Direct response of the crab Carcinus to the movement of the sun.

One line of evidence points to the conclusion that eye responses should be measurable in a crab, in air, which sees only the movement of the sun in a clear sky. It has been shown in the previous paper, as an extension of earlier work with slowly moving striped drums, that eyestalk movements follow a single light in a dark room when the light is moved with low angular velocities round the crab in the range io-20°/hr. For horizontal movements which subtend angles of 0-02-1-o° at the crab's eye, at a lamp velocity of o-oi°/sec., the horizontal eye movement is 20-50% of the light's movement. The light used gave an illumination of 0-05 lux on a surface in the position of the crab's eye. These figures remove any doubt that the crab is able to see the movement of the sun directly. The movement of the full moon, with a maximum illumination of 0-24 lux on the earth's surface, must also be seen by the crab, and possibly some of the brighter stars are also seen directly. Whether the crab responds to these moving lights will depend, not only on whether it sees them, but also on whether there are stationary contrasting objects in the visual field. If the eye begins to follow an isolated moving stimulus, other contrasting objects will move in the opposite direction over the eye, and this will at once diminish the response by an opposite stimulus. Therefore, to make sure of recording a maximum response to celestial movements the animal should see as little as possible besides the object of interest. That several crustaceans give compass reactions to the sun has been known for some time. The most remarkable results are those of Pardi & Papi (1961) in a series of observations on the littoral amphipods Talorchestia and Talitrus. They find that in the absence of any landmarks except the sun these animals are able to head correctly towards the sea when too dry or towards the land when submerged, and that the position of the sun at different times of the day is taken into account by reference to an internal rhythm within the animal. More difficult to explain is the finding that the moon can also be used as a reference in the same orientation response. Papi (i960) explains this by supposing the existence of a separate rhythm which is linked to the moon, but the method outlined in the present paper seems equally effective in providing a compass direction by moonlight without requiring a lunar rhythm. Although fishermen's stories on the subject are common, the only scientific reference to a crab's ability to escape towards the sea when released on land is in an unpublished thesis (van Tets, 1956) quoted by Pardi & Papi (1961), showing that the Pacific shore crab Hemigrapsus utilizes some feature of the path of the sun as a basis for directional escape towards the sea. Drzewina (1908) found that Carcinus released on a beach goes to the nearest shadow.