Abstract
Cephalopods have very conspicuous eyes that are often compared to fish eyes. However, in contrast to many fish, the eyes of cephalopods possess mobile pupils. To increase the knowledge of pupillary and thus visual function in cephalopods, the dynamics of the pupil of one of the model species among cephalopods, the common octopus (Octopus vulgaris), was determined in this study. We measured pupillary area as a function of ambient luminance to document the light and dark reaction of the octopus eye. The results show that weak light (<1 cd/m2) is enough to cause a pupil constriction in octopus, and that the pupil reacts fast to changing light conditions. The t50-value defined as the time required for achieving half-maximum constriction ranged from 0.45 to 1.29 s and maximal constriction from 10 to 20% of the fully dilated pupil area, depending on the experimental condition. Axial light had a stronger influence on pupil shape than light from above, which hints at a shadow effect of the horizontal slit pupil. We observed substantial variation of the pupil area under all light conditions indicating that light-independent factors such as arousal or the need to camouflage the eye affect pupil dilation/constriction. In conclusion, the documentation of pupil dynamics provides evidence that the pupil of octopus is adapted to low ambient light levels. Nevertheless it can quickly adapt to and thus function under brighter illumination and in a very inhomogeneous light environment, an ability mediated by the dynamic pupil in combination with previously described additional processes of light/dark adaptation in octopus.
Highlights
The cephalopods are a molluscan class that differs from other members of the phylum by a number of characters such as the anatomy of the body and the organization of the nervous system
Pupillary reactions were documented in one wild-caught (Tuscan Archipelago of the Mediterranean Sea), female adult common octopus, Octopus vulgaris, with a mantle length of 6.5 cm
The pupil constricted within less than 1 s (Figure 2; all measurements can be found in Supplementary Figure 1 and Figure 3)
Summary
The cephalopods are a molluscan class that differs from other members of the phylum by a number of characters such as the anatomy of the body and the organization of the nervous system. One of the most prominent characteristics of cephalopods are their eyes (for review see Packard, 1972; Messenger, 1979; Land, 1984; Budelmann, 1994, 1996). They are large and often actively scanning the animal’s surrounding. A conspicuous feature of the cephalopod eye is its pupil, which is peculiarly shaped in some species. Pupil shape varies from horizontal to U- or W-shaped in bright light depending
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