Richard K. Zimmer

Chemical Signaling Processes in the Marine Environment

Understanding the mechanisms by which environmental chemical signals, chemical defenses, and other chemical agents mediate various life-history processes can lead to important insights about the forces driving the ecology and evolution of marine systems. For chemical signals released into the environment, establishing the principles that mediate chemical production and transport is critical for interpreting biological responses to these stimuli within appropriate natural, historical contexts. Recent technological advancements provide outstanding opportunities for new discoveries, thus allowing quantification of interactions between hydrodynamic, chemical, and biological factors at numerous spatial and temporal scales. Past work on chemically mediated processes involving organisms and their environment have emphasized habitat colonization by larvae and trophic relationships. Future research priorities should include these topics as well as courtship and mating, fertilization, competition, symbiosis, and microbial chemical ecology. There are now vast new opportunities for determining how organisms respond to chemical signals and employ chemical defenses under environmentally realistic conditions. Integrating these findings within a larger ecological and evolutionary framework should lead to improved understanding of natural physicochemical phenomena that constrain biological responses at the individual, population, and community levels of organization.

PHYSICAL CONSTRAINTS ON ECOLOGICAL PROCESSES: A FIELD TEST OF ODOR-MEDIATED FORAGING

The physicochemical environment can strongly constrain the outcome of ecological interactions such as predation, mating, and competition. This is especially true of processes mediated by the sense of olfaction, because wind and water currents control the dispersal of odor signals and act as ancillary cues during odor plume following. In the field, we examined how variations in the physical and chemical properties of odor plumes would alter the foraging behavior of the blue crab Callinectes sapidus, a common predator/ scavenger in tidal marsh creeks in the southeastern United States. We video-recorded re- sponses of naturally foraging crabs to odor plumes of varying composition and odor release rate (characteristic of clams of differing size). During each trial we presented crabs with an experimental plume that was a mixture of fluorescein-dyed seawater and clam mantle fluid, oyster mantle fluid, or a suite of amino acids, and a control plume which consisted of dyed seawater only. In addition to manipulating the chemical composition and odor release rate of the plume, we allowed flow speed to vary naturally with the tide. We tested for effects of odor composition, odor release rate, and flow speed on the success (i.e., finding the target) and efficiency (i.e., search path direction) of blue crab foraging. Mantle fluid solutions and wounded prey items elicited active search and upstream walking, while control and amino acid solutions had no effect on crab behavior. Odors released at a low rate (either low volume flow or low concentration) elicited fewer responses from crabs, and the resulting search was less efficient and less successful than responses to odors released at higher rates. Ambient current speed also affected both search success and efficiency. There was a decline in search success when current speed in the tidal channel was below 1 cm/s; search success remained constantly high, however, when current speed was above this threshold. Search efficiency was directly proportional to ambient current speeds. Such relationships between hydrodynamic and chemical properties of the environ- ment and foraging success and efficiency suggest that variation in the physicochemical environment can influence the detectability of prey and strategies employed by foragers. These results extend beyond the foraging of marine crustaceans into other olfactory- mediated interactions and habitats.

Odorant receptors and olfactory-like signaling mechanisms in mammalian sperm

Since their discovery in 1991, members of the odorant receptor (OR) family have been found in various ectopic tissues, including testis and sperm. It took, however, more than a decade for the first mammalian testicular ORs to be functionally characterized and implicated in a reproductively relevant scenario. Activation of hOR17-4 and mOR23 in human and mouse sperm, respectively, mediates distinct flagellar motion patterns and chemotactic behavior in various bioassays. For hOR17-4, receptor function and downstream signal transduction events are shown to be subject to pharmacological manipulation. Further insight into the basic principles that govern sperm OR operation as well as into the molecular logic that underlies OR-mediated signaling could set the stage for pioneering future applications in procreation and/or contraception.

