THE POPULATION BIOLOGY OF LIGIA OCEANICA (L. ) (CRUSTACEA : ONISCOIDEA). by Robert I W*llows. 'IV A thesis submitted in accordance with the requirements for the degree of Doctor of Philosophy. February 1984. Department of Pure and Applied Zoology and Wellcome Marine Laboratory University of Leeds. ii An CMDa (IM The dynamics and life history of an isolated popula- tion of Ligia oceanica (L. ) inhabiting a sea-wall exposed to moderate wave action have been studied over a period of three years. Sex ratio, size at maturity, growth, breeding phenology, size and age structure, population density and size, female breeding size, fecundity regression and brood- pouch mortality have been determined. From these data population natality, and schedules of survivorship, fecund- ity and reproductive value are calculated. Although the supralittoral zone of rocky shores is generally regarded as a severe and stressful habitat, the dynamics of the popula- tion exhibit only minor differences between years, and no direct influence of wave exposure can be determined. This population is compared with another nearby but isolated population occupying an harbour wall completely protected from wave action. These populations are presumed to suffer different levels of density-independent mortality due to the wave exposure characteristics of the sites. The sheltered site population shows no significant differences between the two years of study. Interpopulation differences in age and size structure, growth rate, population breeding phenology and breeding sizes are slight. However the shelt- ered population shows increased adult survival and longevity, decreased weight specific fecundity and reproductive alloca- tion, and increased brood-pouch mortality. Because winter survival is high, when mortality due to wave action is exp- ected to be severe, this factor is rejected as an important determinant of the life history. The reduced survival at the exposed site can be interpreted as a consequence of the increased reproductive allocation by individuals at this site. The majority of females are semelparous, but within both populations two distinct waves of breeding activity occurred each year. Larger, faster growing individuals breed at the end of their first ýear, LATE in the season, and have high weight specific fecundities and low absolute iii and weight specific egg sizes. Smaller, slower growing individuals delay reproduction and breed in their second year, EARLY in the season. Although gaining greatly in size through growth the effect on fecundity is offset by a decr- ease in weight specific fecundity and parallel increases in absolute and weight specific egg size. The following trade-offs are recognised for individual female Ligia : between growth and reproduction (within breeding groups), growth and reproduction (between breeding groups), reproduction and survival (between breeding groups), weight specific egg size and weight specific fecundity (bet- ween breeding groups), and between absolute egg size and fecundity (within breeding groups). Two possible life hist- ory strategies are recognised. (i) An ability of a female parent to alter egg size seasonally may optimise total off- spring fitness in the different environments they exper- ience.. (ii) An ability of small, mature females to delay reproduction until the following year may also increase their fitness. By assuming that offspring survival is rela- ted to egg size it is possible to model this tactic. The population breeding phenology is seen as a direct consequ- ence of the life history and has the indirect effect of stabilizing the population. The ability of individual females to vary their life history traits depending on individual circumstances (size and season) may represent a flexible approach to the environment which is not envisioned in the more formal models of life history evolution. iv ACKNOWLEDGMENTS Many thanks are due to my supervisors, Dr. S. L. Sutton and Dr. J. Grahame for providing the opportunity and support to carry out this work. I would like to express my appreciation for the help and encouragement provided by Ian Jardine, Mike Price, Nicola Dimery, Tim Cramp, Sarah Newbury, Arthur Holliday (photography), John Grahame and Stephen Sutton. I am especially grateful to Jonathan Rosewell vvho5e ability and willingness to discuss many diverse aspects of ecology has been to my benefit. I thank Prof. R. McIleill Alexander and Dr. J. R. Lewis for the extensive use of laboratory facilities. The work was carried out under tenure of a studentship from NERC. V CONTENTS Page ABSTRACT ii ACKNOWLEDGMENTS iv ABBREVIATIONS ix CHAPTER 1: LIFE HISTORIES AND WOODLICE 1- INTRODUCTION 1 2. LIFE HISTORY STRATEGIES 2 i. r- and K-selection and related matters 2 ii. Bet-hedging and stocha tic models 7 iii. Reproductive effort and optimality models 8 3. WOODLOUSE POPULATIONS AND LIFE HISTORIES 12 i. Population regulation 13 ii. Life histories is CHAPTER 2: POPULATION DYNAMICS AT AN