Fundamental Niche Narrows through Larval Stages of a Filter-Feeding Marine Invertebrate

AbstractOntogenetic niche theory predicts that resource use should change across complex life histories. To date, studies of ontogenetic shifts in food niches have mainly focused on a few systems (e.g., fish), with less attention on organisms with filter-feeding larval stages (e.g., marine invertebrates). Recent studies suggest that filter-feeding organisms can select specific particles, but our understanding of whether niche theory applies to this group is limited. We characterized the fundamental niche (i.e., feeding proficiency) by examining how niche breadth changes across the larval stages of the filter-feeding marine polychaete Galeolaria caespitosa. Using a no-choice experimental design, we measured feeding rates of trochophore, intermediate-stage, and metatrochophore larvae on the prey phytoplankton species Nannochloropsis oculata, Tisochrysis lutea, Dunaliella tertiolecta, and Rhodomonas salina, which vary 10-fold in size, from the smallest to the largest. We formally estimated Levins's niche breadth index to determine the relative proportions of each species in the diet of the three larval stages and also tested how feeding rates vary with algal species and stage. We found that early stages eat all four algal species in roughly equal proportions, but niche breadth narrows during ontogeny, such that metatrochophores are feeding specialists relative to early stages. We also found that feeding rates differed across phytoplankton species: the medium-sized cells (Tisochrysis and Dunaliella) were eaten most, and the smallest species (Nannochloropsis) was eaten the least. Our results demonstrate that ontogenetic niche theory describes changes in fundamental niche in filter feeders. An important next step is to test whether the realized niche (i.e., preference) changes during the larval phase as well.