Colonial, or modular, organisms are fascinating because each module can experience its own life history while the colony as a whole shares resources. This means that when a module dies it can actually be beneficial to the whole colony. The death of an older module can mean resources are allocated to younger, more vital modules which, in turn, can increase colony reproduction and hence colony fitness.
Most of our knowledge about these types of organisms comes from plants and although there are many marine examples of colonial organisms, there has been little testing of ideas about module mortality and its effects on colony fitness in these animals.
Karin Svanfeldt and her PhD supervisors Keyne Monro and Dustin Marshall have been working to redress the balance. Karin has been studying the colonial bryozoan Watersipora subtorquata as part of her PhD and she was interested in testing some ideas about selection on module longevity in this species and seeing how it compared with what we know about plants.
Karin used un-analysed data from a field experiment where she had manipulated food availability and flow rate. Karin also used data from another experiment where competition between different species was manipulated by settling Watersipora on either bare PVC plates or on plates where other animals were already growing.
Modules in colonial animals are called zooids and in the bryozoan Watersipora, growth and new zooids appear at the edge of the colony. Over time, the zooids in the centre of the colony lose colour and irreversibly senesce. Zooid senescence is visible as the appearance of a grey inner circle of older, dead zooids that expands as the colony grows. This meant that Karin was able to track individual zooids over time to provide measures of zooid longevity and also get data on the reproductive output of colonies to use as a measure of colony fitness. Karin measured reproductive output as either the number of new zooids or the number of ovicells per colony.
This data enabled Karin and her supervisors to ask the question: “does having a shorter zooid lifespan mean increased fitness for the colony as a whole?” Or, put another way: “is module longevity under selection?”
They found that, ‘Yes’ module longevity is under selection and that the strength of selection varies with environmental conditions, which is what has been found in numerous studies looking at modular plant species.