Population effects of resource allocation strategies in asexually reproducing annelids

Background/Question/Methods

Deciding between growth and reproduction is fundamental in determining life history strategies. How many resources are allocated to somatic growth and/or reproduction is a developmental decision with consequences that extend onto its ecological context. This topic has been explored and modeled for sexually reproducing species, but the role of asexual reproductive phases in species with facultative or alternate reproductive modes has been mostly ignored. We explored resource allocation strategies and their effect on population growth using a system modeled after naid annelids, a group of segmented freshwater worms that can reproduce asexually by paratomic fission. Naids grow longer by adding segments at their tail-end; when food is abundant, they are can undergo paratomic fission: a new head and tail are intercalated in a mid-body segment termed fission zone and the worm splits in two daughter individuals. How do they allocate resources to either terminal growth or fission? We propose two alternate hypotheses: A) a constant fixed proportion of available resources is devoted to each process; or B) allocation varies with the length of the worm. We built an individual growth-based model to generate predictions under each hypothesis, and used it to explore the effect of energy input and location of the fission zone along the body, which determines how the current resources are divided among the daughter worms.

Results/Conclusions

By tallying population sizes resulting from a single initial founder worm after a fixed amount of time for each particular combination of model parameters, our model predicts that, under hypothesis A, population size will increase at a rate dependent solely on available energy, irrespective of where a fission zone is placed along the body. However, under hypothesis B, placing a fission zone within certain ranges greatly enhances population growth, implying that fixing the proportion of resources inherited by each of the daughter worms will only matter if growth/reproduction allocation varies with size. To contrast our in silico results with a living system, we quantified location and distribution of fission zones in laboratory worm populations of Pristina leidyi (Annelida:Naididae) and found that they are indeed preferentially placed at a specific location that is within the range predicted under hypothesis B. We conclude that naid worms do not use a fixed  allocation strategy, but instead vary resource allocation to growth and reproduction depending on their current length, so that shorter worms invest more on growth and longer ones allocate more to reproduction.