MORE SNAILS, MORE TRAILS?: THE RELATIONSHIP BETWEEN TRACE AND TRACEMAKER DENSITIES

Can the density of traces in a given area be used as a proxy for the density of tracemakers living there? If so, trace fossils could be used to estimate abundances of organisms in settings where traces, but not body fossils, preserve well. The relationship between the density of the gastropod Batillaria minima and its traces was examined in the muddy intertidal zone of a lagoon on San Salvador Island, the Bahamas. The number of individuals found within a quadrat was compared to the number of trails that crossed the quadrat’s boundaries. These observations (n = 176) were taken daily over a period of about two weeks. The daily tidal cycle erased the previous day’s traces allowing for collection of new data during the next tidal cycle. There was only weak positive correlation between snail and trace densities (r = 0.43). The correlation increases to 0.59 by log-transforming the data. In some cases, high densities of individuals can be found without correspondingly high trace densities. This might result from snails clumping together in space (perhaps to access a shared resource such as food or shelter) for a period of time while not moving much, producing few traces. Observations with tagging individual snails as well as speed measurements suggest that B. minima can disperse around the mudflat as quickly as a few tens of centimeters a day. In sum, although trace density in an area can predict density of tracemakers to an extent, the latter can often be underestimated.