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Notice: ©1983 California Malacozoological Society. This manuscript is an author version with the final publication available and may be cited as: Mook, D. (1983). Homing in the West Indian granulata Gmelin, 1791. The Veliger, 26(2), 101-105.

THE VELIGER © CMS, Inc., 1983 The Veliger 26(2):101-105 (October 3,1983)

Homing in the West Indian Chiton Acanthopleura granulata Gmelin, 1791

by

DAVID MOOK

Harbor Branch Foundation, Inc., R.R. 1, Box 196, Ft. Pierce, Florida 33450

Abstract. The homing behavior of the chiton Acanthopleura granulata was studied on high-, moderate-, and low-energy rocky shores in the Bahamas. on low-energy shores tended to home more frequently than chitons on higher-energy shores. Chitons on higher-energy shores tended to make more frequent nighttime excursions from their home and to travel greater distances than chitons on low-energy shores, possibly because lower food availability on higher-energy shores may force the chi tons to increase their grazing effort.

INTRODUCTION were generally subjected to surf of one to several meters in height. These shorelines were usually wetted by spray HOMING IS A very common phenomenon in some groups and wave swash for a large part of the day. Moderate­ of mollusks. It has been described in several of energy shorelines (Stations 5, 8) were exposed to some gastropods (McFARLANE, 1981; WELLS, 1980; COOK, waves and spray at high tides and during windy weather. 1979; COLLINS, 1977; MACKAY & UNDERWOOD, 1977; Low-energy shorelines (Stations 2, 3) were exposed to and others) and chitons (GLYNN, 1970; THORNE, 1968). little or no wave or spray activity and were wetted only Acanthopleura granulata Gmelin, 1791, a West Indian chi­ at flood tides when water covered the substrate. ton, is very common on the intertidal limestone shores in Individual Acanthopleura granulata and their attach­ the Bahamas. Individuals generally remain stationary on ment sites (=homes) were marked with nail polish. The their rock substratum during the daylight hours and for­ following day, the marked homes were checked to deter­ age for endolithic and surficial algal food at night (GLYNN, mine whether the chi tons were present or absent from 1970). Unlike that of some other species of chi tons, the where they were marked the previous day. If the chiton foraging of A. granulata does not seem to be influenced by was absent from the home it occupied the previous day, tidal height (NEWELL, 1979). Upon completion of its for­ the was located (if possible) and its new site of aging activity, A. granulata often returns (homes) to its attachment (home) was marked. The distance between the original site of attachment. Because individuals do not original and new home was measured. If the animal could always come to rest exactly on their old site of attachment not be located, it was counted as absent. Observations and often orient themselves differently (GLYNN, 1970), the were made from two (Indian Cay) to six (San Salvador) term "homing" as used in this study is not being used in consecutive days. the strictest sense (COLLINS, 1977). In this study, I de­ Because many chitons (especially in low- and moderate­ scribe the differences in homing behavior of individual energy zones) were generally at their original home when Acanthopleura granulata inhabiting high-, moderate-, and observed each day, observations also were made on nights low-wave energy shorelines in the Bahamas. when low tides occurred to determine whether those an­ imals were foraging at night and returning to their homes MATERIALS AND METHODS or simply were not leaving their homes at all. The dis­ In order to compare homing activity of Acanthopleura tances between the foraging chi tons and their homes were granulata on high-, moderate-, and low-energy shorelines, measured. Personal safety considerations prevented night­ observations were made on various rocky limestone (Kar­ time observations on high-energy shores. renfeld) (GARY et al., 1974) shorelines in the Bahama A Student's t test was used to compare distances trav­ Island chain (Figure 1). High-energy zones (Stations 1, eled between chi tons tAcanthopleura granulata) in low­ 4, 6, 7) were on shorelines exposed to the open ocean and energy areas and chitons in moderate-energy areas. Be- Page 102 The Veliger, Vol. 26, No.2

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Figure 1 Localities where observations were made. I: Indian Cay, near West End, Grand Bahama Island. 2, 3, 4: Black Rock, near Moors Island, Abaca Chain. 5, 6: North Point, San Salvador Island. 7, 8: Barkers Point, San Salvador Island.

