Macropinna microstoma and the Paradox of Its Tubular Eyes
The opisthoproctid fish Macropinna microstoma occupies lower mesopelagic depths in Monterey Bay and elsewhere in the subarctic and temperate North Pacific. Like several other species in the family, Macropinna has upward-directed tubular eyes and a tiny, terminal mouth. This arrangement is such that in their upright position, the visual field of these highly specialized eyes does not include the mouth, which makes it difficult to understand how feeding takes place. In situ observations and laboratory studies reveal that the eyes of Macropinna can change position from dorsally-directed to rostrally-directed, which resolves the apparent paradox. The eyes are contained within a transparent shield that covers the top of the head and may provide protection for the eyes from the tentacles of cnidarians, one of the apparent sources of the food of Macropinna.Abstract
Video frame-grab of Macropinna microstoma at a depth of 744 m, showing the intact, transparent shield that covers the top of the head. The green spheres are the eye lenses, each sitting atop a silvery tube. Visible on the right eye, just below the lens on the forward part of the tube, is the external expression of a retinal diverticulum. The pigmented patches above and behind the mouth are olfactory capsules. High-definition video frame grabs of Macropinna microstoma in situ are posted on the web at: http://www.mbari.org/midwater/macropinna.
Lateral views of the head of a living specimen of Macropinna microstoma, in a shipboard laboratory aquarium: (A) with the tubular eyes directed dorsally; (B) with the eyes directed rostrally. The apparent differences in lip pigmentation between (A) and (B) are because they were photographed at slightly different angles. (A) was shot from a more dorsal perspective and it shows the lenses of both eyes; the mouth is not sharply in focus. (B) shows only the right eye, with the lips in sharper focus.
Chapman's (1942) mesial view of the left eye of Macropinna microstoma. Abbreviations: RS = rectus superior, L = lens, OS = obliquus superior, OI = obliquus inferior, RIN = rectus internus, RI = rectus inferior, RE = rectus externus, OP = optic nerve.
Contributor Notes
Associate Editor: J. F. Webb.