|7. ANNOTATION | TEXT | FACSIMILE | INDEX|
DIMORPHISM AND SIZE DISTRIBUTION IN VELELLA and PHYSALIA
By ROBERT BIERI
(Reprinted from Nature, Vol. 194 pp. 1333, October 24,1959)
WOODCOCK (1) attributed right- and left-handedness in Physalia, the Portuguese man-of-war, to a selective advantage in avoiding entrapment in windrows of Sargassum weed and floating debris. This selective advantage was presumed to be due to differences in sailing patterns through convection cells in the surface water of the northern and southern hemispheres. The absence of Sargassum from the South Atlantic and the paucity of debris in the barren, blue waters in which Physalia is characteristically found would seem to invalidate this hypothesis. On the other hand Woodcock's arguments concerning food concentration by the convection cells and sailing patterns are more convincing. Woodcock I continued to favour the hypothesis that there are significant differences in abundance of the two forms between the northern and southern hemispheres. Other authors (3),(4) have commented on this problem; however, none has pointed out that the more extensive literature on Velella, long known to be dimorphic (5), shows no statistically reliable difference between the abundance of the two forms in the' northern and southern hemispheres.
The fact that Agassiz (6) found only left-handed specimens present in more than two thousand Velella collected along the shores of the north-west Atlantic while Chun' found 71 left-handed and 6 righthanded Velella off Africa in the north-east Atlantic would indicate an east-west or zonal difference. Chun's (7) findings axe confirmed by the results of Moser (8). In a recent paper, Savilov (9) reported left and righthanded specimens of Velella from the north-west and south-west Pacific. Of more than 250 specimens examined by the author from the north-east Pacific all were left-handed. Thus, if there is an east-west difference in relative abundance of the two forms, the results available to date indicate that the situation in the Pacific is the reverse of that in the Atlantic.
Savilov advances a hypothesis that appears to solve this problem. In the northern hemisphere, lefthanded specimens of Velella move to the left of the wind direction due to the anticyclonic wind circulation over the ocean. The left-handed specimens are therefore concentrated along the outer edges of the distribution. The right-handed Velella move to the right of the wind direction and are concentrated in the centre of the distribution. Thus one should find the left-handed specimens near shore. In the anticyclonic wind circulation of the southern hemisphere the left-handed Velella are concentrated in the centre
of the distribution with the right-handed specimens more abundant along the borders of the distribution. The only results which weaken this argument are the exclusively left-handed specimens taken by me off California. ;Many of these were collected more than 300 miles off shore.
Savilov (9) found large specimens of Velella most abundant in the region of 40' N. lat. in the Kuroshio Extension. Young and larval forms were common in the south and far western parts of the Pacific. He attributed this size distribution to the wind and current patterns. An alternative explanation follows.
My studies of Velella off California, extending over a period of six years (unpublished results), show a marked seasonal appearance of Velella at the surface. This is confirmed by a careful examination of the previously published literature. 'The post-larval specimens first appear at the surface in very late December or early January and continue to reach the surface through the spring. The largest specimens are found in late autumn and early winter.
Examination of the track of the Vityaz shows that the stations in the Kuroshio Extension were occupied from July to November when neither larval nor very young forms would be present. The southern and faxwestern stations were occupied in December, January, February, July and August when the largest specimens are rare if present at all. Larval and young specimens would be present in December, January and February. Thus the size distribution Savilov describes in general terms can be more easily explained by seasonal differences. The data he presents do not allow a more precise analysis of the problem. One might also expect to find mean length differences between local populations because of different sea surface temperatures. However, these would be much smaller than those due to seasonal appearance and growth.
From the above it is apparent that the origin and occurrence of the different sizes and morphological forms of Velella and Physalia are not yet satisfactorily explained. Of the several variables that appear to be involved, seasonal appearance and growth have not been properly considered in previous reports.
R0BERT BIERI Antioch College, Yellow Springs, Ohio.
(1) Woodcock, A. H., J. Mar. Res., 5,106 (1944).
(2) Woodcock, A. H., Nature, 178, 253 (1956).
(3) Fontaine, A., Notes Sat. Hist. Soc. Jamaica, 64, 61 (1964).
(4) Totten, A. K., and Mackie, G. 0., Nature, 177, 290 (1956).
(5) Eschscholtz, F.. 'Systeni der Acalephen eine ausfürliche Beschrelbung aller Medusenartigen Strahlthiere' , 190 (F. Dummler, Berlin, 1829).
(6) Agassiz, A., Mem. Mus. Comp. Zool. Harr. Coll., 8, 1 (1883).
(7) Chun, C., Ergeb. Plankton Exped., 2, (1897). Moser, F., Deut. Südpolar-Exped., 17 (Zool. 9), 1 (1925).
(8) Savilov, A. I., Doklady Akad. Alauk SSSR, 122, 1014 (1958).
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