Brian G. Gall

Tetrodotoxin levels in larval and metamorphosed newts (Taricha granulosa) and palatability to predatory dragonflies.

Some populations of the newt Taricha granulosa possess extremely high concentrations of the neurotoxin tetrodotoxin (TTX). Tetrodotoxin is present in adult newts and their eggs, but has been assumed to be absent from the larval stage. We tested larval and metamorphosed juveniles for the presence of TTX and evaluated the palatability of these developmental stages to predatory dragonfly nymphs. All developmental stages retained substantial quantities of TTX and almost all individuals were unpalatable to dragonfly nymphs. Tetrodotoxin quantity varied greatly among individuals. When adjusted for mass, TTX concentrations declined steadily through metamorphosis. Several juveniles were palatable to dragonflies and these individuals had significantly lower TTX levels than unpalatable juveniles. These results suggest that despite previous assumptions, substantial quantities of TTX, originally deposited in the embryo, are retained by the developing larvae and metamorphosed juveniles and this quantity is enough to make them unpalatable to some potential predators.

Female newts (Taricha granulosa) produce tetrodotoxin laden eggs after long term captivity

We investigated the presence of tetrodotoxin (TTX) in the eggs of wild-caught newts (Taricha granulosa) at capture and again after one, two, and three years in captivity. Females initially produced eggs that contained quantities of TTX similar to previous descriptions of eggs from wild-caught adults. After the first year in captivity, the egg toxicity from each female declined, ultimately remaining constant during each of the successive years in captivity. Despite declining, all females continued to produce eggs containing substantial quantities of TTX during captivity. The decline in toxicity can not be attributed to declining egg mass but may be the result of the abbreviated reproductive cycle to which the captive newts were subjected in the lab. Finally, an estimate of the amount of TTX provisioned in the entire clutch from each female is similar to the quantity of TTX regenerated in the skin after electrical stimulation. These results, coupled with other long-term studies on the maintenance and regeneration of TTX in the skin, suggests an endogenous origin of TTX in newts.

Interfamily variation in amphibian early life-history traits: raw material for natural selection?

The embryonic development and time to hatching of eggs can be highly adaptive in some species, and thus under selective pressure. In this study, we examined the underlying interfamily variation in hatching timing and embryonic development in a population of an oviparous amphibian, the rough-skinned newt (Taricha granulosa). We found significant, high variability in degree of embryonic development and hatching timing among eggs from different females. Patterns of variation were present regardless of temperature. We also could not explain the differences among families by morphological traits of the females or their eggs. This study suggests that the variation necessary for natural selection to act upon is present in the early life history of this amphibian.

Fine‐scale selection by ovipositing females increases egg survival

One of the most important defenses for the eggs of ovipositing female organisms is to avoid being laid in the same habitat as their predators. However, for most organisms, completely avoiding an offsprings predators is not possible. One mechanism that has been largely overlooked is for females to partition an oviposition site into microhabitats that differ in quality for offspring survival. We conducted a series of experiments to examine whether female newts avoid microhabitats utilized by their offsprings primary predator, caddisfly larvae. Female newts avoided laying eggs near predatory caddisflies and shifted egg laying upward in the water column when provided with a vertical dimension. Caddisflies were attracted to chemical stimuli from female newts and their eggs, yet primarily used benthic areas in experimental chambers. Finally, results from a field experiment indicate that the behavioral strategy employed by female newts increases offspring survival. This subset of non-genetic maternal effects, micro-oviposition avoidance, is likely an important yet underexplored mechanism by which females increase offspring survival.

Complex predator–prey interactions between the rusty crayfish (Orconectes rusticus) and invertebrate and vertebrate prey within their native range

Crayfish can have profound effects on native species where they are introduced, yet few studies have examined the role of crayfish predation on species within their native ranges. We examined interactions between predatory rusty crayfish (Orconectes rusticus) and invertebrate and vertebrate prey in Southeast Indiana, where the crayfish are a native species. We conducted multiple experiments, which included assessing the predation efficiency of rusty crayfish on salamander larvae (Eurycea cirrigea) and aquatic isopods (Lirceus fontinalis), the role of substrate in prey survival, prey responses to chemical stimuli from crayfish, and the distribution of each species in a natural stream setting. We found that crayfish almost completely eliminated isopods, but the addition of even a simple gravel substrate dramatically increased survival. Although the size of the crayfish had no effect on isopod survival, salamander larvae were more likely to survive in the presence of smaller crayfish. Neither prey species responded to chemical stimuli from crayfish, yet field observations indicated that crayfish and isopods inhabit different microhabitats within the stream. Finally, the interaction between crayfish and salamander larvae is complex with the crayfish size and the natural distribution of size classes in streams likely reducing the larval predation risk and permitting coexistence. These results indicate that although crayfish are major predators on both invertebrates and vertebrates, coexistence over evolutionary time has likely led to multiple behavioral adaptations to avoid these predators.

