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Factors Determining the Patchy Distribution of the Pacific Sand Dollar, Dendraster Excenticus, in a Subtidal Sand-Bottom Habitat

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Factors Determining the Patchy Distribution of the Pacific Sand Dollar, Dendraster Excenticus, in a Subtidal Sand-Bottom Habitat FACTORS DETERMINING THE PATCHY DISTRIBUTION OF THE PACIFIC SAND DOLLAR, DENDRASTER EXCENTICUS, IN A SUBTIDAL SAND-BOTTOM HABITAT A Thesis Presented to the Faculty of California State University, Stanislaus through Moss Landing Marine Laboratories In Partial Fulfillment Of the Requirements for the Degree Master of Science in Marine Science By Tamara Lea Voss December 2002 DEDICATION To my family for their constant love and unending support. Thank you. iii ACKNOWLEDGMENTS As with all accomplishments, they are never completed alone. I wish to thank the Moss Landing Marine Laboratories community, fellow classmates who enthusiastically offered their help in the field, and their time with in the lab, and the MLML professors who generously shared their wisdom and experience. I would like to thank my thesis committee: Drs. Stacy Kim, Kenneth Coale, Pamela Roe, and Gary Greene for their help and support during my long tenure at MLML. I especially wish to thank Stacy for her woulderful guidance and patient compasswn. The Mary Stewart Rogers Fellowship from California State University, Stanislaus, provided partial funding for this work. iv TABLE OF CONTENTS PAGE Dedication....................................................................................... m Acknowledgements............................................................................ IV List of Tables.................................................................................... VI List of Figures................................................................................... vn Abstract... vm Introduction...................................................................................... I Materials and Methods......................................................................... 9 Results............................................................................................ 17 Discussion... 22 Literature Cited................................................................................... 30 Tables.............................................................................................. 36 Figures............................................................................................. 46 Appendix.......................................................................................... 53 v LIST OF TABLES TABLE PAGE I Sampling schedule including site, date, location, sample type, . ... 36 and number of replicates. 2 Statistical results for Del Monte Beach core samples................. 37 3 Mean and standard deviation of groupings per core.................. 38 4 Statistical results for Coast Guard Jetty core samples................ 39 5 Sediment characteristics inside and outside Dendraster excentricus 40 beds. 6 Statistical results for mean grain size, inside versus outside the sand 41 dollar beds. 7 Statistical results for settlement experiments............................ 42 8 The mean and standard deviation of Dendraster excentricus......... 43 larvae that metamorphosed, remained as plutei, or were lost from the experiment. 9 Category groupings that resulted from post-hoc comparisons........ 44 of the settlement experiments vi LIST OF FIGURES FIGURE PAGE 1 Oral and Aboral views of Dendraster excentricus test... 46 2 Dendraster excentricus beds comparing shoreward . ... 4 7 and seaward edges. 3 Perpendicular and parallel alignment of sand dollars within. .. 48 a bed. 4 Competent Echinopluteus of Dendraster excentricus, . 49 showing adult rudiment within larval gut area. 5 Aboral view of a newly metamorphosed juvenile . .. 50 Dendraster excentricus. 6 Map of Monterey Bay, California showing the location of.......... 51 the two study sites. 7 Seasonal adult Dendraster excentricus density at the Del Monte... 52 Beach site. vii ABSTRACT The Pacific sand dollar, Dendraster excentricus, can form dense assemblages in the shallow sandy subtidal. The distribution of beds of D. excentricus is patchy along the Pacific coast, and factors controlling the distribution of sand dollars are not well understood. The distribution of sand dollar beds at two sites within Monterey Bay was evaluated for infaunal community structure. Infaunal organisms were determined to belong to one offour groups (burrowers, predators, tube-builders, or Mollusca). These groups were examined for their effects on one another and on sediment stability, both inside and outside of the adult sand dollar bed. Additionally, sand dollar plutei that were competent to metamorphose were offered several substrata to determine what type of settlement cue is necessary for successful settlement and metamorphosis to take