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Date Permissions Signed


Date of Award

Spring 2022

Document Type

Masters Thesis

Department or Program Affiliation


Degree Name

Master of Science (MS)



First Advisor

Arellano, Shawn M.

Second Advisor

Fullerton, Heather

Third Advisor

Kodner, Robin

Fourth Advisor

Bingham, Brian L., 1960-


Marine invertebrates form specific associations with bacterial communities that are different from their environment, change throughout their development, and shape evolutionary and ecological processes. The bathymodiolin (Mytilidae) mussel Gigantidas childressi lives at deep-sea methane seeps and relies on methanotrophic endosymbionts for its nutrition. Its larval life, however, is spent feeding in the water column. Upon metamorphosis at a suitable seep habitat, methanotrophic bacteria rapidly colonize gill cells and the juvenile mussel switches to symbiont-derived energy. To determine if the microbiome of the G. childressi changes during these transitions, the V3/V4 region of the 16S rRNA gene was sequenced to census the bacterial diversity within G. childressi across early development. Like larvae of other deep-sea taxa, diversity was relatively low in all samples, but results show strong evidence for the re-organization of the larval and juvenile microbiomes based on stage-specific shifts in both habitat and nutritional mode. In planktotrophic (feeding) larvae, the microbiome is influenced in part by the environment, with key ASVs assigning to environmental generalists such as Pseudomonas and Sphingomonas. Other taxa were specific to pediveligers and veligers and were not found in juveniles or the surrounding water. Chloroplast sequences were detected in most swimming larvae, potentially as a component of the larval diet in the form of eukaryotic phytoplankton in the gut. Our results also suggested that initial infection of chemoautotrophic symbionts occur before settlement and metamorphosis in pediveligers of G. childressi, which were enriched for several potential chemoautotrophic symbionts that were found in juveniles. In early juveniles, the microbiome is mainly composed of several strains of methanotrophic symbionts known to inhabit G. childressi. Unexpectedly, we also detected thiotrophic, sulfur-oxidizing symbionts related to the SUP05 cluster, a well-known clade of symbionts in other Bathymodiolin species. We also present the first indirect evidence of the heavy-oil degrading Cycloclasticus as a symbiont of G. childressi. We suspect that the unexpected diversity of symbionts in early juveniles follows a successional pattern where diversity decreases as the mussel grows and matures, eventually resulting in the 1-2 dominate MOX phylotypes commonly observed in adults.




symbiosis, larval development, larval ecology, microbiome, microbial ecology, gigantidas childressi


Western Washington University

OCLC Number


Subject – LCSH

Microbial ecology; Mytilidae; Larvae--Ecology




masters theses




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