Reproductive Success in Antarctic Marine Invertebrates
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When referring to this work, full bibliographic details including the author, title, awarding institution and date of the thesis must be given e.g. AUTHOR (year of submission) "Full thesis title", University of Southampton, name of the University School or Department, PhD Thesis, pagination http://eprints.soton.ac.uk UNIVERSITY OF SOUTHAMPTON FACULTY OF SCIENCE School of Ocean and Earth Science Reproductive Success in Antarctic Marine Invertebrates By Laura Joanne Grange (BSc. Hons) Thesis for the degree of Doctor of Philosophy July 2005 Dedicated to my Mum, Dad, Sam and my one and only Mike. UNIVERSITY OF SOUTHAMPTON ABSTRACT FACULTY OF SCIENCE SCHOOL OF OCEAN AND EARTH SCIENCE Doctor of Philosophy REPRODUCTIVE SUCCESS IN ANTARCTIC MARINE INVERTEBRATES By Laura Joanne Grange The nearshore Antarctic marine environment is unique, characterised by low but constant temperatures that contrast with an intense peak in productivity. As a result of this stenothermal environment, energy input has a profound ecological effect. These conditions have developed over several millions of years and have resulted in an animal physiology that is highly stenothermal and sometimes closely coupled with the seasonal food supply, e.g. reproductive periodicity and food storage. Therefore, Antarctic marine animals are likely to be amongst the most vulnerable species worldwide to environmental modifications and can be regarded as highly sensitive barometers for change. Reproductive success is a vital characteristic in species survival and evaluation of change in reproductive condition with time key to identifying vulnerable taxa. Characterising reproductive success with time is a major requirement in predicting species response to change and the early stages of species loss. Some invertebrates are highly abundant in shallow water sites around the Antarctic and form conspicuous members of the Antarctic benthos. Three common echinoderms and one nemertean were sampled from sites adjacent to the British Antarctic Survey’s Rothera Research Station, Adelaide Island, on the West Antarctic Peninsula between 1997-2001. Reproductive patterns were determined by histological analyses of gonad tissue. This study provided further evidence for inter-annual variation in Antarctic gametogenic development, which appeared to be driven to some extent by trophic position and reliance on the seasonal phytoplankton bloom. The largest variation in reproductive condition was demonstrated for the detritivorous brittle star, Ophionotus victoriae. The seasonal tempos of this echinoderm have been attributed in part, to the seasonal sedimentation events common in the high Antarctic. The reproductive patterns in the scavenging starfish, Odontaster validus and the predatory nemertean, Parborlasia corrugatus showed less inter-annual variation. The de-coupling of these invertebrates from the intensely seasonal phytoplankton bloom appeared to partially account for the reproductive trends observed. The lack of inter-annual variation in the reproduction of the filter-feeding sea-cucumber, Heterocucumis steineni, was somewhat counterintuitive, although problems with sample processing probably accounted for the majority of this anomaly. Echinoderms were also collected during the Antarctic summer field seasons in 2003 and 2004. A series of fertilisation success studies were undertaken comparing the adaptations in an Antarctic and an equivalent temperate starfish to achieve optimal numbers of fertilised eggs, and elemental analyses were used to estimate the nutritional and energetic condition of the bodily and reproductive tissues in two Antarctic echinoderms. Fertilisation studies indicated that Antarctic invertebrates require 1-2 orders of magnitude more sperm to ensure optimal fertilisation success. These sperm tended to be long-lived and capable of fertilising eggs 24+ hours after release. The study suggested that synchronous spawning, aggregations and specific pre-spawning behaviour are employed to help counter the deleterious effects of sperm limitation. The Antarctic eggs and sperm were also highly sensitive to even small modifications in temperature and salinity, affecting the number of eggs fertilised. Such stenothermy is of particular relevance if the 1-2ºC rise in global temperature, predicted over the next century, is realised. Biochemical composition of body components of two species of Antarctic echinoderm indicated a significant difference