Studies on reflexion of light from silvery surfaces of fishes, with special reference to the bleak, Alburnus alburnus

Abstract

Plates I-V and Text-figs. 1-12)The problem of how a fish can make itself invisible in the natural light-conditions in an aquatic environment is discussed with particular reference to the silvery surfaces of fish.In fish which we have examined, the silvery surfaces are of two types: (1) an argenteum which consists of long thin crystals of guanine whose reflecting surfaces are approximately parallel with the surface of the fish; (2) layers of guanine crystals lying either on the inner surfaces of the scales or in the subdermis-these crystals are not, in general, orientated with their reflecting surfaces parallel with the surfaces of the fish, and are much broader than those of the argenteum.Methods are described by which the orientation of the crystal planes with respect to the planes of the scales on which they lie can be determined.The orientation of the crystals of type 2 in different parts of the body is described for the horse mackerel, Trachurus trachurus (L.), and for the bleak, Alburnus alburnus (L.).For the bleak it is shown that although the planes of the crystals are often very much inclined with respect to the planes of the scales, the long axes of the crystals are always approximately parallel with the planes of scales. The inclination of the crystals, therefore, is away from the scales across their short axes.Measurements of the light transmitted by silvery scales of the bleak show that they reflect light strongly when this falls obliquely on the crystals which they contain and that they are most transparent to light which strikes the scales in a direction perpendicular to the reflecting planes of the crystals.The high reflectivity of most of the fish to light striking its surfaces at normal incidence is explained by the overlap of the silvery parts of different scales and by the argenteum underneath the scales.

To see and not be seen:the visibility of predator and prey with respect to feeding behaviour

“…Clupeids adapt their pigmentation to the sea floor and refuse to pass over mismatching ground (Breder 1959, Nicol 1961. In agreement with Denton & Nicol (1965a, b, 1966, these results show that herring have evolved an excellent strategy for staying invisible not only to horizontally scanning, but also to downward scanning eyes. When the herring is located 30 to 90" below the mysid, the prey is fairly visible for the predator but the predator is nearly invisible to the prey.…”

“…Such silvery fish are only conspicuous when viewed from below ( Fig. 6; 210 to 330") (Denton & Nicol 1965a, b, 1966. Some mesopelagic fish, cephalopods, and euphausiids compensate for this silhouetting by means of downwelling, biogenic light produced by the luminophores on their ventral sides (Clarke 1963, Hastings 1971, Young & Roper 1976.…”

Effects of exogenous thyroid hormones on visual pigment composition in coho salmon (Oncorhynchus kisutch)

“…Crypsis in freshwater streams is accomplished by earthy and dark coloration patterns that match the substrate, therefore prey detection would be optimized by having VPs matched to the spectral background. In the open ocean, crypsis is accomplished by being transparent, or silvery, to reflect the monochromatic background light (Denton and Nicol, 1965;Johnsen, 2002;McFallNgai, 1990). The long-wavelength shift in MWS cone λ max would offset the VP improving the detection of brightly colored prey.…”

Spectral and spatial properties of polarized light reflections from the arms of squid (Loligo pealeii) and cuttlefish (Sepia officinalisL.)

“…To determine the physical basis of the optical properties of the arm stripes, the fine structures of the iridophores were studied using electron microscopy. Denton and Nicol (Denton and Nicol, 1965). Due to the tendency of the dermal tissue to lose its transparency over time, the collection of spectra and images occurred within 5·h following the sacrifice of the animal.…”