Hermit Crabs: Their Distribution, Abundance, Population Structure and Associations in the Southern Benguela
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Deep water parapagurid hermit crabs: their distribution, abundance, population structure and associations in the Southern Benguela Amy Grace Wright A dissertation submitted in fulfilment of the degree of Master of Science in the Department of Biological Sciences, Faculty of Science, University of Cape Town University of Cape Town Supervisor: Emeritus Professor Charles L. Griffiths January 2017 1 The copyright of this thesis vests in the author. No quotation from it or information derived from it is to be published without full acknowledgement of the source. The thesis is to be used for private study or non- commercial research purposes only. Published by the University of Cape Town (UCT) in terms of the non-exclusive license granted to UCT by the author. University of Cape Town Acknowledgements “It is a dangerous business, Frodo, going out your door…” It’s an adventurous business to be sure, to set off on an adventure like an MSc by dissertation, working on animals very few people have studied before, and none of which I would see alive. And what sort of adventure would it be without a little danger, a little rain and a little fire? I could not have gone there and back again without the guidance, support, humour and wisdom of my ever patient supervisor, Emeritus Professor Charles Griffiths. I am forever grateful for your lessons on science, life and joy. To my merry band of fellow Team Hermit conspirators – Jannes, Arno, Jackson and Robyn – thank you for your hard work and enthusiasm for interesting questions, zoanthid “goo” and decapods in general. Hats off to Lara Atkinson for her organisation and advice, to Tracey Fairweather for data, proof reads and guidance, and to the collection team from DAFF, who go to sea come-what-may and who started collecting “monkey nuts” all those years ago. Thanks to my fellow detainees in the postgraduate lab, especially Jess and Mark, who kept me sane and challenged me in equal measure. Since this thesis would have remained but a dream without the support of my funders, I must thank the NRF, the Marine Research Institute of UCT and the Cape Tercentenary Foundation. I am especially grateful to my parents, for their support, love and enthusiasm for my dreams, and to the rest of the family, for listening to two solid years of hermit talk. And finally, to Hilton – thank you for your boundless love, your endless humour and your patient support. 2 Plagiarism Declaration I, Amy Grace Wright, hereby declare that the work on which this thesis is based is my original work (except where acknowledgements indicate otherwise) and that neither the whole work nor any part of it has been, is being, or is to be submitted for another degree in this or any other university. I authorise the University to reproduce for the purpose of research either the whole or any portion of the contents in any manner whatsoever. I confirm that the uploaded files are the correct versions for the Library/ OpenUCT. A supporting paper is included in Appendix 2 for consideration by the examiner. As a co-supervisor of the project undertaken by TPA Botha, I participated in all of the laboratory work involved, and played a role in the conceptualisation and management of the project. Signature: ________________ Date: ___2017-02-16________ 3 Contents Abstract ........................................................................................................................... 5 Chapter 1 ......................................................................................................................... 6 Hermit crabs, their shelters and their associates: a review of gastropod shell use and Cnidarian associations Chapter 2 ....................................................................................................................... 25 Population structure and morphology of South African deep-water parapagurid hermit crabs Introduction ............................................................................................................................. 26 Methods ................................................................................................................................... 28 Results ...................................................................................................................................... 32 Discussion ................................................................................................................................. 46 Chapter 3 ....................................................................................................................... 54 Distribution and abundance patterns of deep water Parapaguridae hermit crabs (Crustacea: Decapoda: Anomura) along the west and south coasts of South Africa Introduction ............................................................................................................................. 54 Methods ................................................................................................................................... 57 Results ...................................................................................................................................... 64 Discussion ................................................................................................................................. 75 Chapter 4 ....................................................................................................................... 82 Long-term changes in distribution and abundance of South African hermit crabs of the family Parapaguridae (Crustacea: Decapoda: Anomura) Introduction ............................................................................................................................. 83 Methods ................................................................................................................................... 86 Results ...................................................................................................................................... 88 Discussion ................................................................................................................................. 93 Conclusions .................................................................................................................... 98 References ................................................................................................................... 104 Appendices .................................................................................................................. 121 Appendix 1: Trawl survey cruise summary by year ................................................................ 121 Appendix 2: Supporting Paper ............................................................................................... 123 4 Abstract The parapagurids Sympagurus dimorphus (Studer, 1883) and Parapagurus bouvieri (Stebbing, 1910) dominate South African deeper-water benthic invertebrate communities. Samples from the 2016 DAFF bi-annual hake demersal research trawl surveys provided data on population structure and morphology. Since these species forego the use of “traditional” gastropod shells in favour of symbiotically-associated Epizoanthus pseudoshells, these data were also used to investigate parameters of pseudoshell association. S. dimorphus to have a female-biased sex ratio of 1:1.5, and P. bouvieri of 1:1. The proportion of males in both species increased with cephalothoracic shield (CL) length. S. dimorphus males had positive allometric major right chelipeds, while P. bouvieri cheliped lengths were positively allometric in males, and negatively allometric in females. All collected P. bouvieri occurred in pseudoshells, but S. dimorphus showed a real shell occupancy rate of 3.39% (n = 38). A positive relationship was found between CL size and real shell occupation frequency (r2 = 0.96). S. dimorphus pseudoshell zoanthid polyps increased in number as a function of both total shell volume (R2 = 0.73) and colony height (R2 = 0.61), with polyps present on every available part of the shell. No such relationship was found for P. bouvieri, presumably because the polyps are distributed in a linear pattern around the back and outside of the pseudoshell. Juvenile parapagurids appear to select for Naticidae original shells, with Euspira napus being the dominant original shell species for both S. dimorphus (45.45 %) and P. bouvieri (39.00 %). Questions are raised about whether these hermits exist along an evolutionary continuum of traditional shell reliance, whereby the species utilise their pseudoshells differently - P. bouvieri appears to show a stronger mutualistic symbiosis with its exclusive pseudoshell species than S. dimorphus, and may have lost the ability to change shelters entirely, utilising the pseudoshell as a brood pouch covering only the abdomen, rather than as a shell in which to retract to escape predation. Data from 1987 – 2014 surveys were used to map the distribution patterns and densities of these species in the southern Benguela, and to investigate changes in the populations over time. As it stands, no correction factor need be applied to South African parapagurid catches as a result of the 2003-2004 change in survey gear. However, further work is needed. S. dimorphus occurred at a significantly greater mean sample mass (287.88 kg.km-2) than P. bouvieri (31.37 kg.km-2). The bulk of sample mass occurred within the West Coast Benguela upwelling region, with very few parapagurids caught over the Agulhas Bank, which may be too shallow for