A genetic approach to the conservation of marine sponges in the Western Indian Ocean, with emphasis on the Mascarene Archipelago
Thesis (PhD)--Stellenbosch University, 2019.
ENGLISH ABSTRACT: Marine sponges are an important component of benthic ecosystems globally. They have evolved to adapt to very different environments, thus resulting in a global distribution. However the vast diversity of sponges is still largely unknown and undescribed. Sponges lack complex morphological characteristics, and this makes it difficult to describe their taxonomic diversity. Skeletal and spicule analysis allows for morphological identification, but these characteristics have certain limitations. With new molecular techniques, the field of sponge taxonomy has recently accelerated, with studies unravelling the classification of sponges with different lineages of same species observed across various geographic regions. This highlights that sponge diversity is vastly underestimated using traditional morphological methods. Consequently, recent studies are using an integrated approach combining morphological taxonomy with molecular techniques to obtain more information on the taxonomic and systematic classification of sponges. The Mascarene Islands comprise of a group of three islands (Rodrigues, Mauritius and Reunion) which emerged from the ocean at different time periods over the past eight million years. Due to its isolation in the middle of the Western Indian Ocean, the Mascarene Islands provide a natural laboratory for phylogenetic and phylogeographic studies. Sponges are considered as the oldest living metazoans and at a regional scale, no studies have been undertaken on sponges in the Mascarenes. This PhD therefore assessed the biodiversity of sponges collected from the shallow waters of all three Mascarene Islands using both morphological and molecular approaches. The samples were identified based on morphological characteristics (spicule and skeletal). With morphological methods, the highest sponge diversity was observed in Mauritius, followed by Reunion and Rodrigues. Two main factors may explain the lowest sponge diversity in Rodrigues, firstly the small size of the island compared to the other two, and secondly its more recent emergence from the ocean. In order to validate the results obtained from the morphological taxonomy, molecular tools were then utilised to assess the biodiversity of the Mascarene sponges. The phylogenetic relationships were established and compared with similar georeferenced species available on GenBank using both mitochondrial CO1 and nuclear 28S markers. The results generally validated the morphological classification, but cryptic lineages were observed for several species thus confirming that the morphological taxonomy underestimated the true sponge biodiversity present. The global phylogenetic analyses revealed that the Mascarene sponges were largely isolated from other species found outside the Mascarene region. A regional phylogenetic reconstruction was undertaken using concatenated CO1 and 28S sequences obtained from the Mascarene sponges, with main findings showing that the sponges found in Mauritius are phylogenetically closest to those from Reunion. Lastly, a phylogeographic study was carried out for the lemon sponge, Leucetta chagosensis, which is a widely distributed sponge species in the Indo-Pacific region. Six major lineages were observed in the phylogenetic tree produced, which reinforced the hypothesis that sponges tend to adapt in their respective geographical locations with physical barriers shaping their evolution. Overall, the sponge biodiversity in the Mascarene region was described using both morphological and molecular methods and their phylogenetic relationships were assessed in the context of global sponges. This study provides an important baseline for understanding sponge biodiversity and evolution in the Mascarene Islands, as well as highlight potential conservation implications. Furthermore, the use of both morphological taxonomy coupled with molecular phylogenies have been shown to be crucial for better understanding the relationship between sponges as well as assessing their diversity.