ORIENTATION TO SHELL MOVEMENT BY TRICOLOR (GIBBES) (, , ) 1)

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BRIAN A. HAZLETT Department of Zoology, University of Michigan, Ann Arbor, Michigan, U.S.A.

INTRODUCTION

The frequency of occurrence of social interactions will be influenced by orga- nisms' locomotory movements with regard to each other. As individuals move close to one another, the probability of more complex communication increases. In assessing the social behavior of an organism, its initial orientation to a conspecific must be considered. Hermit crabs inhabit empty gastropod shells and much of their behavior is influenced by shell occupation (Hazlett, 1970, 1971; Kinosita & Okajima, 1968; Reese, 1962, 1963; Volker, 1967). Several workers have considered the question of visual orientation of hermit crabs towards shells. Ten Cate (1930) and Rabaud (1941) both reported that while crabs oriented visually towards small objects in their environment, they could not discriminate shape at a distance. Reese (1962) and Hazlett (1971) independently observed that visual orientation does not occur in the glaucothoe stage but is operative in the first crab instar. Hazlett (1966a) found that Calcinus tibicen (Herbst) tended to move toward a shell facing away and avoid a shell facing toward the crab. Reese (1963) found greater movement towards shells which visually contrasted with the background in Pagllrlls sal7771elif (Stimpson).

While watching groups of Clibanariitf tricolor in tide pools, appeared that orientation of crabs toward snail shells (occupied by either snails or hermit crabs) was influenced by size relationships and locom.o,tory movement of the snail shell. The particular sequence of behaviors that suggested the experiments outlined below was that of a large individual of C. tricolor (i) moving toward a much . smaller, moving shell (snail or crab), (2) touching it with one ambulatory leg, (3) the movements ceasing, and (4) the large crab moving on without further interaction. This sequence was seen a number of times. Interactions between crabs of very different sizes was surprising in view of earlier observations on Clibanarius

1) Contribution from the Pine Channel Marine Laboratory, Little Torch Key, Florida. Thanks are given to Stan Becker for his aid during the course of the research and to Ernie Reese, John Childress, and Dan Rittschof for their commentson the manuscript. 272 vittatu.r (Bosc) (Hazlett, 1968) which indicated that such interactions were rare. Since shell movement was not previously considered in the visual orientation of marine hermits toward shells, the following experiments were conducted.

MATERIALSAND METHODS

The experiments were carried out in the field, in a series of tide pools along the west shore of West Summerland Key, Florida near the base of the old Bahia Honda bridge. The crabs were tested during January and February 1973, only at low tides, when the tide pools were largely isolated from wave action. Tests were made primarily during the time of rising tide since Clibanariu.r tricolor becomes more active as the tide brings new water into the tide pools.

Fig. 1. Most common path of locomotion for Clibanariu.r tricolor (Gibbes) on uniform substrate (solid line). Movement toward presented gastropod shell represented by dashed line. Movement of the gastropod shell indicated by the small arc. Scale line 5 mm.

Each test consisted of placing a shell of Cel'ithiopsiJ emer.ronii (C. B. Adams) (12 mm long, 6 mm wide at widest part) which was glued to the end of a glass rod (5 mm diameter, 33 cm long) in front of a crab and the crab's movements recorded. The shell was placed facing the crab, two cm away, usually to the right of the crab. Crabs which were relatively isolated were chosen for testing. When moving on a flat, uninterrupted surface in the tide pools, individuals of C. tricolor usually move in a line about 30° to the left of a line parallel to the anterior- posterior axis of the crab (see fig. 1). When a crab was on an uneven surface, the test shell was placed in a position such that it was judged unlikely that the crab would have crawled in that direction. Before each presentation, the crab to be tested was ca,tagorized as smaller, larger or the same size (about four mm cephalothorax length) as a crab which could have inhabited the test shell. For each test, the shell was either held stationary or was moved. The movements were