Chromulina pusilla Butcher, a Dominant Member of the Ultraplankton

Abstract

RECENT researches have stimulated interest in marine nannoplankton. Hydrographical evidence suggested that the bulk of organic production off Plymouth might have been due to autotrophic flagellates rather than to diatoms¹. The pigment-extraction method indicated that during frequent sampling more than 90 per cent of the total mass of phytoplankton passed through the meshes (40 µ × 50 µ) of a fine net, and that on the only two occasions for which results are given, about 40 per cent passed through Whatman No. 2 filter paper². The small size of ultraplankton organisms (about 5 µ; Thorson³ gives references to the authors of this unfamiliar but convenient term) does not prevent their use as food by important members of the zooplankton, for a pure culture of a flagellate less than 2 µ in size has supported growth in Calanus⁴. Flagellates of about 10 µ or less form the essential food of oyster larvæ^5–7, and hence probably of most of the planktotrophic larvæ which are met with in the development of more than 70 per cent of all benthic invertebrates⁸.