Optical Scanning in the Lateral Eyes of the Copepod Copilia

Abstract

The lateral eyes of the female Copilia are exceptionally large for a creature of its size but have very few receptors. These receptors appear to scan the image plane of the anterior lenses of the eye, but until now there has been no behavioural evidence linking their movement positively with the functioning of the eyes, though there is some (disputed) evidence that it is merely a side-effect of peristalsis. On channel-capacity grounds we might not have expected optical scanning to occur in biological systems, so any positive evidence for scanning, in the engineering sense, is particularly interesting. Optical measurements of the position of the image plane, in live intact female Copilia quadrata, generally corroborate Exner's (1891) finding that an image is in focus at the plane of the distal ends of the receptors, though the position of best focus seems slightly in front of a second lens, behind which the receptor structure lies. An alternative optical schema of Wolken and Florida (1969) is disproved. The position of focus satisfies minimal optical conditions for scanning. Movements of a coloured stripe pattern, used as a test object for the optical measurements, elicited the putatively scanning movements of the receptor structure, suggesting strongly that these really are an intrinsic part of normal visual functioning, not an epiphenomenon. It was also discovered that intermittent swimming movements of the limbs tend to be preceded by these photoreceptor movements. Both findings support the hypothesis that Copilia's eye is an optical scanning system.

Vertical distribution and diel migration in the iridescent copepods of the family Sapphirinidae:a unique example of reverse migration?

“…Males of Copilia have only the median eye in which the 3 cups are together with 2 smaller lenses (Elofsson 1966, 1969, Moray 1972, Land 1984. The lateral eyes are of the 'scanning' type which have been suggested to have an image-forming function (Gregory et al 1964, Elofsson 1969, Wolken & Florida 1969, Downing 1972, Land 1981, 1984). If we assume that the well-developed eyes of the sapphirinids, particularly in the females, are used for searching for conspecific mates, then the male iridescence can be considered as a mechanism, at least in part, of increasing male visibility.…”

The Evolution of Eyes

“…Its eyes are constructed st.rangely (Figures lg and 3b), and they move to and fro in the longitudinal plane, thus scanning the water in front of the animal (Exner 1891). Each eye has two lenses that are arranged like a telescope: A large, long focal length "objective" lens forms an image on or close to a second, short focal length "eyepiece" lens immediately in front of the cluster of five to seven photoreceptors (Wolken & Florida 1969;Downing 1972). The second lens and receptors move together as a unit during scanning.…”

“…Each eye has a field of view only about 3° wide, and within this the image is probably not further resolved. Although the eyes move back and forth through about 14° (Downing 1972), they still only scan a minute fraction of the space around the animal (Figure 3b), which raises the obvious question, What can Copilia find to look at? Moray (1972) points out that although the scanning movements are horizontal, the predominant movements of the plankton on which Copilia feeds are vertical.…”

Tête Mates 2

“…Using a low-power microscope we could hardly avoid seeing scanning. This was virtually proved by my colleague Tony Downing, on our second visit, when he showed that the moving inner lenses do lie at the plane of focus of the large anterior fixed lenses, which had been a matter for doubt (Downing 1972). By that time, if there was an alternative to scanning we could not see it.…”