Ontogeny of salinity tolerance and evidence for seawater-entry preparation in juvenile green sturgeon, <Emphasis Type="Italic">Acipenser medirostris</Emphasis> |
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Authors: | Peter J Allen Maryann McEnroe Tetyana Forostyan Stephanie Cole Mary M Nicholl Brian Hodge Jr" target="_blank">Joseph J CechJr |
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Institution: | (1) Department of Wildlife, Fish, and Conservation Biology, University of California, 1 Shields Avenue, Davis, CA 95616, USA;(2) School of Natural and Social Sciences, State University of New York, Purchase, NY 105777, USA;(3) Department of Wildlife, Fisheries and Aquaculture, Mississippi State University, Mississippi State, MS 39762, USA |
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Abstract: | We measured the ontogeny of salinity tolerance and the preparatory hypo-osmoregulatory physiological changes for seawater
entry in green sturgeon (Acipenser medirostris), an anadromous species occurring along the Pacific Coast of North America. Salinity tolerance was measured every 2 weeks
starting in 40-day post-hatch (dph) juveniles and was repeated until 100% survival at 34‰ was achieved. Fish were subjected
to step increases in salinity (5‰ 12 h−1) that culminated in a 72-h exposure to a target salinity, and treatment groups (0, 15, 20, 25, 30, 34‰; and abrupt exposure
to 34‰) were adjusted as fish developed. After 100% survival was achieved (134 dph), a second experiment tested two sizes
of fish for 28-day seawater (33‰) tolerance, and gill and gastrointestinal tract tissues were sampled. Their salinity tolerance
increased and plasma osmolality decreased with increasing size and age, and electron microscopy revealed three types of mitochondria-rich
cells: one in fresh water and two in seawater. In addition, fish held on a natural photoperiod in fresh water at 19°C showed
peaks in cortisol, thyroid hormones and gill and pyloric ceca Na+, K+-ATPase activities at body sizes associated with seawater tolerance. Therefore, salinity tolerance in green sturgeon increases
during ontogeny (e.g., as these juveniles may move down estuaries to the ocean) with increases in body size. Also, physiological
and morphological changes associated with seawater readiness increased in freshwater-reared juveniles and peaked at their
seawater-tolerant ages and body sizes. Their seawater-ready body size also matched that described for swimming performance
decreases, presumably associated with downstream movements. Therefore, juvenile green sturgeon develop structures and physiological
changes appropriate for seawater entry while growing in fresh water, indicating that hypo-osmoregulatory changes may proceed
by multiple routes in sturgeons. |
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