Ultrastructure and function of the gastrointestinal tract in marine herbivorous fishes

The structure of the gastrointestinal tract (GIT) of temperate and tropical marine herbivorous fishes was described using histological and ultrastructural approaches. Radial distribution of endosymbiotic microbiota in the hindgut, and the association of microbiota with degradation of algal fragments were also examined. The overall aim was to provide a detailed description of the GIT in these fishes to improve understanding of gut function. Substantial variation in gut structure was observed among study species. In general, villus length, thickness of musculature, thickness of mucus, cell types present and microvillus length indicated that morphology of the anterior GIT was consistent with enzyme secretion and lipid absorption, while the posterior GIT was consistent with protein absorption. Secretory cells lined the stomach, exocrine cells with long microvilli lined the pyloric caeca and anterior GIT, and absorptive cells, with long microvilli, invaginations and pinocytotic vesicles, lined the posterior GIT. The relationships between the radial distribution of the endosymbiotic microbiota in the hindgut and gut structures suggested that some protein was microbiallyderived. Greatest surface area in the anterior and posterior GIT correlated with previously determined regions of endogenous and exogenous (microbial) digestion. Overall, gut structure was generally most similar between study fish species from the same family, demonstrating that phylogeny strongly influences GIT morphology. Microbial density was generally greatest close to the mucosal surface, where bacteria were often attached to the glycocalyx covering microvilli. Endosymbiotic microbiota were near, attached to, and degrading algal fragments in the posterior GIT, showing that bacteria are more directly involved with algal breakdown than previously thought. Microbial degradation was only observed in the outer cell wall of algal fragments, not in cortex cell layers. Degradation of algal fragments associated with bacteria was extensive in some fishes, with clear zones of hydrolysis surrounding bacteria in species consuming phaeophytes, although exceptions to this trend suggest that study species employ different digestive strategies. The microbiota in some host fish species appear to utilise both algal cell contents and cell wall, whereas in other species the microbiota appear to ferment mainly algal cell contents, and the host fish supplements the resulting short-chain fatty acids with nutrition derived from endogenous digestion of protein-rich epiphytes such as cyanobacteria. The results presented in this thesis show that diet alone cannot be used as an indicator of digestive processes and gut function, and demonstrate that the relationships between diet, GIT structure, hindgut microbiota and digestive strategy are complex in marine herbivorous fishes, and can differ even between species that appear similar ecologically.

Name thesis:degree_name

PhD

Level thesis:degree_level

Doctoral

Discipline thesis:degree_discipline

Biological Sciences

Grantor dc:publisher

ResearchSpace@Auckland

Year dc:date.issued

2016

Author dc:creator

• Johnson, Kathryn

Advisors dc:contributor.advisor

• Clements, K
• White, L

Statement dc:rights

• Items in ResearchSpace are protected by copyright, with all rights reserved, unless otherwise indicated. Previously published items are made available in accordance with the copyright policy of the publisher.

Licence dc:rights.uri

• https://researchspace.auckland.ac.nz/docs/uoa-docs/rights.htm

Handle dc:identifier.uri

https://hdl.handle.net/2292/30939

OAI identifier oai:identifier

oai:researchspace.auckland.ac.nz:2292/30939