in the composition between the male and female gonad, particularly in the Antarctic brittle star Ophionotus victoriae. The ovaries contained a much larger proportion of lipid compared to the testes, and demonstrated a distinct seasonality in composition. Higher levels of lipid were observed in the ovary during the austral winter coincident with a period of reproductive investment and maturing oocytes in the gonad. O. victoriae exhibited lower amounts of lipid in the late austral spring suggesting the removal of mature oocytes from the ovary through spawning. The seasonality in composition and the high levels of lipid and protein measured in the ophiuroid gut tissue, suggested the gut might play a role in providing material and energy for metabolic function and possibly gametogenesis; higher lipid levels were apparent during the period of seasonal phytodetrital flux. The role of the pyloric ceaca in asteroids as a nutrient storage organ was also evident in the high levels of both protein and lipid observed in this bodily component in the star fish, Odontaster validus. Table of Contents Chapter 1. Introduction 1.1 The Antarctic Ecosystem 1-10 1.2 Benthic Flora and Fauna 10-12 1.3 Antarctic Reproduction 12-22 1.4 Inter-annual Variation 22-25 1.5 Project Aims 25-26 Chapter 2. Long-term Reproductive Cycles 2.1 Introduction 27-35 2.2 Method 36-51 2.3 Results 52-98 2.4 Discussion 99-127 Chapter 3. Fertilisation Kinetics 3.1 Introduction 128-136 3.2 Methods 137-153 3.3 Results 154-178 3.4 Discussion 179-196 Chapter 4. Tissue Composition and Condition 4.1 Introduction 197-204 4.2 Methods 205-217 4.3 Results 218-249 4.4 Discussion 250-258 Chapter 5. Synopsis 259-266 Bibliography 267-319 Appendices Tables and Figures 320-364 Grange et al., (2004) Reprint Back Cover Figures Chapter 1. Introduction Fig. 1.1.1 Adapted from Clarke and Johnston (2003). Map of the Southern Ocean showing the mean position of the Polar Front (the Antarctic Convergence) and broad scale bathymetry around the Antarctic continent. Regions of East and West Antarctica and the Antarctic Peninsula are also indicated, including the South Orkney Islands. Chapter 2. Long-term Reproductive Cycles Fig. 2.2.1 The British Antarctic Survey Rothera Research Station and shallow water sampling sites. Fig. 2.2.2 The brittle star Ophionotus victoriae (A), the starfish Odontaster validus (B), the nemertean Parborlasia corrugatus (C) and the sea cucumber Heterocucumis steineni (D) respectively. Fig. 2.2.3 The location of the Rothera sediment trap off Trolval Island and the CTD sampling site in Ryder Bay. Fig. 2.2.4 Ophionotus victoriae. Disc diameter measurements showing the aboral and oral view (A), and male (B) and female (C) dissections identifying the gut and gonad tissue. Fig. 2.2.5 Odontaster validus. Variation in body size caused by internal water content (A), measurements of radial length (R) and body radius (r) using an aboral and oral view (B) and a dissected individual describing the position and appearance of the gonad and pyloric caeca (C). Fig. 2.2.6 Parborlasia corrugatus. Retracted length (mm) is measured and the everted proboscis labelled (A). A 5mm cross-section of the nemertean is labelled to identify regions of the gonad (B). Fig. 2.2.7 Heterocucumis steineni. Animal length (mm) is measured (A) and an individual dissected to illustrate the position of the gonad (B). The gut and cloaca are also labelled. Fig. 2.2.8 Ophionotus victoriae. Female (A) and male (B) histological sections identifying progressive stages in gametogenic development. PO = previtellogenic oocyte, VO = vitellogenic oocyte, N = nucleus, BP = by-products, SC = spermatocytes, SG = spermatogonia and SZ = spermatozoa. Fig. 2.2.9 Odontaster validus. Female (A) and male (B) histological sections identifying progressive stages in gametogenic development. PO = previtellogenic oocyte, VO = vitellogenic oocyte, SC = spermatocytes, SG = spermatogonia and SZ = spermatozoa. Fig. 2.2.10 Heterocucumis steineni. Female (A) and male (B) histological sections identifying progressive stages in gametogenic development. OG = oogonia, PO = previtellogenic oocyte, VO = vitellogenic oocyte, SG = spermatogonia and SZ = spermatozoa. Fig. 2.2.11 Parborlasia corrugatus. Female (A) and male (B) histological sections identifying progressive stages in gametogenic development. PO = previtellogenic oocyte, VO = vitellogenic oocyte, SC = spermatocytes, SG = spermatogonia and SZ = spermatozoa. Fig. 2.3.1 Rothera Time Series Water Sampling Programme (RaTS) environmental data (1997-2001). Julian Day