Abstract
Oviposition site location may be dependent on the costs and benefits provided to both the offspring and the female. Resource availability, presence of predators, and competition are challenges that the offspring will be faced with, whereas females will be constrained by predation risk and the likelihood of successful mating. This review will examine the factors that influence site selection in odonates. Understanding the trade-offs that drive natural selection and behavior will help to inform management decisions and conserve ideal habitat for odonate oviposition.
Introduction
Successful oviposition is vital for the continuation of odonate species. Site selection for oviposition sites, which is influenced by many environmental and behavioral factors, will determine reproductive success and offspring survival (Guillermo-Ferreira and Del-Claro 2011). Visual and olfactory cues allow terrestrial adults to detect a suitable habitat that will ideally provide aquatic larvae with the highest chance of survival (Matushkina and Lambret 2011). Oviposition site selection may account for the costs and benefits that the female and offspring will endure as well as the environmental cues that signify a suitable environment for egg development (Thornton and Switzer 2015). While many researchers have studied the effects of specific factors on oviposition in odonates, few have compiled this information to create a comprehensive synthesis of the many variables affecting the distribution of oviposition sites in odonates. It is crucial to understand the trade-offs that drive natural selection and behavior in order to effectively manage and protect these species.
Visual and Olfactory Cues
Odonates rely on visual and olfactory cues from the environment to determine suitable oviposition sites.
Polarotaxis is the main habitat selection cue in odonates. Odonates locate bodies of water by detecting the horizontal polatrization of light reflected from the water surface using photoreceptor cells in their highly developed compound eyes (Kriska et al. 2009). Odonates can be deceived by artificial surfaces such as oil lakes, asphalt roads, dark colored cars, gravestones, and black plastic foil which reflect high levels of the horizontally polarized light (Sigutova et al. 2015) Unable to distinguish these unsuitable habitats from a suitable body of water, odonates may lay their eggs on the artificial substrate, resulting in complete mortality of their offspring. Trophic state of a freshwater ecosystem can also influence water polarization patterns thus influencing habitat selection (Sigutova et al. 2015). Mesotrophic ponds, which have an intermediate level of productivity, were found to be more attractive than eutrophic ponds which have higher levels of vegetation and algae (Sigutova et al. 2015).
Females also receive olfactory cues emitted by prey and use these signals to determine suitable habitat to oviposit. Odonate antennae possess olfactory sensilla (Piersanti et al. 2014). These antennal olfactory sensilla show excitatory responses to prey odor (Piersanti et al. 2014). The ability to detect chemical cues from prey allows the female place her offspring in a habitat with abundant prey thus increasing the chances of survival for their offspring. Studies show that some species of damselflies respond to chemical cues of fish predators (McGuffin and Baker 2011)
Dispersal Behaviors
Odonates exhibit different dispersal behaviors depending on the costs and benefits associated with each method. Females may oviposit all of their eggs in a single location, or disperse them over a larger area composed of several sites in a behavior called bet hedging (Thornton and Switzer 2015). Their choice in dispersal behavior is often dependent on egg predation and interspecific competition among larvae (Thornton and Switzer 2015). For odonates that have exhibited bet hedging behavior, oviposition site locations were at least a few meters apart and tandem individuals continued moving during oviposition, causing a distribution of eggs over a large area (Thornton and Switzer 2015). Bet hedging alleviates the risk of putting all of the reproductive output in a single location. This may be a more beneficial behavior in environments with unpredictable disturbances such as flooding (Wissinger). Bet hedging may also decrease competition amongst larval offspring (Thornton and Switzer 2015).
Other species of odonates will lay all of their eggs in a single location. Although there has been little research to explain this behavior, one study found that females will spend less time ovipositing when they do not have a male guarding them (Thornton and Switzer 2015). This observation may suggest that females are at higher risk of predation when not in tandem, therefor it is more beneficial to lay all of their eggs at once in order to guarantee that the eggs make it into the water before the female is susceptible to predation.
Endophytic and Exophytic Oviposition
Some female odonates oviposit endophytically while others lay their eggs under water. Several species of dragonflies and nearly all species of damselflies oviposit endophytically, meaning they oviposit in plant tissue (Harabis et al. 2015). There is evidence that some species of dragonflies show particular preference in plant species and parts of the plan for oviposition (Matushkina and Lambret 2011). This oviposition behavior requires special adaptations. Species that oviposit endophytically have a well-developed cutting ovipositor that is equipped with receptors that aid in detecting suitable sites (Matushkina and Lambret 2011). The benefits of endophytic oviposition include reduced risk of egg desiccation and freezing and protection from predation. Consequently, it is a time consuming and energetically expensive behavior (Harabis et al. 2015).
Underwater oviposition is a special subtype of endophytic oviposition. In contrast, odonates that extrude fertilized eggs onto the surface of the water exhibit exophytic oviposition. There is a strong tendency to oviposit underwater at sites with high risks of predation and egg parasitism. Higher proportions of eggs laid exophytically are parasitized which results in higher egg mortality (Harabis et al. 2015). This may be attributed to the searching behavior of parasidic wasps, which is limited by their inability to move underwater (Corbet 1999). While underwater oviposition lowers the risk of parasitism, a greater proportion of eggs laid underwater will fail to develop compared to those laid on the surface of the water (Harabis et al. 2015). The deeper the eggs are laid, the lower their hatchability (Harabis et al. 2015). Figure 3, adapted from Harabis et al. (2015) depicts overall mortality, proportion of developing eggs, and proportion of parasitized eggs in relation to egg position above or below water and level of egg parasitism in the environment. The figure demonstrates that the proportion of eggs parasitized is significantly higher for eggs laid above the water surface than for eggs laid underwater. The highest proportion of eggs parasitism was recorded for eggs laid right above the surface of the water, and the degree of egg parasitism decreased with depth under water (Harabis et al. 2015).
Environmental Influences
The starting time of oviposition is highly correlated with weather (Ishizawa 2012). Weather, which determines the number of suitable oviposition days, has been shown to affect fecundity of populations (Hassall and Thompson 2008). Precipitation, cloud cover, and low temperatures increase stress and restrict development (Hassall and Thompson 2008).
Habitat Preference
There are several behavioral strategies odonates use to place eggs in a suitable habitat and provide offspring with the greatest chance of survival. In areas that are prone to flooding, odonates may select substrates that are more likely to be flooded earlier by rainfall (Lambret et al. 2015). This decreases the risk of desiccation while the eggs are vulnerable and immobile thus increasing hatching success (Lambret et al. 2015). Also, females may preferentially oviposit in low quality ponds because of the high abundance of competitors at the high-quality ponds (Sigutova et al. 2015).
A study by Thornton and Switzer (2015) observed one species of dragonfly that preferred to oviposit in relatively deep water at the edge of surface vegetation and algae mats, or over submerged vegetation. The choice to oviposit in dense vegetation may decrease risk of predation, while ovipositing in open water may decrease risk of desiccation (Buskirk and Sherman 1985). In order to gain both benefits, individuals may oviposit within the vegetation near the edge of open water (Thornton and Switzer 2015).