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