Charles L. McKenney

Mysid crustaceans as potential test organisms for the evaluation of environmental endocrine disruption: a review

Anthropogenic chemicals that disrupt the hormonal systems (endocrine disruptors) of wildlife species recently have become a widely investigated and politically charged issue. Invertebrates account for roughly 95% of all animals, yet surprisingly little effort has been made to understand their value in signaling potential environmental endocrine disruption. This omission largely can be attributed to the high diversity of invertebrates and the shortage of fundamental knowledge of their endocrine systems. Insects and crustaceans are exceptions and, as such, appear to be excellent candidates for evaluating the environmental consequences of chemically induced endocrine disruption. Mysid shrimp (Crustacea: Mysidacea) may serve as a viable surrogate for many crustaceans and have been put forward as suitable test organisms for the evaluation of endocrine disruption by several researchers and regulatory bodies (e.g., the U.S. Environmental Protection Agency). Despite the long-standing use of mysids in toxicity testing, little information exists on their endocrinology, and few studies have focused on the potential of these animals for evaluating the effects of hormone-disrupting compounds. Therefore, the question remains as to whether the current standardized mysid endpoints can be used or adapted to detect endocrine disruption, or if new procedures must be developed, specifically directed at evaluating hormone-regulated endpoints in these animals. This review summarizes the ecological importance of mysids in estuarine and marine ecosystems, their use in toxicity testing and environmental monitoring, and their endocrinology and important hormone-regulated processes to highlight their potential use in assessing environmental endocrine disruption.

The influence of insect juvenile hormone agonists on metamorphosis and reproduction in estuarine crustaceans.

Abstract Comparative developmental and reproductive studies were performed on several species of estuarine crustaceans in response to three juvenile hormone agonists (pyriproxyfen, methoprene and fenoxycarb). Larval development of the grass shrimp, Palaemonetes pugio, was greater than two orders of magnitude more sensitive to disruption by methoprene and fenoxycarb than was embryonic development. Developing larvae of the mud crab, Rhithropanopeus harrisii, exhibited reduced metamorphic success at lower concentrations of methoprene and pyriproxyfen than grass shrimp larvae. These responses suggest that the more rigidly controlled metamorphic process in crabs is more sensitive to compounds acting as endocrine disruptors than is the more flexible metamorphic pattern in shrimp. The final crab larval stage, the megalopa, was more sensitive to methoprene and fenoxycarb exposure than earlier zoeal stages. Mud crab larvae exposed to fenoxycarb had reduced biomass and lipid content, particularly triglycerides and sterols. Concentrations of fenoxycarb which reduced the reproductive capacity in single life-cycle exposures of the estuarine mysid, Americamysis bahia, were similar to those concentrations which inhibited metamorphosis in grass shrimp. Juvenile mysids released by exposed adults and reared through maturation without further exposure produced fewer young and had altered sex ratios (lower percentages of males) at lower parental-exposure concentrations than directly affected parental reproduction. These transgenerational responses may well be a product of irreversible effects during developmental exposures which become apparent following maturation and initiation of reproduction. These findings support using a functional approach as an appropriate screening procedure to evaluate potential environmental endocrine-disrupting chemicals in aquatic environments.

Ecdysteroid Responses of Estuarine Crustaceans Exposed Through Complete Larval Development to Juvenile Hormone Agonist Insecticides

Abstract Fenoxycarb and pyriproxyfen are insecticides that gain their toxicity by specifically acting as insect juvenile hormone agonists (JHA), and so are endocrine disruptors by design and effectively prevent larvae from maturing into adults. Efforts to assess the environmental effects of JHAs on nontarget populations of invertebrates have resulted in the utilization of several established estuarine crustacean models. This work was conducted to test the hypothesis that the mortality, inhibition of development and decreased fecundity reported previously in these animals from JHA exposure coincides with abnormal circulating titers of ecdysteroids. Gravid female grass shrimp (Palaemonetes pugio) and mud crabs (Rhithropanopeus harrisii), species with different developmental plasticity and JHA tolerances, were collected and held at wet lab conditions (20 ppt salinity, 25°C) until larval release. Larvae were collected <12 hr after hatch and exposed to JHAs during a static renewal test through end of development with seawater or nominal concentrations of JHA previously shown to induce significant developmental delays and/or decreased body weights. Larvae were subsampled (10 larvae/sample, n = 2 to 8) at each developmental stage, lyophilized, and ecdysteroids extracted by homogenization in 80% methanol and elution from C18 Sep-Pak cartridges with 25%, 60% and 100% methanol to capture the polar, free, and apolar conjugates, respectively, and then quantified by ELISA. As was expected significant differences in successful completion of development (larval survival), developmental duration, and growth (dry weight) were observed. These physiological perturbations were linked with significantly altered ecdysteroid titers, supporting a newly emerging theory that juvenoids possibly act as anti-ecdysteroids through a novel molecular mechanism involving inhibition of ecdysteroid signaling.

