Abstract:
Symbiosis is a key driver of evolutionary innovation, shaping biodiversity through morphological, physiological, and behavioral adaptations. This is particularly evident on coral reefs, where intricate ecological networks are driven by close species interactions. However much of this complexity remains undocumented, especially among cryptic reef-associated invertebrates like coral-dwelling gall crabs (Cryptochiridae), obligate symbionts of scleractinian corals inhabiting morphologically diverse domiciles in their hosts. While their diversity is increasingly known, the functional roles, selective pressures, and physiological underpinnings of their symbiotic lifestyle remain largely unexplored. To better understand their symbiotic ecology and potential significance for coral reef ecosystems, this dissertation takes a multidisciplinary approach investigating cryptochirid population dynamics, adaptive trait evolution, and the molecular foundations of their coral-associated lifestyle. Chapter 2 presents a two-year monitoring effort based on fate-tracking of gall crabs and their host colonies. The results highlight unexpectedly high cryptochirid turnover, settlement preferences for inhabited hosts, resilience amidst moderate bleaching, and population collapse following host mortality. These dynamics reflect specialized life history strategies shaped by host dependency and suggest that gall crabs help channel coral-derived resources into higher trophic levels via their larval output. Chapter 3 combines multivariate trait analysis and phylogenomic reconstruction to investigate fluorescence evolution in cryptochirids, showing variation in expression levels and the presence of four morphologically distinct fluotypes. These patterns suggest fluorescence as an ecologically relevant trait that may be shaped by microhabitat-driven selective pressures, with a potential function in camouflage. Chapter 4 uses comparative transcriptomic analysis to explore the molecular basis of fluorescence by comparing fluorescent and non-fluorescent cryptochirid taxa inhabiting distinct microhabitats on the same coral host. While the molecular origin of fluorescence remains unresolved, clear physiological differences indicate habitat-driven adaptation in their metabolic strategies. These findings position cryptochirids as a model for understanding how obligate symbiosis shapes evolutionary trajectories, physiological divergence, and ecological dynamics. Their specialized traits emerge through reciprocal interactions with coral hosts, with microhabitat morphology as key locus for evolutionary interaction between host and symbiont. These discoveries highlight the hidden contributions of symbiotic invertebrates to reef functioning and the urgency of incorporating them into reef conservation and biodiversity assessment efforts.

Bio:
Susanne Bähr began working on coral-associated invertebrates during her Master’s research at the University of Groningen. Over the past four years at the King Abdullah University of Science and Technology (KAUST), she has investigated the symbiotic ecology and evolutionary adaptations of coral-dwelling gall crabs (Cryptochiridae). Her PhD combined long-term ecological monitoring, phylogenomics, fluorescence imaging, and transcriptomics to explore how obligate symbiosis shapes evolutionary trajectories and ecological persistence.