Matthew R. Gilg

Thermal constraints for range expansion of the invasive green mussel, Perna viridis, in the southeastern United States

Cold temperatures are thought to be among the most important determining factors of geographic distribution for tropical and sub-tropical marine invertebrates. The Asian green mussel, Perna viridis, has been introduced into coastal waters of Florida where its current distribution is hypothesized to be limited by low temperatures during winter. Lethal and sub-lethal effects (heat shock protein/Hsp70 expression) of cold water and air temperatures were analyzed in two size classes of P. viridis from Florida in an effort to determine the effects of current and forecasted temperatures on the potential for range expansion. Mussels were exposed to water temperatures of 14, 10, 7 and 3°C for up to 30 days, or to air temperatures of 14, 7, 0 and -10°C for periods of 2 hr. Mortality was significantly increased at all water and air temperatures ≤14°C. No differences in mortality rates were observed between small (15-45 mm) and large (75-105 mm) size classes except after exposure to 7°C air, in which small mussels had higher mortality. Significant increases in Hsp70 expression were observed after a 2-hour exposure to 10°C water, but Hsp70 expression was not significantly increased at any temperatures in which mortality was not also significant. The temperature threshold for survival in this population appears to be between 10 and 14°C, suggesting that under current conditions P. viridis may already be at the northern edge of its potential range in the United States. If water temperatures increase with global climate change, northerly flowing currents may permit range expansion as temperatures allow.

Spatio-Temporal Settlement Patterns of the Non-Native Titan Acorn Barnacle, Megabalanus coccopoma, in Northeastern Florida

Abstract Individuals of the non-native titan acorn barnacle, Megabalanus coccopoma, have been documented at several locations in the southeastern United States in the past eight years, but little work has been done on the basic biology of M. coccopoma in U.S. waters, nor has anyone documented active settlement in local areas. We collected recently-settled M. coccopoma spat at sites within the main channel and some of the adjacent feeder creeks of the Intracoastal Waterway of northeastern Florida to compare spat abundance over time, between channel and creek environments, and among sites. Spat were present almost exclusively between April and July with most settlement occurring relatively close to either St. Augustine Inlet or Matanzas Inlet. No spat were observed at any of the feeder creek collection sites; the lower salinity there may be intolerable to M. coccopoma larvae.

Comparison of larval thermal maxima between Fundulus heteroclitus and F. grandis

Fundulus heteroclitus and F. grandis are resident salt marsh fishes that overlap in distribution over a narrow range in northeastern Florida. The objective of the present study was to examine whether the limits of the species’ ranges could be explained by differences in thermal tolerance. Two populations of each species were collected and then spawned in the laboratory, and 9-day-old larvae were used for critical thermal maxima trials. Mean LOE temperatures of larvae ranged from 43.04 to 43.65°C and showed little difference between species. Therefore, differences in high temperatures experienced cannot account for the differences of the distributions of the two species. Condition-specific competition may play a greater role in determining the observed range of the two species.

Abundance and Distribution of Larval and Juvenile Fundulus heteroclitus in Northeast Florida Marshes

Abstract Larvae and juveniles of Fundulus heteroclitus (Mummichog), commonly occur in small, water-filled depressions on the intertidal marsh surface during low tide. Previous work has shown that larger juveniles are typically found at lower elevations on the marsh surface, while small larvae are more abundant in the high marsh. The present study compared the abundance and size distributions of larval and juvenile Mummichog between relatively low- and high-elevation sites on the marsh surface at three locations in Northeastern Florida. Fundulus heteroclitus were both more abundant and larger in size at low-elevation sites than at high-elevation sites following the size-selective marsh-use pattern shown in other locations.

