David H. Funk

Climate Change and the Life Histories and Biogeography of Aquatic Insects in Eastern North America

One of the most important factors affecting the life history characteristics and biogeography of aquatic insects is temperature (Sweeney, 1984). Insects are poikilothermic (cold-blooded) animals whose metabolism, rate and magnitude of growth, development, and overall behavioral activities respond significantly to thermal change on a diel, seasonal, and annual basis (Ward and Stanford, 1982). Despite this sensitivity to temperature, most aquatic insect species can be found in aquatic habitats over a broad geographic area that includes a wide range of thermal regimes. Obviously, these aquatic insect species possess bioenergetic, developmental, and/or behavioral mechanisms that enable conspecific populations to survive and reproduce in very different environmental conditions.

Selenium bioaccumulation and maternal transfer in the mayfly Centroptilum triangulifer in a life-cycle, periphyton-biofilm trophic assay.

Selenium contamination in aquatic ecosystems provides management challenges because bioaccumulation in animals is largely a function of dietary exposure, whereas regulatory entities have traditionally focused on direct water to organism interactions. Selenium is known to be readily absorbed by primary producers and can potentially biomagnify in food webs and elicit adverse effects in higher trophic levels. However, selenium bioaccumulation in the invertebrate prey of many predatory animals is poorly understood. Here, we used 75Se (as selenite) as a radiotracer to characterize Se bioaccumulation into natural periphyton biofilms and subsequent dietary and maternal transfer in the mayfly, Centroptilum triangulifer, in a life-cycle assay. On average periphyton biofilms bioconcentrated selenium 1113 (+/-430)-fold following 7-9 days of exposure to a range of environmentally relevant dissolved concentrations (2.4-13.9 microg L(-1)). Mayflies grown to adulthood on these diets further biomagnified Se with trophic transfer factors averaging 22 (+/-0.4)-fold in postpartum maternal tissues. Adults then transferred 46.5 (+/-8.8)% of their body burdens to eggs with an observed reduction in fecundity for mayflies fed on diets greater than approximately 11 microg g(-1). These results suggest that at environmentally feasible dietary Se concentrations insects are potentially affected by Se exposure, and that the current presumption that insects are simply conduits of Se to higher trophic levels is inaccurate.

Semivoltinism, Seasonal Emergence, and Adult Size Variation in a Tropical Stream Mayfly (Euthyplocia hecuba)

Life history characteristics of the stream mayfly Euthyplocia hecuba (Hagen) (Polymitarcyidae:Euthyplociinae) were studied over a 4-yr period in two tropical streams (Río Tempisquito and Quebrada Marilin) draining primary evergreen forest in northern Costa Rica. Larvae burrow in the substratum of the stream, living under small to large cobbles that are firmly embedded in the stream bottom. Euthyplocia hecuba is sexually dimorphic and one of the largest mayflies of Central America, with maximum larval size of 149 mg (dry mass) for females and 35 mg for males. Mature female and male larvae are about 40% larger in Q. Marilin relative to R. Tempisquito. Subimagoes emerge about 1 h before dawn (∼0400 h); the molt to the imago (or true adult stage) occurs ∼20 min later. Adult emergence and reproduction occurs mainly between June and November. Females mate and then oviposit in riffle areas just before dawn, with the time period gradually changing from 0446 and 0507 h in June to 0508 and 0529 h in October. We found that size of adult males and females gradually declined during the emergence period. Fecundity averaged 984 in October (range 364-2851), with eggs being very large for a mayfly (0.46 × 0.4 mm; 0.017 mg dry mass). The proportion of adult biomass allocated to eggs (reproductive effort) averaged 0.51 but varied according to female size. Median egg development time was 113, 55, and 31 d at 15, 20, and 25°C. The hatch success of eggs was >90% at 15 and 20°C, only 68% at 25°C, and 0% at 10 and 30°C. First instar larvae are relatively large for a mayfly, having a head width of 0.11 mm and a body length of 1.30 mm. Seasonal changes in larval size distribution suggest that larval development takes ∼22 mo to complete and may involve a developmental quiescence or diapause during the first year. A 2-yr life history appears to be the most parsimonious hypothesis given a 2-mo egg development time, the occurrence of two size cohorts of larvae throughout most of the year, and the temporal pattern of maximum and average larval sizes over the course of the sampling period. Our data suggest that seasonality in the developmental dynamics of larvae underlies both the seasonal emergence and decline in dry mass of both adult males and females during the emergence period. The data provide the first evidence for semivoltinism in a tropical mayfly.

Trophic transfer of Cd from natural periphyton to the grazing mayfly Centroptilum triangulifer in a life cycle test.

