Optical properties of the iridescent organ of the comb-jellyfish Beroë cucumis (Ctenophora)

Using transmission electron microscopy, analytical modeling, and detailed numerical simulations, the iridescence observed from the comb rows of the ctenophore Beroë cucumis was investigated. It is shown that the changing coloration which accompanies the beating of comb rows as the animal swims can be explained by the weakly-contrasted structure of the refractive index induced by the very coherent packing of locomotory cilia. The colors arising from the narrow band-gap reflection are shown to be highly saturated and, as a function of the incidence angle, cover a wide range of the visible and ultraviolet spectrum. The high transparency of the structure at the maximal bioluminescence wavelength is also explained.

Figure 1

(Color online) The ctenophore Beroë cucumis is a transparent marine animal which can be recognized by its ellipsoidal shape and the comb rows, which are characteristic of ctenophores and run along the length of its body. These comb rows are iridescent, with rainbow colors passing down the length of the animal as it swims. This animal typically reach sizes from 6 to $10\mathrm{cm}$. Image courtesy of Dr. Kevin Raskoff, California State University, Monterey Bay, USA.

Figure 2

A section through the comb row of the ctenophore Beroë cucumis, showing the structure responsible for the iridescence. The regular packing of the cilia is apparent, each one being found at a node of an orthorhombic lattice. Some of the internal structure of the cilia is visible; from an optical point of view this can be described as a central pair of microtubules (the “central microtubules pair”), here treated as a single unit, which is surrounded by nine other microtubules distributed evenly and at a constant distance from the central microtubules pair.

Figure 3

A transmission electron micrograph of a section through the color-producing structure found in the comb-row of the ctenophore Beroë cucumis at a higher magnification than in Fig. 2. The internal structure of the cilia comprises eleven microtubules, but the central pair are fused and were treated as a single entity.

Figure 4

(Color online) Model for the reflectance of the tightly packed cilia found in the combs of Beroë cucumis. The two translation vectors ${\vec{a}}_1$ and ${\vec{a}}_2$ have the respective lengths $d_1=215\mathrm{nm}$ and $d_2=195\mathrm{nm}$, and the angle $\theta$ between them is 77°. The diameter $d$ of the high-refractive circle is $73\mathrm{nm}$, and the size of the microtubules forming the central pair is $h=40\mathrm{nm}$.

Figure 5

(Color online) The special illumination situation considered for modeling. The sun, the ctenophore, and the observer are in the same plane, normal to the seawater surface, and the animal is swimming upwards. At an incidence close to the Brewster angle on the sea surface, some transverse-electric light is reflected, while the transverse-magnetic waves are fully transmitted into the sea.

Figure 6

(Color online) Reflectance of the model structure of the cilia bundle in the comb rows of Beroë cucumis, in TM (above) and TE (below) polarizations, for various angles of incidence $\theta$. (a) $\theta =0$; (b) $\theta =15°$; (c) $\theta =30°$; (d) $\theta =45°$; (e) $\theta =60°$.

Figure 7

(Color online) Transmission of the tightly packed cilia at the specific wavelength of $489\mathrm{nm}$, which corresponds to the maximal bioluminescence of Beroë cucumis. The transmission is nearly perfect for emergence angles below the high-reflection band.

Figure 8

(Color online) Transmission of a multilayer obtained by reorganizing the material in the cilial structure into a binary periodic planar multilayer with the same period as the original two-dimensional structure. The low transmission gap is found under the same angle as the two-dimensional structure, but the average transmission is slightly weaker, for lower angles.

Figure 9

(Color online) Transmission efficiency of the tightly packed cilia as a function of the luminescence wavelength. The efficiency is measured for emergence angles in the range $\pm \mathrm{\Delta }\theta$, where $\mathrm{\Delta }\theta =10°$.