JELLY ROLES: THE IMPORTANCE OF GELATINOUS ZOOPLANKTON IN PHANEROZOIC MARINE ECOSYSTEMS

Gelatinous zooplankton (“jellyfish” or “gelata”), include the medusae of scyphozoan, hydrozoan, and cubozoan Cnidaria, Ctenophora, and the chordate tunicate groups Thaliacea (pyrosomes, salps, and doliolids) and Appendicularia (larvaceans). They have an estimated global biomass of 0.038 Gt C, about half of that estimated for marine fish. Marine biologists have developed a new appreciation for the importance of these forms in marine food webs (Hays et al. 2020 TREE), including recognition that the food value of jellyfish has been underestimated. Although by volume they have generally low energy content, they can be digested much faster than other prey items, can occur in high densities and may have large body sizes (=large overall calorie count). Most importantly, the energy cost of capturing them is low compared to more active prey. The energy content of jellyfish can also be increased if their stomachs and oral arms contain the remains of their own prey. Leatherback turtles and sunfish are well known jellyfish specialists, but gelatinous zooplankton are also eaten by a diverse range of vertebrate and invertebrate taxa, including marine mammals, fish, crabs, snails, squid, other jellyfish, and deep-sea octopi. Shorebirds, arthropods, annelids, gastropods, and other taxa also scavenge on stranded jellyfish, and decomposing gelata can contribute considerable carbon and nitrogen to deep water food webs during localized “jelly falls.” Cnidarian medusae and ctenophores have a well-documented fossil record back to Stage 3 of the Cambrian, which scattered reports of possible earlier forms. Benthic tunicates are also known from the Early Cambrian; planktonic tunicates lack a known fossil record, but phylogenetic studies suggest that the common ancestor of tunicates was free living—and therefore would have been part of the gelata at that time. Gelata have thus been components of the marine biota and played a role in marine food webs throughout the Phanerozoic. Comparisons with modern predators and with robotic grippers for sampling soft marine organisms suggest the functional morphology for scooping gelata. Based on this, potential Paleozoic predators on jellyfish include fish, cephalopods, megalograptid and pterygotid eurypterids, and radiodonts.