Help identifying mollusk - Oxnard, CA, USA

In Channel Islands Harbor, Oxnard, CA, a mollusk resembling an oyster has garnered attention, particularly from those familiar with the area since the 1970s. Observations indicate that these mollusks began appearing roughly a decade ago, coinciding with the decline of local starfish populations. This correlation raises questions about their potential classification as an invasive species or as organisms thriving in an altered ecosystem. Community insights and expertise are sought to accurately identify this mollusk and understand its implications for the local marine environment.