Complex sentence: The larvae are retained inside the body cavity until they are well-advanced; when they are released they settle on the seabed within a few minutes, so groups of this sea squirt are found growing quite close to each other.
Split into simpler sentences: The larvae are retained inside the body cavity until they are well-advanced. When they are released they settle on the seabed within a few minutes, so groups of this sea squirt are found growing quite close to each other.