a
laboratory food chain based on selenized yeast. Oligochaetes
readily accumulated Se from mixtures of selenized and control
yeasts. The protocol for dosing oligochaetes for pupfish feeding
studies included long-term (at least 28 days) feeding of a
low-ration of yeast mixtures to large batches of oligochaetes.
Oligochaetes were dosed at five Se levels in a 50-percent dilution
series. Pupfish were simultaneously fed Se-dosed oligochaetes
and exposed to a series of Se concentrations in water
(consisting of 85 percent selenate and 15 percent selenite) to
produce exposures that were consistent with Se concentrations
and speciation in pupfish habitats. The nutritional characteristics
of oligochaete diets were consistent across the range of
oligochaete Se concentrations tested.
The life-cycle exposure started with laboratory-cultured
juvenile pupfish that were exposed to Se through sexual maturation
and reproduction (150 days; F0 exposure). The Se exposure
continued with eggs, larvae, and juveniles produced by
Se-exposed parents (79 days; F1 exposure). Selenium exposure
(water and diets), Se bioaccumulation (whole-body and eggs),
and toxicity endpoints (juvenile and adult survival and growth;
egg production and hatching success, larval survival and deformities)
were documented throughout the life-cycle study.
Selenium concentrations in water (as much as 52 micrograms
per liter [μg/L]) and diets (as much as 53 micrograms
per gram [μg/g], on a dry weight basis) bracketed
concentrations reported in pupfish habitats. Juvenile F0
pupfish rapidly accumulated Se and bioaccumulation models
indicated that pupfish had reached more than 97 percent of
maximum whole-body Se concentrations by the time they
reached reproductive maturity. Adult pupfish accumulated
whole-body Se concentrations that averaged about 40 percent
of those in the oligochaete diets. Selenium concentrations in
eggs and F1 juveniles were similar to or slightly greater than
Se concentrations in F0 adults. Juvenile F0 pupfish contained
selenomethionine fractions (62–71 percent of whole-body Se)
greater than the average reported for wild pupfish from the
Imperial Valley (53 percent).
Selenium exposure had minimal effects on survival or
growth of juvenile and adult pupfish. There was evidence of
toxic effects on pupfish in the highest Se treatment (Se–5),
including reduced growth of F0 and F1 juvenile pupfish
(17–21 percent less than controls) on some sampling dates.
These growth reductions did not persist to subsequent sampling
dates, but reduced growth of F1 pupfish in the Se–5
treatment was associated with reduced survival (12 percent
less than controls).
Egg production was greatest
in the controls and decreased with increasing Se exposure,
reaching a minimum (51 percent less than controls) in the Se–4
treatment, but egg production was reduced by only 24 percent
in the Se–5 treatment, a lesser reduction than in other Se
treatments except Se–1. There was no statistically significant
overall effect of Se treatment on mean pupfish egg production,
reflecting large variation among replicates and among sampling
dates. However, comparisons of daily mean egg production
for 23 sampling dates indicated that egg production in each of
5 Se treatments was significantly less than controls on multiple
(3–7) sampling dates, but no mean for any Se treatment
was significantly greater than controls on any date. Significant
reductions in daily egg production occurred mainly during the
middle of the study and egg production increased in several Se
treatments during the final 2 weeks of the study. These results
suggest that pupfish egg production, although a highly variable
endpoint, was adversely affected by elevated Se exposure.
Neither egg hatching success nor survival of F1 larvae
indicated clear evidence of Se toxicity. Egg hatching success did
not differ significantly among treatments, with means ranging
from 84–91 percent. The frequency of morphological deformities
(primarily spinal deformities) was greater in larvae 10 days
post-fertilization (dpf) from a preliminary reproduction study
than in older larvae (14 dpf) from the main reproduction study.
The frequency of larval deformities was generally greater in Se
treatments than controls, but mean frequencies did not differ
significantly among treatments. Survival of F1 larvae to 21 dpf
was not reduced significantly by parental Se exposure, but the
Se–5 treatment had the lowest larval survival (84 percent), and
lowest combined egg hatching and larval survival (76 percent).
Results of the Se treatments indicate that pupfish were
insensitive to Se toxicity through most of their life cycle.
Consistent toxic effects on survival and growth of juvenile
and adult pupfish (defined as at least 10 percent reduction
compared to controls) occurred only in treatment Se–5, which
had a mean dietary Se concentration of 52 μg/g and a mean
pupfish whole-body Se concentration of 27 μg/g. These apparent
toxicity thresholds for growth and survival rank among
the least sensitive chronic Se toxicity values reported for
nonreproductive endpoints in freshwater fish. Comparisons of
these thresholds to surveys of Se concentrations in the Imperial
Valley suggest that risks of Se toxicity are low in pupfish
habitats. The dietary threshold was about twice as high as the
greatest mean Se concentrations reported in midge larvae from
seven sites in the Imperial Valley. Whole-body thresholds were
greater than mean whole-body Se concentrations reported
for field-collected pupfish from three sites and for the sailfin
molly (Poecilia latipinna), a potential bioaccumulation surrogate
for pupfish, from seven sites.
Reduced egg production, although highly variable, was
the most sensitive response of pupfish to Se exposure. Toxic
effects on egg production (reductions of 24–51 percent relative
to controls) occurred in the four highest Se treatments,
corresponding to reproductive toxicity thresholds of 7.3 μg/g
for Se in diet, 3.4 μg/g in pupfish (whole body), and 4.4 μg/g
in pupfish eggs. These thresholds are substantially lower than
published Se toxicity values for reproductive effects in other
freshwater fish (for example, 17–24 μg/g in eggs). Reduced
egg production has not been reported as a sensitive endpoint
in Se toxicity studies, although abnormal ovarian development
has been reported in Se-exposed fish, and reduced egg production
has been reported as a sensitive response of other Cyprinodon
pupfish to other environmental stressors.
Selenium concentrations in tissues of pupfish, mollies,
and diet items from Imperial Valley sites frequently exceeded
concentrations associated with reduced pupfish egg production
in the laboratory study. Reduced egg production may limit the
ability of pupfish populations to persist and recover in Se-contaminated
habitats in the Imperial Valley and elsewhere in their
limited range. However, these apparent risks of Se toxicity are
not supported by recent surveys of desert pupfish populations in
the Imperial Valley. These surveys indicated that desert pupfish
made up a small, but variable, component of fish communities
in Imperial Valley habitats, including sites with increased levels
of Se exposure, and that pupfish distribution and population
density indicated no clear relationships with Se concentrations
in diets or fish tissues. Additional studies could determine the
role of egg production in the maintenance and recovery of desert
pupfish populations in Se-contaminated habitats.