NEBALIOPSIS TYPICA 



By H. Graham Cannon, Sc.D., f.r.s. 

 Beyer Professor of Zoology in the Victoria University of Manchester 



(Plate XV) 

 THE FORM OF THE BODY 



IN a recent report which has just reached this country, Linder (1943) has called in doubt the shape 

 and configuration of Nebaliopsis typica which I described in an earlier Discovery Report (193 1) and 

 illustrated by photographs of a specimen which I stated (loc. cit. p. 201) was complete and undamaged. 

 The only specimen which, previous to my report, had reached the surface intact, I stated on the 

 authority of Dr Odhner of the Riksmuseum, Stockholm, was most probably lost. Dr Linder has now 

 found the missing specimen, and it is this that he maintains represents the normal appearance of this 

 rare deep-sea crustacean. My specimen was labelled F 2, and was illustrated by three untouched photo- 

 graphs on plate xxxii, and text-figs, i and 2, based on these photographs. Linder's chief criticism 

 is that the cephalothorax is distended, more especially in the posterior part, while the carapace has 

 remained unaffected (loc. cit. p. 5). As a result, part of the thorax is not covered by the lateral carapace, 

 and the length of the carapace relative to the rest of the body is abnormally small. The difference is very 

 marked by measuring Linder's figure and my own. A comparison shows that the length of the cephalo- 

 thorax in his specimen is about 54 % that of the carapace, while in my specimen it is 96 %. Obviously 

 such a large difference calls for further investigation. 



I pointed out (loc. cit. p. 202) that measurements of all the specimens I had, together with that of the 

 specimen we may now call Linder's, established the existence of a considerable variation in the length 

 of the carapace and left the matter at that. I took the view that there was no doubt about it as, while 

 Linder's specimen and mine were both presumably as perfect as could be, his had a large carapace and 

 mine had a small one. I was relying for details of Linder's specimen on a sketch which had been 

 published by Ohlin (1904, fig. i). From this, however unsatisfactory it might have been in other 

 respects, there was no doubt about the length of the carapace— it was relatively long. The question to 

 be settled is whether the relative shortness of the carapace of my specimen F2 is abnormal. 



Linder's explanation (loc. cit. p. 6) is that at the great depths at which Nebaliopsis lives, there must 

 be inside the body an enormous pressure. This is a point which no one will dispute. However, he then 

 says that as the specimen is brought to the surface in the collecting gear this pressure inside, acting in 

 all directions, enlarges the body so that by the time atmospheric pressure is reached at the surface the 

 body is completely distended. This distension will occur to different degrees in different parts, and 

 Linder assumes that the carapace is not affected by the pressure. Hence it retains its normal size while 

 the body becomes bloated, and thus the carapace appears abnormally short. 



Now this argument represents a widespread fallacy, a fallacy which has arisen first from the known 

 fact that specimens of deep-sea fish are occasionally completely distended when they reach the surface, 

 and secondly from the persistent and erroneous belief that sea water is much more dense at great 

 depths than at the surface. Actually Linder's argument is valid, and then to a very uncertain degree, 

 only if the body of Nebaliopsis contained gas. 



As long as the body contains no bubble of gas, no such distension as Linder describes can take place. 

 Now there is no reason to suppose that the body of Nebaliopsis contains gas any more than one may 

 expect to encounter gas on opening up a lobster. Its body can be looked upon very largely as a mass of 

 aqueous liquid and, moreover, a liquid closely similar in its physical properties to sea water. Now sea 



