Cycliophoran dwarf males break the rule: high complexity with low cell numbers.

ABSTRACT

Complexity of metazoan bodyplans is commonly assumed to be correlated to the absolute number of cells and the number of cell types present in a species (1). Sexually mature individuals of the smallest free-living animals have a minimum of several hundred somatic cells, and only secondarily simplified parasitic or commensal species range below this threshold. Males of the two hitherto described representatives of the phylum Cycliophora (2), with a body length of about 40 microm, are among the smallest existing free-living metazoans, yet they exhibit an amazingly complex bodyplan. Herein, we show that only a few dozen cells account for this complexity. We conclude therefore that metazoan complexity is not obligatorily correlated with body size or with the overall cell number of an individual. Accordingly, the earliest multicellular animals on Earth, which most probably were small individuals, may have had more complex bodyplans than commonly assumed.