Optical Performance in Transmittance and Reflectance

9 Clarke T.A. Feeding habits of stomiatoid fishes from Hawaiian waters. , 10 Hopkins T.L. Trophic ecology of the stomiid (Pisces: Stomiidae) fish assemblage of the eastern Gulf of Mexico: strategies, selectivity and impact of a top mesopelagic predator group. 11 Kenaley C.P. Exploring feeding behaviour in deep-sea dragonfishes (Teleostei: Stomiidae): jaw biomechanics and functional significance of a loosejaw. 11 Kenaley C.P. Exploring feeding behaviour in deep-sea dragonfishes (Teleostei: Stomiidae): jaw biomechanics and functional significance of a loosejaw. Figure 1 Light Microscopy of Dragonfish Teeth Show full caption (A) Head of a freshly collected specimen showing the row of teeth erecting outward from the jaw. Arrow points to the barbel. Image taken of the head in filtered seawater and captured using a polarizing filter. (B) Close-up, showing optical evidence of concentric layers and hollowness of the tooth. Image taken in filtered seawater with an immersive lens. Arrow points to the striations seen on the concave side. (C) Tooth imaged in seawater with color line behind to demonstrate transparency. The arrow indicates an air bubble within the hollow cavity of the tooth due to dissection from the jaw. Image captured with a polarizing filter. (D) Tooth under fluorescence excited at a broadband excitation 440–490 nm and collected with a long-pass filter (>515 nm). This image shows little fluorescence. Image taken of a dry specimen in air. Deep-sea dragonfish, belonging to the family Stomiidae, are apex predators that typically feed on smaller myctophiform and gonostomatid fishes.Thus, they have proportionately enormous jaws capable of a special mechanism of opening and closure referred to as “loosejaw.” This mechanism allows the dragonfish to open its jaw to a greater extent than a conventional jaw of similar size, and enables it to ingest preys up to 50% of its own size.This design allows for effective predation as the dragonfish can ingest large prey for sustained energy, which are often scarce in the deep sea. It has also been reported that the jaw muscles of the dragonfish are weakand therefore that the large fangs must be sharp, as confirmed by their slim and pointed outline ( Figure 1 ), to pierce prey effectively. These teeth, although “large” relative to the jaw, and in fact sticking out of it in many cases, are difficult to see because they range from translucent to transparent for light depending on whether in air or water, respectively.

Figure 2 Optical Properties in Transmittance and Reflectance Show full caption (A) μ-CT scan of a longitudinal cross-section of a representative tooth. The color mapping indicates relative density signifying degree of mineralization. Hotter colors (red) are more dense while cooler colors (blue) are less dense. Dashed lines represent relative locations where analysis was performed. (B) Transmittance for three sections of one tooth (tip, middle, base) with respect to wavelength. (C) Reflectance for three sections (tip, middle, base) with respect to wavelength. See also Figure S1 In terms of spectral translucency, the teeth analyzed in filtered seawater showed light transmittance that changed gradually with the color spectrum, increasing gradually from∼38% in the blue range to ∼73% of light in the red range ( Figure 2 B). This is a significant increase of transmittance when compared with analysis done in air, which showed transmittance around ∼32% of light in the blue range to ∼35% of transmittance in the green-red range ( Figure S1 ). Although a limited amount of this difference could come from technical challenges of capturing light transmittance in water rather than air, the difference is likely to originate mainly from differences of refractive indices between the inside of the tooth material and the outside medium (water versus air). In water, the interfaces with the tooth material show a smaller difference of refractive index than for air, causing light to be less diffracted sideways when penetrating and existing the tooth, resulting in it becoming more transparent than in air. To analyze the effect of tooth material (tooth thickness and relative mineralized density) on the transparency, we performed hyperspectral analysis at three different locations along the tooth (tip, middle, base) considering that the micro-computed tomography (μ-CT) data ( Figure 2 A) clearly showed a gradient structure along the length of the tooth. The base is thicker and less mineralized (indicated by the cooler colors) while the tip is thinner and more mineralized (indicated by the hotter colors). The tip transmits less light in the blue range and more light in the red range when compared with the base and middle sections. As expected, all sections of the tooth transmit less light in the blue range, which is due to an increase in Rayleigh scattering that is inversely proportional to the fourth power of wavelength.

12 Li L.

Ortiz C. Biological design for simultaneous optical transparency and mechanical robustness in the shell of placuna placenta. 13 Carter J.G.

Schneider J.A. Condensing lenses and shell microstructure in Corculum (Mollusca: Bivalvia). 14 Johnsen S.

Widder E.A. Transparency and visibility of gelatinous zooplankton from the Northwestern Atlantic and Gulf of Mexico. 14 Johnsen S.

Widder E.A. Transparency and visibility of gelatinous zooplankton from the Northwestern Atlantic and Gulf of Mexico. The transparency of the teeth of the deep-sea dragonfish Aristostomias scintillans, in terms of both level of transparency and spectral difference, is comparable with that of the shell of the shallow-water oyster Placuna placenta. In this shell, transmittance ranges from ∼20% in the blue range up to ∼80% in the red range, which is homogeneous spatially for various thicknesses (71–660 μm).P. placenta is a shallow-water oyster. Transparency in shallow waters where sunlight is abundant is critical for camouflage but also for sunlight to reach photosymbionts that could live under shell structures.At such great depths where the dragonfish is found, there is only light from bioluminescence, which causes the effectiveness for transparency to increase dramatically.For example, the contrast threshold for a cod's eye increases from 0.02 at light intensities found at 200 m to 0.5 at 650 m, where it would be unable to detect tissues with a transparency greater than 50%.This suggests that the transparent teeth, while comparable with a shallow-water oyster, are much more effective in the greater depths of the ocean.

While transmittance is an indicator of transparency by measurement of how much light can pass through a material without being lost (e.g., via scattering, absorbance), it is also valuable to analyze reflectance. Reflectance is important from an ecological standpoint as it signifies the amount of light that would shine back from the tooth surface and can therefore illuminate the fish's presence. Reflectance was analyzed with hyperspectral imaging at three positions similar to those for transmittance (tip, middle, base) to differentiate between structural effects along the tooth. The teeth were analyzed against a white background, and since they are translucent the background influences the reflectance. Reflectance near 100% indicates how closely matched to the background the material is (i.e., the more transparent it is). Anything less than 100% reflectance suggests that the light was lost due to various mechanisms such as scattering and absorbance. The teeth analyzed showed reflectance in filtered seawater from 39% of light in the blue range to 86% of light in the red range ( Figure 2 C). The base of the tooth had the greatest reflectance of 70% in the blue range and 86% in the red range. Again, reflectance in the blue range was decreased due to an increase in Rayleigh scattering for blue light. The reflectance analysis shows that the teeth match fairly well to the white background and therefore are not likely to reveal the presence of the dragonfish to either prey or predator.