18.4: Transient and clonal ("true") metazoans

Although we often think about developmental processes as restricted to multicellular organisms, there are versions that involve organisms that exist in both unicellular and transient multicellular forms. Because they are simpler, we can learn important and relevant lessons from studying transient metazoans.^[594] Forming a transient multicellular organism requires that single celled organisms cooperate with one another, they get social.
The ability of individuals to cooperate, through processes such as quorum sensing and community effects, enables them to tune their responses so that they are appropriate and useful. Social interactions also make it possible for them to produce behaviors impossible for isolated individuals. Once individual organisms develop (evolutionarily) the ability to cooperate, new opportunities and challenges (cheaters) emerge. There are strategies that can enable an organism to adapt to a wider range of environments, or to become highly specialized to a specific environment, through the production of increasingly complex behaviors. Many cooperative strategies can be adopted by single celled organisms, but others require a level of multicellularity. Multicellularity can be transient – a pragmatic response to specific conditions, or it can be (if we ignore the short time that gametes exist as single cells) permanent, allowing the organism to develop the range of specialized cell types needed to build large, macroscopic organisms with complex and coordinated behaviors. We can divide multicellularity into two distinct types, aggregative and clonal. These appear to have arisen independently in a number of lineages .^[595]

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