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INTRODUCTIONRegression Analysis of Fresh-water Gastropods Distribution Along the San Marcos River
• Existing State of Knowledge– Most invasion gastropods
were introduced by humans in aquarium trade
– Snails are an intermediate host for liver fluke parasite which is fatal to small native fish
• 12 different species of snails (Diaz and Alexander)
• For our study’s focus:• Melanoides
tuberculata• Tarebia granifera• Hydrobiidae
Amnicola limosa• Marisa cornuarietis
Snails of the San Marcos River
Melanoides tuberculata(red-rimmed melania)
• Were first called “Philippine Horn of Plenty” ion 1937
• From southern Asia
Tarebia granifera
• Makes up 60% if bottom-dwelling invertebrates of the river
• Displaces native snails• Also from southern Asia
Hydrodiidae Amnicola
• Spire has four whorls• Aperture is dextral• Pathenogenic: females
produce eggs without involvement of males
Marisa cornuarietis(giant ram-horn)
• From South America• About 1 inch in diameter
Snail Anatomy
• The shell:• Grows by secreting
shell material at the edge of the mantle
• Opening of the shell is called the aperture
Sinistral
dextral
Snail Anatomy
• Movement:• The foot is the muscle that
projects from the shell• Slime track of mucous• Not known if movement is
dependent on waves of muscular contractions
• “hunching” used foot to pull shell forward
Snail Anatomy
• Feeding and Digestion• Vegetarian: eating
chiefly off lawns of living algae, dead plant material and some dead animal matter
• Greater activity and food intake at higher temperatures
Predators/Parasites
• Greatest enemy are the fish: suckers, perch
• Some ducks• Invertebrates
• Leeches• Ostracods
• Intermediate host for parasite: Centrocestus formosanus• Fatal to fountain darter• Originates in Asia
Etheostoma fonticola
• Broader Ecology Context• Invasive species drive
out native species
• Research QuestionsHow do snails feed? breed? respire? move?
Do these aspects determine where the snail resides in the river, and to how much effect?
What significant do the invasive species pose on the native species of the ecosystem?
HypothesisThere will be no difference in snail abundances among
different flow regimes, substrate type/size, and/or depth.
Materials and Methods
• Location– San Marcos River at the
falls of Spring Lake near Salt Grass
• When– 3 separate days in March– From 11am-4pm
• How• Materials
• Two meter sticks
• Thermometer• GPS device• Orange• Under Water
Digital Camera• Frame
• How• Technique
• Measure average depth
• Measure surface flow
• Photograph plot for snail count
• Statistical Design • Regression analysis: analyzing several
variables, with a focus on the relationship between the dependent (number of snails and an independent variable (depth, current, substrate size)
RESULTS
Fig. 1. Regression analysis, number of snails present as a function of water depth
Fig. 2. Regression analysis, number of snails present as a function of water current
Fig. 3. Regression analysis, number of snails present as a function of substrate size
Fig. 4. Regression analysis of all three variables compared.
Fig. 4. Number of snails present in areas with and without vegetation. Note, only 14 of the 35 sites had vegetation.
DISCUSSION
• Hypothesis:• There will be no difference in snail
abundances among different flow regimes, substrate type/size, and/or depth.
• Compared Results– Found in abundance by
Kimmel, and Diaz and Alexander.
– Grantham showed results that support rams-horn have a preference for macrophytes
• Limitations of the study• Time
• Limited space• Knowledge• Equipment
• Broader Ecology Meaning• Insight to why this
invasive species is so successful
• Influence to endangered fountain darter