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NATURE 



[August 26, 1920 



There are many other- problems of the plankton in 

 addition to quantitative estimates — probably some 

 that we have not yet recognised — and various interest- 

 ing conclusions may be drawn from recent planktonic 

 observations. Here is a case of the introduction and 

 rapid spread of a form new to British seas. 



Biddnlphia sinensis is an exotic Diatom which, 

 according to Ostenfeld, made its appearance at the 

 mouth of the Elbe in 1903, and spread during suc- 

 cessive years in several directions. It appeared sud- 

 denly in our plankton gatherings at Port Erin in 

 November, 1909, and l\as been present in abundance 

 each year since. Ostenfeld in 1908, when tracing its 

 spread in the North Sea, found that the migration to 

 the north along the coast of Denmark to Norway 

 corresponded with the rate of flow of the Jutland 

 current to the Skager Rak, viz. about 17 cm. per 

 second — a case of plankton distribution throwing light 

 on hydrog-raphy — and he predicted that it would soon 

 be found in the English Channel. Dr. Marie Lebour, 

 who recently examined the store of plankton gather- 

 ings at the Plymouth Laboratory, finds that, as a 

 matter of fact, this form did appear in abundance in 

 the collections of October, 1909, within a month of the 

 time when, according to our records, it reached Port 

 Erin. Whether or not this is an Indo-Pacific species 

 brought accidentally by a ship from the Far East, or 

 whether it is possibly a new mutation which appeared 

 suddenly in our seas, there is no doubt that it was 

 not present in our Irish Sea plankton gatherings 

 previous to 1909, but has been abundant since that 

 year, and has completely adopted the habits of its 

 English relations, appearing with B. mobiliensis in 

 late autumn, persisting during the winter, reaching 

 a maximum in spring, and dying out before summer. 

 The Nauplius and Cypris stages of Balanus in the 

 plankton form an interesting study. The adult 

 Ijarnacles are present in enormous abundance on the 

 rocks round the coast, and they reproduce in winter 

 at the beginning of the year. The newly emitted 

 young are sometimes so abundant as to make the 

 water in the shore-pools and in the sea close to the 

 shore appear muddy. The Nauplii first appeared at 

 Port Erin in 1907 in the bay gatherings on February 22 

 (in 1908 on February 13), and increased with ups and 

 downs to their maximum on April 15, and then de- 

 creased until their disappearance on April 26. None 

 were taken at any other time of the year. The Cypris 

 stag-e follows on after the Nauplius. It was first 

 taken in the bay on April 6, rose to its maximum on 

 the same day with the Nauplii, and was last caught 

 on May 24. Throughout the Cypris curve keeps 

 below that of the Nauplius, the maxima being 1740 

 and 10,500 respectively. Probably the difference 

 between the two curves represents the death-rate of 

 Balanus during the Nauplius stage. That conclusion 

 I think we are justified in drawing, but I would not 

 venture to use the result of any haul, or the average 

 of a number of hauls, to multiply by the number of 

 square yards in a zone round our coast in order to 

 obtain an estimate of the number of young barnacles 

 or of the old barnacles that produced them ; the 

 irregularities are too great. 



To my mind it seems clear that there must be 

 three factors making for irregularity in the distribu- 

 tion of a plankton organism : 



(i) The sequence of stages in its life-history, such 

 as the Nauplius and Cypris stages of Balanus. 



(2) The results of interaction with other organisms, | 

 as when a swarm of Calanus is pursued and devoured | 

 by a shoal of herring. I 



(3) Abnormalities in tirhe or abundance due to the | 

 physical environment, as in favourable or unfavour- 1 

 able seasons. , | 



NO. 2652, VOL. 105] 



And these factors must be at work in the open 

 ocean as well as in coastal waters. 



In many oceanographical inquiries there is a double 

 object. There is the scientific interest and there is 

 the practical utility — the interest, for example, of 

 tracing a particular swarm of a Copepod like Calanus, 

 and of making out why it is where it is at a particular 

 time, tracing it back to its place of origin, finding 

 that it has come with a particular body ot water, and 

 perhaps that it is feeding upon a particular assem- 

 blage of Diatoms ; endeavouring to give a scientific 

 explanation of every stage in its progress. Then 

 there is the utility — the demonstration that the migra- 

 tion of the Calanus has determined the presence of a 

 shoal of herring or mackerel that are feeding upon it, 

 and so have been brought within the range of the 

 fisherman and have constituted a commercial 

 fishery. 



We have evidence that pelagic fish which congre- 

 gate in shoals, such as herring and mackerel, feed 

 upon the Crustacea of the plankton, and especially 

 upon Copepoda. A few years ago when the summer 

 herring fishery off the south end of the Isle of Man 

 was unusually near the land, the fishermen found 

 large red patches in the sea where the fish were 

 specially abundant. Some of the red stuff brought 

 ashore by the men was examined at the Port Erin 

 Laboratory, and found to be swarms of the Copepod, 

 Temora longicornis ; and the stomachs of the herring 

 caught at the same time were engorged with the same 

 organism. It is not possible to doubt that during 

 these weeks of the herring fishery in the Irish Sea the 

 fish were feeding mainly upon this species of Copepod. 

 Some ten years ago Dr. E. J. Allen and Mr. G. E. 

 BuUen published some interesting work from the 

 Plymouth Marine Laboratory demonstrating the con- 

 nection between mackerel and Copepoda and sunshine 

 in the English Channel ; and Farran " states that 

 in the spring fishery on the West of Ireland the food 

 of the mackerel is mainly composed of Calanus. 



Then again, at the height of the summer mackerel 

 fishery in the Hebrides in 1913, we found " the fish 

 feeding upon the large Copepod, Calanus flnmarchi- 

 cus, which was caught in the tow-net at the rate of 

 about 6000 in a five minutes' haul, and 6000 was also 

 the average number found in the stomachs of the fish 

 caught at the same time. 



These were cases where the fish were feeding upon 

 the organism that was present in swarms — a mono- 

 tonic plankton — but in other cases the fish are clearly 

 selective in their diet. If the sardine of the French 

 coast can pick out from the micro-plankton the minute 

 Peridiniales in preference to the equally minute 

 Diatoms which are present in the sea at the same 

 time, there seems no reason why the herring and the 

 mackerel should not be able to select particular species 

 of Copepoda or other large organisms from the macro- 

 plankton, and we have evidence that they do. Nearly 

 thirty years ago the late Mr. Isaac Thompson, a con- 

 stant supporter of the Zoological Section of thjs Asso- 

 ciation and one of the honorary local secretaries for 

 the last Liverpool meeting, showed me in 1893 that 

 young plaice at Port Erin were selecting one particular 

 Copepod, a species of Jonesiella, out of manv others 

 caught in our tow-nets at the time. H. Blegvad '* 

 showed in 1916 that young, food-fishes, and also small 

 shore-fishes, pick out certain species of Copepoda 

 (such as Harpacticoids) and catch them individually — 

 either lying in wait or searching for them, A couple 



12 Jnurn. Mar. Biol. Assof., vol. viii. (1909'), pp. 394-406. 

 1* Conseil Internat. Bull. Trimestr., 1902-8, " Planktonique," p. 89. 

 I'l " Soo'ia Runiana," Hi., Linn. Soc. Journ., Zoology, vol. xxxiv., 

 p. 05. 1918. 

 1' Rep. Danish Biol. Stat., vol. xxiv., 1916. 



