Date of Award

6-2026

Degree Name

MS in Biological Sciences

Department/Program

Biological Sciences

College

College of Science and Mathematics

Advisor

Kristin Hardy

Advisor Department

Biological Sciences

Advisor College

College of Science and Mathematics

Abstract

The California shellfish industry’s near-exclusive reliance on the non-native Manila littleneck clam (Ruditapes philippinarum) creates an economic vulnerability to climate-driven disturbances, which can cause mortality events that eradicate up to 80% of stocks. This singular reliance on an introduced species highlights a need for the evaluation of native alternatives like the Pacific littleneck clam (Leukoma staminea). Our goal was to assess the feasibility of using the native Pacific littleneck in commercial aquaculture by comparing their performance to the non-native Manila littlenecks across the following conditions: 1) a five-month field outplant trial at high and low intertidal positions on a shellfish farm in Morro Bay, CA, 2) lab-simulated commercial storage/transport conditions (4ºC air), and 3) laboratory-based heat and anoxia stress challenges. For both species, we monitored growth and survival rates during the farm grow-out period and compared survivability and immune capacity (lysozyme activity; total hemocyte counts, THC) during industry-standard cold-air storage and environmental stress conditions (heat, anoxia, combination). We found that Manila littlenecks had lower mortality rates (6.1%) than Pacific littlenecks (11.6%) while outplanted in the high tide zone, although mortality rates were comparable - but on the higher end (11.4-13.9%) - for both species grown in the low tide zone. Growth rates were universally higher in Manila littlenecks outplanted at the farm. During cold-air storage, Manila littlenecks outperformed Pacific littlenecks in trials of both baseline clams (LT50: 23d vs 9d) and post-farm clams (LT50: 19-22d vs 9-12d), indicating greater post-harvest durability in the non-native species. Our comparison of immune capacity demonstrated no significant differences in THC between the native and non-native species during cold air storage, heat challenge or anoxia stress. Only during the most extreme stress challenge - combined heat and anoxia - do we see higher hemocyte counts in the Manila littlenecks. Lysozyme activity, however, was higher in the native Pacific littlenecks, but only in the clams used in the cold-storage experiment. We did not see any differences in lysozyme between species, or as a result of tidal height, in clams assessed after the farm grow-out period. Taken together, these data indicate that the native Pacific littleneck is a viable secondary candidate for farm diversification, however, it should serve to complement rather than replace Manila littlenecks due to their slower growth rates, increased mortality rates on the farm, and reduced tolerance for surviving commercial storage/transport conditions.

Notes

I am co-advised by Dr. Kristin M. Hardy and Dr. Kevin M. Johnson 

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