Growth rates ofHeterophrys marina(heliozoa) onChrysochromulina polylepis(prymnesiophyceae)

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Abstract
Chrysochromulina polylepis caused in the spring of 1988 one of the most severe toxic blooms in the Kat-tegat/Skagerrak area ever recorded. Though the bloom was not massive in terms of biomass its unialgal nature indicated that it had a strong and unspecific toxin. Microscopy did not reveal any likely predators on C. polylepis. However adding a sample from bloom water in the second stage in a chemostat with C. polylepis in the first stage gave growth of a heliozpan Heterophrys marina which grew well on C. polylepis. Growth studies were performed in cell culture chambers. Maximal growth rates were measured at concentrations of C. polylepis around 2000 cells/ml, growth rates decreased at increasing concentrations and zero growth or negative growth were found at concentrations of P-limited C. polylepis above 75 000 cells/ml H. marina were in this situation without visible axopods and non spherical. Transferring these cells into fresh medium with low numbers of C. polylepis gave normal growth after 2-3 days. The growth rate was fairly temperature dependent. H. marina had variable growth on other phytoflagellates, however it did not grow with a diatom or Alexandrium tamarense as food source. The particulars of the biology of H. marina shows an organism with considerable adaptive flexibility. These adaptations can be seen as changes in cell volume, rapid conversion from/to seden-tary/planktonic life style, multiple fission without cell growth and wide range in food preference. It may be particularly suited as predator on C. polylepis as C. polylepis were seemingly attracted to cells of H. marina concentrating in its vicinity.