TACTILE STIMULI IN THE SOCIAL BEHAVIOR OF PAGURUS BERNHARD US (DECAPODA, PAGURIDAE) by

BRIAN A. HAZLETT 1) (Department of Zoology, University of Michigan, Ann Arbor, Mich., U.S.A.) (With 7 Figures) (Rec. 21-VI-1969)

INTRODUCTION

In a series of papers on the social behavior of hermit crabs (HAZLETT, 1966a, b, I968a, b, c; HAZLETT & BOSSERT, 1965, 1966), much emphasis was placed on the importance of visual stimuli. A number of agonistic displays were described and their communicatory functions investigated. However, the chemical and tactile modalities are of potential importance in these crabs as in other marine crustaceans (COHEN & DIJKGRAAF, I96i; BARBER, Ig6I). The use of some sort of tactile-proprioceptive information has been impli- cated in the shell selection behavior of hermit crabs by the work of REESE (1962) and V6LKER (1967). The following series of experiments were carried out to more completely outline the type of stimuli that are important in the social behavior of one species of hermit crab. The species studied was Pagurus bernhardus (L.), the common hermit crab of Northern European waters. Several aspects of the social behavior of this species have been reported elsewhere (HAZLETT, 1967, 1968a, b, d). The specimens used in the following experiments were collected by dredge in the waters near Fiskebackskil, Sweden and were maintained at the Kristineberg Zoological Station in Io-I5 gallon aquaria supplied with running sea water. The bottoms of the aquaria were covered with calcareous sand. Animals were fed pieces of fish or mollusc daily. The experiments were done during day- light hours in June, July and August of 1968, when the natural daylength is quite long in Sweden. Animals were always discarded after use in any one test to eliminate experimentally induced differences in test animals.

1) Contribution from the Kristineberg Zoological Station, Fiskebäckskil, Sweden. This work was supported by a grant from the National Institutes of Health, No. MH-02847. Thanks are given to Drs B. OAKLEY,A. PROVENZANO,J. MARKHAM, and W. HERRNKIND for their comments on the manuscript. 21

The social behavior patterns investigated are grouped into three categories: sexual behavior, shell fighting behavior, and agonistic behavior other than shell fighting (= general agonistic behavior). The particular acts studied and experiments carried out will be described under these headings.

METHODS AND RESULTS

GENERAL AGONISTIC BEHAVIOR

Cheliped Flicks.

As is true for a number of species in the family Paguridae (HAZLETT, T966a, b), individuals of Pagurus bernhardus can often be observed to move rapidly one or both cheliped mani out-and-back through an arc of about 30°. Only the manus is moved and only very rarely is the limb held in the "out" position for any appreciable length of time. The act is most common when a crab that was withdrawn into its gastropod shell is grasped by another crab. The execution of "flicks" appeared to increase the probability of the grasping crab releasing the shell of the flick-executing crab. The flicks can be executed by either cheliped one at a time or by both simultaneously.

Visual Inhibition of Flicks.

While observing interacting crabs, it was seen that flicking sometimes continued for several seconds after the offending crab had released the "flicker". However, flicking was never observed when the touched crab was up in its shell in a position which exposed the eyestalks. Thus it appeared that visual input might inhibit flicking. It had been found (see below) that flicks could be elicited by touching the surface of the appendages of a with- drawn crab with a glass rod drawn out to a point about one millimeter in diameter. To test the influence of light on flick execution crabs were tested under several conditions. Ten crabs were touched with a glass pointer in a standard pattern (see below): ( i ) when withdrawn into their gastropod shells, and (2) when they were up in the aperture of the shell with eyestalks visible to the experimenter; animals were touched in these two positions under three conditions: when the animals were (r) normal, (2) after blinding animals (five specimens) by cutting off both eyestalks about half way down, and (3) when animals (five specimens) were in an aquarium placed in a photographic darkroom, with only the red safety light on (to enable observa- tion of crab movement). Flicks were often executed by animals withdrawn in their shells when touched with the glass pointer. But, repeated series of touches on the appen-