42.8 Thursday, Jan. 5 Turning Mechanisms in Cubomedusae EICHINGER, J. M.*; SATTERLIE, R.A.; University of North Carolina Wilmington firstname.lastname@example.org
Box Jellyfish (Class: Cubozoa) are strong, active swimmers capable of evading hazardous obstacles and orienting to photic stimuli. Directional swimming is accompanied by asymmetric deformations of the velarium, a striated muscle sheet encircling the bell opening. Radial, buttress-like muscles, termed frenula, are presently thought to serve only a stabilizing structural role during swimming. We hypothesize the frenula serve a greater function in turning behavior than previously thought. During a turning event, frenular contractions in the radial axis result in shortening of the bell height as the velarium is pulled upward towards the bell apex. Asymmetrical contractions would produce directional fluid ejection from the bell opening, which would propel the medusa into a turn. To investigate this behavior confocal-light microscopy, electron microscopy, videography and electrophysiological techniques were employed. Our results demonstrate that the frenulum is the most densely-innervated swim muscle in representative medusae of two cubozoan families (Carybdeidae; Tripedaliidae). The frenular nerve net also has a distinct radial orientation in parallel with muscle cell orientation, whereas subumbrellar and velarial neurons are distributed more randomly in non-directional networks independent of muscle cell orientation. Frequency dependent neuromuscular facilitation occurs in all three muscle sheets, but our results suggest the frenulum is a functionally unique muscle sheet. Contraction patterns and spontaneous, powerful contractions at high swimming frequencies suggest that the frenula may be important components in generating asymmetric velarial contractions to produce turning in cubomedusae.