OBSERVATIONS ON THE ESCAPE RESPONSE IN NASSARIUS VIBEX (SAY), (MOLLUSCA: GASTROPODA) 1 ROBERT H. GORE Institute of Marine Science, University of Miami ABSTRACT Observations were made on the escape response of Nassarius vibex. This reaction, consisting of a violent flipping movement, is used by the snail whenever certain predatory sea stars or molluscs make contact with it. Contact situations involving the gastropods Fascialaria tulipa, F. hunteria, and the sea star Luidia alternata with N. vibex elicited the reaction and were investigated. The escape response is discussed and a possible relation- ship between righting behavior and escape response is suggested. INTRODUCTION The escape reaction that is known to occur in prosobranch snails of the genus Nassarius commonly found in the intertidal region, has received particular, though infrequent mention in the literature. Tryon (1882) makes first brief mention of this reaction when he states simply that some "Nassas" spring up and throw themselves over when disturbed. Bauer (1913) reported more completely on this reaction with N assa (=N assarius) reticuLata, and described the flipping reaction as a series of somersaults when the snail was stimulated on the metapodial tentacles by the sea star Astropecten bispinosus. Weber (1924; in Carthy, 1958) illustrated the movement of N. mutabilis as a twisting, side-to-side forward leap instead of a somersault. Hoffman (1930; in Kohn, 1961) said that this response can be induced when the sea star is some distance away, but Bauer (1913) held that contact between sea star and snail was necessary. All three authors agree that contact with the tube feet of asteroids would elicit this response in the snail. Hoffman (1930; in Kohn, 1961) also found that picric acid or chloroform, among other chemicals, similarly elicited an escape reaction in Nassa reticuLata. The escape reaction of molluscs that encounter predatory sea stars or predatory molluscs has been documented by Bullock (1953), Margolin (1964 a, b), Clark (1958), Peters (1964), and Gonor (1965). Gonor described a predator-prey relationship involving Nassarius Luteostoma and its predator Natica chemnitzii. A comparison in the literature of N. Luteostoma, N. reticuLata, and N. mutabilis shows that the escape response is somewhat similar in all three species. It differs only in whether the snail somersaults or leaps diagonally forward. 'Contribution No. 692 from the Institute of Marine Science, University of Miami. 424 Bulletin of Marine Science [16(3) I observed that Nassarius vibex also utilizes an escape reaction when it is stimulated by either the sea star Luidia alternata Say or by the gastropods Fascio/aria hunteria Perry or F. tulipa Linnaeus. This reaction consists of continual thrusts by the foot, which is stretched out until it is very thin. The foot then is used as a lever to push the snail over in a series of somer- saults. The foot swings from side to side as the snail somersaults. This results in a violent flipping motion. These observations, and others in the literature, suggest that the somer- sault or flipping response is a definite escape mechanism used by Nassarius vibex when it is stimulated by potential predators. The purpose of this paper is to report on investigations of both the stimuli and reactions associated with the escape reaction. In addition, a possible relationship between escape and righting behavior is suggested. I am indebted to Drs. Warren Wisby and Arthur Myrberg, and Mr. William Herrnkind, who offered many valuable suggestions and critically read the manuscript. In addition, I gratefully acknowledge the criticisms of Drs. F. M. Bayer and H. B. Moore. Their help was very much appreciated. MATERIALS AND METHODS Experiments were carried out in flowing seawater on a water table and in two 75-liter aquaria. The aquaria were filled with 4 em of sand to allow the snails to burrow. Fluorescent lights were located about 3 m over the tanks and table. Each tank held approximately 300 N. vibex from 5 to 15 mm long. Tests consisted of repeated introduction of various sea stars and molluscs, as well as body fluids from these animals, in the immediate vicinity of N. vibex. Any reactions by Nassarius to sea stars or snails were noted. Table 1 summarizes the results together with reactions noted to other animals presented to Nassarius outside the test situation. REACTIONS TO TEST ANIMALS Individuals of three species of sea star were placed singly into each tank and the water table consecutively in order to observe the flight response of N. vibex to these animals. Repeated introduction of the sea stars resulted in the varied reactions seen. When a 12-cm specimen of Echinaster sentus (Say) was introduced there was no flipping response. This sea star moved slowly compared with Nassarius. The only response noted in the snails throughout repeated introduction was that Nassarius moved from the sea star's path as it approached. Often