Large cluster of oysters. Every leg of the main jetty that was underwater in high tide was covered in these. It seems some of their shells are a bit cracked to reveal yellow flesh.
A smaller, black cluster of other bivalves was also adjacent to this patch; their observation is https://www.inaturalist.org/observations/2669743
Oyster shells scatter the beach
Smooth Pink Scallop, chlamys rubida
Tide was going out and rocks were exposed, Cabrillo inner beach, near the outlet of the salt marsh and boat launch ramp