Trichoplax - Reproduction

Reproduction

The Placozoa normally propagate asexually, dividing in the middle to produce two roughly equal-sized daughters. These remain loosely connected for a while after fission. More rarely, budding processes are observed: spherules of cells separate from the dorsal surface; each of these combines all known cell types and subsequently grows into an individual of its own.

Sexual reproduction is thought to be triggered by excessive population density. As a result, the animals absorb liquid, begin to swell, and separate from the substrate so that they float freely in the water. In the protected interior space, the ventral cells form an ovum surrounded by a special envelope, the fertilization membrane; the ovum is supplied with nutrients by the surrounding syncytium, allowing energy-rich yolk to accumulate in its interior. Once maturation of the ovum is complete, the rest of the animal degenerates, liberating the ovum itself. Small unciliated cells that form at the same time are interpreted to be spermatozoa. It has not yet been possible to observe fertilization itself; the existence of the fertilization membrane is currently taken to be evidence, however, that it has taken place. Putative eggs have been observed, but they degrade at the 32–64 cell stage. Neither embryonic development nor sperm have been observed.

Usually even before its liberation, the ovum initiates cleavage processes in which it becomes completely pinched through at the middle. A ball of cells characteristic of animals, the blastula, is ultimately produced in this manner, with a maximum of 64 cells. Development beyond this 64-cell stage has not yet been observed.

Trichoplax lack a homologue of the Boule protein that appears to be ubiquitous and conserved in males of all species of animals tested in preparation a 2010 paper published in PLoS Genetics. If its absence implies the species has no males, then perhaps its "sexual" reproduction may be a case of the above-described process of regeneration combining cells separated from two separate organisms into one.

Due to the possibility of its cloning itself by asexual propagation without limit, the life span of Placozoa is potentially infinite; in the laboratory, several lines descended from a single organism have been maintained in culture for an average of 20 years without the occurrence of sexual processes.