Sex and flow: the consequences of fluid shear for sperm-egg interactions

SUMMARY Fertilization is a complex interaction among biological traits of gametes and physical properties of the fluid environment. At the scale of fertilization (0.01–1 mm), sperm encounter eggs while being transported within a laminar (or viscous) shear flow. Varying laminar-shear in a Taylor-Couette flow tank, our experiments simulated important aspects of small-scale turbulence within the natural habitats of red abalone (Haliotis rufescens), a large marine mollusk and external fertilizer. Behavioral interactions between individual cells, sperm–egg encounter rates, and fertilization success were quantified, simultaneously, using a custom-built infrared laser and computer-assisted video imaging system. Relative to still water, sperm swam faster and moved towards an egg surface, but only in comparatively slow flows. Encounter rate, swim speed and orientation, and fertilization success each peaked at the lowest shear tested (0.1 s–1), and then decayed as shear increased beyond 1.0 s–1. The decay did not result, however, from damage to either sperm or eggs. Analytical and numerical models were used to estimate the propulsive force generated by sperm swimming (Fswim) and the shear force produced by fluid motion within the vicinity of a rotating egg (Fshear). To first order, male gametes were modeled as prolate spheroids. The ratio Fswim/Fshear was useful in explaining sperm–egg interactions. At low shears where Fswim/Fshear>1, sperm swam towards eggs, encounter rates were pronounced, and fertilization success was very high; behavior overpowered fluid motion. In contrast, sperm swimming, encounter rate and fertilization success all decayed rapidly when Fswim/Fshear<1; fluid motion dominated behavior. The shears maximizing fertilization success in the lab typically characterized natural flow microenvironments of spawning red abalone. Gamete behavior thus emerges as a critical determinant of sexual reproduction in the turbulent sea.

Patterns and Processes of Larval Emergence in an Estuarine Parasite System

Trematode parasites in intertidal estuaries experience constantly varying conditions, with the presence or absence of water potentially limiting larval transport between hosts. Given the short life spans (≤24 h) of cercariae, emergence timing should be optimized to enhance the probability of successful transmission. In the present study, field measurements and laboratory experiments identified processes that regulate the emergence of cercariae from their first intermediate snail hosts in an intertidal marsh. Larvae emerged over species-specific temperature ranges, exclusively during daylight hours, and only when snails were submerged. The three factors operate over different temporal scales: temperature monthly, light diurnally (24-h period), and water depth tidally (12-h period). Each stimulus creates a necessary condition for the next, forming a hierarchy of environmental cues. Emergence as the tide floods would favor transport within the estuary, and light may trigger direct (downward or upward) swimming toward host habitats. Abbreviated dispersal would retain asexually reproduced cercariae within the marsh, and local mixing would diversify the gene pool of larvae encysting on subsequent hosts. In contrast to the timing of cercarial release, emergence duration was under endogenous control. Duration of emergence decreased from sunrise to sunset, perhaps in response to the diminishing lighted interval as the day progresses. Circadian rhythms that control cercarial emergence of freshwater species (including schistosomes) are often set by the activity patterns of subsequent hosts. In this estuary, however, the synchronizing agent is the tides. Together, exogenous and endogenous factors control emergence of trematode cercariae, mitigating the vagaries of an intertidal environment.

Neuroecology, Chemical Defense, and the Keystone Species Concept

Neuroecology unifies principles from diverse disciplines, scaling from biophysical properties of nerve and muscle cells to community-wide impacts of trophic interactions. Here, these principles are used as a common fabric, woven from threads of chemosensory physiology, behavior, and population and community ecology. The “keystone species” concept, for example, is seminal in ecological theory. It defines a species whose impacts on communities are far greater than would be predicted from its relative abundance and biomass. Similarly, neurotoxins could function in keystone roles. They are rare within natural habitats but exert strong effects on species interactions at multiple trophic levels. Effects of two guanidine alkaloids, tetrodotoxin (TTX) and saxitoxin (STX), coalesce neurobiological and ecological perspectives. These molecules compose some of the most potent natural poisons ever described, and they are introduced into communities by one, or only a few, host species. Functioning as voltage-gated sodium channel blockers for nerve and muscle cells, TTX and STX serve in chemical defense. When borrowed by resistant consumer species, however, they are used either in chemical defense against higher order predators or for chemical communication as chemosensory excitants. Cascading effects of the compounds profoundly impact community-wide attributes, including species compositions and rates of material exchange. Thus, a diverse array of physiological traits, expressed differentially across many species, renders TTX and STX fully functional as keystone molecules, with vast ecological consequences at multiple trophic levels.

