Obligate planktotrophy in the Götte’s larva of Stylochus ellipticus (Platyhelminthes)
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Polyclad flatworms are a diverse and emerging model system for developmental biologists, yet development remains poorly understood for many species. One limitation of polyclads as a model system has been the lack of reliable methods for culturing planktotrophic polyclad larvae to metamorphosis. There are conflicting statements in the literature about which types of polyclad larvae require food to complete development. We developed simple methods for rearing planktotrophic flatworms to metamorphosis and tested the effects of food type and concentration on their development. The flatworm Stylochus ellipticus develops from small (~65 μm) eggs into an obligately planktotrophic Götte’s larva. In this species, development to metamorphosis requires high concentrations (50,000 cells/ml) of the unicellular alga Rhodomonas lens as a food source. High concentrations of two other algal species (Dunaliella tertiolecta and Isochrysis galbana) were successfully ingested by larvae but failed to result in development to metamorphosis. We provide estimates of larval development times and initial descriptions of late larval and early juvenile forms in S. ellipticus. These data provide an important contrast between larval development in obligate planktotrophs and development in non-feeding species. Other indirect developing flatworm species may also be reared using these methods, allowing broader examination of polyclad developmental patterns than previously possible.