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Gulf of Maine Organisms
Littorina spp.
Algal Species
Carcinus maenas

Nerita Versicolor Experiment
Abstract Introduction
Methods
Results
Discussion

Experimental Nerita versicolor individuals placed into e phermeral pools high in the intertidal may have emigrated for many reasons. Food availability, salinity, temperature, and possibly fish and bird predation are all viably possible explanations as to why the numbers dropped so suddenly. Fish and bird predation are unlikely however, due to the complete absence of any remains of the missing snails. Observations of Nerita versicolor have shown an absence of predation by gulls, crabs, or fish (Bovbjerg, R.V. 1984).

It is more likely that they moved due to intolerance for physical stresses. Escape is the most likely response to environmental stresses by motile organisms in the high intertidal (Knox, G.A.). The p values of the χ² analysis show strong motivation behind the efflux of nerites from the pool. The high rate of activity being contributed to predation is refuted by the strong positive correlation between the outward movement rates between all three pools. The effects of predation would normally not be so evenly distributed among the three populations, nor would they account for an influx in the third pool on the third day. Competition for food resources has been shown to be virtually non-existent in intertidal nerites (Bovbjerg, R.V. 1984 & Kolpinsky, M.C. 1964), presumably ruling food availability out as an explanation . The patterns reflected in the retention rates over time imply a physical stress as the motivating force for the change in nerite density rather than a biological interaction.

The Nerita versicolor retention rate increases as salinity again begins to increase. The drop in salinity over night can be attributed to the rainfall experienced on the nights that the experiment was conducted. The samples following those nights had the highest emigration rates. The salinities on those days were at their lowest. The salinity and retention rate are highly correlated due to the preferred saline level of Nerita versicolor As the salinity drops due to rainfall the nerites have the most motivation to leave the brackish water. As the salinity level begins to go up due to wave exposure, the nerites are more willing to stay in the area. Pool three has the only exception to this due to the influx of organisms on the last day. This pool had the largest area, so it is possible that probability based on pool area alone accounted for this anomaly. It is more likely, however that the nerites moved back into pool three as a result of that pool's position relative to the upper tidal mark. Nerita versicolor has been shown to prefer higher tidal distributions than other nerites, moving to the top of and over the high tidal mark (Bovbjerg, R.V. 1984). Temperature may have accounted as well for the difference in behavior.

A further study to determine Nerita versicolor's motivation for movement in the intertidal should be conducted to fully asses distribution patterns of the species. Additional replicate pools and a more extended time period may reveal a more positive correlation between physical factors and nerite movement. Bovjberg (1984) claims however, that reversion to a normal distribution via directed movement can occur within 4 days of experimental change of nerite distribution so further time may reveal little more information. Photoperiod effects on nerites might also be studied via dark and light period measurements, as time of day highly effects distribution (Bovbjerg, R.V. 1984). Temperature measurements of the ocean, tidal pools, and organisms themselves combined with a more in depth look at salinity values may lead to a more comprehensive understanding of dispersal motivation of Nerita versicolor throughout intertidal Bermuda.