Graduation Year

2026

Document Type

Campus Only Senior Thesis

Degree Name

Bachelor of Arts

Department

Environmental Analysis

Reader 1

Veronica Padilla Vriesman

Reader 2

Kathleen Purvis-Roberts

Abstract

The California mussel (Mytilus californianus) serves as a valuable environmental archive for reconstructing climate and oceanographic change due to the calcified layers in its shell. This study investigated whether shell morphology varies across gradients of anthropogenic influence along the California coast, hypothesizing that pollution-driven stress would produce significant, potentially non-linear morphological responses. Twelve sites were selected and characterized from a master dataset with over 96 sampling locations, then categorized using a tiered pollution index integrating pollution source proximity (Tier 1), activity intensity (Tier 2), and environmental context (Tier 3). Index scores ranged from 11 (Bodega Marine Reserve, a protected area with minimal human impact) to 38 (Santa Monica Beach, a heavily urbanized area), with sites characterized as Low (11-20), Moderate (21-32), or High (33+) pollution impact. Shell morphology measurements from 1916 specimens were analyzed using one-way ANOVA and Kruskal-Wallis tests, revealing highly significant differences among shell index and pollution levels (p = 1.03x10-12). The relationship was nonlinear, with moderately impacted sites exhibiting higher shell index values than both low and high impacted sites. This pattern is consistent with the Intermediate Disturbance Hypothesis, suggesting moderate stress may enhance shell growth while high pollution levels suppress calcification. Pollution level was partially confounded with latitude however (low-impact sites in Northern California and high-impact sites in Southern California), indicating that observed patterns may also reflect natural biogeographic variation. These findings demonstrate that shell morphology responds to anthropogenic influence in complex, threshold-dependent ways and highlights its potential as a biogenic indicator of environmental stress.

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