Diversity and evolution of coral-dwelling gall crabs (Cryptochiridae: Opecarcinus)

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Abstract

Coral reefs are believed to have the greatest biodiversity of all ecosystems on earth, with numerous species living in a symbiotic relationship with invertebrate hosts. Brachyuran crabs are one of the most diverse taxa in coral reef communities and several species exhibit obligate associations. This thesis aimed to uncover the evolution and diversification of coral-dwelling gall crabs (Cryptochiridae), with a particular focus on the circumtropical genus Opecarcinus, to gain insights into the evolutionary history of these crabs living in obligate symbiosis with Agariciidae corals. Furthermore this thesis briefly discusses the relationship between coral colony size/depth and assemblages of coral-dwelling symbionts (including gall crabs).
Chapter 1 provided a brief introduction to symbiosis within coral reef ecosystems and offered general background information on symbiotic gall crabs.
Chapter 2 revealed the diversity and historical biogeography of Opecarcinus using a global dataset of DNA sequences. Multiple species delimitation tests were applied to characterize species diversity, leading to the identification of at least 16 putative new species. A Bayesian molecular clock time-calibrated phylogeny was conducted to estimate the divergence time, placing the time to the most recent common ancestor (tMRCA) at 15−6 Mya (middle Miocene—late Miocene). Furthermore, a Dispersal-Extinction Cladogenesis model (DEC) was employed to infer the ancestral area and dispersal history, indicating that the Indo-West Pacific was the most probable ancestral area, from where the Opecarcinus lineage colonized the Western Atlantic and subsequently speciated into a new species. The inference suggested that Opecarcinus likely crossed the East Pacific Barrier from the Indo-West Pacific to the Atlantic before the complete closure of the Isthmus of Panama ca. 2.8 Mya.
Chapter 3 further extended the discovery of biodiversity in Chapter 2, explored the factors driving fluctuation of diversification rates, and revised the genus Opecarcinus. Utilizing morphological and molecular data we described 16 new species to science. Additionally, the genus Luciades is now considered a junior synonym of Opecarcinus and the monotypic species L. agana was transferred to the genus Opecarcinus. This chapter also presented a comprehensive overview of Opecarcinus, covering aspects such as diversity, morphological characteristics, host corals, symbiotic relationships, and distribution ranges. To assess diversity patterns over time a Lineage-through-time (LTT) analysis was conducted, and we illustrated how fluctuations of glacial sea levels during the Pleistocene might have influenced the diversification rates of Opecarcinus.
Chapter 4 focused on the evolutionary history of symbiotic crab lineages within Thoracotremata (a clade of crabs in the Brachyura). We assembled and characterized new complete mitochondrial genomes of three gall crab species: Kroppcarcinus siderastreicola, Opecarcinus hypostegus and Troglocarcinus corallicola. By utilizing concatenated genes, including 13 protein-coding genes and two ribosomal RNA genes, we reconstructed a phylogenetic tree encompassing 75 thoracotreme species. The results indicated that symbiotic crab lineages evolved independently at least three times within the Thoracotremata. Additionally, selective pressure analyses were conducted, revealing signs of adaptation to a symbiotic lifestyle in these crab families (Cryptochiridae, Pinnotheridae, certain Varunidae) through positive selection pressure on protein-coding genes. However, the specific roles and functions of these genes remain unclear, highlighting the need to address it in future.
Chapter 5 investigated the correlation between non-branching coral colony size/depth and symbiont assemblages based on fieldwork data in the Caribbean. Visual surveys were carried out to detect the presence of coral-dwelling symbionts in three non-branching coral species (Porites astreoides, Siderastrea siderea, and Agaricia agaricites) at two depths (6 m and 15 m) along the leeward side of Curaçao. Statistical analyses indicated no correlation between colony size and symbiont assemblages. However, a significant decrease in symbiont assemblage with depth was observed in P. astreoides and S. siderea, but not in A. agaricites. Additionally, a high proportion of colonies hosted no more than two symbiotic species and this trend increased with the depth.
Chapter 6 is a synthesis of the primary outcomes presented in this thesis, along with a discussion of the implications of these findings. Moreover, I also discussed potential avenues for further research that could enhance our understanding of the mechanisms driving stable symbiotic relationships in coral reef ecosystems.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • University of Groningen
Supervisors/Advisors
  • van der Meij, Sancia, Supervisor
  • Eriksson, Britas Klemens, Co-supervisor
Award date4-Jun-2024
Place of Publication[Groningen]
Publisher
DOIs
Publication statusPublished - 2024

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