Discussion

           Schinus terebinthifolius can have a significant negative impact on the survival of a range of unique mobile invertebrate taxa, depending on S. terebinthifolius fruit density and the condition of the fruits (crushed or intact). Although the patterns were often the same, with the highest density of crushed fruits having the greatest impact on survival, each animal reacted differently to the treatments. Likewise, multiple trials with the brine shrimp documented similar variability. From this it can be inferred that the presence of S. terebinthifolius can affect a wide range of estuarine invertebrates.

            Schinus terebinthifolius fruits proved to be lethal for S. quadridentata, I. obsoleta, and A. salina, while having no significant effect on the survival of P. armatus. Furthermore, for I. obsoleta and A. salina, only exposure to the highest tested density of crushed S. terebinthifolius fruits caused a significant negative effect on their viability. This negative effect could be due to the fact that the mud snail I. obsoleta has an external shell, in which it can close itself off from the outside environment for short periods of time, potentially reducing the surface area of the organism that is exposed to the toxin for prolonged periods of time.

           Although the second trial of A. salina showed no significant impact of S. terebinthifolius on survival, it is still believed that S. terebinthifolius had a negative effect on A. salina given the results from the other two trials. The low p values for trials 1 and 3 both showed S. terebinthifolius significantly impacted A. salina’s survival. This skew in the data from trial 2 could have been a result of differences in age, size, or condition of the purchased brine shrimp. These measurements were not collected at the time of the trials, but it is common to purchase brine shrimp that range in age from four days to two weeks old and vary in size from 2 – 10 mm. All specimens are simply sold as brine shrimp and any variation depends on the distributors.

            Interestingly, P. armatus was the only organism tested that showed no significant impact due to the presence of S. terebinthifolius fruits. One possible explanation for this difference is the variation in water volumes among the organisms. While the treatments were kept the same, the volume of water used for the P. armatus trials was greater than the trials with other organisms. This greater water volume could have been significant in that the toxic chemicals from the S. terebinthifolius would be more diluted in comparison. It was also postulated that the exoskeleton of P. armatus helped to slow the accumulation of toxins within the tissues of the organisms, thus giving it an advantage during the bioassays. Previous research has demonstrated that the absorption of certain metals, such as zinc and lead, is slower through the exoskeleton than in the soft tissues (Dallinger and Rainbow, 1993). All of the organisms in this study possessed exoskeletons, but the exoskeleton of a brine shrimp, an isopod, or a snail is different from a crab exoskeleton. A crab exoskeleton is a fused carapace, which creates less exposed soft tissue surface area. Also, the exoskeleton of a crab is both tanned and calcified, which renders the cuticle impermeable (Langston and Bebianno, 1998). While a snail also possesses a calcified shell, the exposure of the soft tissue in this organism is much higher once the snail begins crawling and feeding. It is thought that P. armatus could become more vulnerable after molting to the toxins from the fruits and show an increased mortality rate. This hypothesis is based on other studies demonstrating increased absorption of polycyclic aromatic hydrocarbons during the molt cycle of blue crabs (Neff, 2002). This enlarged absorption is due to increased permeability of the soft shell, and additional water uptake during time of the molt. All of our crabs had hard, calcified shells. Further research would need to be conducted to see if the lack of a calcified exoskeleton could significantly reduce the survival of P. armatus by additional intake of the toxin through the organism’s outer membranes.

            One question that deserves attention is whether the inhibitory chemicals in Schinus terebinthifolius are soluble in water. Although not directly tested, we assume we are dealing with a water soluble chemical, as the crushed fruits did change the water color from clear to slightly red. If not soluble in water, then the organisms might only be impacted if coming in direct contact with the fruits. Such impact was likely in the small containers used in our trials. In the field, it is possible the allelopathic chemicals are indirectly important to the survival of our invertebrates, even if the chemicals do not directly kill them. If these chemicals primarily influence the surrounding species, especially prey species, then it interacts through indirect effects, not direct interference (Inderjit and Weiner, 2001). Inderjit and Weiner (2000) suggested that inhibitory chemicals alter the abiotic components of the environment, such as availability of nutrients, which ultimately can affect the survival of the species that come into contact with the allelopathic plant. However, this theory would need further testing to show whether it holds true with respect to our plant-animal interactions, since the test subjects are able to travel in their environment.

            Our study documents the negative effect of fruits of the exotic Brazilian pepper on marine invertebrates, which are important prey sources for higher taxa and serve as an example of how an exotic plant species can cause cascading effects through an ecosystem. As was observed with multiple salt marsh plant species (Donnelly et al., 2008), the overall impact of S. terebinthifolius fruits varied with the species tested. In order to predict more accurately which other Indian River Lagoon species could be negatively impacted by the fruits of S. terebinthifolius, further research should be done to determine the chemical composition of the toxin and determine how each taxa responds to specific fractions of it. Once this is known, researchers should be able to predict which species the toxin will be able to enter and harm, due to varying membrane permeability, which will increase our understanding of the effect that the S. terebinthifolius invasion has on estuarine systems.

Acknowledgements