October 14, 1922] 



NA TURE 



527 



The Fauna of the Sea-Bottom. 1 

 By Dr. C. G. Joh. Petersen, Director of the Biological Station, Copenhagen. 



CTUDIES of the fauna of the sea-bottom are of 

 '--' essential zoological significance, and many 

 scientific questions as well as important fishery 

 interests depend upon them. The subject, hew 

 is so extensive that I must confine myself mainly 

 to the different methods adopted for the investigation 

 of the fauna of the sea-bottom. 



Since 1S83 I have investigated Danish waters by 

 means of the dredge ; it was my task then to give 

 on charts the distribution, especially in the Kattegat, 

 of everv single species 01 marine animal, to under- 

 stand the laws ruling the distribution of the animals 

 on the sea-bottom (the cruises of the gunboat Hancli). 

 Different specialists had each a group of animals 

 to work out, and a great number of charts were 

 printed, but I did not feel quite content with my 

 first publication, although something was cleared off 

 by that method. The method was, and is still, the 

 usual one for such investigations. 



Many years later the question was put before me : 

 Why does the plaice in the western Limfjord grow 

 very slowly, but very quickly in the middle of the 

 Limfjord ? The answer required first of all a 

 quantitative investigation of the amount of plaice- 

 food in both places. A small bottom-sampler on a 

 pole, long enough to reach the bottom in the shallow 

 fjord was made in the 'nineties of last century and 

 proved that much food was to be found in both 

 places ; the difference in growth-rate of the plaice 

 was found later on to depend not only upon the 

 amount of food on the bottom, but also upon the 

 different number of plaice living there on each square 

 mile. The idea of overcrowding for sea-fishes was 

 introduced for the first time. 



Many years later I constructed a new bottom- 

 sampler fastened to a wire ; this I have used every- 

 where in Scandinavian waters down to a depth of 

 300 fathoms. It was my idea at first to compare 

 the amount of fish-food per sq. metre in the Limfjord 

 with the amount of fish-food per sq. metre in our 

 remaining waters by means of the bottom-sampler ; 

 but I soon found it difficult to compare the animals 

 from one water with those of another ; in some places 

 the animals were small and of great value for fishes, 

 in others the animals were bigger and built up of 

 carbonate of lime (chalk) mainly, and with a great 

 content of water ; chalk and water being of course 

 of little importance as food in the sea for other animals, 

 I realised that I should compare, first of all, the 

 amount of food in places with the same kind of animal 

 population, and I had to map out these places. 



The bottom-sampler taught me that we have 

 about eight such different animal communities in 

 Danish waters from o to 300 fathoms, character- 

 ised by numerous large and characteristic animals. 

 They may be echinoderms, bivalves, Crustacea, etc., 

 but are .ill animals living mainly on detritus, not 

 rapacious animals. These last named are necessarily 

 always scarcer than the more peacefully living animals, 

 as the grass-feeding animals on the dry land are 

 more common than tigers and lions. 



One thing puzzled me in the beginning very much ; 

 the bottom-sampler showed in many hauls the most 

 uniform content in the sieves in the same animal 

 ■community, then suddenly it came up filled with 

 quite different animals, Modiola modiolus, Trophonia, 

 Ophiophohs aculeata, etc., without any corresponding 

 difference in the depth or in the nature of the bottom. 

 How is this to be explained ? 



Section D (Zoology) of the British 



1 Sept. 7. 

 NO. 2763, VOL. I ioj 



All these new organisms were animals living not 

 in the bottom like ordinary animals, but above the 

 bottom, originally fixed to a small stone or a shell, 

 as on a heath we may find lichens on stones, not 

 heather, or as in a beech-wood, on stones we find 

 mosses, not flowers ; strong currents mav help to 

 nourish such an epi-fauna with its often enormously 

 rich animal life. Every object on the sea-bottom, 

 a stone, a shell, a wreck, living plants, may give 

 rise to such epi-faunas ; within the same community 

 on the level sea-bottom there is the same epi- 

 fauna, but in different communities different epi- 

 faunas may be found. The epi-fauna is, as a rule, 

 scattered over the bottom in spots, and it is not 

 always easy, in single cases, to say what is the reason 

 for its existence ; it is therefore not possible to 

 give its distribution on a chart : you may give it 

 on the spots where you have found 'it, but you never 

 will be able to give all the spots existing on the 

 bottom. On rocky coasts the epi-faunas are dominat- 

 ing ; the coral reefs are a kind of epi-fauna, built up 

 mainly of chalk and water ; they are of very little 

 importance as fish-food. 



In contradistinction to the distribution of the epi- 

 faunas the communities of the level sea-bottom are of 

 a very uniform distribution, in such localities as in 

 Danish waters and in the North Sea. Their distribu- 

 tion may be easily mapped out, and their content 

 of fish-food and other animals quantitatively deter- 

 mined. We have taken thousands of samples, each 

 of T V sq. metre, with the bottom-sampler in Denmark, 

 and they have nearly always shown several animals, 

 worms, bivalves, Ophiurida?, etc., -in each ; only one 

 or two samples of them have shown no animal content. 

 By means of the bottom-sampler we may, there- 

 fore, using the most frequently occurring organisms, 

 easily map out the communities of the level sea- 

 bottom, and determine its content of fish-food. 



The theory of probability will indicate the degree 

 of accuracy ; many samples will give, as a rule, more 

 and more exactitude. We may determine how the 

 number of organisms varies at different seasons and 

 in different years. If we examine what the fishes 

 eat of these organisms we may determine whether 

 they are good or bad areas for tins or that species of 

 fish., and may get a fair idea of the productivity of 

 the sea-bottom as a whole, not forgetting that all 

 the small, fast-growing, short-living animals are 

 often to be reckoned as yearly production, whereas 

 the bigger, longer living animals must be reckoned 

 by means of another method. We may get an idea 

 of the whole metabolism of the sea — but I must not 

 go too far in mentioning these problems. 



With a good steamer I could in one month map 

 out the whole of the North Sea as to its animal 

 communities. I would take between 500 and 1000 

 samples spread over the whole ot tin \'c nth Sea, out to 

 the 100-fathom line, about one or two per hour. Using 

 a bottom-sampler of 0-2 sq. m. I should then have 

 taken up only the animals of an area at 100-200 

 sq. m., but I am sure that I should get all the species 

 of the common uniformly-distributed animals of the 

 whole area, and I should be able to give a rough- 

 sketched map of the animal communities. If we 

 used a bottom-sampler on a heath only once, we 

 should catch heather, and so in the sea, I should not 

 catch many rare animals, but I do not care for 

 rare animals ; the main thing is to know the animals 

 that make up the great bulk of the bottom popula- 

 tion, to know their distribution and their weight per 

 square mile. If you wish for greater exactitude than 