In streams, periphyton biofilms are important sinks for trace metals such as cadmium and are primary food sources of many invertebrate consumers. To study Cd trophic transfer, we produced differentially contaminated diets by exposing natural periphyton to environmentally relevant dissolved Cd ranging from 0 to 10 microg L(-1) for 6-7 days using a radiotracer approach. On average, periphyton grown during three different seasons bioconcentrated Cd similarly--approximately 1315 (+/-442) -fold above dissolved concentrations. However, mayfly larvae (Centroptilum triangulifer) raised on these differentially contaminated diets (first instar through adulthood) had significantly higher trophic transfer factors from periphyton grown in Aug and Nov 2008 (4.30 +/- 1.55) than from periphyton grown in Jan 2009 (0.85 +/- 0.21). This Cd bioaccumulation difference is only partially explained by apparent food quality and subsequent growth differences. Taken together, these results suggest that primary producers at the base of food webs drive metal bioaccumulation by invertebrate grazers.

Taxonomy and genetics of the parthenogenetic mayfly Centroptilum triangulifer and its sexual sister Centroptilum alamance (Ephemeroptera:Baetidae)

Abstract Allozymes were used to examine population genetic structure and species boundaries in the clonal, obligately parthenogenetic Centroptilum triangulifer and its sexual sister species Centroptilum alamance from 3 sites in Pennsylvania and 1 site in North Carolina, USA. Genotype frequencies in field populations of C. triangulifer showed numerous departures from Hardy–Weinberg expectations (mostly heterozygote excesses) and significant linkage disequilibrium at most testable locus combinations, as expected for a clonal parthenogen. A total of 51 distinct clones of C. triangulifer was identified, some of which were found at multiple sites (separated by >700 km in one case). Each stream contained from 7 to 25 clones and clonal frequencies at a given site varied greatly over time. Allelic patterns in laboratory hybrids suggest that parthenogenesis in C. triangulifer is diploid and automictic, and that crossing over is suppressed. In contrast, C. alamance populations had only a few Hardy–Weinberg departures (all heterzygote deficiences) and little or no linkage disequilibrium. No published work provides characters to distinguish the 2 species, but our data support retention of specific status for C. triangulifer and C. alamance because: 1) consistent, fixed allelic differences were found over a broad geographic area (>700 km) with no evidence of introgression despite co-occurrence (sympatry) in the same stream, 2) laboratory hybridization experiments demonstrated the existence of postzygotic barriers to gene flow between the species, and 3) measures of both interspecific and intraspecific genetic similarity were comparable to those found between other mayfly species. Phylogenetic analysis of the genetic data suggests speciation preceded the transition to obligate parthenogenetic reproduction in C. triangulifer. Morphological examination of genetically identified specimens enabled us to describe subtle but consistent differences that can be used to distinguish the species. Thus, what initially appeared to be single populations with skewed sex ratios (7, 11, and 24% male) at 3 of our study sites was, in fact, 2 populations at each site—1 exclusively female (C. triangulifer) and the other a normal, sexual (1:1 sex ratio) population (C. alamance). A similar approach, combining detailed genetic and breeding experiments with close morphological study, could help resolve taxonomic problems in some of the numerous other bisexual/parthenogenetic mayfly taxa.

Population genetics of the burrowing mayfly Dolania americana : geographic variation and the presence of a cryptic species

The genetic structure of Dolania populations in South Carolina, Alabama, and Florida was examined using starch gel electrophoresis. All study populations except Alabama consisted entirely of D. americana Edmunds and Traver. Electrophoresis revealed that the Alabama site contained both D. americana and an undescribed species (D. sp. nov.), with the latter taxon predominating. Individuals of the two taxa had nearly fixed allelic differences at two loci (Est 4 and Gda). The extent of genetic differentiation between populations of the two taxa in the same river or between adjacent river systems was significantly greater than between D. americana populations separated by large geographic distances. Populations of both species were characterized by relatively normal levels of polymorphic loci (average = 23%) but unusually low levels of heterozygosity (average = 0.03). Genetic variation between the Florida and South Carolina populations of D. americana was statistically significant. There was also some indicatio...

