Alatina alata box jellyfish monthly migrations in Hawai’i: Lunar and physical oceanographic triggers

Abstract

Alatina alata, a species of box jellyfish, is found in distinct localities throughout tropical zones in the Atlantic, Pacific, and Indian Oceans, and is responsible for the most severe stings reported in Hawaiian coastal waters. Alatina alata is unique among species of box jellyfish (Cubozoa) in that it is only observed in shallow shoreline waters during lunar period-specific monthly mass aggregations and beaching events, which occur 8–10 days after each full moon. At other times of the lunar cycle, this species has been documented in pelagic zone surveys and is not observed close to shore (0 to 5 meter deep area, 0–100 m from shoreline). To assess potential ecological and environmental triggers for the synchronized shoreward migration behavior of Alatina alata, we investigated 14 years of beach count data in relation to lunar transit, luminosity, tides, and ocean currents using: a 14-year time series of Alatina alata pre-dawn counts; morphometric and reproductive status analyses; three full-transect nighttime shipboard observations; modeling studies; and additional supporting data. Model simulations show a persistent anticyclonic eddy in the study area, which may act as a source point for the jellyfish. Additionally, in situ night-long observations over three separate months revealed for the first time that Alatina alata actively swam en masse from over one mile offshore towards the shore before moonrise. Over 500 animals were observed and all showed strong shoreward swimming behavior perpendicular to the prevailing current. While the monthly populations each demonstrated heterogeneity with regard to size and beaching time, there was a slight trend towards the beaching of larger jellyfish first. Despite this heterogeneity in size, all animals were sexually mature adults replete with gametes or freshly spent. For the first time, we document that a key number of hours of darkness between twilight and moon rise correlate with the migration, and we hypothesize that this lunar cycle specific event triggers gonadal maturation in adult jellies, and maturation then triggers migrations to the shallow reef zone for spermcast mating.