What Firefly Squids See The Retina of Watasenia scintillans

Watasenia scintillans is a small species of squid belonging to the family Enoploteuthi-dae. The adults reach only 60 mm in body length and spend most of their time living in semidarkness at a depth of about 400m. Watasenia is unique in possessing three different visual pigment chromophores, which combine with a protein (opsin) to form three different visual pigments. Each photoreceptor cell contains only one of these visual pigments, but two types of photoreceptors receive light from the same part of the visual field. This is compelling evidence that Watasenia is capable of distinguishing different wavelengths of light (Michinomae et al., 1994).

The ventral region of Watasenia's retina is highly specialized (Seidou et al., 1990, 1995; Michinomae et al., 1994). There are some unusually long (600 mm) photore-ceptors and an arrangement of rhabdoms in well-defined layers, which have been compared to the banked retina of certain deep-sea fishes (Denton and Locket, 1989). The nonspecialized part of Watasenia's retina appears similar in cross-section to any other cephalopod retina, with rhabdoms consisting of microvilli aligned in two orthogonal planes, and a visual pigment with a (wavelength of maximum sensitivity) of 484 nm and retinal (A1) as the chromophore. The outer layer of the ventral retina is similarly organized (figure 9.22), except that 4-hydroxyretinal (A4) is the chro-mophore, with a 1nax that is blue shifted to 470nm (the a cells of Michinomae et al., 1994). In deeper layers of the ventral retina there are two additional sets of rhabdoms (from b and g cells) with orthogonal microvilli offset by 45 deg from those of the distal rhabdoms; and the most proximal layer has a fourth, loose type of rhabdom produced by § cells. The latter contain a visual pigment with dehydroretinal (A2) as its chromophore, with a 1nax red shifted to 501 nm (Michinomae et al., 1994). The overlying

Figure 9.22

Diagram of the ventral region of the retina of the firefly squid, Watasenia scintillans, to illustrate the arrangement of the outer segments of the four photoreceptor cell types (a, b, g, and S) with depth. (a) Block stereogram of the retinal elements. (b) Cross-sectional patterns at the levels indicated in (a). (c) Representation of the relative positions of the outer segments of the four cell types. The scale at the right indicates the depth from the surface of the retina. The longest cells become much narrower distally, producing smaller-sized rhabdoms (a'). The parallel lines indicate the orientation of microvilli. BM, basement lamina; N, nucleus in photoreceptor cell body. (Reprinted with permission from Michinomae et al., 1994.)

Figure 9.22

Diagram of the ventral region of the retina of the firefly squid, Watasenia scintillans, to illustrate the arrangement of the outer segments of the four photoreceptor cell types (a, b, g, and S) with depth. (a) Block stereogram of the retinal elements. (b) Cross-sectional patterns at the levels indicated in (a). (c) Representation of the relative positions of the outer segments of the four cell types. The scale at the right indicates the depth from the surface of the retina. The longest cells become much narrower distally, producing smaller-sized rhabdoms (a'). The parallel lines indicate the orientation of microvilli. BM, basement lamina; N, nucleus in photoreceptor cell body. (Reprinted with permission from Michinomae et al., 1994.)

rhabdoms containing A4 act as a short-wavelength cutoff filter, shifting the l^« of the A2-containing cells another 50nm, to 550nm, which coincides with the peak wavelength of bioluminescence produced by green photophores in the ventral skin (figure 9.23) (Seidou et al., 1995).

It is therefore interesting to note that any color sensitivity in the eyes of blue-water oceanic species has been predicted to be a blue-green opponent system, since the presence of yellow substances in the water is always low, and red light varies the most in quantity with depth in the water column (Lythgoe, 1976). However, the fact of the basic polarization sensitivity of the cephalopod retina raises questions about the role of different wavelength sensitivities in the firefly squid's visual system.

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