Trends in nematocyst abundance provide a mechanism for venom metering in the ‘upside-down’ jellyfish Cassiopea (Cnidaria: Scyphozoa: Rhizostomeae)

Cassiopea spp. exhibit mixotrophy through photoautotrophic symbiosis with zooxanthellae and heterotrophic prey capture via venom. Despite this, venom composition data remain limited. We present a 406.6 Mbp draft genome for Cassiopea andromeda, annotated using transcriptomes from aposymbiotic tissues. This genome supported proteomic mapping of venom from nematocysts of C. andromeda and Cassiopea xamachana from pristine and polluted mangroves in Brazil and a pristine mangrove in the Florida Keys. MS/MS data revealed no qualitative interspecific differences in toxin homologs, which were predominantly haemostasis-disrupting, suggesting venom specialization. Toxin abundance and nematocyst counts were higher in specimens from pristine environments, despite similar nematocyst types and sizes across samples. Environmental condition, rather than species or location, influenced venom abundance. We propose that modulation of nematocyst density may serve as an unrecognized mechanism for venom metering in adult Cassiopea.