Interactions Among Salinity, Temperature, and Age on Growth of the Estuarine Mysid Mysidopsis Bahia Reared in the Laboratory Through a Complete Life Cycle. I. Body Mass and Age-specific Growth Rate

ABSTRACT A broad range of salinity-temperature conditions (salinities from 3―31‰ and temperatures from 19-31°C) significantly influenced growth rates and subsequent body mass of the estuarine mysid Mysidopsis bahia reared in the laboratory from the first free juvenile stage through first brood production by the mature adult. Not only did salinity and temperature significantly interact to affect mysid growth, but a highly significant three-factor salinity-temperature-age interaction modified growth in this species. Response surfaces depict that maximum body mass was obtained after 4 weeks at temperatures between 24 and 29°C and in salinities (S) above 19‰. Optimal salinity-temperature conditions for growth of M. bahia are correlated with both its resistance patterns to these dominant environmental factors and its distribution pattern in estuarine waters. Canonical analysis of the empirical data produced an absolute maximum dry weight at 26‰ S and 27°C after 4 weeks of growth. Salinity conditions accounting for optimal growth are in close agreement with the isosmotic point (24‰ S) for this species, suggesting reduced growth efficiency concurrent with osmotic stress, particularly hypoosmotic stress. Maximum growth rates of mysids reared under a broad salinity-temperature range occurred during the second week, just prior to maturation, suggesting that changing levels of reproduction are correlated with modifications in mysid growth over time.

Variations in larval growth and metabolism of an estuarine shrimp Palaemonetes pugio during toxicosis by an insect growth regulator

Abstract 1. Exposure of the estuarine shrimp, Palaemonetes pugio , to a juvenile hormone analogue (⩾8 μ g methoprene 1 −1 ) throughout larval development inhibited successful completion of metamorphosis. 2. Methoprene exposure retarded growth in early larval stages and postlarvae, but enhanced growth in premetamorphic larvae. 3. Respiration rates of early larvae were elevated by methoprene exposure, but not so older larvae or post larvae. 4. Lower net growth efficiency ( K 2 values) in methoprene-exposed early larvae suggests that increased metabolic demands reduced assimilated energy available for growth. 5. Modifications in O:N ratios of premetamorphic larvae and postlarvae suggest that methoprene altered substrate utilization patterns during metamorphosis.

Growth, lipid class and fatty acid composition in juvenile mud crabs (Rhithropanopeus harrisii) following larval exposure to Fenoxycarb®, insect juvenile hormone analog.

This study examines the effects of Fenoxycarb on larval growth, and lipid class and fatty acid composition in first crabs of the mud crab Rhithropanopeus harrisii reared through total larval development in nominal water concentrations from 1 to 100 microg/l. In first crabs of R. harrisii, dry weight (microg) decreased significantly (P < 0.05) from 228.8+/-38.2 microg (n = 9) in the controls to 131.8+/-10.1 microg (n = 4) in animals exposed throughout larval development to 100 microg/l. A significant (P < 0.05) reduction was found between total lipid content in the controls and first crabs reared at concentrations greater than 50 microg/l. In relative terms (% dry weight), different lipid classes predominated in the controls and the various fenoxycarb exposure concentrations. There were no significant (P > 0.05) differences among the treatment groups in phospholipid level, while the triglyceride content was significantly lower in crabs exposed to 10 and 100 microg/l. No significant differences in the percent of free fatty acids were found in crabs exposed to 1-10 microg/l and the controls. Free sterols in crabs exposed to concentrations higher than 10 microg/l were below the detection limit. Control animal fatty acid profiles were dominated by palmitic, stearic, and oleic acid, accounting for 48% of total fatty acids (TFA). The fatty acid composition of crabs exposed to 100 microg/l significantly (P < 0.05) differed from the controls. The results suggest that fenoxycarb has substantial effects on growth, lipid class and fatty acid composition in developing larvae of R. harrisii at water concentrations greater than 10 microg/l.

