Planaxids

Planaxids are a group of marine bivalve mollusks, belonging to the subclass Heterodonta. They are characterized by their asymmetrical shells, with one valve being larger and more convex than the other. This asymmetry is a key feature that distinguishes planaxids from other bivalves. The shells of planaxids are typically elongated and can vary in size, ranging from a few centimeters to over a meter in length. The inner surface of the shells is often covered in a layer of nacre, which provides strength and protection.

Planaxids are found in a variety of marine environments, including shallow coastal waters, deep-sea trenches, and even in the abyssal plains. They are known for their ability to burrow into the sediment, using their siphons to create a water current that helps them filter-feed on plankton. Some species of planaxids are also known to be hermaphroditic, meaning they have both male and female reproductive organs.

The taxonomy of planaxids is complex, with many species and genera described over the years. However, recent molecular studies have suggested that the group may be more closely related to other heterodont bivalves than previously thought. Despite this, the exact evolutionary relationships of planaxids remain a topic of ongoing research.