Chemosensory Ca2+ dynamics correlate with diverse behavioral phenotypes in human sperm

In the female reproductive tract, mammalian sperm undergo a regulated sequence of prefusion changes that “prime” sperm for fertilization. Among the least understood of these complex processes are the molecular mechanisms that underlie sperm guidance by environmental chemical cues. A “hard-wired” Ca2+ signaling strategy that orchestrates specific motility patterns according to given functional requirements is an emerging concept for regulation of sperm swimming behavior. The molecular players involved, the spatiotemporal characteristics of such motility-associated Ca2+ dynamics, and the relation between a distinct Ca2+ signaling pattern and a behavioral sperm phenotype, however, remain largely unclear. Here, we report the functional characterization of two human sperm chemoreceptors. Using complementary molecular, physiological, and behavioral approaches, we comparatively describe sperm Ca2+ responses to specific agonists of these novel receptors and bourgeonal, a known sperm chemoattractant. We further show that individual receptor activation induces specific Ca2+ signaling patterns with unique spatiotemporal dynamics. These distinct Ca2+ dynamics are correlated to a set of stimulus-specific stereotyped behavioral responses that could play vital roles during various stages of prefusion sperm-egg chemical communication.

Controlled Field Release of a Waterborne Chemical Signal Stimulates Planktonic Larvae to Settle

Settlement rates and distributions of planktonic larvae are critical determinants of population dynamics in marine and freshwater benthic communities. On the basis of the principles of solute diffusion from a porous material, chemical-releasing collectors (CRCs) were engineered and tested in an estuary. Significantly more barnacle larvae (Balanus amphitrite) were found to colonize collectors emitting trace amounts of the synthetic peptide analog, glycylglycyl-L-arginine (5 3 10 28 M), than those emitting either seawater or an organic enrichment (glycyl-glycyl- L-histidine) control. The inductive compound is similar in structure to peptide signal molecules that have been shown to elicit settlement under laboratory conditions and are naturally released by adult barnacles and oysters. The potent effects of subtle changes in seawater chemistry may thus warrant careful attention as putative agents mediating habitat colonization. For a half-century or more, the pervasive perception that

Sperm chemotaxis, fluid shear, and the evolution of sexual reproduction

Chemical communication is fundamental to sexual reproduction, but how sperm search for and find an egg remains enigmatic. For red abalone (Haliotis rufescens), a large marine snail, the relationship between chemical signaling and fluid motion largely determines fertilization success. Egg-derived attractant plumes are dynamic, changing their size and shape in response to unique combinations of physical and chemical environmental features. Attractant plumes that promote sexual reproduction, however, are limited to a precise set of hydrodynamic conditions. Performance-maximizing shears are those that most closely match flows in native spawning habitats. Under conditions in which reproductive success is chronically limited by sperm availability, gametes are under selection for mechanisms that increase sperm–egg encounter. Here, chemoattraction is found to provide a cheap evolutionary alternative for enhancing egg target size without enlarging cytoplasmic and/or cell volume. Because egg signaling and sperm response may be tuned to meet specific fluid-dynamic constraints, shear could act as a critical selective pressure that drives gamete evolution and determines fitness.

LARVAL SWIMMING OVERPOWERS TURBULENT MIXING AND FACILITATES TRANSMISSION OF A MARINE PARASITE

Planktonic cercariae (parasite larvae) of digenetic flatworms (Himasthla rhigedana) encyst up to 100% of intermediate host populations. Toward explaining such high prevalence, larval behavior and passive-transport processes were evaluated experimentally for their roles in waterborne parasite transmission. Using a new application of laser and digital video imaging technologies, we quantified cercarial movements in still water and in simulated field flows. In still water, downward swimming in response to light, irrespective of intensity or source, and gravity brought larvae to the bottom three-times faster than gravitational sinking alone. A 33% elevation in temperature (18–24°C) caused a 71% increase in swim speed. In flume flows characteristic of southern California salt marshes (u* = 0.2 cm/s, occurring >80% of the time), vertical larval distributions were highly bottom skewed. The mean downward swim speed (0.59 cm/s at 24°C) was three times faster than turbulent fluctuations (w′ = 0.23 cm/s), indicating...