Temporal and Spatial Activity Patterns of the Intertidal Chiton Mopalia muscosa Suanne Yvonne Smith Hopkins Marine Station of Stanford University Pacific Grove, California 93950 Running Title: Activity Patterns of Mopalia muscosa Send all proofs and correspondence to: Suanne Yvonne Smith page 1 S. Y. Smith Activity Patterns of Mopalia muscosa Page 2 INTRODUCTION The study of animals in the field often reveals aspects of behavior not discoverable from observations on captive animals. Behavioral studies on intertidal organisms in the field are usually carried out during periods of low tide. Observations at high tide are more difficult to make, particularly at night, and animal behavior under these conditions remains largly unexplored. A search of the literature revealed no such work done on chitons. This study, carried out on Mopalia muscosa (Gould, 1846), was directed toward revealing its temporal and spatial patterns of activity. Attention was also focused on inter- and intraspecific interactions involved in habitat partitioning, and on the possible occurrence of individual homesites and territories or home ranges. Homing ability in some Polyplacophora has been noted previously (Crozier 1921; Thorne, 1967, 1968). FIELD STUDY SITE AND METHODS The present study was carried out during the Spring of 1974 on the rocky shores of Mussel Point, Pacific Grove, California. Mopalia muscosa is common here, particularly in the mid tide zone. The site selected was a surface 1.2 x 1.3 m on a sculptured granitic outcropping, occurring between the +1.8 and +4.2 foot tide level on the eastern edge of the point.Surf conditions at the site during high tide varied from a gentle surge to breaking waves. The macroalgae occurring in the study site were Gigartina papillata S. Y. Smith Activity Patterns of Mopalia muscosa page 3 (C. A. Agardh) J. G. Agardh, Petrocelis franciscana Setchell and Gardner, ndocladia muricata (Postels and Ruprecht) J. G. Agardh, Iridaea flaciida (Setchell and Gardner) Hollenberg and Abbott, Pelvetia fastigiata (J. G. Agardh) DeToni, Gigartina leptorhynchos J. G. Agardh, and Gigartina crymbifera (Kutzing) J. G. Agardh in order of decreasing percent cover. Immediately adjacent to this area, the rocks were covered with Pelvetia and here the relative abundance of Mopalia muscosa dropped. The resident animals in the study area, in order of decreasing numbers of individuals, included the annelid Phragmatopoma californica (Fewkes, 1889), the anemone, Anthopleura elegantissima (Brandt, 1835), the snail Tegula funebralis (Adams, 1854), hermit crabs (Pagurus spp.), the young limpets, Collisella digitalis (Rathke, 1833) and Collisella scabra (Gould, 1846), the chitons Mopalia muscosa and Cyanoplax hartwegii (Carpenter, 1855), the anemone nthopleura xanthogramiica (Brandt, 1835), the chiton Nuttallina californica (Reeve, 1847), and the crab Pachygrapsus crassipes Randall, 1839. Herbivores occurring on the adjacent rocks consisted primarily of Cyanoplax and the lmipets Collisella pelta (Rathke, 1833) 1833, C. limatula (Carpenter, 1864), and Notoacmea scutum (Rathke, 1833). None of these latter limpet species occurred in the area where M. muscosa was studied. Tegula were found in approximately equal densities in and adjacent to the study site, whereas the numbers of C. digitalis and C. scabra declined in the neighboring areas. The activity of eight individuals of Mopalia muscosa in the study site was monitored over a five-week period from April 28 to May 30, 1974. Marking the chitons was unnecessary due to distinctive individual differences in algal growth on their plates. The substrate was also S. Y. Smith Activity Patterns of Mopalia muscosa page 4 unmarked, and natural landmarks were used as reference points to note the Positions of animals. The chitons could, in this way, be observed with minimal disturbance. An initial 48-hour watch indicated that the Mopalia moved only at night, therefore an intensive surveillance of the chitons was conducted from just before dusk to shortly after dawn, with intermittent daytime observations. The night time observations were made at intervals of 15 minutes to one hour. For each observation the chitons' new positions and the distances traveled from their previously noted positions were recorded. These night observations were made with a flashlight, using a red filter to reduce disturbance during their dark cycle. Observations on submerged animals were made with the use of a wet suit, face plate, and snorkle. DIURNAL AND TIDAL ACTIVITY PATTERNS The data on movements are summarized in Fig. 1; original data are on file in the library at Hopkins Marine Station. During daylight hours and when exposed by the tide, Mopalia muscosa were not active, and were in fairly protected locations. Six of the eight sites were adjacent to a sandy bottom and in a depression, crevice, or at the base of a rock. Individuals were often partially or completely buried by the sand. Another daytime site was under or next to Anthopleura elegantissima and Anthopleura xanthogrammica. After foraging, the eight individuals in the rocky area homed for 69 out of the 71 excursions monitored. Homing is degined here as the return to the exact site the individual left aarlier that night. In one instance, a rock washed into the home of a foraging S. Y. Smith Activity