Bioconcentration and Biotransformation of Selenite versus Selenate Exposed Periphyton and Subsequent Toxicity to the Mayfly Centroptilum triangulifer

Little is known about the bioaccumulation dynamics, biotransformation processes, or subsequent toxicity to consumers of dissolved selenite (SeO3) versus selenate (SeO4) uptake into aquatic primary producer communities. To address these data gaps, we examined SeO3 and SeO4 bioconcentration into complex freshwater periphyton communities under static and static-renewal conditions. Further, we explored periphyton biotransformation of Se species using X-ray absorption near edge structure (XANES) spectroscopy analysis and changes in the periphyton associated microbial consortium using denaturing gradient gel electrophoresis (DGGE). Last, we fed differentially treated periphyton to the mayfly Centroptilum triangulifer in full life cycle exposures to assess toxicity. Selenite exposed periphyton readily bioconcentrated Se while, in contrast, initial periphyton uptake of SeO4 was negligible, but over time periphyton [Se] increased steadily in conjunction with the formation of dissolved SeO3. XANES analyses revealed that both SeO3 and SeO4 treated periphyton biotransformed Se similarly with speciation dominated by organo-selenide (∼61%). Mayfly survival, secondary production, and time to emergence were similar in both SeO3 and SeO4 treated periphyton exposures with significant adverse effects at 12.8 μg g(-1) ((d.w.) secondary production) and 36 μg g(-1) ((d.w.) survival and development time). Overall, dissolved selenium speciation, residence time, and organisms at the base of aquatic food webs appear to be the principal determinants of Se bioaccumulation and toxicity.

A stressful shortness of breath: molting disrupts breathing in the mayfly Cloeon dipterum

Abstract: Molting is a stressful event in insect development. When an insect molts, the individual discards its exoskeleton and sheds and renews the interior lining of substantial portions of the respiratory (tracheal) system. We profiled for the first time the disruptive pattern of respiration during the molting process in larvae of the mayfly Cloeon dipterum (Ephemeroptera:Baetidae). Molting induces a precipitous drop in O2 consumption immediately followed by a surge in O2 consumption that appears to be compensatory in nature. Postmolt metabolic suppression is consistently observed during which O2 consumption rates lag relative to those of nonmolting larvae. Furthermore, the magnitude of respiratory disturbance during the molt increases as a function of temperature. Increasing temperatures increase molting frequency and the apparently stressful nature of the molt itself. Thus, the insect molt appears to be a previously unappreciated route by which warming conditions may affect aquatic insects.

Elevated major ion concentrations inhibit larval mayfly growth and development

Anthropogenic disturbances, including those from developing energy resources, can alter stream chemistry significantly by elevating total dissolved solids. Field studies have indicated that mayflies (Order Ephemeroptera) are particularly sensitive to high total dissolved solids. In the present study, the authors measured 20-d growth and survivorship of larval Neocloeon triangulifer exposed to a gradient of brine salt (mixed NaCl and CaCl2 ) concentrations. Daily growth rates were reduced significantly in all salt concentrations above the control (363 µS cm(-1) ) and larvae in treatments with specific conductance >812 µS cm(-1) were in comparatively earlier developmental stages (instars) at the end of the experiment. Survivorship declined significantly when specific conductance was >1513 µS cm(-1) and the calculated 20-d 50% lethal concentration was 2866 µS cm(-1) . The present studys results provide strong experimental evidence that elevated ion concentrations similar to those observed in developing energy resources, such as oil and gas drilling or coal mining, can adversely affect sensitive aquatic insect species.

Part 2: Sensitivity comparisons of the mayfly Centroptilum triangulifer to Ceriodaphnia dubia and Daphnia magna using standard reference toxicants; NaCl, KCl and CuSO4.

Criteria for establishing water quality standards that are protective for 95% of the native species are generally based upon laboratory toxicity tests. These tests utilize common model organisms that have established test methods. However, for invertebrates these species represent mostly the zooplankton community and are not inclusive of all taxa. In order to examine a potential under-representation in emerging aquatic invertebrates the US Environmental Protection Agency has cultured a parthenogenetic mayfly, Centroptilum triangulifer (Ephemeroptera: Baetidae). This study established a 48h acute and a 14-day short-term chronic testing procedure for C. triangulifer and compared its sensitivity to two model invertebrates, Ceriodaphnia dubia and Daphnia magna. Toxicity tests were conducted to determine mortality and growth effects using standard reference toxicants: NaCl, KCl and CuSO4. In 48-h acute tests, the average LC50 for the mayfly was 659mgL(-1) NaCl, 1957mgL(-1) KCl, and 11μgL(-1) CuSO4. IC25 values, using dry weight as the endpoint, were 228mgL(-1) NaCl, 356mgL(-1) KCl and 5μgL(-1) CuSO4. C. triangulifer was the most sensitive species in NaCl acute and chronic growth tests. At KCl concentrations tested, C. triangulifer was less sensitive for acute tests but was equally or more sensitive than C. dubia and D. magna for growth measurements. This study determined C. triangulifer has great potential and benefits for use in ecotoxicological studies.