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Population Genetic Structure and Connectivity of the Abundant Sea Urchin, Diadema Setosum Around Unguja Island (Zanzibar)

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Population Genetic Structure and Connectivity of the Abundant Sea Urchin, Diadema Setosum Around Unguja Island (Zanzibar) Södertörns Högskola Population genetic structure and connectivity of the abundant sea urchin, Diadema setosum around Unguja island (Zanzibar) Uppsats VT 2009 Författare:Josefine Larsson Handledare: Oskar Henriksson & Mats Grahn 1 Abstract The distribution and abundance of many coral reef organisms are affected by their predator’s distribution and abundance. Loss of predators may cause a shift in species compositions that will cascade down to other ecological processes on the reef. One example of a shift like this is the growing sea urchin populations inhabiting the coral reefs of East Africa. Areas with high fishing pressure often have large populations of sea urchins. The large populations of sea urchins have a negative impact on the reef ecology both by their grazing and bio‐erosion as well as on fish growth and the recovery of fish populations. Previous population genetic studies conducted on Diadema setsosum, using mtDNA and allozymes, found genetic structuring between populations on a large geographical and evolutionary scale. The aim of this study was to examine the genetic population structure of the sea urchin Diadema setosum, at four sites around Zanzibar. We used the amplified fragment length polymorphism (AFLP) technique, a fast and effective method with high resolution. The long term objective is to understand the migration pattern and colonization of D. setosum to facilitate possible management actions. We found a significant genetic structuring of D. setosum hence the populations can not be considered panmictic. The reason behind this structure does not seem to be based on the geography nor size. One possible explanation might be that the structure lies on a larger geographical scale than we have studied, further studies around the Western Indian Ocean may reveal this. Another explanation may be that the structuring is due to differences in spawning time between the different phenotypes and an analysis of gonad maturations may give information about this. To find the reasons behind the observed genetic structure is of great importance for management of the sea urchins and therefore the management of whole reef ecosystems. Key words: Diadema setosum, population structure, Zanzibar, AFLP 2 Table of Contents Introduction ......................................................................................................................................... 4 Material and Methods ......................................................................................................................... 6 Study area ................................................................................................................................................. 6 Study sites ................................................................................................................................................. 7 Sampling .................................................................................................................................................... 8 Lab ............................................................................................................................................................. 9 Morphological parameters ................................................................................................................... 9 Dissection .............................................................................................................................................. 9 Sex determination ................................................................................................................................. 9 DNA extractions .................................................................................................................................... 9 Quantifying DNA ................................................................................................................................. 10 AFLP analysis ....................................................................................................................................... 10 Data analyses .......................................................................................................................................... 11 Results ............................................................................................................................................... 11 Amplification and primer combinations .................................................................................................. 12 Genetic variation ..................................................................................................................................... 13 Discussion .......................................................................................................................................... 16 Acknowledgements ........................................................................................................................... 18 References ......................................................................................................................................... 19 3 Introduction The beautiful and highly diverse coral reef ecosystems are one of the most productive ecosystems on earths and they provide food and income for millions of people. The coral reef ecosystems are endangered, due to over‐exploitation, pollution, sedimentation and costal development (Wilkinson 2004). One large problem facing the coral reefs is overfishing, the excessive fishing cause problems for the whole reef ecosystem. One example of this problem is the large populations of sea urchins, inhabiting the coral reefs of East Africa. As the predator populations are decreasing due to the intensive fishing, the sea urchin population are increasing and causing a problem (McClanhan 1998). One of the most abundant species of sea urchins is the long spined Diadema setsoum (Leske). D. setsoum play a major role in the bentic ecology and the bio‐erosion of the coral reef as it has one of the highest bio‐erosion and herbivory rates among the sea urchins (Carrerio‐Silva and McClanahan 2001). The Tanzania coast and its islands are surrounded by fringing and patchy coral reefs including highly productive biotopes as mangroves, sea grass beds and estuaries, with a high biodiversity of marine animals and plants. The people of this area utilize marine ecological goods and services (fish is one of the most important resources) for survival and as a source of income. The demand for these goods and services has increased and the extensive utilisation of the coastal resources has negative impacts on the coastal ecosystem with the effect of a significant decline in the productivity and biodiversity of marine fisheries. The inshore fishing areas and several of the shallow reefs surrounding the coasts of Tanzania and Zanzibar are considered overexploited and degraded (Sesabo & Tol 1999, Jiddawi & Öhman 2002, Silva 2006). A population’s genetic variation and evolutionary potential are influenced by a number of factors; gene flow/exchange of alleles for example homogenizes allele frequencies as well as the degree of isolation and effective population size. Gene flow between populations in different geographically areas will be restricted by isolation by distance (Hardy & Vekemans 1999) hence, these populations will have different genetic structures due to random genetic drift (Carvalho 1993 and Herbert 2000). For benthic sessile organism the pelagic larval stage is a crucial step for connectivity and gene flow, given that the passive drift with ocean currents will assist the dispersal over both small and large geographical distances (Cowen & Sponaugle 2009). Consequently the duration of the larval stage is an important factor for how far an organism can spread; this determines the gene flow between populations (Strathmann 1978). Knowledge about gene flow and genetic structure among populations has important implications not only for ecology and evolutionary biology but also for conservation and management of biodiversity (Cowen & Sponaugle 2009). The distribution and abundance of many coral reef organism’s are affected by their predator’s distribution and abundance and fish is one of the most important predators (Neudecker 1979, Wellington 1982, Hay et al. 1983, Hay 1984a,b, Lewis 1986, McClanahan 1989, McClanahan & Muthiga 1989 and Carreiro‐Silva & McClanahan 2001), high level of fishing will affect the whole coral reef ecosystem (Jiddawi & Öhman 2002). Consequently a loss of predators may cause a 4 shift in species compositions due to the fact that the competitive interactions (both intra and inter specific) will change in the absence of predators, cascading down to other ecological processes on the reef (McClanahan & Shafir 1990). One example of a shift like this is the growing sea urchin populations inhabiting the coral reefs. Reefs with a high level of fishing activities often have large populations of sea urchins (Hay 1984a and McClanahan & Muthiga 1988) because the major predators of the sea urchins; trigger fish (Balistid) and terminal‐male wrasses (Labridae) are being extensively fished (McClanahan 1995, 1998, 2000). The large populations of sea urchins have negative impact on the reef ecology by their grazing and bio‐ erosion (Dumas et al. 2007) as well a negative impact on fish growth and the recovery of fish populations, even on reefs where the fishing is reduced or eliminated (McClanahan 1995). When the urchins feed on the reef they remove a large proportion of calcium
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