Riding Langmuir circulations and swimming in circles: a novel form of clustering behavior by the scyphomedusa Linuche unguiculata

Abstract

Linuche unguiculata (Schwartz) seasonally forms patches in the Caribbean Sea and Indo-Pacific Ocean. Eighteen patches of medusae varying from about 500 m2 to nearly 1 km2 in area, were documented along the Belize barrier reef in March and April 1987, April 1988, and March and April 1990. The shape of each patch and the inter-medusa distances varied with wind velocity. At low wind speed (<4 m s-1) patches were elliptical or circular and the individual medusae were separated by distances of 0.5 m, whereas at higher speeds windrows were evident and medusae were closer together. Windrows probably form by horizontal advection owing to convergence by Langmuir circulations. Because individual patches might exist for up to 4 mo as they drift downwind, and because winds of sufficient speed to produce Langmuir circulations do not always occur, a mechanism is necessary to maintain patch integrity during calms. In situ observations and in vitro video recording showed that the medusae swam in horizontal, near-surface, circular, clockwise trajectories. Although swimming speed was relatively high (up to 8 cm s-1). net displacement velocity can be low (<1 cm s-1). Thus, circular swimming probably reduces cluster breakup. Patch formation probably improves reproductive success by reducing sperm dilution.