By linking surface waters to the deep ocean, the mesopelagic zone (200–1000 m) plays crucial ecological roles in biogeochemical cycling, feeding apex predators, and sustaining a massive daily migration of organisms. The importance of this expansive ecosystem cannot be fully understood without accurately estimating its biomass. Several recent studies have estimated the biomass of mesopelagic fishes, yet the biomass of zooplankton is far less understood. The towed vehicle “Deep-See” was designed to obtain in situ acoustical (1–410 kHz), optical, and environmental measurements of mesopelagic fish and invertebrates. Dense patches of jellyfish were observed in images collected by Deep-See in the New England slope sea during Summer 2019, revealing the presence of gelatinous biomass potentially masked by more strongly scattering organisms (e.g., fish, krill) in narrowband shipboard acoustic surveys. To quantify this “hidden” biomass acoustically will require understanding the physics of sound scattered by these organisms. Using a full 3-D scan of an individual, and simpler approximate shapes, the Distorted Wave Born Approximation (DWBA) was used to model the target strength of the common mesopelagic jellyfish Solmissus. Images were used to predict the volume scattering spectrum of an observed Solmissus patch and compared with acoustical measurements.