this sea star would crawl directly over Nassarius, but in most cases, the snails avoided contact. A 14-cm specimen of Astropecten duplicatus Gray also elicited little response. This sea star moved more rapidly than Echinaster, but in only 1966] Gore: Escape Response in Nassarius vibex 425 TABLE] REACTION OF Nassarius vibex TO CONTACT WITH 8 SPECIES OF ANIMALS COLLECTED FROM VARIOUS AREAS IN SOUTH FLORIDA Modeof Reaction Animaltested Placecollected feeding elicited1 Placetested Luidia alternata intertidal carnivore 3,2 Lab & field Astropecten alligator 200 fathoms carnivore ],2,3 Lab A. duplicatus intertidal carnivore 1,2 Lab & field Echinaster sentus fringe reef carmvore 1,2 Lab Fasciolaria tulipa intertidal carnivore 3,2 Lab & field F. hunteria intertidal carmvore 3,2 Lab Natica canrena sub-littoral carnIvore 2,3 Lab Strombus gigas sub-littoral herbivore 1 Lab 'The reaction elicited most is listed first. 1= wilhdrawal inlo shell 2 == active avoidance 3 = flip response two instances was there actual contact between the snails and Astropecten. In both instances the snail bumped into the stationary sea star and then jerked away with a partial somersault. This response may have resulted from contact with the numerous spines along the arm of the sea star. At times, Astropecten also crawled over Nassarius but the snail showed no escape response. A 10-cm specimen of Luidia alternata elicited considerable response. As soon as it had settled to the bottom it began to move rapidly across the sand. Snails which were buried in a resting position emerged immediately before the path of the sea star; usually as soon as the tentacles of the animal touched the extended siphons of the snails. Some snails emerged, however, as soon as Luidia touched bottom. Most Nassarius, if touched on the propodium or siphonal area with tentacles, or, especially the tube feet of Luidia, immediately began violent flipping movements. A few snails simply moved rapidly without flipping from out of the sea star's path or fled before it. But these latter, when touched by tube feet or tentacles of Luidia, would immediately make the flipping response, often turning over five to ten times before righting. Less often they would turn right or left and move rapidly away from the sea star. The gastropod's siphon would be held high and waved over the shell throughout these encounters. Some snails that flipped did so directly into the sea star's path and were overrun. Luidia never stopped when this happened but continued to move quickly across the sand and around the tank. Snails trapped under the arms either continued the flipping action or withdrew deeply into their shells until after the asteroid had passed. Sea stars that touched the posterior of the foot of N. vibex, especially in the region of the metapodial tentacles, rarely elicited a flipping response. The snail seldom varied its direction when stimulated in this manner. The flipping response in N. vibex appeared to be evoked most whenever a 426 Bulletin of Marine Science [16(3) tentacle or, more often a tube foot, of Luidia came in contact with the frontal region of the snaiL This was easily seen in those snails which, in their movement from the sand, emerged directly in front of the sea star and touched its arms. The flipping response was then quick and violent, taking the snail 25 to 30 em away from Luidia. Nassarius then righted itself and continued moving in a normal manner. Wells (1958) has determined that the predacious neogastropod Fascio- laria hunteria will feed on N. vibex to some extent. I have also seen Fascio- laria tulipa feed on Nassarius both in the field and in the laboratory. Each of these species was introduced into the Nassarius tank with similar results. F. tulipa, when either dropped into the water or set quietly on the tank bottom, caused immediate emergence of all Nassarius in its vicinity. They heaved up out of the sand, began exploratory movements with the siphon and moved away from Fasciolaria. There were some Nassarius that moved towards Fasciolaria, or because they emerged near Fascialaria, moved closely parallel with it. When these individuals made contact with Fascialaria with their siphon or propodial tentacle, immediate flipping occurred. The somersaulting response was especially pronounced when the siphon or the ventral surface of the foot of Fascialaria touched any soft part of Nassarius. Fifteen minutes after the fasciolarid snail was introduced, the tank walls became covered with Nassarius as they moved upward to congregate at the water surface. This, and the fact that emergence of N. vibex occurred first in the area where Fascialaria had settled and then at the opposite end of the tank shortly thereafter, implies a good chemosensory ability in N. vibex to detect "odors" carried by water current. REACTIONS TO ANIMAL FLUIDS I observed that Nassarius would often flip in a section of the tank or water table where no Fascialaria was present.