place. Larviphagy, the cannibalism oflarvae of D. excentricus by its adults, was evaluated as another factor important in determining initial distribution of sand dollar beds. The results of the study indicate that the distribution of sand dollar beds are initially established and maintained by settlement processes. vi:ii. INTRODUCTION Range and Distribution The Pacific sand dollar, Dendraster excentricus, (Figure 1) is an irregular echinoid belonging to the Order Clypeasteroida. Patchy beds of D. excentricus exist from Alaska to Baja California, covering the sand bottoms of both sheltered bays and open coast areas (Chia, 1969; Merrill & Hobson, 1970; Birkeland & Chia, 1971; Timko, 1975, 1976; Niesen, 1977; Cameron & Rumrill, 1982; Highsmith, 1982). D. excentricus is found intertidally along its northern distribution and in shallow subtidal waters along its southern distribution (Chia, 1969; Birkeland & Chia, 1971; Timko, 1975; Niesen, 1977; Cameron & Rumrill, 1982). Subtidal sand dollars live in an inclined position with their anterior edge partially buried in the sand (Chia, 1969, Timko, 1975, 1976; Smith, 1981; Morin et al., 1985). D. excentricus can form dense aggregations of up to several hundred individuals per square meter (Chia, 1969; Birkeland & Chia, 1971; Smith, 1981; Highsmith, 1982; Morin et al., 1985). These beds may exist for decades at the same location, much longer than the life span of the individuals, which is approximately eight years (Jensen, 1969; Birkeland & Chia, 1971; Highsmith, 1982). The shoreward edge of a sand dollar bed seems to remain in the same location on the bottom throughout the year. Although periods of high surf can temporarily displace large numbers of sand dollars, the bed margin is reestablished within a few days after the storm (Morin et al., 1985). The shoreward edge is located where major sand movement stops, just outside the breaker line. The seaward edge is more distinct than the shoreward edge (Figure 2). It moves out during the winter until the bed reaches irs peak width in 1 January, when it starts migrating back shoreward (Morin et al., 1985). Pisaster brevispinus and P. giganteus, seastars that prey on D. excentricus in this environment, are found along the seaward edge of the sand dollar bed, and may influence its location (Birkeland & Chia, 1971; Morin et al., 1985). Habitat Merrill and Hobson (1970) defmed four habitats that D. excentricus occupies: (1) bay, (2) tidal channel, (3) protected outer coast, and (4) exposed outer coast. Individuals of the "bay" type assume a horizontal position, lying flat on the sand or burrowing just below the surface of the sand. The substratum consists of fme, poorly sorted sand, usually with an overlying layer of detritus. Most individuals occur in water 0.6 to 1.2 meters deep. During unfavorable conditions, such as exposure during falling tides or when heavy rains decrease salinity, individuals of D. excentricus will burrow deeper into the sands rather than move to deeper water. The "tidal channel" habitat includes strong tidal currents, and clean, well-sorted sand, with most individuals living in the inclined position. Populations of D. excentricus · within tidal channels can be found with either their lateral axis perpendicular to the surge current (Figure 3a), or parallel to the surge current (Figure 3b). When the sand dollar's lateral axis is perpendicular to the surge current its oral surface faces upstream, and when it's lateral axis is parallel to the surge current, its oral surface faces either right or left across the current. Sand dollars of the "protected outer coast" maintain an inclined position with their anterior edge buried in the sand, and align their lateral axis parallel to the strong 2 onshore-offshore sweep of the current surge, which is perpendicular to the weaker long-. shore transport (Merrill & Hobson, 1970) (Figure 3b ). These beds tend to have sharp seaward margins, which exhibit the maximum density in 6 to 12 meters of water; the bed extends shoreward to approximately just outside the breaker line. Sand dollars of the "exposed outer-coast" are not found close to the shore and are completely buried (Merrill & Hobson, 1970). According to Merrill and Hobson (1970) this situation extends north from Point Conception into Oregon and Washington, and less is known about the individuals found in these locations than those in the other three habitat types. Natural History Dendraster excentricus uses three different methods to feed, depending upon the type of food being handled. Food items include particles <50J.l1Il, non-motile material, and motile prey (Timko, 1976). Transportation of particles <50J.l1Il in size toward the mouth is by ciliary currents. Tube feet are used to grasp and push non-motile material, >50J.l1Il in size, toward the food grooves. Small active prey are trapped by the spines, which enclose them, and create a cone-like trap. The large bidentate
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