Periwinkle snail: effects on plant biomass

Written by Donna McDowell

On the 22nd of September, it had been two weeks since the cageing experiment of periwinkle snail Littoraria irrorata on saltmarsh cordgrass Spartina alterniflora was initiated. Today was the day for the class to go outside and record the findings of the experiment. The weather was great and all were happy to be in the field. Students went out to untie the tops of the cages and unveil the findings.

Once cages were opened the teams then had to count the number of snails, measure height (mm) of 5 plants within each experiment, count radulations per plant, and assess % damage. The three densities of snails in the experiments were: 13, 600, and 1200. Sometimes within the experiments there were unfortunate victims. We found dead shrimp, fiddler crabs, and a mummichog. After all the data was collected the snails were returned to the marsh to live their happy fungi-eating lives.






The mean plant height was highest for the medium treatment, second for high, and third for lowest. Also, the mean number of radulations was greater for the medium treatment, 19.2, than the high treatment, 8.8, probably due to more marshgrass being alive to count the radulations. The percentage damage, of course, was greatest for the high treatment, 82%, and lowest for the low treatment, 2%.

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The waters are teeming with plankton!

Written by Amber Wilkinson

This week, September 8, we looked at plankton in the water column 0.5 um and larger. Before conducting two plankton tows we discussed diel vertical migration and larval transport. Diel vertical migration is a behavioral pattern in which organisms come up to the surface at night and down during the day.

The collection method was new to me. Plankton tows I have done in the past were off the side of docks and boats that are in neutral. This method used a bongo net which consisted of two plankton nets side by side resembling bongo drums. The bongo layout allows for replicate samples for each tow. Tows were conducted for 5 minutes. Tow 1 sampled the top meter of the water, and tow 2 sampled the entire water column (an oblique tow). The boat speed for our plankton tow was 2 knots. We sampled against the current and in the wake of the boat. If you want to measure the volume of water sampled, it is not recommended to sample in the wake of the boat because the prop creates turbulence, disrupting the sample area.

After a 5 minute tow, the net was rinsed in the water by holding the net’s ring and dipping the net in the water vertically 2-3 times to get most of the sample down in the cod end. The cod end is the end of the net where the sample collects. Our nets have mesh openings in the cod end to allow for more water flow. Before conducting another tow, the net was rinsed for 30 seconds in the water without the cod end to minimize contamination between samples.

To keep a lot of water out of our sample jars (more condensed samples) the contents from the cod end can be rinsed into a sieve than from there into the sample jars. For the purpose of our class, we looked at our samples in Pyrex dishes (9x13in).

Back in the lab, we observed the samples under dissecting microscopes. The diversity amazed us, there were fewer species near the top. The surface tow contained crab larvae and a skeleton shrimp. An interesting observation was that the crab larvae followed the light from the microscope when we were looking at the different species in the petri dishes. This may suggest why we found so many crab larvae near the surface. Also, there was greater diversity throughout the water column. Juvenile shrimp, mature smaller shrimp, larval fishes, parasitic isopods, crab larvae, and a lot of plant matter were in the oblique tow.

Community Dynamics and Settlement

Top-down regulation of Spartina alterniflora (saltmarsh cordgrass) by the periwinkle snail
September 1, 2009
Written by Kelli L. Edwards
The salt marsh community is regulated by a trophic cascade. Although some communities are typically structured in a pyramid, some consist of an inverted pyramid. These inverted pyramids are most likely to be associated with pristine aquatic communities, where there are much larger animals and fewer trophic levels. Despite the differences, each trophic cascade consists of the primary producers, primary consumers, secondary consumers, and so on.

Each one of these trophic cascades is regulated by either top-down regulation or bottom-up regulation or both. Top-down control is determined by how many predators or consumers are present, while bottom-up control is determined by how many resources are present (Silliman and Bertness, 2002). Plant biomass is greatly controlled by predators and their feeding ativities. Beyond other environmental factors such as nutrient availitiblity and salinity, top-down control may be a key determinant of marsh grass growth.

In a 2002 paper, Silliman and Bertness investigated the effect of top-down regulation with the periwinkle snail Littoraria irrorata.

This snail feeds on a fungus which grows on the Spartina grass. With enough snails, they can devour a Spartina population. The periwinkle snail does have natural predators which help to indirectly regulate the consumption of Spartina.
For our class activity we investigated the principle of top-down regulation. We are to assess the effects of density-dependent snail grazing on Spartina grass growth by maintaining constant snail densities in isolated cages. Snails were put into cages at low (13 per meter square), medium (600 per meter square) and high (1200 per meter square) densities. Over the course of the next 2 weeks, we will monitor the growth and consumption of Spartina alterniflora.

Low density:

Medium density:

High density: