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First published online October 7, 2008
Journal of Experimental Biology 211, 3315-3322 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.018747
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Multifocal lenses in a monochromat: the harbour seal

Frederike D. Hanke1, Ronald H. H. Kröger2, Ursula Siebert3 and Guido Dehnhardt4,*

1 University of Bochum, General Zoology and Neurobiology, ND 6/33, D-44780 Bochum, Germany
2 Lund University, Department of Cell and Organism Biology, Zoology Building, Helgonavägen 3, S-22362 Lund, Sweden
3 University of Kiel, Research and Technology Centre West Coast, Werftstraße 6, D-21542 Büsum, Germany
4 University of Rostock, Institute for Bioscience, Sensory and Cognitive Ecology, Albert-Einstein-Strasse 3, D-18059 Rostock, Germany


Figure 1
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  		Fig. 1. Schematic representation of the experimental setups used to measure harbour seal lenses. (A) IR-Photoretinoscopy under water. A retinoscope (R) with 13 IR-LEDs arranged eccentrically and a knife edge (edge of metal shield in the lens aperture; KE) arranged horizontally was attached to a video camera (C). IR-light entered the eye, was partly reflected and caused variation in brightness in the pupil. The seals were trained to lower their heads into an aquarium from an elevated platform. (B) Schlieren photography. White light (point source, cold light laboratory lamp, 3200 K) was reflected by a beam splitter into the optical axis. The lens, immersed in PBS (pH=7.4, 290 mosmol, 20°C), focused the light beam on a diffuse reflector. Reflected light was focused by the lens on the pinhole and recorded by a camera. (C) Laser scan. The lens was positioned on a holder in the middle of the immersion bath filled with PBS to which a small amount of microparticles was added. A 5 mW green (537 nm) diode-pumped, solid-state laser was used to scan through a meridional plane of the lens. The upwards scattered light was recorded by a video camera.

 

Figure 2
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  		Fig. 2. Harbour seal `Enzo' during underwater photorefractive measurements. The seal immersed its head into an aquarium placing its snout on a red ball (target). This way, the seal's eyes came close to the aquarium's front window. Pupils are almost completely circular because measurements were performed in darkness.

 

Figure 3
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  		Fig. 3. Dissection of harbour seal lenses. (A) Lens lying on the vitreous humour after removal of the cornea and iris, and after cutting the zonular fibre ring. (B) Posterior half of the lens showing the tight coupling between lens and vitreous. The lens was indirectly handled with a pair of forceps holding a small piece of sclera with some attached zonular fibres and the lens. Scale bar, 5 mm.

 

Figure 4
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  		Fig. 4. Photorefractive images of a young (3 years old) harbour seal under water axially refracted with lateral (A) and frontal camera position (B) and an older (12 years old) harbour seal under water (C) and in air (D). On the underwater pictures (A–C), two rings in the centre and one pronounced ring in the periphery can be seen. Corneal cloudiness in the older seal leads to a central stripe in the brightness distribution under water (C), and irregularities in corneal topography to spot-like distortions in air (D) that mask the slightly visible rings.

 

Figure 5
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  		Fig. 5. Results of schlieren photography. For each seal, one typical picture is presented in chronological order of measurement (A, picture of seal 1; B, seal 2; C, seal 3; D, seal 4). Coloured rings and lens sutures are prominent.

 

Figure 6
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  		Fig. 6. Results of laser scanning displaying the longitudinal spherical aberration (LSA) curves of harbour seal lenses measured at 537 nm. (A–D) LSA curves of the right eyes of seal 1 (A), seal 2 (B), seal 3 (C) and seal 4 (D). Long arrows mark peaks, short arrows mark steep declines in back centre distance.

 

Figure 7
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  		Fig. 7. (A) Mean LSA curve of all lenses except for the neonate's lenses. Two peaks at 0.67 R and 0.87 R are visible (long arrows). Note the steep decline in back centre distance in the periphery (short arrow). (B) An example picture of a spherical harbour seal lens deflecting the laser beams. Scale bar, 5 mm.

 

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© The Company of Biologists Ltd 2008