Overcoming gamete dilution in free-spawning zooplankton: How the moon jellyfish, Aurelia sp., exploits the water column to maximize fertilization success

Jellyfish are prevalent and increasing in the marine environment on a global scale. Many species have been linked to significant ecological and economic loss. Studies have addressed factors contributing to jellyfish success but few have examined the fertilization strategies. It is imperative to have a detailed and comprehensive description of fertilization before researchers can adequately address this growing gelatinous presence. The cosmopolitan moon jellyfish, Aurelia sp., is seasonal in the northern Gulf of Mexico (GoM) and served as the model organism in this research. Aurelia sp. are dieoceous but do not copulate. Males spawn sperm strands that females collect for internal fertilization. This research examined the spawning characteristics of the moon jellyfish using novel methodologies to elicit and quantify reproductive strategies occurring at the biological, chemical, and physical interfaces of the marine environment. Measures of sex ratio, gonad index, and maturity index were calculated to describe seasonal fecundity of Aurelia sp. Modified extraction procedures and gamete response assays were conducted to determine the presence of and sperm strand response to a female reproductive chemical signal. Novel scuba techniques helped to describe and quantify the physics of the water column (shear and dilution) and their effects on spawned sperm strands. Laboratory shear chambers were used to quantify the effects of shear on strand viability. Aurelia sp. were abundant and had a high reproductive index from July through October. Females produced a chemical that induced follicle release of sperm and arrested sperm strand activity. Sperm strand viability significantly decreased over time and led to reduced fertilization success. Shearing from surface waters and from within the pycnocline may facilitate horizontal sperm strand transfer without significant viability loss. This work suggests that the interaction of endogenous factors (biology and chemistry of Aurelia sp.) with exogenous factors (physical water column) may result in high fertilization success, especially within the pycnocline.