ROSENTHAL and CHESS: PREDATOR-PREY RELATIONSHIP 



side Mission Bay, San Dieg-o County (lat 32° 

 45'30" N; long 117°14'30" W), and the other 

 was from the study site off Pt. Loma. Asteroid 

 tube feet were selected as the biotic stimuli be- 

 cause of the known effectiveness of this tissue in 

 eliciting avoidance reactions in other inverte- 

 brate species (Bullock, 1953). Coarse, washed 

 sand grains were used as the abiotic control. 

 Purple urchins were placed individually into 

 glass bowls which contained seawater, and then 

 an asteroid tube foot, or sand grain, was dropped 

 onto the urchin's test. Each urchin was tested 

 for a 2-min period using a different stimulus 

 on each run. Fresh seawater was placed into 

 the bowls prior to each test. The tube feet of 

 D. imbricata elicited gaping and erection of pur- 

 ple urchin globiferous pedicellariae in 28 out of 

 30 test animals. These 28 individuals had an 

 average reaction time of 8 sec, with a range be- 

 tween 2 and 20 sec. None of the S. purpuratus 

 displayed gaped globiferous pedicellariae when 

 either Pisaster giganteus tube feet or sand grains 

 were presented to them. 



Additional asteroids that were found in the 

 sublittoral zone off Pt. Loma were also tested. 

 The tube feet from Astrometis sertulifera, Pa- 

 tiria miniata, and Pycnopodiu helianthoides 

 evoked the pedicellariae response in S. purpiir- 

 atus; however, no response was elicited when 

 eight other asteroid species were tested (Table 

 1). It is interesting to note that the former 

 three species were the only other sea stars, be- 

 sides D. imbricata, that have been observed feed- 

 ing on live S. purpuratus off Pt. Loma. 



Table 1. — A list of sea stars found off Pt. Loma, Cal- 

 ifornia. All 12 species were individually used to test 

 the globiferous pedicellariae response in S. purpuratus. 



Species 



Reaction 



Astrometis sertulifera 

 Astropecten armafus 

 Dermasterias imbricata 

 Henricia leviuscuta 

 lAnckia columbiae 

 Mediaster aequalis 

 Patiria miniata 

 Pisaster brevispinus 

 Pisaster giganteus 

 Pisaster cchraceus 

 Orlhasterias koehleri 

 Pycnopodia helianthoides 



+ 



+ 



+ 



+ 



pedicellariae response 

 no pecJicellariae response 



The erection and gaping of the globiferous 

 pedicellariae initially occurred only on the area 

 of the urchins' test which was directly stimu- 

 lated by the sea star's tube foot. Jensen (1966) 

 found that a single tube foot from the sea star, 

 Marthasterias glacialis, activated the globiferous 

 pedicellariae only in a restricted area on the test 

 of the sea urchin, Psanimechinus miliaris, 

 whereas the sea star arm tip caused a res]3onse 

 from all globiferous pedicellariae. In contrast, 

 we found that the arm tip of D. imbricata acti- 

 vated the pedicellariae of S. purpuratus only 

 in the region of the stimulus. The defensive 

 response and recognition of predatory stimuli 

 was so acute in S. purpuratus that an arm of 

 a P. giganteus placed on one side of a purple 

 urchin's test, and a D. imbricata arm positioned 

 on the opposite side elicited a response from the 

 globiferous pedicellariae only in the area of 

 leather star contact. 



Defensive use of globiferous pedicellariae by 

 sea urchins when disturbed by predatory aster- 

 oids has been described by Prouho (1890), Jen- 

 nings (1907), Jensen (1966), Mauzey et al. 

 (1968), and Rosenthal and Chess (1970). Jen- 

 sen (1966) reported that the poison contained 

 in globiferous pedicellariae of Psammechinus 

 miliaris was not strong enough to paralyze a M. 

 glacialis; however, it did have an irritating ef- 

 fect on the sea star which caused it to retreat 

 from the urchin. We found that in some lab- 

 oratory situations, globiferous pedicellariae bites 

 on the arms of D. imbricata caused localized 

 withdrawal of gills or papulae, and a shortening 

 or retraction of the affected arm. Despite this 

 irritation, 90 Sr of all the leather stars we found 

 feeding on purple urchins had from one to over 

 300 sea urchin pedicellariae attached to their 

 epidermis. 



Certain groups of aquatic organisms have been 

 observed to respond to chemical signals or alarm 

 substances emitted by injured conspecifics (von 

 Frisch, 1941; Pfeiffer, 1963; Snyder and Sny- 

 der, 1970). Recently, Snyder and Snyder (1970) 

 found that the tropical sea urchin, Diadema aji- 

 tillarum, exhibited an alarm or escape response 

 when stimulated with the juices of injured mem- 

 bers of its own species. Laboratory and field 



213 



