Field biology of placozoans (Trichoplax): distribution, diversity, biotic interactions.

The goal of this review is to highlight what little is known, and point to the bulk of what is yet to be learned, about the natural history of placozoans in the field-in order to stimulate a broader search for placozoans and a fuller exploration of their distribution, diversity, and all other aspects of their enigmatic lives. The documented geographic distribution of placozoans lies mostly in the nearshore, warm, marine waters of the tropics and subtropics. Although placozoans have long been viewed as benthic organisms, they can be more readily collected from the water column, well above the sea bottom. The full life-history of placozoans is unknown, including the nature of this abundant pelagic phase and all details of sexual reproduction and development. We note observations on the biota associated with placozoans in field collections, in particular the other regular members of the microcommunity in which placozoans occur on our collecting plates and on some factors influencing this assemblage. Among the animals found are some potential predators against which placozoans appear to be defended, although the mechanisms are still to be examined. Also yet to be uncovered is the full breadth of diversity in this phylum, certainly underrepresented by its single named species. We report here greatly expanded distributions for known haplotypes and fresh specimens that include a new haplotype, and we review the evidence that many more almost certainly await discovery. We also describe some methods for collecting and handling these small, fragile animals.