Pieces of sponge are able to regenerate into whole new sponges. Asexual reproduction occurs by budding or by fragmentation. The buds may remain attached to the parent or separate from it, and each bud develops into a new individual. Freshwater sponges, as well as several marine species, form resistant structures called gemmules that can withstand adverse conditions such as drying or cold and later develop into new individuals. Gemmules are aggregates of sponge tissue and food, covered by a hard coating containing spicules or spongin fibers. Sexual reproduction also occurs. Most sponges are hermaphroditic, the same individual producing eggs and sperm, but in some species the sexes are separate. The larvae are flagellated and swim about freely for a short time. After settling and attaching to a suitable substrate, the larvae develop into young sponges.