The animals that make up a colony of sea creatures contribute to its motion depending on their size.
Complex colonies called physonect siphonophores — relatives of jellyfish — are formed of many individuals that propel the colony using jets of water. John Costello at Providence College in Rhode Island and his team collected colonies of one physonect species (Nanomia bijuga) and photographed them as they swam. Individuals worked together to drive the colony around, but did not all contribute equally. Smaller, weaker colony members steered the swimming colony, and their more powerful neighbours provided the thrust.
Nature Commun. http://dx.doi.org/10.1038/ncomms9158 (2015)
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How colonies of sea animals swim. Nature 525, 161 (2015). https://doi.org/10.1038/525161b