Entoprocta - Reproduction and Life Cycle

Reproduction and Life Cycle

Most species are simultaneous hermaphrodites, but some switch from males to females as they mature, while individuals of some species remain of the same sex all their lives. Individuals have one or two pairs of gonads, placed between the atrium and stomach, and opening into a single gonopore in the atrium. The eggs are thought to be fertilized in the ovaries. Most species release eggs that hatch into planktonic larvae, but a few brood their eggs in the gonopore. Those that brood small eggs nourish them by a placenta-like organ, while larvae of species with larger eggs live on stored yolk. The development of the fertilized egg into a larva follows a typical spiralian pattern: the cells divide by spiral cleavage, and mesoderm develops from a specific cell labelled "4d" in the early embryo. There is no coelom at any stage.

In some species the larva is a trochophore which is planktonic and feeds on floating food particles by using the two bands of cilia round its "equator" to sweep food into the mouth, which uses more cilia to drive them into the stomach, which uses further cilia to expel undigested remains through the anus. In some species of the genera Loxosomella and Loxosoma, the larva produces one or two buds that separate and form new individuals, while the trochophore disintegrates. However, most produce a larva with sensory tufts at the top and front, a pair of pigment-cup ocelli ("little eyes"), a pair of protonephridia, and a large, cilia-bearing foot at the bottom. After settling, the foot and frontal tuft attach to the surface. Larvae of most species undergo a complex metamorphosis, and the internal organs may rotate by up to 180°, so that the mouth and anus point upwards.

All species can produce clones by budding. Colonial species produce new zooids from the stolon or from the stalks, and can form large colonies in this way. In solitary species, clones form on the floor of the atrium, and are released when their organs are developed.