FLATWORM PREDATION ON JUVENILE FRESHWATER MUSSELS A Thesis Presented to the Graduate College of Southwest Missouri State University In Partial Fulfillment of the Requirements for the Master of Science Degree By Angela Marie Delp July 2002 i FLATWORM PREDATION OF JUVENILE FRESHWATER MUSSELS Biology Department Southwest Missouri State University, July 27, 2002 Master of Science in Biology Angela Marie Delp ABSTRACT Free-living flatworms (Phylum Platyhelminthes, Class Turbellaria) are important predators on small aquatic invertebrates. Macrostomum tuba, a predominantly benthic species, feeds on juvenile freshwater mussels in fish hatcheries and mussel culture facilities. Laboratory experiments were performed to assess the predation rate of M. tuba on newly transformed juveniles of plain pocketbook mussel, Lampsilis cardium. Predation rate at 20 oC in dishes without substrate was 0.26 mussels·worm-1·h-1. Predation rate increased to 0.43 mussels·worm-1·h-1 when a substrate, polyurethane foam, was present. Substrate may have altered behavior of the predator and brought the flatworms in contact with the mussels more often. An alternative prey, the cladoceran Ceriodaphnia reticulata, was eaten at a higher rate than mussels when only one prey type was present, but at a similar rate when both were present. Finally, the effect of flatworm size (0.7- 2.2 mm long) on predation rate on mussels (0.2 mm) was tested. Predation rate increased with predator size. The slope of this relationship decreased with increasing predator size. Predation rate was near zero in 0.7 mm worms. Juvenile mussels grow rapidly and can escape flatworm predation by exceeding the size of these tiny predators. Captive culture for even a few weeks might improve the survival of propagated juveniles if flatworm predation also occurs in nature. This abstract is approved as to form and content ___________________________________ Chairperson, Advisory Committee Southwest Missouri State University ii FLATWORM PREDATION ON JUVENILE FRESHWATER MUSSELS by Angela Delp A Thesis Submitted to the Graduate College of Southwest Missouri State University in Partial Fulfillment of the Requirements for the Degree of Master of Science July 2002 Approved: ____________________________________ Chairperson ____________________________________ Member ____________________________________ Member ____________________________________ Associate Vice President for Academic Affairs and Dean of the Graduate College iii ACKNOWLEDGEMENTS I first recognize my mentor Dr. Chris Barnhart. He has given me more confidence in myself and helped me develop my analytical abilities and writing skills. Dr. Barnhart is enthusiastic about science, constantly questioning the biological interactions in the world around him. He never ceases to amaze me with his seemingly unlimited interests and intelligence. I am proud to say I have worked with this outstanding malacologist. Thanks to Dr. John Havel and Dr. Alicia Mathis for being on my thesis committee. Dr. Havel helped me identify an ostracod I used in one of my pilot studies. Dr. Mathis helped me with the design of my alternative prey experiment. They both are exceptional scientists and have offered me valuable advice. I am honored to have been their pupil. I think Havel and Mathis are irreplaceable assets to this school. I received useful flatworm information from Dr. Jurek Kolasa and Dr. George Shinn. Kolasa had helped me identify the predator organism, when I was starting out. I was able to ask him any questions I had about the flatworms via email. Shinn informed me on the egg capsule structure of Macrostomum. He gave me some key references to help me with some questions I had. This thesis is dedicated to my mom, dad, sister, and grandparents. They have been loving and supportive throughout my life. I hope that I have influenced them as positively as they have influenced me. The Graduate College and the Department of Biology at Southwest Missouri State University provided funding for this project. Christian Hutson collected mussels for my project. Melissa Shiver, Shannon Bigham, Jeremy Myers, and Matt Stone assisted in the laboratory. Kristen Pattinson and Tina Tamme helped with identifying and culturing zooplankton. iv TABLE OF CONTENTS ABSTRACT .....................................................................................................................ii ACKNOWLEDGEMENTS ............................................................................................iv LIST OF TABLES AND FIGURES ...............................................................................vi INTRODUCTION ........................................................................................................... 1 METHODS ...................................................................................................................... 5 Effects of substrate on predation ............................................................................... 7 Effects of alternative prey on predation ..................................................................... 8 Effects of flatworm size on predation ......................................................................... 9 RESULTS ...................................................................................................................... 10 Flatworm identification ............................................................................................ 10 Occurrence of flatworms........................................................................................... 11 Culturing flatworms ................................................................................................. 12 Effect of substrate on predation ............................................................................... 13 Effect of alternative prey on predation .................................................................... 13 Effect of flatworm size on predation ........................................................................ 13 DISCUSSION ................................................................................................................ 14 Effect of substrate on predation ............................................................................... 14 Prey preference ........................................................................................................ 15 Effect of size on predation rate ................................................................................ 17 LITERATURE CITED .................................................................................................. 20 TABLES AND FIGURES ............................................................................................. 25 v LIST OF TABLES AND FIGURES Table 1. Rates of predation by Macrostomum tuba ..................................................... 25 Figure 1. Macrostomum flatworm ................................................................................ 26 Figure 2. Microstomum flatworm ................................................................................. 27 Figure 3. Extraction of glochidia .................................................................................. 28 Figure 4. Polyurethane foam used as substrate ............................................................. 29 Figure 5. Mesostoma flatworm ..................................................................................... 30 Figure 6. Penis stylet of Macrostomum tuba ................................................................ 31 vi INTRODUCTION This study concerns the predatory behavior of flatworms in the Class Turbellaria of the Phylum Platyhelminthes. The phylogeny of Platyhelminthes is poorly understood and there is currently no universally accepted classification of the phylum (Tyler, 1999). The Class Turbellaria is probably not a monophyletic group, but it is retained in most recent classifications pending further clarification of relationships. Within the Turbellaria, the species considered in this study have historically been classified in the Order Rhabdocoela. Like Turbellaria, Rhabdocoela is also no longer thought to be a phylogenetic grouping, and the species in this taxon have recently been distributed into 3 orders, the Catenulida, Macrostomida, and Neorhabdocoela (Tyler, 1999; Pechenik, 2000). Nevertheless, small turbellarians with linear guts are still commonly referred to as rhabdocoels (="linear gut"). Small Turbellaria (~4 mm or smaller), which are mainly rhabodocoels, are also referred to as "microturbellaria". Species having larger body size (mainly triclads and polyclads) are referred to as "macroturbellaria" (Pennak, 1989; Tyler, 1999). There are approximately 400 species of freshwater microturbellaria, of which approximately 150 occur in North America (Kolasa, 2001). Diversity in lakes and ponds in temperate regions can range from 20 to 60 species in a single habitat, depending on nutrient availability and other factors (Kolasa, 2001). Freshwater turbellarians can be found in a variety of habitats including ponds, lakes, streams, and temporary pools. Most are negatively phototactic and many occupy interstitial spaces in benthic habitats (Pennak, 1989). The habitat of microturbellarians ranges from cold areas like Igloolik, NWT, Canada (68o north latitude) to warm tropical areas to caves and underground 1 waters. As many as 40,000 individuals per square meter can occur in littoral zones of oligotrophic lakes (Kolasa, 2001). Turbellaria may reproduce either asexually or