A genetic approach to the conservation of marine sponges in the Western Indian Ocean, with emphasis on the Mascarene Archipelago
Date
2019-12
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : Stellenbosch University
Abstract
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.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
Description
Thesis (PhD)--Stellenbosch University, 2019.
Keywords
Sponges -- Ecology, Sponges -- Identification, Marine biodiversity conservation, Sponges -- Western Indian Ocean -- Mascarene Archipelago, Phylogeny -- Analysis, UCTD