Mesophotic coral reefs generally begin at about what depth?

Mesophotic coral reefs generally begin at about 30 meters depth. They occupy the dimly lit transition below typical shallow reefs, where enough sunlight remains for some photosynthetic corals and algae to survive.

The word mesophotic combines roots meaning “middle light.” The exact upper boundary is not universal: local water clarity, latitude, reef structure, and seasonal conditions can shift it. Many descriptions place the upper mesophotic zone around 30 to 60 metres, although some researchers use a broader range.

These reefs are not simply shallow reefs viewed from farther down. Light levels decline sharply with depth, temperatures can differ, and the community often changes from coral-dominated habitat toward greater sponge abundance. Some mesophotic species are specially adapted to low-light conditions or are found almost nowhere else.

The depth is also a practical dividing line for exploration. Conventional scuba diving can reach the upper part only with specialized training and equipment, while deeper surveys often use technical diving, remotely operated vehicles, or autonomous underwater vehicles. That difficulty has left many mesophotic habitats understudied.