AUTECOLOGY, LIFE-HISTORY AND TOXICOLOGY OF THE SILICOFLAGELLATE DICTYOCHA-SPECULUM (SILICOFLAGELLATA, DICTYOCHOPHYCEAE)

Growth and development of cultured skeleton-bearing and naked stages of the heterokont flagellate Dictyocha speculum Ehrenberg (Silicoflagellata, Dictyochophyceae) were examined. The bloom-forming naked stage is easily cultivated and shows optimum growth at salinities of 15-25 parts per thousand, within a temperature range of 11-15-degrees-C. A maximum growth rate of 1 division/24 h was found in cultures. The skeleton-bearing stage is difficult to maintain in culture: growth is slow and dense cultures were never obtained. The salinity optimum of this stage as deduced from field data is somewhat higher than that observed for the naked stage, whereas the temperature requirements are similar. Multinucleate cells occurred in cultures of the naked stage. Cells analogous to the multinucleate cells were derived from the skeleton-bearing cells and subsequently observed to transform into naked cells. The multinucleate cells are possibly resting stages. Measurements of the DNA content of the skeleton-bearing and the naked cells showed equivalent ploidy levels in both. It has not been possible to demonstrate a toxic effect for the naked stage in laboratory experiments. Fish-kills in Denmark in 1983, associated with a bloom of D. speculum, were therefore most likely caused by the generation of anoxic conditions.