Black Purse Shell

Isognomon californicus

Isognomon californicus is a small bivalve that typically grows to a length of around one to one and a half inches. Its appearance is often flat and thin, with a flaky exterior and a pearly interior. This species can be distinguished from others by its unique shape and texture, making it a notable find for divers. It has the ability to wedge itself onto rocks, securing its position and preventing other species from dislodging it. This behavior is particularly useful in its natural habitat, which consists of brackish environments near the high tide line, where it is often found in crevices. Isognomon californicus is endemic to the Hawaiian Islands, and divers can encounter it in these specific coastal areas. The species is rarely found in calm or steady waters, preferring more dynamic environments.  Its diet primarily consists of detritus, which it filters from the surrounding water. Breeding habits are not detailed in the sources, but as a bivalve, it likely engages in external fertilization. Encountering this species while diving can be intriguing, as it often blends into its rocky surroundings, requiring keen observation to spot.