Abstract
Gonad maturation processes were studied in Pseudocalanus spp. females from Georges Bank and Cape Cod Bay (northwest Atlantic) using a combination of morphological analysis and experiments. For light microscopy of the oocytes, females of different maturation stages were preserved immediately after capture. The maturation processes during the spawning cycle were described from observations of live females which were exposed to feeding and starvation at two temperatures, 8 and 15°C, for 12 days. The gonad morphology of these females was examined in 24 h intervals, and spawning events were recorded. Both light microscopy and whole animal observation revealed that oocytes during maturation change in shape and size, in the morphology of the nucleus, and in the appearance of the ooplasm. Due to these modifications of oocyte morphology and due to oocyte migration, the morphology of the gonads changed distinctly during a spawning cycle. Five oocyte development stages were identified by light microscopy and related to a macroscopic system of four gonad development stages, that can be applied to whole animals and allows the identification of females ready to spawn. The experiment showed that food and temperature had strong effects on gonad maturation processes. High proportions of mature females were found when food was available, whereas the proportion of immature females increased shortly after exposure to starvation. Compared to 15°C, gonad maturation at 8°C was prolonged and thus spawning frequency was lower. The final maturation processes at food saturated conditions were slower than the embryonic development, and no indication was found that mature oocytes are stored in the diverticula waiting to be released. The duration of the interspawning interval would thus be determined by the duration of final oocyte maturation which is dependent on both temperature and food supply.
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Acknowledgements
I appreciate the support by P.H. Wiebe and P. Alatalo during the experimental work at the Wood Hole Oceanographic Institution. Thanks are also due to the crew and the scientists on R.V. "Endeavour" and R.V. "Delaware", especially to E. Durbin and his group for collecting zooplankton for the experiments, and Mark Ohman for species identification. This study profited from discussions with E. Durbin and J.A. Runge, and S.B. Schnack-Schiel who commented on a previous version of this manuscript. I also thank three anonymous referees for their helpful comments. The author was funded by the Deutsche Forschungsgemeinschaft (German Science Foundation) and through NSF grant (OCE 9806565). The experiments comply with the current laws of the USA. This is publication no 10882 of the Woods Hole Oceanographic Institution, USA.
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Communicated by O. Kinne, Oldendorf/Luhe
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Niehoff, B. Gonad morphology and oocyte development in Pseudocalanus spp. in relation to spawning activity. Marine Biology 143, 759–768 (2003). https://doi.org/10.1007/s00227-003-1034-7
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DOI: https://doi.org/10.1007/s00227-003-1034-7