cause an F rnax test indicated that distance data were not value of samples taken from low- and moderate-energy normal, all distance data were transformed using a log zones. transformation. Caloric values of substrata in a moderate- (Station 8) RESULTS and low- (Station 2) energy zone were compared. Samples of the limestone substrate were taken by scrubbing the From a total of 162 observations, about 43% of the Acan­ substrate with a wire brush and rinsing the loosened lime­ thopleura granulata had moved from the point at which stone and its associated endolithic and surficial algae into they were marked the previous day. Far fewer chitons a jar. The samples were kept frozen until they were re­ were found at the point where they were marked in high­ turned to the laboratory where they were filtered, dried, energy zones (15%) than in moderate- (56'70) or low- (93'70) and ground into a powder. Samples were mixed with ben­ energy zones (Figure 2). Although only a small number zoic acid (50% acid, 50% sample) and their caloric value of chitons observed in low-energy zones were found absent measured in a Phillipson microbomb calorimeter (PHIL­ from their home the following day, nighttime observations LIPSON,1964). showed that 59% of the marked chitons in low-energy Because the topography and density of the substratum zones were away from their home at night. All of these were similar on all islands studied, caloric values are re­ chi tons had returned to their home by morning. In mod­ ported in calories per milligram of substratum rather than erate-energy zones, about 70% of the marked chi tons were in calories per area of substratum. Irregularities in the observed away from their home at night, but only 56% of substratum surface made it difficult to remove limestone these chi tons had returned back to their home by morning evenly from a specific area. Twelve replicates were done (Figure 3). Since nighttime observations could not be made for each sample. A t test was used to compare the caloric in high-energy zones, it is not known whether the chitons D. Mook, 1983 Page 103

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O'-----L..-....J-----L---'----'------'- Low Moderate High Energy Energy Energy o Moderate N=44 N=86 N=32 Low Energy Energy Figure 2 N=13 N=23 Percent of chitons found at marked sites one day after marking. N = total number of observed. Figure 3 Percent of chitons observed away from site of marking at night in moderate- and low-energy zones. Solid line inside bar indi­ cates percentage of these animals which had returned to the marked area the following morning. that were present at the point of marking the following day had homed or simply had not moved during the night. Acanthopleura granulata in low-energy zones which were not observed at their home the following day (only three) farther (6-65 em) from their home than chi tons in low­ were observed from 5 to 35 em away from their home. In energy zones (2-30 em; P < 0.05) (Table 1). moderate-energy zones, chi tons which had not returned to Caloric values of the substrate at the moderate-energy their home by the following day were observed 5 to 500 site were significantly lower (x = 0.1767 caljmg) than the em from their home (Table 1). No distance measurements caloric value of the substrate at the lower energy site (x = were done with high-energy-zone chitons because only a 0.5056 caljmg; P < 0.05). small percentage of the marked chi tons in the high-energy zone were found the following day. The fact that marked DISCUSSION chi tons were not found within several meters of their home suggests that either these chitons could have traveled far­ The observations in this study suggest that Acanthopleura ther from their home than moderate-energy-zone chitons granulata living in lower-energy zones may have a greater and relocated in areas that were inaccessible, or they were tendency to stay attached to their home than those living removed by predators or wave action. A t test detected no in higher-energy zones. When chitons in lower-energy significant difference between the distances of low-energy­ zones do leave their home to graze, they tend to travel zone chitons and moderate-energy-zone chi tons from their shorter distances and to have a greater tendency to return home. The reason no difference was detected could be due to their home than do chitons in moderate- and high­ to the small number of individuals that did not home in energy zones. the low-energy zone; this would make a significant dif­ The reasons for development of homing in chitons are ference difficult to detect. At night, chi tons (Acanthopleura unclear. Several explanations, such as protection from dis­ granulata) in moderate-energy zones ranged significantly lodgment (LINDBERG & DWYER, 1983; COLLINS, 1977), Page 104 The Veliger, Vol. 26, No.2

Table 1 ited on a temporal basis (NEWELL et al., 1971). NEWELL (1979) suggested that when food availability was lower, Distance (ern) that chitons were found away from the feeding effort may have had to increase in order for the point of marking (home) the night and day following organism to continue to get enough food to sustain itself. marking. NS means t value not significant at P = .05. In the case of the chitons examined in this study, the lower caloric value of the moderate-wave (and possibly high-) Day after Night after marking marking energy substrata may force animals in these zones to for­ age at more frequent intervals and to travel longer dis­ Low energy tances (increase grazing effort) in order to obtain the nec­ Range (ern) 5-35 2-30 x (n) 18.3 (3) 12.00 (11) essary calories to sustain themselves. Moderate energy Other factors in addition to wave exposure or food Range (ern) 5-500 6-65 availability may affect homing behavior in chi tons. Ele­ X(n) 61.12 (32) 36.22 (9) ments such as inter-island variability, differences in sub­ t value 1.37 (NS) 2.73 strate morphology, and differences in substrate productiv­ ity have not been addressed in this study and would be interesting subjects for future studies with Acanthopleura granulata.