Do all portable cases constructed by caddisfly larvae function in defense

Abstract The portable cases constructed by caddisfly larvae have been assumed to act as a mechanical defense against predatory attacks. However, previous studies have compared the survival of caddisflies with different cases, thereby precluding an analysis of the survival benefits of “weaker” case materials. The level of protection offered by caddisfly cases constructed with rock, stick, or leaf material, as well as a no-case control, was investigated against predatory dragonfly nymphs (Anax junius Drury (Anisoptera: Aeshnidae)). A valid supposition is that the cases made of stronger material are more effective at deterring predators. Yet, observations revealed that there was no difference in survival between the case types. All caddisflies with a case experienced high survival in comparison to caddisflies removed from their case. In addition, larvae with stick-cases experienced fewer attacks and captures by dragonflies. These results showed that the presence of a case, regardless of the material used in its construction, offers survival benefits when faced with predatory dragonfly nymphs.

Otter Predation on Taricha granulosa and Variation in Tetrodotoxin Levels with Elevation

Abstract Tetrodotoxin (TTX) is a low molecular weight neurotoxin that is found in a wide variety of taxa. Tetrodotoxin blocks voltage-gated sodium channels, preventing the propagation of action potentials and inducing paralysis in susceptible animals. Taricha granulosa have been documented to possess TTX in high quantities and are preyed upon by snakes of the genus Thamnophis. However, recent observations of predation events on T. granulosa by otters were documented in a high-elevation population just outside of Crater Lake National Park, Oregon. We quantified TTX levels in this population as well as 3 populations in Crater Lake National Park using a Competitive Inhibition Enzymatic Immunoassay. We further compared these high elevation populations to a known high-toxicity population from Benton County, Oregon. We found that the populations in Crater Lake have lower levels of TTX relative to populations outside of the lake, and that all high-elevation locations have relatively low levels of TTX. We then analyzed previously published whole-newt TTX levels and elevation, and found that there is a significant negative relationship. However, there is a non-significant relationship between whole-newt TTX levels and elevation when examining elevations below 500 m. This further exemplifies the potential for novel predation and previously unidentified selective pressures in high-elevation newt populations.

The alarm cue obstruction hypothesis: isopods respond to alarm cues, but do not respond to dietary chemical cues from predatory bluegill

Predator avoidance behaviours occur when prey detect a predator but the predator has not yet detected and identified prey. These defences are critical because they prevent predation at the earliest possible stages when prey have the best chance of escape. We tested for predator avoidance behaviours in an aquatic macroinvertebrate (Caecidotea intermedius; order Isopoda) in a series of three experiments. The first experiment attempted to determine if isopods possess alarm cues by exposing them to stimuli from macerated conspecifics. We then exposed isopods to kairomones from non-predatory tadpoles (Rana catesbiana) and predatory fish (Lepomis macrochirus) that had been fed a benign diet. Finally, we exposed isopods to kairomones of predatory fish that had been fed a diet exclusively of isopods. We found that isopods did not respond to any kairomone cues or dietary cues from any potential predator, but did reduce activity in response to alarm cues. These results suggest that isopods exhibit predator avoidance responses toward chemical cues in a limited setting (they do not respond unless the information suggests an attack has occurred in the immediate past) or that bluegill have the ability to modify or mask the alarm cues from their prey.

Toxic Prey and Predator Avoidance: Responses of Toxic Newts to Chemical Stimuli from a Predator and Injured Conspecifics

Abstract Predator avoidance behavior minimizes predation risk because the prey never enters the perceptual field of the predator. For toxic species, avoiding potential predators indiscriminately is unlikely to be the best predator avoidance strategy because many predators will be incapable of consuming the toxin. We investigated the role of chemical-based avoidance behavior in predator-prey interactions between a toxic prey species, the Rough-skinned Newt (Taricha granulosa), and its major predator, the Common Garter Snake (Thamnophis sirtalis). Terrestrial newts avoided predator kairomones only when the predator had recently consumed a conspecific, but did not avoid damage-released alarm cues. These results indicate newts may be able to chemically differentiate between toxin-resistant and toxin-sensitive snakes in the same population and avoid only those snakes actively preying upon their cohort.

Only fear the fatal foe: predation risk assessment by eastern newts (Notophthalmus viridescens) in response to common snapping turtles and other potential predators

Many organisms utilize toxic or noxious compounds as a means of deterring predation. Eastern newts (Notopthalmus viridescens), along with other species in the family Salamandridae, possess a potent neurotoxin called tetrodotoxin (TTX). Although TTX can serve as an effective antipredator mechanism in species of newts with high concentrations (e.g., Taricha), eastern newts have relatively low levels of toxicity in comparison to those species, and it may not serve as an effective antipredator mechanism against all threats. In this case, they may benefit rather by utilizing behavioral changes to avoid initial contact with predators. We tested for predator-avoidance behavior in newts by exposing individuals to kairomones from various predators. We recorded activity patterns of newts when they were exposed to cues from potential predators including bullfrogs (Lithobates catesbeiana), water snakes (Nerodia sipedon) and snapping turtles (Chelydra serpentina), as well as a non-predator (bullfrog tadpoles), and a control (deionized water). Newts reduced activity when exposed to snapping turtle stimuli, but did not change activity when exposed to any other chemical cues. We verified that newts interact with this predator by trapping snapping turtles found in ponds from which newts were collected. Finally, we used turtles caught during this sampling to test whether they are an actual predator of newts and whether newts shift microhabitat use when exposed to this predator. In each replicate, turtles consumed newts, and newts spatially avoided the snapping turtle, relative to a control. The results of these experiments indicate newts rely on predator-avoidance behavior to reduce the probability of being consumed by snapping turtles, but do not reduce activity in response to other potential predators that may only consume them rarely.