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A note on the elongated eye of the octopus Vitreledonella richardi

  • Michael F. Land (a1)

Extract

The elongated eye of Vitreledonella richardi is described from two specimens caught at 200 m in the North Atlantic. It is shown that the cylindrical shape, with the lens placed centrally, results in a reduced field of view in the horizontal plane. It is argued that the unusual shape is an adaptation which minimises the silhouette of the eye as seen from below, and is part of the animal's camouflage strategy.

Introduction

The majority of fish and cephalopods have two hemispherical eyes, each of which has a field covering roughly the 180° on one side of the animal (Walls, 1942; Packard, 1972). The commonest exceptions to this rule are animals with so-called ‘telescopic’ eyes, found in many species of deepsea fish (e.g. Benthalbella, Argyropelecus, Opisthoproctus, Scopelarchus, see Marshall, 1954; Locket, 1977) and some cephalopods (Amphitretus, see Chun, 1915; Histioteuthis sp., see Young, 1975a). These eyes are tubular, with the lens at the open end of the tube, and a retina with a reduced field of view at the other. The eyes usually point upwards, and it is generally agreed that their shape is determined by the need to concentrate sensitivity and resolution into the rather narrow field above the animal where the residual daylight is still adequate for visual detection. It is 200 times darker in the downward direction (Denton, 1990), and so there must be some depth at which upward vision works, but downward vision does not. These are not the only kind of tubular eyes, however. In some cephalopods, but not to my knowledge in any fish, there are eyes of tubular appearance but with the lens not at the end, but in the centre of the tube (Figure 1).

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References

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Chun, C. 1910. Die Cephalopoden. I. Oegopsida. ‘Valdivia’ Report, XVIII, pt. 1. Jena: Fischer.
Chun, C. 1915. Die Cephalopoden. II. Myopsida, Octopoda. ‘Valdivia’ Report, XVIII, pt. 2. Jena: Fischer. [Both parts translated and reprinted, 1975. Israel Program for Scientific Translations. Jerusalem: Keter Publishing House.]
Denton, E.J. 1990. Light and vision at depths greater than 200 metres. In Light and life in the sea (ed. Herring, P.J. et al), pp. 127148. Cambridge: Cambridge University Press.
Locket, N. A. 1977. Adaptations to the deep-sea environment. In Handbook of sensory physiology, vol. VII/5 (ed. Crescitelli, F.), pp. 67192. Berlin: Springer Verlag.
Marshall, N.B. 1954. Aspects of deep sea biology. London: Hutchinson.
Packard, A., 1972. Cephalopods and fish: the limits of convergence. Biological Reviews, 47, 241307.
Walls, G.L., 1942. The vertebrate eye and its adaptive radiation. Bloomfield Hills, Michigan: Cranbrook Institute. [Reprinted, 1963. New York: Hafner.]
Young, J.Z., 1962. The retina of cephalopods and its degeneration after optic nerve section. Philosophical Transactions of the Royal Society of London (B), 245,118.
Young, R.E., 1975a. Function of the dimorphic eyes of the midwater squid Histioteuthis dofleini. Pacific Science, 29, 211218.
Young, R.E., 1975b. Transitory eye shapes and the vertical distribution of two midwater squids. Pacific Science, 29, 243255.

A note on the elongated eye of the octopus Vitreledonella richardi

  • Michael F. Land (a1)

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