Diet, prey selection, and individual feeding rates of the jellyfish Lychnorhiza lucerna (Scyphozoa, Rhizostomeae)

Large scyphomedusae can deplete zooplankton communities when occurring in high densities and the assessment of their trophic impacts relies on basic information of the species' feeding habits. We quantified in situ gut contents of the South American jellyfish Lychnorhiza lucerna Haeckel, 1880 and described the procedures to determine the diet, prey-selectivity patterns, and feeding rates of this species. Specimens were collected between 2008 and 2011 from surface waters along the southeastern coast of Brazil (23 degrees-25 degrees S, 45 degrees-48 degrees W), where they were immediately preserved simultaneously with plankton samples near aggregations of medusae. Most prey items (similar to 70%) were extracted from the central cruciform stomach by rinsing, although similar to 16% remained in the gastric cavity even after several rinses. Non-digestive body regions (oral arms and umbrellar canals) accounted for a small proportion of the prey found (<10%). Calanoid copepods were the most abundant (53%) prey, followed by cyclopoid (15.1%) and poecilostomatoid (11.4%) copepods and bivalve veligers (similar to 7%). The dietary composition was mostly similar to the proportional abundances in the surrounding mesozooplankton. As medusa size increased, the proportion of calanoids increased, but dietary diversity decreased. The ingestion rates quantified did not supply the species minimum carbon requirements as estimated from oxygen consumption rates; therefore, nutritional resources (e.g., dissolved and particulate organic matter) in addition to mesozooplankton must be considered in further studies. We estimated that from 110 to 102,871 copepods were ingested daily by medusae (5-30cm diameter), which indicates the species have one of the highest feeding rates among scyphomedusae. Therefore, the aggregations of L. lucerna along the southwestern Atlantic coast must be better studied to understand what are the predatory impacts and the role of this species in local production process.