DISTRIBUTION AND MICROHABITAT SELECTION OF HEMIGRAPSUS OREGONENSIS (DANA) AND PACHYGRAPSUS CRASSIPES RANDALL IN ELKHORN SLOUGH, MONTEREY COUNTY, CALIFORNIA A Thesis Presented to the Graduate Faculty of California State University, Hayward In Partial Fulfillment of the Requirements for the Degree Master of Science in Biology By Mark C. Sliger March 1982 Copyright © 1982 by Mark C. Sliger ii ABSTRACT The vertical distribution and habitat selection of two species of grapsid crabs Hemigrapsus oregonensis (Dana) and Pachygrapsus crassipes Randall along the main channel bank of Elkhorn Slough, Monterey County, California was investigated. While the vertical distribution of the two crab species was found to overlap, H. oregonensis typically occupied burrows in the lower region of the bank and P. crassipes was usually found in burrows located in the upper bank or in bank slumps located on the lower mudflat. Substratum and tidal elevation were found to be the most important factors influencing crab distribution along the banks of Elkhorn Slough. Both H. oregonensis and P. crassipes had similar resistance to desiccation abilities, however smaller members of each crab species were more susceptible to desiccation. · Hernigrapsus oregonensis was found to be able to tolerate silty-clay water while P. crassipes was highly susceptible to small, unconsoli­ dated mud particles. iii DISTRIBUTION AND MICROHABITAT SELECTION OF HEMIGRAPSUS OREGONENSIS (DANA) AND PACHYGRAPSUS SIPES RANDALL IN ELKHORN SLOUGH, MONTEREY COUNTY, CALIFORNIA By Nark C. Sliger Date: ' .,.( 7. -i"-- . ...: ~-- 17 [ iv ACKNOWLEDGMENTS There are a great number of people without whose help this work would not have been completed. Financial support was provided by Sea Grant #R/CZ-45. I wish to thank the members of my committee, Drs. James Nybakken, Pamela Roe and Gregor Caillier for their continued support through the duration of this research. I am especially grateful to Dr. Pamela Roe for her contagious vitality, inspiration and encouragement during times of crisis. Thanks to Dr. Ann Hurley for providing assistance with statistical analysis and experimental design during the critical preliminary phase of the study. I am grateful to Dr. John Oliver for his stimulating conversations and cr ical review of the initial draft. Chris Jong deserves special thanks for providing friendship, thought-provoking ideas and valuable assistance in collecting data. Her undaunted spir contributed greatly to the successful comp tion of this research. I wish to express my gratitude to numerous iends at the Moss Landing Marine Laboratories who supported and encouraged me throughout this study. In particular, I am indebted to Signe Johnsen for her help with computer analysis; Sheila Baldridge, the librarian, for locating needed references; Rosie Stelow for editing the manuscript; v . vi and Lynn McMasters for her exceptional illustrations. I also greatly appreciate the assistance of Joy Milhaven, Neal Scanlon and Fred Lauber with the field work. Special thanks to the "Benthic Bubs" for the camaraderie and help in unraveling some of the mysteries of Elkhorn Slough. My sincere appreciation to Valerie Breda, Debbie Fellows and Melanie Mayer for their advice and emotional support. Finally, I extend the deepest appreciation to my family for their understanding, love, and support. TABLE OF CONTENTS ABSTRACT o o • • iii ACKNOWLEDGMENTS v LIST OF TABLES . ix LIST OF FIGURES x· INTRODUCTION . 1 METHODS AND MATERIALS 4 Study Area 4 Crab Distribution . 5 Vertical Bank 5 Bank Slumps 6 Physical Properties of the Channel Bank 7 Erosion 7 Exposure 10 Field Experiments 11 Tidal Height Preference 11 Bank Region Preference 13 Substratum Transferal 13 Laboratory Experiments 14 Desiccation 14 Tolerance to Silty-Clay Water 15 RESULTS 16 vii viii Page Crab Distribution . 16 Vertical Bank 16 Bank Slumps 17 Physical Properties of the Channel Bank 17 Field Experiments 19 Tidal Height Preference 19 Bank Region Preference . 20 Substratum Transferal 20 Laboratory Experiments 21 Desiccation 21 Tolerance to Silty-Clay Water 21 DISCUSSION . 22 SUMMARY 30 LITERATURE CITED 31 TABLES . 35 FIGURES 39 - LIST OF TABLES Table Page 1. Mean densities and mean differences in densities of Pachygrapsus crassipes and Hemigrapsus oregonensis collected in 0.25 m2 quadrats in Elkhorn Slough from December 1979 to May 1980 . .. 35 2. Summary of stat tical analysis for the physical charac tics of the upper and lower bank regions of the main channel bank of Elkhorn Slough . 36 3. Summary of chi-square tests for tidal height preferences of Pachygrapsus crassipes and Hemigrapsus oregonensis . 37 ! 4. Summary of chi-square tests for the bank region preferences of Pachygrapsus crassipes and Hemigrapsus oregonensis . 