Optics of the eyes ofPhronima and other deep-sea amphipods

Summary

• 1. Phronima, and many other oceanic hyperiid amphipods, have double eyes in which the medial (dorsal) part has a small field of view and large facets, and the lateral (ventral) part a large field and small facets (Fig. 6).

• 2. The eyes are of the apposition type. Each crystalline cone forms an image at its proximal tip (Fig. 3), which in most hyperiids abuts directly onto the rhabdom. The exception is the medial eye ofPhronima where light from the image is conveyed via a narrow light-guide to the retina 5 mm ventral to the eye surface (Fig. 2). Living eyes all show pseudopupils (Fig. 4).

• 3. It is shown by interference microscopy that the crystalline cones inPhronima eyes are inhomogeneous (Figs. 7–8, 11–13). Medial eye cones have a V-shaped refractive index gradient, densest at the distal tip (1.40). Lateral cones are dense at both distal (1.38) and proximal (1.45) ends, with a less dense zone in between. Ray tracing (Figs. 9 and 13) confirms that images are formed at the proximal ends of both types of cone.

• 4. Measurements made on the pseudopupil and from ray tracing confirm that in the medial eyes the acceptance angles of single light-guides are larger than the inter-ommatidial angles by a factor of 8 (Δp=3.5 °, Δφ=0.44 °). This means that as many as 60 rhabdoms in each medial eye view the same point in space. There is no such discrepancy in the lateral eyes, where Δp≃15 ° and Δφ is 9–12 °. The entire field of view of each medial eye (approximately 10.5 by 11 °) is about the same as that of a single ommatidium in the lateral eye.

• 5. It is argued in the Discussion that, whilst a high Δp: Δφ ratio would be a disadvantage if a textured background had to be resolved, it is a good solution to the problem of locating single small objects in dim light,provided that all the rhabdoms that view the same point in space pool their signals. A substrate for such pooling appears to exist in a plexus overlying the lamina of the medial eye, which is not present in the lateral eye lamina (Fig. 16).

• 6. The structure of the eyes of the deep-livingCystisoma, which has no lateral eyes, differs fromPhronima in that the retina is curtain-like, with widely separated, unscreened rhabdoms, instead of being a small dark condensed structure. It is suggested that these represent alternative ways of making eyes that are minimally visible to potential predators (Fig. 17).