3/- 



NATURE 



[November 28, 1912 



This organism proved on further investigation to 

 be Ainphidiniuin operculatutn, Clap, and Lachm., 

 which had never before been found in British seas, 



anrl vrrv rnn^h- nnvwhr-re fl«e. And '."■■'" b"T- ii wn<; 



ir'v^ 



v»^ 



in vast amount for weeks at a time"; and I am 

 inclined to think it may be abundant on other beaclies 

 also. Several naturalists have told me since I men- 

 tioned this matter to the Linnean Society last year that 

 they are convinced that they have previously seen 

 Amphidinium patches on the shore, and had taken 

 them for diatoms. But one of the most extraordinary 

 points about the occurrence of 

 this little Dinoflagellate is that it 

 seems to alternate in time on the 

 beach with almost pure cultures of 

 certain common diatoms, such as 

 Navicula amphishaena, Bory, and 

 A^. digito-radiata, Greg. (see 

 Figs. 4 and 5). 



During 191 1 the history of these 

 coloured patches on Port Erin 

 beach was as follows : — 

 April 7 to May i. — Amphidinium 

 and a few 

 diatom - 

 (Navicula). 

 June 3 to July 22. — Diatoms (sonn- 

 Navicula, 

 others Pleuro- 

 sigma). 

 September 9 and 10. — Amphidinium 

 in abund- 



ance ; diatoms 

 absent. 

 September 16 to 18. — Diatoms 

 (Navicula). 

 October 2 to 26. — .Amphidinium iii 

 abundance; 

 diatoms absent. 

 October 28 to November i. — No 

 Amphidinium present. 

 Novernber 2. — .Amphidinium (three small patches). 



During the remainder of the winter no patches 

 were found, but by the beginning of April Amphi- 

 dinium had reappeared in force and monopolised the 

 beach for a couple of weeks. It was then replaced 



- See Journ. Linn. Soc, vol. .vvxii., No. 212, October, 1511, 



NO. 2248, VOL. 90] 



by diatoms for a few days, and in the latter part of 

 April, 1912, the alternation took place no fewer than 

 four times, ending with a couple of weeks in May, 

 when neither organism was present. Amphidinium' 

 reappeared on May 15, and was present 

 more or less during the greater part of 

 ' be summer, except in the drier intervals 

 I July and August, when it was absent. 

 1 10m September 14 onwards it has again 

 been present in larger or smaller patches, 

 and I have examined living samples sent 

 from Port Erin, up to the last day of 

 October ; but, curiously enough, the indi- 

 viduals in these recent gatherings differ 

 considerably in shape, size, and some 

 other minor points from the Amphid'miiim 

 opcrculatum we had been examining in 

 such quantity at Port Erin during the 

 previous year. 



I am not of opinion as yet that this 

 difference indicates more than a form of 

 the same species — possibly seasonal or 

 due to age or nutrition, or sorne other 

 ' nvironmental influence; and the variation 

 ■ Iocs not affect the broad phenomenon of 

 I he striking alternation of the two very 

 ilil'ferent kinds of organisms, diatoms and 

 I )inonagellates, in vast quantity. Although 

 It may not be possible yet to give any 

 detailed explanation, the facts seem to 

 I !■ i)oint to the probability that the cause of 

 I he phenomenon is a physiological one, 

 .md that the explanation may consist in showing 

 that each organism in turn in its metabolism exhausts 

 or alters some essential constituent of the environ- 

 ment, so as to prevent its own continued existence, in 

 quantity, at that spot, but leaves the ground suitable, 

 or even favourable, to the physiological needs of the 

 other set of competing organisms. 





bumiiii;r I'll... 7. — Ui.Uu.n jihyio pLiiiklc 



gatherings, 

 ographs to the skill of my friend Mr. Edwin Thompst 



Possibly we have a similar phenomenon on a more 

 extended scale in the well-known seasonal variations 

 of the plankton of the open sea, where during spring 

 and summer the main constituent groups of organisms 

 are Diatoms, Dinotlagellates, and Copepoda, succeed- 

 ing one another in that order. (Figs. 6 and 7 show 