Patterns of Mopalia muscosa page 5 chiton and blocked it. This individual settled about 7 cm away, and then consistently homed to this new site for 5 observed journeys over the following 15 days. The only other occasion a chiton failed to home occurred when one individual traveled 20 cm beyond it customary range of 40 cm from home. Instead of returning, it stayed at a site similar to its old home, about 35 cm away, and subsequently was lost from the study area. Foraging occurred only at night, when the chitons were submerged or awash. Not all of the animals monitored moved every night: during the 14 nights of observation the total number of times a particular individual left home varied from 4 to 14, with a mean of 9. If the rock was submerged or awash by an incoming tide at the onset of dusk, the animals moved away from their homes. If the study site was dry, activity was delayed until the chitons were wetted by the incoming tide. The chitons began the return journey to their homesites at such a time that they were back at their homesites before sunrise or before a receeding tide uncovered them, whichever came first. Thus no animal in the study site was left either conspicuously exposed to avian predators or stranded in a position where desiccation rates were high. Similar activity patterns were found for Chiton tuberculatus Linne by Crozier (1921) and for Acanthozostera gemmata (Blainville) by Thorne (1967, 1968). Figure 1 illustrates other intriguing variations in activity. More individuals were active at spring tides (May 7, May 20 and May 21, 1974) than at neap tides (April 29 and May 9, 1974). There was a period of inactivity during high tide at night when the individuals were away from home (May 3, May 7 and May 20, 1974). In addition, some individuals remained out, but inactive, during the high low tides that occurred in the middle of the night. (April 29 and May 9, 1974); these individuals were Activity Patterns of Mopalia muscosa S. Y. Smith page 6 located at the lowest vertical positions in the study area and during these low tides were never entirely uncovered. On one occasion an individual was found away from its home during a low tide that left it totally uncovered. That particular animal halted movement until the next in¬ coming tide, whoih occurred while it was still dark. These observations suggest that the chitons predict the time and perhaps the height of low tides. Westersund (1974) found evidence of endogenous tidal rhythms in his work with Mopalia muscosa, which could explain the animals' ability to anticipate coming events. He also found the extent of their activity reflected the magnitude of the tides. This is corroborated by my field observations. HOME RANGES, DIET, AND INTERACTIONS WITH NEIGHBORS Fifty-three chiton journeys were coninually monitored while an ont additional 18 journeys were monitored sporadically (due to adverse weather conditions). Figures 2a and 2b show the various pathways the individuals followed. The distances traveled from the homesites ranged from 5 to 50 cm, most being between 25 and 40 cm. Only on one occasion, already described, did an individual move farther than 50 cm from his home. Each Mopalia muscosa appeared to have a series of established pathways within a radius of 50 cm from their homesites. These pathways were followed fairly closely and consistently, though they did not always appear to coincide with any obvious contours of the substrate. The same path, was normally used on both the outgoing journey and the return home, though this was not always the case. In general, a chiton left home, traveled to a certain location within its home range, and remained there "until it was time" to return home. The locations to which foraging chitons traveled S. Y. Smith Activity Patterns of Mopalia muscosa page 7 usually had much macroalgae present; yet some locations were void of any macroalgae. On several occasions, feeding on Gigartina papillata was observed. Quantitative studies of the diet of Mopalia muscosa were not under¬ taken, but the gut contents of 5 chitons from very similar habitats in areas adjacent to the sutyd site were analyzed qualitatively. The guts contained a wide variety of algal material along with small amounts of animal matter and a lot of sand. Those algae that appeared most often were identified (with the assistance of Dr. Isabella A. Abbott) as Gigartina spp., Cladophora sp., Endocladia muricata (Postels and Ruprecht) J. G. Agardh, and Hildenbrandia occidentalis Setchell. Also included were Petrocelis franciscana, (author), Chaetomorpha sp. and a variety of blue-green and coralline algae. Boolootian (1964) also found M. muscosa to be a non¬ specific herbivore, feeding on what was available, along with small amounts of animal matter he believed was accidentally ingested. The home ranges of individual chitons, as determined by the paths they followed, did not appear to overlap. Only on 2 out of the 63 chitons journeys monitored did a chiton enter into another's home range. On one of these occasions the resident individual left its home, approached the intruder's girdle and remained there for about half an hour. The resident chiton then left the intruder and traveled a wide semicircle of about a 50 cm radius in its home range before returning home again. The intruder remained for a little over