California Sea Grant College Program

Significant declines in California abalone populations have been attributed to Withering syndrome (WS), a fatal disease affecting abalone throughout southern California. The desire to prevent the spread of WS to northern California abalone stocks necessitates further research on both the contributions of environmentally relevant stressors towards the development of WS, and a treatment for the disease.

Using nuclear magnetic resonance (1H NMR) based metabolomics; we have investigated the influence ofbacterial infection, temperature, and food availability, both individually and in combination, on the metabolic status of the California red abalone (Haliotis rufescens). Food limitation caused dramatic reductions in all classes of foot muscle metabolites, while at the same time, metabolite levels within the digestive gland were preserved or increased. In addition, food limitation along with the additional stress of elevated seawater temperature led to greater metabolic perturbations in both tissue types than those observed under food limitation alone. WS-RLP infection and food reduction resulted in many of the same metabolic changes within the tissues studied although the effects of infection alone were more modest. This study also showed that NMR metabolomics data correlates well with classic histology, supporting the use of unique methodologies in characterizing both normal and pathological events in marine species.

Classical histology along with 1H NMR metabolomics were then used to evaluate treatment of WS-RLP infected abalone with oxytetracycline (OTC) medicated feed at both 13.4±1.2°C and 17.3±1.3°C. While the histological observations demonstrated the highly efficacious nature of OTC treatment in combating WS-RLP infection at both temperatures, NMR metabolomics showed differences between treated and untreated animals only at the elevated temperature. Principal component analyses (PCA) of the NMR spectra revealed that the most significant metabolic changes observed were related to the post-treatment times at which the animals were sampled, regardless of OTC treatment. Animals at both temperatures showed decreased levels of all identified amino acids and carbohydrates with concurrent increases in the levels of taurine, glycine­ betaine, and homarine.

In addition, drug residue concentrations, WS-RLP burden, and WS-associated pathological changes were assessed for both OTC-treated and untreated abalone at 17.3°C. Unusually high OTC concentrations and long depuration times were observed within the digestive gland while drug depuration occurred rapidly from foot muscle. Digestive gland depuration conformed to a two-compartment model, with terminal phase elimination half lives of approximately 22 days. Cohabitation trials with WS-RLP infected abalone following a 10-day OTC-treatment showed reduced susceptibility to re­infection for up to 88 days after treatment. This finding suggests that OTC residues retained within the digestive gland confer long-term resistance to pathogen. This study also provides a further understanding of the tissue distribution and residue depletion, of OTC in abalone during treatment and has also allowed the documentation of the sequential metabolic changes that occur during the pre-clinical phases of the disease at both 13.4° and 17.3°C. The information provided will help increase the efficiency of OTC therapy and minimize the potential impacts of drug residues entering the environment from culture facilities.