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UBC Theses and Dissertations

Modeling of semelparous/iteroparous polymorphism in Botryllus Schlosseri Omielan, John


This thesis describes and models the age of first sexual reproduction in Botryllus schlosseri. B. schlosseri is a sessile, colonial fouling organism that lives mainly in the low intertidal zone of temperate waters. In Eel Pond, Woods Hole, Massachusetts, semelparous (reproduces only once) and iteroparous (reproduces several times) morphs apparently co-exist. A survey of previous life-history models is given, but none of them can be used very effectively on B. schlosseri. Instead, a dynamic programming model is presented that models, with a good fit to field data, the age of first sexual reproduction in each morph separately. The model makes several predictions. First, the life history characteristics, particularly the age of first sexual reproduction, are near equilibrium. Secondly, the growth reduction after winter affects the optimal age of sexual reproduction throughout the year, suggesting that colonies can tell the time of year through the water temperature or the photoperiod. Thirdly, the primary cause of variance in the age of first sexual reproduction is the spatial variation in the environment. Finally, iteroparous colonies continue reproducing every generation once they begin, not necessarily due to a physical constraint, but simply because it is optimal for them to do so. Next, the coexistence of the morphs was looked at. An important factor is Botryl-loides leachi, a closely related competitor, that in the summer and early fall overgrows semelparous colonies but not iteroparous colonies. However, since B. leachi was only introduced into Eel Pond about 30 years ago, an interesting question is whether the current conditions (i.e. coexistence) is stable or not. A survey was taken of previous models that look at the coexistence of species (the two morphs are treated as separate species). Although these models indicated that the two morphs can co-exist, none of them are accurate enough to be able to confidently determine if the coexistence is stable. Although a dynamic programming model would be precise enough, too much unavailable data would be required.

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