EXPOSED SITE AT ROBIN HOODIS BAY 1. I14TRODUCTION is 2. SITE DESCRIPTION 19 3. POPULATION DENSITY AND STRUCTURE 21 i. Sampling methods 21 ii. Results 22 a. Spatial distribution 22 b. Sex ratio and maturity 24 c. Population age and size structure 26 d. Adult population density 27 e. Breeding phenology 28 4. POPULATION SIZE 30 i. Sampling methods 30 ii. Testing the assumptions 31 a. Loss of marks 32 b. Initial marking mortality 32 c. Age of marks 32 vi Page CHAPTER 2 (Continued) d. Constant survival 33 e. Bias in sex ratio 33 f. Size bias 33 g. Dispersal 33 iii. Results 34 iV. Discussion 35 5. FECUNDITY 37 i. Methods 37 ii. Results 37 a. Fecundity regressions 37 b. Brood pouch mortality 38 6. NATALITY AND RECRUIT DENSITY 40 i. Estimation of natality 40 ii. Recruit density 41 7. SURVIVORSHIP AND LIFE TABLES 43 8. FERTILITY AND REPRODUCTIVE VALUE 46 9. DISCUSSION 48 CHAPTER 3: POPULATION DYNAMICS AT A SHELTERED SITE AT WHITBY, AND COMPARISON TO ROBIN HOOD*S BAY 1. INTRODUCTION 56 2. SITE DESCRIPTION AND SAMPLING METHODS 58 3. RESULTS 59 i. Sex ratio and maturity 59 ii. Population age and size structure 60 iii. Breeding phenology and size at reproduction 60 iV. Fecundity 61 a. Whitby 61 b. Comparison between Whitby and 63 Robin Hood's Bay c. Brood pouch mortality 64 v. Field growth rates 65 vi. Survivorship . 66 4. DISCUSSION 67 vii Page CHAPTER 4: POPULATION ENERGETICS OF LIGIA OCEANICA AT ROBIN HOOD'S BAY 1. INTRODUCTION 72 2. THE DATA 74 i. Water, ash and caloric content 74 ii. Field growth 77 iii. Assimilation efficiency 79 iv. Moulting 80 V. Reproduction 83 vi. Respiration 84 3. THE MODEL as i. The methodology 85 a. Growth, Pg 86 b. Respiration, R 86 C. Moulting, Ex 87 d. Reproduction, Pr 88 e. Excretion, U 88 f. Consumption, C and Egestion, F 89 ii. The results 89 iii. Evaluation of the model 90 4. DISCUSSION 95 CHAPTER 5: COMPARISON OF FECUNDITY, REPRODUCTIVE ALLOCATION AND EGG SIZE BETWEEN BREEDING GROUPS AND POPULATIONS 1. INTRODUCTION 102 2. METHODS AND STATISTICAL TREATMENT 103 3. RESULTS 104 i. Fecundity 104 a. Differences between breeding groups 104 b. Differences between populations 104 ii. Reproductive allocation 104 a. Differences between breeding groups 104 b. Differences between populations 108 viii CHAPTER 5 (continued) Page iii. Egg size 108 a. Differences between breeding groups 108 b. Differences between populations 109 iv. Variation in fecu ndity, reproductive 110 allocation and eg size 4. DISCUSSION 115 CHAPTER 6: POPULATION BREEDING PHENOLOGY AND INDIVIDUAL FITNESS 1. INTRODUCTION 119 2. BREEDING PHENOLOGIES 120 i. occurrence 120 ii. Population regulation 121 iii. Individual patterns of brood production 122 in Ligia oceanica a. The evidence 122 b. Discussion 124 iv. A general explanation 126 3. DOES LIGIA HAVE A STRATEGY? 129 i. Seasonal tactics 130 a. Variation in egg size and fecundity 130 b. Explanations 131 ii. Breeding strategy 134 4. POSTSCRIPT 139 BIBLIOGRAPHY 140 APPENDIX 164 ix ABBREVIATIONS Abbreviations of test statistics follow Sokal & Rohlf (1969,1981) unless stated otherwise. Energy flow terminology follows I. B. P. nomenclature (Petrusewicz & MacFadyen, 1970). S. I. units have been used throughout. The following abbreviations have been used in the text : R. H. B. Robin Hood's Bay DW Dry weight AFDW Ash-free dry weight ANOVA Analysis of variance ANCOVA Analysis of covariance other abbreviations are defined where used in the text. Tt if ý 'po 'v- M, -.,. ., vs, -Ole The sea-slater Ligia oceanica (Linnaeus, 1767), male, about 22mm head to telson length. This specimen illustrates the marking technique des- cribed in Chapter 2. Small drops of cyanoacry- late ester are applied from a syringe needle to the dorsal segments, followed by small drops of cellulose paint. Four marks are illustrated red, yellow, purple and orange. This animal has recently moulted the posterior part of its exoskeleton. GREENAWAY I know so well this turfy mile, These clumps of sea-pink withered brown, The breezy cliff, the awkward stile, The sandy path that takes me down. To crackling layers of broken slate Where black and flat sea-woodlice crawl And isolated rock pools wait Wash from the highest tide of all. I know the roughly blasted track That skirts a small and smelly bay And over squelching bladderwrack Leads to the beach at Greenaway. Down on the shingle safe at last I hear the slowly dragging roar As mighty rollers mount to cast Small coal and seaweed on the shore, And spurting far as it can reach The shooting surf comes hissing round To heave a line along the beach Of cow$ries waiting to be found. Tide after tide by night and day The breakers battle with the land And rounded smooth along the bay The faithful rocks protecting stand. But in a dream the other night I saw this coastline from the sea And felt the breakers plunging white Their weight of water over me. There were the stile, the turf, the shore, The safety line of shingle beach With every stroke I struck the more The backwash sucked me out of reach.