Oysters typically have spring and fall spawning peaks, although intermittent summer spawns occur. Fertilization is external. Fertilized eggs develop sequentially into trocophore , veliger and pediveliger larvae. Pediveligers have a foot which they use to explore the substrate upon which they eventually must settle. The time from fertilization to settlement is about two weeks, during which time the larvae drift with the currents. The larvae are thus the means of dispersal for this otherwise sessile organism. Larvae must find a suitable hard substrate, such as the shells of other oysters, upon which to settle. Newly attached oysters are called spat, which develop into juveniles and then into adults. Oysters are protandric hermaphrodites, meaning they reproduce first as males and later as females.