38 'i ix LIST OF FIGURES Figure Page 1. Map of Elkhorn Slough showing position of study site . 39 2. Photograph bank slumps at the study site in Elkhorn Slough during low tide 41 3. Diagram of the device used in the substratum erodibility experiment . 43 4. Artificial substratum cage against the main channel bank of the study site during low tide . 45 5. Size frequency diagram of all Hemigrapsus oregonensis collected from mud burrows of the upper bank region . 47 6. Size frequency diagram of all Hemigrapsus oregonensis collected from mud burrows of the lower bank region . 49 7. Size frequency diagram of all Pachygrapsus crassipes collected from mud burrows of the upper bank region . 51 8. Size frequency diagram of all Pachygrapsus crassipes collected from mud burrows of the lower bank region . 53 9. Results of five Vertical Height Preference/ Species Interaction experiments . 55 10. Tolerance to desiccation of Hemigrapsus oregonensis and Pachygrapsus crassipes 57 11. Regression of desiccation survival time of Hemigrapsus oregonensis against crab size as determined by carapace width . 59 12. Regression of desiccation survival time of Pachygrapsus crassipes against crab s e as determined by carapace width 61 13. Tolerance to silty-clay water 63 X INTRODUCTION While the study of the distribution and habitat preference of brachyuran crabs has been generally limited i . to works which concentrated on the burrowing, depos feed- i: .r' I , I , ing ocypoid crabs of the genus Uca (for a review, see i ~ Crane, 1975), a number of investigators have focused on the crabs of the family Grapsidae. For example, Bacon (1971) studied the distribution of Cyclograpsus insularum and Cyclograpsus lavauxi and found substratum and tidal elevation important factors for habitat selection. Sub- stratum and behavior were suggested by Abele (1973) as important factors in limiting the distribution in Florida of six species of closely associated grapsid crabs of the genus Sesarma. Salinity has been found to affect the distribution of several species of grapsid crabs (Snelling, 1959; Jones, 1976; Seiple, 1979) and Kikuchi et al. (1981) found that the distribution of the pebble crab (Gaetice depressus was correlat with beach elevation. Although MacGinitie (1935) f st noted the occurrence of the grapsid crabs Hemigrapsus oregonensis, Hemigrapsus nudus and Pachygrapsus crassipes among rocks or in inter- tidal mud burrows in Elkhorn Slough, California and additional studies (Knudsen, 1964; Ricketts and Calvin, 1968; Batie, 1974) have shown that Pacific coast populations of 1 2 the three grapsid crabs generally form distinct distribu- tional patterns, few authors have explored experimentally the biological and physical parameters influencing the distribution and habitat selection of these crabs. Hiatt (1948) suggested that the observed habitats of H. oregonensis, H. nudus and P. crassipes along the coast of California were influenced by substratum and desiccation. He in- vestigated the relative abilities of the three grapsid crabs to withstand siccation and found that ~· crassipes and H. nudus apparently had greater tolerances to desiccation than H. oregonensis. Low (1970) found that the divergent habitat preferences of H. oregonensis and H. nudus ~ I were influenced by different physiological tolerances I~ to muddy water and low oxygen concentrations. Hemigrapsus oregonensis outlived H. nudus when both spec s of crabs I. were placed in flasks filled with low oxygenated, muddy \ water. His results were consistent with Hiatt's (1948) suggestion that morphological differences in the respira- tory systems of H. oregonensis, H. nudus and P. affected the ability of the di erent cies to survive on fine particulate substratum. Willason (1981) found that the distribution and coexistence of P. crassipes and H. oregonensis a southern Californian saltrnar were primarily the result of predation. By preying on H. P. crassipes restrict that crab spec s to 3 lower intertidal areas. He suggested that a possible release mechanism for H. oregonensis was the inability of small P. crassipes to cope with the stressful estuarine environment. Hemigrapsus oregonensis (Dana) and Pachygrapsus crassipes Randall form an important part of the invertebrate macrofauna of Elkhorn Slough, Monterey County, California. Both species of crabs occupy similar habitats, generally found in burrows located in the intertidal mudflat or pickleweed Salicornia virginica) marsh. The present study examined the abundance and distribution of H. oregonensis and P. crassipes along the intertidal mud banks of this embayment and sought to answer questions concerning the limits of their distribution and habitat selection and how these m~y be influenced by tolerances to environmental variables and crab agonistic behavior.