Members of many invertebrate groups live symbiotically with unicellular algae, but the symbiosis between corals and dinoflagellate algae (zooxanthellae) is especially interesting because it occurs in all species of tropical reef-building corals (see reviews by Droop, 1963;Yonge, 1963;McLaughlin and Zahl, 1966). Moreover, a significant effect of the algae on the physiology of corals has been clearly demonstrated and quantified : Corals with symbiotic algae calcify many times faster in light than in darkness, while corals which have lost their zooxanthellae calcify at rates which are slower and unaffected by light (Kawaguti and Sakumoto, 1948;Goreau, 1959;Goreau and Goreau, 1959). In the light, photosynthesis by zooxanthellae must somehow lead to higher rates of calcification by corals.
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.
for assistance in obtaining samples. Deutsche Forschungsgemeinschaft (Schi277/10) and the Human Frontier Science Program (HFSP RGP0221/2001-M) are gratefully acknowledged. AGC thanks NSF Grant EAR-9814845 for support of this project at an early stage.
The phylogenetic placement of the morphologically simple placozoans is crucial to understanding the evolution of complex animal traits. Here, we examine the influence of adding new genomes from placozoans to a large dataset designed to study the deepest splits in the animal phylogeny. Using site-heterogeneous substitution models, we show that it is possible to obtain strong support, in both amino acid and reduced-alphabet matrices, for either a sister-group relationship between Cnidaria and Placozoa, or for Cnidaria and Bilateria as seen in most published work to date, depending on the orthologues selected to construct the matrix. We demonstrate that a majority of genes show evidence of compositional heterogeneity, and that support for the Cnidaria + Bilateria clade can be assigned to this source of systematic error. In interpreting these results, we caution against a peremptory reading of placozoans as secondarily reduced forms of little relevance to broader discussions of early animal evolution.
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