Quantification of Reproductive Isolating Barriers Between Two Naturally Hybridizing Killifish Species

Understanding the relative importance of various reproductive barriers to the early stages of speciation is an essential question in evolutionary biology. The closely related killifishes Fundulus heteroclitus and F. grandis occasionally hybridize in a small region in coastal Northeastern Florida showing that while barriers to reproduction exist, they are incomplete. The objective of this study was to elucidate barriers to reproduction between F. heteroclitus and F. grandis in the lab, as well as to quantify their strengths and relative contributions to reproductive isolation. Pre-zygotic (mating and fertilization) and post-zygotic (hatching) barriers were investigated by performing a variety of choice and no-choice laboratory mating experiments. Under no-choice conditions, barriers to mating had the greatest influence on hybrid production in F. grandis, whereas hatching barriers contributed to the majority of reproductive isolation in F. heteroclitus. Under choice conditions, however, pre-zygotic barriers had the greatest influence on hybrid production in both species. The total reproductive isolation that was observed in females of each species was stronger in F. heteroclitus than in F. grandis, and was nearly complete in F. heteroclitus females under choice conditions and was of moderate strength in F. grandis females. These results reveal an asymmetry in the potential gene flow between these two species, with F. grandis being more likely to hybridize than F. heteroclitus in the absence of environmental influences. No-choice backcrosses were also conducted and showed that at least some F1 hybrids are fertile. The observation that pre-zygotic barriers tend to be stronger than post-zygotic barriers in the early stages of speciation is consistent with similar studies in other organisms.

Genetic diversity of a hitchhiker and prized food source in the Anthropocene: the Asian green mussel Perna viridis (Mollusca, Mytilidae)

Insight into a species’ native and introduced range is essential in understanding the invasion process. Genetic diversity, propagule pressure and environmental conditions all have been recognised as playing a determinant role in invasion success. Here, we aimed to investigate the genetic diversity and population genetic structure (using the COI mtDNA gene region and 22 nDNA microsatellite markers) of the Asian green mussel Perna viridis within its potential native range in Asia and at introduced locations in the USA and the Caribbean. We also analyse genetic data from vessel intercepts and an incursion. By doing so, we aimed to identify genetic signatures that could allow to track vessel samples to their source and provide further insight into potential high-risk invasive populations or areas. Three top hierarchical clusters were identified using the individual-based Bayesian clustering method in STRUCTURE, corresponding to populations in three world regions: (1) USA and Caribbean, (2) India and (3) Southeast Asia. Within Southeast Asia, additional analysis indicate a shallow genetic differentiation of three subgroups consisting of (3a) Thailand, (3b) Taiwan and Hong-Kong, and (3c) a cluster of Singapore–Indonesia samples. Overall, the population structure found in this study suggests that the markers used could be useful in identifying source populations, particularly between the three mains world regions. Most surprisingly however, this study shows that the genetic diversity of samples collected from vessel intercepts and incursions did not differ significantly from established populations in Southeast Asia. In this region, in addition to the high vessel connectivity and number of P. viridis transported, all sampled populations are likely to pose a comparable risk in terms of genetic diversity. The present work represents the most comprehensive population genetic study of P. viridis, and the first to address the potential genetic introduction risk posed by populations of this species. The information and genetic markers in this study constitute a valuable addition to the tools already used to infer on potential high-risk source populations of P. viridis. They should therefore prove useful for biosecurity surveillance and management actions directed at this species.

Testing the relative effectiveness of traditional and non-traditional antifouling substrates on barnacle and macroalgae settlement

Abstract Due to economic impacts, there is considerable interest in determining effective methods for limiting the attachment of marine invertebrates to submerged materials. We tested the effectiveness of traditional and non-traditional coatings on materials used for boat construction to limit the settlement of barnacles and macroalgae. Substrates included fibreglass, fibreglass coated with wax, aluminium, aluminium coated with antifouling paint, aluminium coated with Vaseline®, and aluminium coated with Vaseline® mixed with cayenne pepper. Tiles of each substrate were attached to frames, placed at two sites in the Intracoastal Waterway near Jacksonville, Florida, and collected after one or two successive months in the field. Barnacles as well as macroalgae showed significantly greater settlement on fibreglass than aluminium. Each type of coating tested reduced settlement relative to controls, with the lowest overall settlement of barnacles being observed on aluminium coated with Vaseline®, both with and without the addition of cayenne pepper.