Other aspects of reproduction and colony formation may have biogeographic and evolutionary impacts. One is the possibility that polyploidy (the possession of more than two entire chromosome complements) occurs in corals just as it does in many groups of plants and some groups of animals. Another is the possibility that mutations in body cells can be transferred to sex cells by the formation of cancerous growths or ‘neoplasms’. Neoplasms, which typically consist of many polyps that are presumed to have grown from a single mutant-containing cell, are common in corals and in theory at least raise the possibility of instant new species by sexual reproduction by the mutant polyps. Another type of colony formation occurs when spat or planulae of different genetic origin fuse to form post-fertilisation hybrids or ‘chimeras’. It is common for colonies to form from two or more original spat that fuse as they grow. Similarly, planulae can grow from two or more original eggs that fuse in early development. The outcome, in both cases, is a chimera (a colony that is not what it seems) and these may be post-fertilisation ‘hybrids’ if developed from eggs of different species. Chimeras of many different types (reflecting differing levels of cell integration) are known in many groups of plants and in some groups of animals. However, unless there has been some sort of nuclear DNA fusion or polyploid formation, their reproductive output, if any, will be one or other of the parent species.
The Scleractinia are an ancient group of organisms and, as with many groups of plants, do not have a close genetic control over morphological characteristics. Like many groups of plants, this allows them to have a wide range of life-cycle options in reproduction and dispersal and hence biogeographic and evolutionary change. Both reproduction and dispersal are closely linked to concepts of what species are. Corals, like many major groups of plants, have fuzzy species boundaries: species are not single reproductively isolated units and, because of biogeographic variation, there are no essential distinctions between species, subspecies and hybrids. All such taxonomic units are parts of interconnected genetic patterns, created by ocean currents, and forever changing.