predator avoidance (WELLS, 1980), maximum utilization ACKNOWLEDGMENTS of food resources (MACKAY & UNDERWOOD, 1977), and I wish to thank Richard Houbrick, Paul Mikkelsen, and protection from desiccation (VERDERBER et al., 1983; Robert Virnstein for their helpful comments on the manu­ COLLINS, 1977; and others) have all been suggested for script. I am grateful to Pat Linley for proofreading the other homing mollusks. It is unlikely that protection from manuscript, and I am indebted to Charles Hoskin for his dislodgment is an important reason for homing in Baha­ support during this study. I would like to thank Donald mian Acanthopleura granulata because chitons in high-en­ and Kathy Gerace of the College Center of the Finger ergy zones, where dislodgment is more likely due to high Lakes field station on San Salvador for their hospitality wave activity, have a lesser tendency to home. Whether and help. This is Harbor Branch Foundation Contribu­ homing is an important mechanism for predator avoidance tion No. 339 and CCSL Contribution No. 64-B. was not determined. However, little evidence of predation and few predators (except humans at some sites) were LITERATURE CITED observed at any of the study sites, suggesting that homing may not be important for predator avoidance in the Ba­ COLLINS, L. S. 1977. Substrate angle, movement and orien­ hamas. tation of two sympatric species of , Collisella digitalis It is also not clear whether Bahamian Acanthopleura and Collisella scabra. Veliger 20:43-48. COOK, A. 1979. Homing in the . Malacologia 18: granulata home for protection against desiccation. The fact 315-318. that chi tons home less in high-energy areas, which are GARY, M., R. McAFEE, JR. & C. WOLF. 1974. Glossary of frequently wetted by wave action, could indicate that geology. American Geological Institute. Washington, D.C. homing may be an adaptation to prevent desiccation. xiv + 805 pp. + 52 pp. However, because homing may be accomplished by olfac­ GLYNN, P. W. 1970. On the ecology of the Caribbean chitons tion and/or trail retracing (McFARLANE, 1981; COOK, Acanthopleura granulata Gmelin and Chiton tuberculatus 1979; THORNE, 1968), the reduced homing in high-energy Linne: density, mortality, feeding, reproduction, and growth. Smithsonian Institution Press No. 60: 21 pp. areas may be caused simply by trails and olfactory stimuli LI:-IDBERG, D & K. DWYER. 1983. The topography, formation being washed away by wave action. If homing is impor­ and role of the home depression of Collisella scabra (Gould) tant for the prevention of desiccation, the loss of a trail to (Gastropoda: Acmaeidae). Veliger 25:229-234. home could be of little consequence for chitons in high­ MACKAY, D. A. & A. J. UNDERWOOD. 1977. Experimental wave-energy areas where desiccation is not a major prob­ studies in the intertidal patellid Cellana tramoserica lem. (Sowerby). Oecologia 30:215-237. McFARLANE, I. D. 1981. In the intertidal homing gastropod The lower caloric food value of the moderate-energy Onchidium verruculatum (Cuv.) the outward and homeward substrate zones may explain the more frequent and more trails have a different information content. J. Exp. Mar. distant nighttime excursions of chitons (Acanthopleura Bio!' Eco!' 51:207-218. granulata) in moderate- (and possibly high-) energy zones. NEWELL, R. C. 1979. Biology of intertidal animals. Marine Studies with filter-feeding bivalves (WINTER, 1978) and Ecological Surveys L & D, Faversham. ix + 781 pp. grazing winkles (Littorina littorea Linne, 1758) (NEWELL NEWELL, R. C., V. I. PYE & M. AIlSUNULLAII. 1971. Factors affecting the feeding rate of the winkle Littorina lit/area. et al., 1971) have shown that feeding effort (filtration rates Mar. Bio!' 9:138-144. or radular movement) tended to decrease as food became PHILLIPSON, J. 1964. A miniature bomb calorimeter for small more available. For example, winkles tended to have higher biological samples. Oikos 15:130-139. radula movement rates when their food supply was lim- TIIOR:-IE, M. J. 1968. Studies on homing in the chiton Acan- D. Mook, 1983 Page 105

thozostera gemmata. Aust. J. Mar. Freshwater Res. 19:151­ WELLS, R. A. 1980. Activity pattern as a mechanism of pred­ 160. ator avoidance in two species of acmaeid limpet. J. Exp. VERDERBER, G. W., S. B. COOK & C. B. COOK. 1983. The Mar. BioI. Ecol. 48:151-168. role of the home scar in reducing water loss during aerial WINTER, J. 1978. A review of the knowledge of suspension exposure of the pulmonate limpet Siphonaria alternata (Say). feeding in lamellibranchiate bivalves, with special reference Veliger 25:235-243. to artificial aquaculture systems. Aquaculture 13: 1-33.