. Rapporti di ricerca. Discovery (nave); spedizioni scientifiche; Ocean; Antartide; Isole Falkland. I vecchi stadi 185 lo stato di sviluppo della mensola di uova è nettamente in anticipo del corrispondente stato di acqua oceanica, e mentre questo può solo significare che lo sviluppo sul ripiano è più rapida che in mare aperto^ una più lenta velocità di affondamento in condizioni meno profonde sembra una possibilità chiara. Come confrontato con il mare aperto, tuttavia, le acque poco profonde delle regioni a mensola sono relativamente instabili, essendo soggetti a notevoli turbolenze derivanti attraverso miscelazione verticale, movimenti di marea e

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. Discovery reports. Discovery (Ship); Scientific expeditions; Ocean; Antarctica; Falkland Islands. THE OLDER STAGES 185 developmental state of the shelf eggs is distinctly in advance of the corresponding state in oceanic water, and while this may merely mean that development on the shelf is more rapid than in the open sea^ a slower sinking rate in the shallower conditions seems a distinct possibility. As compared with the open sea, however, the shallow waters of shelf regions are relatively unstable, being subject to considerable turbulence arising through vertical mixing, tidal movements and so on, and it may be that this turbulence alone contributes to the near-surface occurrence of developmental stages that in oceanic water are more commonly encountered at deeper levels. As Fig. 23 shows, wherever advanced eggs containing developing nauplii were encountered in shelf water, they tended to occur nearest the surface where the water was shallowest. OCEANIC WATER -EAST WIND DRIFT >• < WEDDELL DRIFT- XL. Fig. 23. Diagrammatic illustration of the developmental condition of the eggs in shelf and oceanic waters, the vertical egg columns representing the actual condition recorded at every station where observations on egg development were made. It seems clear then that such eggs as we have recorded must in fact have been sinking and were laid above 250 m. With the possible exception of those at Station 540 (Fig. 19), however, the numbers we find at this near-surface level are so consistently small that one hesitates to suggest that it is there, near the surface, whether in oceanic or shelf water, that spawning principally takes place. All in fact that can safely be concluded from the meagre results of our analyses is that some sporadic surface or near-surface spawning, evidently on a very minor scale, does take place in both shelf and oceanic water, and that the eggs so laid do in fact sink, developing as they go down, to hatch at deeper levels. Turning again to F

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• bookcentury1900
• booksubjectocean
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