Figure 1.
Bell contractions induce rhopalial swing.
Due to morphological specializations bell contractions in the box jellyfish Tripedalia cystophora make the rhopalia swing between the relaxed state (a) and the contracted state (b). The corresponding angular change is depicted in c and d. Video analysis of free swimming animals determined the average swing amplitude of 18–19° to occur within 80–100 ms regardless of the swimming direction (see also Table 1). Here a vertical swimming animal is shown but the results were the same for horizontal and 45° upwards swimming. These values match the known spatio-temporal resolution of the upper lens eye (d) suggesting a connection between the biomechanics of the locomotion and refreshing the retinal image. ULE, upper lens eye and LLE, lower lens eye.
Figure 2.
The compression of the 180° horizon above water (green broken line) into a 97° subsurface cone (blue broken line) (a) enable the box jelly to long distance navigate back under the mangrove canopy. b, the contrast line between the canopy and the open sky as seen through Snell's window (without photoadaptation). c, the effect of a swim contraction on the retinal image where activated photoreceptors will light up on a background of inactive receptors.
Figure 3.
Variation in rhopalial swing pattern.
Tracking the rhopalial angle through a single bell contraction show variations in the swing pattern, but the swing amplitude and swing time remain relatively constant (see Table 1). The tracings are synchronized in relation to the onset of the bell contraction, and a contraction of each of the swimming directions is shown. Red trace is the average bell diameter for the three contractions.
Figure 4.
Tracking the rhopalial swing amplitude.
Tracking the amplitude of the rhopalial swing, ΔRA, and the swing time, Δt, in a series of swim contractions (a) showed deviation from the hypothesized sinusoidal swing pattern but most contractions induced a rhopalial swing which matches the spatio-temporal resolution of the upper lens eye (c). It is evident that the bell contraction (a, red trace) induced the rhopalial swing (a, black trace). The pictograms in b represent the orientation of the animal and, red arrows, the swimming direction for each of the tracked contractions. The contraction parameters (duration, degree of contraction, ΔRA and Δt) remained relatively constant (c).
Table 1.
Rhopalial displacement induced by bell contractions.
Figure 5.
Imitating the swim contractions.
Light area in the circles represents the amount of open sky in the visual field. The upper lens eye was exposed to a changing area mimicking the effect of a swim contraction (red trace). Blue trace depicts a typical ERG response to the change in light area of the fiber optic image bundle.
Figure 6.
Electroretinogram response to a moving contrast line.
The electroretinogram (ERG) confirmed the finding that the bell contractions refreshed the retinal image. Even though all changes in light are registered by the photoreceptors, the optimal response and, thereby optimal image contrast, was obtained when the change matched the photoreceptor acceptance angle (a). A shadow was moved in the visual field of the upper lens eye to mimic the contrast line between the mangrove canopy and the open sky, and manipulated to test the neural response to different rhopalial swing amplitudes. There was a marked increase in the ERG response up to a 20° displacement of the shadow after which no significant change in ERG amplitude was detected (b). A similar result was obtained when manipulating the swing time (c); here the ERG amplitude did not increase further when the shadow displacement time was longer than 100 ms. These data match the known spatio-temporal resolution of the ULE and strongly support the hypothesis that the bell contractions refresh the retinal image in the upper lens eye of T. Cystophora. Vertical black lines in b and c represent the amplitude and time of the rhopalial swing measured in a horizontal swimming animal, which is preferred swimming direction of the animal for navigational purposes (interrupted lines, ± standard deviation of the mean). The experiments were performed at two light intensities (solid line, 30 Wm−2sr−1 and interrupted line, 10 Wm−2sr−1, ± standard deviation of the mean, N = 8).