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First published online September 19, 2006
Journal of Experimental Biology 209, 3758-3765 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02431

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The spectral sensitivity of the lens eyes of a box jellyfish, Tripedalia cystophora (Conant)

Melissa M. Coates^1,*, Anders Garm², Jamie C. Theobald², Stuart H. Thompson¹ and Dan-Eric Nilsson²

¹ Hopkins Marine Station, Department of Biological Sciences, Stanford University, Oceanview Boulevard, Pacific Grove, California, 93950, USA
² Department of Cell and Organism Biology, Lund University, Zoology Building, Helgonavägen 3, S-223 62 Lund, Sweden

View larger version (87K): [in a new window]   Fig. 1. (A) Photograph of Tripedalia cystophora. There are four rhopalia located on the sides of the bell (arrow), which alternate with the four groups of tentacles at the corners. The lens eyes point inward, toward the center of the bell. Scale bar, 1 cm. (B) A photograph of a single rhopalium inside the rhopalial niche. Arrows indicate a pit eye and a slit eye. The slit and pit eyes are both identically matched on the other side of the rhopalium. (C) A photograph of an isolated rhopalium. Arrows indicate the upper and lower lens eyes. Scale bar, 200 µm.

View larger version (14K): [in a new window]   Fig. 2. A sample ERG trace showing the response of an isolated rhopalia to a flash of white light, 40 ms in duration, at 5.47x103 W m-2 sr-1 (ND 0.5). Peak response is taken as first peak (a), regardless of polarity. It is assumed that the first response is from photoreceptors and subsequent peaks (b) may be from downstream events.

View larger version (11K): [in a new window]   Fig. 3. A single V-logI curve from a lower lens eye. Data points (open circles) show how the response changed with changing light intensity (I; W m-2 sr-1), while the model fit (solid line) shows the sigmoid shape of this response. For each preparation the V-logI-based model fit was used to calculate spectral sensitivity for that preparation (see text for details).

View larger version (16K): [in a new window]   Fig. 4. Response characteristics change with intensity of the stimulus. Here, an ERG trace showing the response to a flash of white light at 5.47x103 W m-2 sr-1 (ND 0.5, black trace) is noticeably biphasic. However, when the same preparation was stimulated by a much lower intensity (1.73x101 W m-2 sr-1, ND 3.0), the response appears more monophasic and is of smaller magnitude (gray trace). Inset shows onset (1.5 s) and duration (40 ms) of flash.

View larger version (13K): [in a new window]   Fig. 5. Normalized response versus stimulus intensity (W m-2 sr-1) for the lower and upper eyes (V-logI). Values are means ± 1 s.e.m. Solid line, lower eye (N=14); broken line, upper eye (N=8).

View larger version (18K): [in a new window]   Fig. 6. Mean spectral sensitivity curves for both the lower (solid line, open circles, N=14) and the upper (broken line, open triangles, N=8) lens eyes (values are means ± 1 s.e.m.). When plotted on the same graph the similarity, in both the peak sensitivity (500 nm) and the shape of the curves, between the two eye types is apparent.

View larger version (22K): [in a new window]   Fig. 7. Spectral sensitivity data (closed circles; A,B) are well modeled by both the SSH rhodopsin template (A,B) and the GFRKD template (described by Govardovskii et al., 2000) (C,D). For the lower eye the models give an -peak absorbance of 498 nm (A) and 496 nm (C). Both models deviate in the ß-peak range and produce similar correlations (0.927, SSH; 0.903, GFRKD). For the upper eye models yield peaks of 496 nm (B) and 495 nm (D), again with similar correlations (0.909, SSH; 0.887, GFRKD).

View larger version (24K): [in a new window]   Fig. 8. Removing the ß-peak from the GFRKD visual pigment template optimization allows for a much better fit in both eye types (gray lines; correlations=0.984, lower eye; 0.968, upper eye). With invertebrate levels of self-screening (k=0.0067; A,B, black lines) the curves are indistinguishable from the no screening case (A,B, gray lines). However, these fits can be improved further by adding vertebrate level self-screening to the optimization (C,D, black lines); where k= 0.035 µm-1, and l=50 µm for the lower eye photoreceptors and l=35 µm for the upper eye photoreceptors. Here, self-screening has the effect of broadening the absorption curve. For the lower eye (C) the half-width increases to 112.1 nm with the addition of screening, compared to 82.0 nm without screening. For the upper eye (D) the half-width increases to 111.2 nm from 82.8 nm. Correspondingly the correlation values increase to 0.991 and 0.980 for the lower and upper eyes, respectively.

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