Resistance patterns to salinity and temperature in an estuarine mysid (Mysidopsis bahia) in relation to its life cycle

Abstract Survival of the estuarine mysid Mysidopsis bahia was monitored in the laboratory from juvenile through maturation to an adult under various combinations of salinity (3–31 % 0 S) and temperature (19–31°C). Salinity had a greater influence on mysid survival than did temperature. Salinity tolerance was strongly modified by temperature with maximum tolerance at intermediate temperatures. Salinity-temperature interactions on mysid survival varied with age of the mysid. Canonical analysis of weekly resistance patterns produced absolute maximum survival combinations for salinity and temperature which changed over time. The salinity-temperature tolerance ranges described for this estuarine mysid correspond with previously described physiological capabilities and distribution patterns.

Effects of fenoxycarb exposure on complete larval development of the xanthid crab, Rhithropanopeus harrisii.

Pest control agents, such as juvenile hormone analogues (JHA), have been developed to limit effects on non-target organisms that co-inhabit insect pest habitats. Rhithropanopeus harrisii, an estuarine xanthid crab, was used to observe the impacts of the JHA, fenoxycarb, on the pattern of complete larval development as well as survival of larvae and successful metamorphosis to first crab stage. Significant mortality occurred in the first of four zoeal stages (after 2-3 days of exposure) at the highest treatment of 240 microg fenoxycarb/l and in megalopae exposed to 48 microg fenoxycarb/l. The time required to metamorphose to the first crab stage was significantly increased for megalopae in all treatments 48 microg/l. This delay in development was sufficient to significantly prolong the entire developmental period from zoea to crabs. Unexposed larvae developed to crabs in an average of 16 days; larvae exposed to >/=48 microg/l required 19-20 days. Reduced survival and extended duration of developing larval stages in the life history of a benthic invertebrate may alter the population dynamics of these organisms in the estuary.

From organisms to populations : Modeling aquatic toxicity data across two levels of biological organization

A critical step in estimating the ecological effects of a toxicant is extrapolating organism-level response data across higher levels of biological organization. In the present study, the organism-to-population link is made for the mysid, Americamysis bahia, exposed to a range of concentrations of six toxicants. Organism-level responses observed were categorized as no effect, delayed reproduction, reduced overall reproduction, or both reduced overall reproduction and survival. Population multiplication rates of each toxicant concentration were obtained from matrix models developed from organism-level endpoints and placed into the four categories of organism-level responses. Rates within each category were compared with growth rates modeled for control populations. Population multiplication rates were significantly less than control growth rates only for concentrations at which overall reproduction and both reproduction and survival were significantly less than the control values on the organism level. Decomposition analysis of the significant population-level effects identified reduced reproduction as the primary contributor to a reduced population multiplication rate at all sublethal concentrations and most lethal concentrations. Mortality was the primary contributor to reduced population growth rate only when survival was less than 25% of control survival. These results suggest the importance of altered reproduction in population-level risk assessment and emphasizes the need for complete life-cycle test data to make an explicit link between the organism and population levels.

Projecting Population-Level Responses of Mysids Exposed to an Endocrine Disrupting Chemical

Abstract To fully understand the implications of a chemicals effect on the conservation of a species, effects observed at the physiological or individual level must be expressed in terms of the population. Since long-term field experiments are typically not feasible, vital rates such as survival and reproduction of individual organisms are measured in life table response experiments (LTRE) and employed to extrapolate the effects of a pollutant on the population. The population-level response of the mysid, Americamysis bahia, to varying concentrations of methoprene (0, 4, 8, 16, 31, 62 μg/L) was determined using age-structured population models. Models were parameterized from the results of an LTRE conducted throughout the entire mysid life cycle. A density-independent matrix model with time invariant demographic parameters was developed to measure the change in population growth rate, λ, with change in methoprene concentration. The values of λ were greater than one for all methoprene concentrations, indicating that populations exposed to the concentrations reported here would not become extinct. However, a general decrease in λ occurred with increasing methoprene concentration and would result in reduced population sizes. Sensitivity and decomposition analyses were conducted to determined the relative roles of the vital rates on altered population growth rates and determined that impaired reproduction was the primary influence on the observed decrease in λ. The model constructed was a useful tool for linking the individual-level effects to the population-level consequences of methoprene exposure on mysids, as well as defining the mechanism (reduced reproduction) responsible for the observed effects on population.