an hour before moving off to a new site. Collisella pelta were absent from the study area but were abundant on rocks in adjacent areas, indicating possible competitive exclusion. Connor (1974) demonstrated that C. pelta exhibits an avoidance reaction to M. muscosa pushing C. pelta when it encountered the limpet in its territory. S. Y. SMith Acti Patt alia muscosa page 8 BEHAVIOR IN OTHER HABITATS Limited observations were made on six M. muscosa located in a tide pool adjacent to the main study site, and on two individuals located on a cement substrate immediately above this pool. The six individuals in the tide pool did not home or follow regular pathways, but rather wandered throughout the pool. At dawn, they settled in sites out of direct sunlight and there spent the day. Activity at low tide during the night decreased in the pool but did not halt altogether as in the chitons of the rocky habitat which were exposed to air during low tide. The two chitons located on the cement substrate above the tide pool, like the rocky habitat individuals, returned to specific homes after foraging at night during the high tides. They, too, generally followed specific pathways within limited home ranges. None of the eight chitons was active during the day and all showed a tidal rhythm in some degree. SUMMARY 1. The movements of eight Mopalia muscosa inhabiting the rocky intertidal were followed during 14 days over a one month period (April 28-May30). All were regularly covered and uncovered by the tides. 2. The animals moved about only at night when submerged or awash. They were inactive during the day and when exposed by low tides at night. 3. Each animal occupied a spcific home site when inactive; home sites on rocks near or in sand, or adjacent to sea anemones (Arthopleura spp.). 4. In foraging, individuals normally followed specific pathways leading no more than 50 cm from the home site, and returned along the same path. 5. Diet consisted mainly of red algae; the animals appear to be relatively unselective herbivores. page 9 S. Y. Smith Activity Patterns of Mopalia muscosa 6. A competitive herbivore, Collisella pelta, appeared to be excluded from the home range of M. muscosa. 7. Animals observed in a tidepool showed similar basic activity patterns but failed to home or completly halt movement during low tides at night, as seen in those directly exposed to the tidal cycle. ACKNOWLEDGMENTS My thanks go to Christopher Harrold and Dr. Robin Burnett for their assistance and advice, and Dr. Isabella A. Abbott and Bob Sellers for help in algae identifications. Most of all I would like to extend my sincere appreciation to Charles Baxter for his undying enthusiasm and unlimited advise. S. Y. Smith Activity Patterns of Mopalia muscosa page 10 LITERATURE CITED Boolootian, Richard A. 1964. On growth, feeding and reproduction in the chiton Mopalia muscosa of Santa Monica Bay. Helgolander wiss. Meeresunters. 11: 186-199; 3 figs. (December 1964) Connor, Michael Stewart 1974. Niche apportionment among the chitons Cyanoplax hartwegii and Mopalia muscosa, and the limpets Acmaea limatula and Acmaea pelta under the brown alga Pelvetia fastigiata. The Veliger. Crozier, W. J. "Homing" Behavior in Chiton. Amer. Natur. 55: 276-281; 1 fig. 1921. Thorne, M. J. 1967. Homing in the chiton Acanthozostera gemmata (Blainville). Proc. R. Soc. Queensl. 79(9): 99-108; 1 fig. Thorne, M. J. 1968. Studies on homing in the chiton Acanthozostera gemmata. Aust. Journ. mar. Freshwat. Res. 19(2): 151-160; 3 figs.; 2 plts. (December 1968) Westersund, Kristen R. 1974. Phototaxis and a tidal rhythm in the chiton Mopalia muscosa (Mollusca: Polyplacophora). The Veliger Activity Patterns of Mopalia muscosa S. Y. Smith FIGURE CAPTIONS Fig. 1: Date, time, light, tidal height, and movements of 8 M. muscosa during ten 14 hr. periods of observation. Positions of animals were recorded at intervals of 15 min. to 1 hr. throughout the 14 hours for these ten dates. Four additional nights these chitons were observed only the observations were sporadic. This additional data is included in an Appendex on file at Hopkins Marine Station. Each vertical white bar above the axis indicates the percentage of the total number of animals moving sometime within that hour: the dark vertical bar shows the average distance these animals moved during that hour. Below the line the vertical white bar indicates the percentage of the total number of animals away from their homes, which does not always equal the percentage moving. Absence of bars means all animals were examined and found to be immobile at their home sites. Fig. 2: Rocky intertidal study area: (a) shows the position of home sites (oval areas) and respctive home ranges with regularly followed pathways, shown in broken lines, in the study area. Note most of the home sites are in the sandy regions (heavily stippled areas bearing diagonal lines). (b) is another view of section A, both views are at approximately a 45° angle looking into the study area. a 44 12 - 2005. 2402- 112- o2- Voo- etee e500. tidal egele Je nat al (etet 2 1 1 5 S. J. S P- FULL PGE 2 holed meding t tided 2 nat hana 76 total madig 4141 64) 1 ale (fet) Clsht be) 1 18 12 1 240p 242 p 41 He — 7120 E 0 e o — E — Aux distenct mosiel ADE. D.STONCE MODES her tet e toud (em) lank te, . :5. S 1 + 27 . . - .. - .. :4. 1 5.