Revisiting the Trophic Role of Doliolids. Are Doliolids Detritivores?

Gelatinous zooplankton play a crucial role in pelagic marine food webs, however, due to methodological challenges and persistent misconceptions of their importance, the trophic role of gelatinous zooplankton remains poorly investigated. This is particularly true for small gelatinous zooplankton including the marine pelagic tunicate, Dolioletta gegenbauri. D. gegenbauri and other doliolid species occur persistently on wide subtropical shelves where they often produce massive blooms in association with shelf upwelling conditions. In contrast to salps that are usually associated with low particle waters, doliolids are most abundantly located in particle and nutrient-rich waters associated with phytoplankton blooms and aggregated detritus. As efficient filter feeders and prodigious producers of relatively low-density organic-rich aggregates, doliolids are understood to contribute significantly to shelf production, pelagic ecology, and pelagic-benthic coupling. Utilizing Molecular Gut Content Analysis (MGCA) and Stable Isotope Analysis (SIA) approaches, the trophic interactions of doliolids were explored during bloom and non-bloom conditions on the South Atlantic Bight continental shelf. Based on MGCA, D. gegenbauri at all life stages ingested a wide range of prey types and sizes, but exhibited selectivity for larger prey types including microzooplankton and metazoans. Experimental growth studies confirmed that metazoan prey were ingested but did not support the conclusion that they were digested and assimilated. Stable isotopic composition (¹³C and ¹⁵N) of wild caught animals were consistent with a detrital-based diet. From a trophic perspective doliolids may function as detritivores rather than planktivores with the unique trophic twist that their feeding behavior functions to create the detritus they consume.