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How do warming temperatures and disease affect abalone? Examining digestive physiology and withering syndrome in Haliotis

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The physiological underpinnings of many marine host-pathogen systems are poorly understood, and even less is known about how climate change will affect the physiology of these systems. In the abalone and Rickettsiales-like organism (RLO) system, the RLO infects abalone digestive tissues and leads to extreme starvation and a characteristic “withering” of the gastropod foot. The withering syndrome-causing RLO (WS-RLO) has led to large declines in most abalone species on the northeastern Pacific coast, but disease resistance levels are species-specific and dependent on environmental temperature.

In order to clarify differential susceptibility in species whose geographic ranges overlap substantially (such as the highly susceptible Black Abalone and relatively resistant Green Abalone), I estimated the phylogenetic relationships of northeastern Pacific Haliotis species using concatenated data from two mitochondrial and two nuclear regions. The most susceptible species form a monophyletic grouping apart from the other northeastern Pacific taxa, implying that WS susceptibility could be a derived trait that traces back to the last common ancestor of this hypothesized clade, and that patterns of WS resistance and susceptibility have a quite deep evolutionary basis.

I then determined differences in gene expression in the digestive gland, the site of infection, in infected and uninfected Pinto Abalone over seven months. Pinto Abalone showed changes in gene expression at seven months post-infection, compared to naïve conspecifics. Differentially expressed gene groups were identified that likely correspond to advanced disease state and to general stress response of being held in captivity.

The third chapter examined the effects of heat stress on metabolic rate, digestibility, and digestive enzyme activities in Red Abalone and Pāua. Metabolic rates were significantly elevated in heat treatment animals. Digestibility did not significantly change, but this may be balanced by changes in transit time and total intake. Digestive enzyme activities changed independently, with amylase remaining the same, maltase increasing, and trypsin decreasing.

In summary, WS susceptibility follows a phylogenetic pattern. Genes associated with immune and hunger are upregulated, but protein transport and reproduction are downregulated in infected Pinto Abalone guts. Heat stress leads to metabolic rate increases, digestibility decreases, and digestive enzyme changes in abalone.

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