Gastropods Associated with Laminaria hyperborea and Saccorhiza polyschides in a Norwegian Kelp Forest Comparison of Sampling and In Situ Imaging Techniques

The snail fauna (class Gastropoda) associated with the two kelp species Laminaria hyperborea and Saccorhiza polyschides was sampled during September and December 2009, and March 2011. Over 1000 gastropod individuals comprising 35 identified taxa were found in the 29 kelp individuals sampled in total. The attractiveness for settlement for gastropods of L. hyperborea was clearly higher than S. polyschides, which also had very little epigrowth compared to L. hyperborea. Highest number of gastropod individuals were found on lamina and the highest number of species were found on haptera for both kelp species. High variation in gastropod numbers was found both between kelp individuals and between seasons. Detailed still images were taken of the kelp before sampling, for comparison to the sampled material. 81 % of the gastropod fauna visible in images was identified to either species or family, but only 17 % of the actual number of gastropods (sampled) were visible in images. Visibility of gastropods in images decreases with increasing complexity of the habitat (i.e. kelp part), both structural complexity in the kelp or added complexity by epigrowth. The ratio of gastropods identified from images to gastropods identified from sampled material (image detection success), showed high variation between gastropod taxa, mainly dependent on gastropod size, degree of camouflage or conspicuousness, and what habitat (i.e. kelp part) they live in.