Ichnofossil morphology as a response to resource distribution: Insights from modern invertebrate foraging

A number of surficial horizontal ichnofossils have non-random, simple geometric morphologies such as spirals or meanders. These paths may result from an organismʹs interactive response to the perceived spatial distribution of resources on the seafloor. This hypothesis was tested by examining the movements of foraging invertebrates in response to the location, shape and internal density gradient of food patches in a controlled laboratory environment. Freshwater pond snails (Planorbis planorbis), marine intertidal crabs (Hemigrapsus nudus) and microscopic soil nematodes (Caenorhabditis elegans) were placed in experimental arenas free of competitors and containing food patches of varying shapes and density distributions, and allowed to move and forage freely. Their movements were recorded at 1-minute intervals, and any visible tracks and trails were photographed. Pathways were digitally reconstructed using the recorded movement data, and along with the photographs were digitally analyzed. The total path length, path tortuosity, polygon area, step number, average step length and average turning radius within the resource patches were compared to the values for movement outside the resource patches. All three invertebrates engaged in area-restricted searching when within patches, consisting of a larger step number, higher path tortuosity, larger polygon area, greater turning radius, and a shorter step length. The behavior of these three invertebrate species suggests that resource distribution, in terms of patch morphology, acts as a strong control on forager movement and path morphology. Area-restricted searching as a distinct movement style may have evolved in response to the appearance of patchy resource distributions on the seafloor during the Cambrian substrate revolution.