Abstract
We examined variability of oil-sac volume in fifth copepodites (C5) of Calanus finmarchicuscollected over and around Georges Bank during winter and spring 1997. Volumes were estimated from video images taken of live specimens. Maximum sac volume increases with prosome length and volume, showing a progressively higher apparent maximum (AM) at greater length. In small C5 (prosome length PL=1.9 mm, prosome volume PV=0.31 mm3) a maximum 28% of body capacity was occupied by oil. In large C5 (PL=2.7 mm, PV=1·1 mm3) the maximum was 50% of body capacity. Thus, large C5 appear to have a disproportionate advantage in lipid storage, potential duration of diapause and condition after diapause. Oil volume variation traced a seasonal progression through winter–spring in the “Year of Calanus”, 1997. In January and February most animals, probably the remainder of the 1996 resting stock, were small-bodied with oil volumes scattered from zero to the AM. March specimens were larger with oil storage less than half their AM. April specimens from deeper than 100 m, and presumably of the first spring generation, were mostly longer than 2.4 mm PL with oil sacs mostly >80% of AM, whereas those from shallower than 40 m were mostly shorter and scattered across the available range of oil sac volumes. May and June specimens occupied the entire size range. Larger spring specimens were more typically deep, mostly with oil volumes greater than 75% of AM. Shallow collections were mostly small specimens, particularly in the Gulf of Maine, and evenly distributed from no oil to the AM. Spring stations over the northeast peak and flanks of the Georges Bank showed lesser distinctions between deep and shallow collections. With less depth available, there was less sorting by age, size, and oil-sac fullness, but shallow vs. deep distinctions were usually in the same direction.
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