Most cells experience an active and variable fluid environment, in which hydrodynamic forces can affect aspects of cell physiology including gene regulation, growth, nutrient uptake, and viability. The present study describes a rapid yet reversible change in cell morphology of the marine dinoflagellate Ceratocorys horrida Stein, due to fluid motion. Cells cultured under still conditions possess six large spines, each almost one cell diameter in length. When gently agitated on an orbital shaker under conditions simulating fluid motion at the sea surface due to light wind or surface chop, as determined from digital particle imaging velocimetry, population growth was inhibited and a short-spined cell type appeared that possessed a 49% mean decrease in spine length and a 53% mean decrease in cell volume. The reduction in cell size appeared to result primarily from a 39% mean decrease in vacuole size. Short-spined cells were first observed after 1 h of agitation at 20 Њ C; after 8 to 12 d of continuous agitation, long-spined cells were no longer present. The morphological change was completely reversible; in previously agitated populations devoid of long-spined cells, cells began to revert to the long-spined morphology within 1 d after return to still conditions. During morphological reversal, spines on isolated cells grew up to 10 m·d Ϫ 1 . In 30 d the population morphology had returned to original proportions, even though the overall population growth was zero during this time. The reversal did not occur as a result of cell division, because singlecell studies confirmed that the change occurred in the absence of cell division and much faster than the 16-d doubling time. The threshold level of agitation causing morphology change in C. horrida was too low to inhibit population growth in the shear-sensitive dinoflagellate Lingulodinium polyedrum. At the highest level of agitation tested, there was negative population growth in C. horrida cultures, indicating that fluid motion caused cell mortality. Small, spineless cells constituted a small percentage of the population under all conditions. Although their abundance did not change, single-cell studies and morphological characteristics suggest that the spineless cells can rapidly transform to and from other cell types. The sinking rate of individual long-spined cells in still conditions was significantly less than that of short-spined cells, even though the former are larger and have a higher cell density. These measurements demonstrate that the long spines of C. horrida reduce cell sinking. Shorter spines and reduced swimming would allow cells to sink away from turbulent surface conditions more rapidly. The ecological importance of the morphological change may be to avoid conditions that inhibit population growth and potentially cause cell damage.

In high latitude planktonic ecosystems where the prymnesiophyte alga Phaeocystis pouchetii is often the dominant primary producer, its importance in structuring planktonic food webs is well known. In this study we investigated how the base of the planktonic food web responds to a P. pouchetii colony bloom in controlled mesocosm systems with natural water enclosed in situ in a West Norwegian fjord. Similar large (11 m 3 ) mesocosm studies were conducted in 2 successive years and the dynamics of various components of the planktonic food web from viruses to mesozooplankton investigated. In 2002 (4 to 24 March), 3 mesocosms comprising a control containing only fjord water; another with added nitrate (N) and phosphate (P) in Redfield ratios; and a third with added N, P, and cultured solitary cells of P. pouchetii, were monitored through a spring bloom cycle. In 2003 (27 February to 2 April) a similar set of mesocosms were established, but cultured P. pouchetii was not added. As expected, during both years, addition of N and P without addition of silicate resulted in an initial small diatom bloom followed by a colonial bloom of P. pouchetii (600 to 800 µg C l -1 ). However, the hypothesis that addition of solitary cells of P. pouchetii would enhance subsequent colony blooms was not supported. Interestingly, despite the large production of Phaeocystis colonial material, little if any was transferred to the grazing food web, as evidenced by non-significant effects on the biomass of micro-and mesozooplankton in fertilized mesocoms. Separate experiments utilizing material from the mesocosms showed that colonies formed from solitary cells at rates that required only ca. 1% conversion efficiencies. The results are discussed from the perspective of future research still required to understand the impact of life cycle changes of this enigmatic phytoplankter on surrounding ecosystems. KEY WORDS: Phaeocystis pouchetii · Mesocosms · Nutrients · Fjord · Biocomplexity Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 321: 2006 gelatinous polysaccharide 'skin' (Chen et al. 2002). Solitary cells may be either motile or non-motile, and are typically 3 to 9 µm in diameter (Rousseau et al. 1994). This unusually large range of sizes between colonies and solitary cells (ca. 6 to 11 orders of magnitude in biovolume) can significantly alter material flow among trophic levels and export from the upper ocean (Wassmann et al. 1990, Lancelot et al. 1998. Furthermore, each stage is thought to function in different ways in order to reduce losses to either small or large zooplankton and viruses, and thus Phaeocystis spp. effectively function as dual species (Weisse et al. 1994, Smaal & Twisk 1997, Hamm et al. 1999, Jacobsen 2000, Verity 2000, Jakobsen & Tang 2002, Tang 2003.The dual life history of colonial and solitary cell stages was described over 50 yr ago (Kornmann 1955), and the dominant morphology appears to alter the ecosystem function from a regenerative system (solitary cells) ...

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Calanoid copepod eggs have a robust, chitinous outer chorion, which makes field-collected, formaldehydefixed eggs difficult to penetrate with stains or molecular probes. Egg development studies in copepods have involved physical, chemical, and enzymatic treatments to remove the chorion. We present an efficient, onestep method for staining copepod eggs with the fluorescent nucleic acid stains DAPI and PicoGreen®. Nuclei in treated eggs are clearly visible for examination and counting with compound and confocal microscopy, so that eggs can be rapidly classified with respect to developmental stage. The method is effective for eggs of Calanus, Metridia, and Centropages. Both stains were effective after 24-h exposure for eggs that were fixed from several days to 8 years. Early stages are distinguished by complete counts of nuclei. A blastula phase and gastrulation are distinctive. Calanus pacificus and Calanus marshallae embryos spend more than half the development period in the later 'gray ball' and limb bud stages. Stage classification by this method is useful for studies of copepod egg mortality in the field.