258 DISCOVERY REPORTS 



In the lined shore crab, Pachygrapsiis crassipes Randall, Hiatt (1948) finds pronounced variation 

 in the frequency of the moult with temperature, and well-marked periods of inhibited ecdysis 

 coinciding with the winter months. 



The myriads of chitinous integuments shed by the moulting krill, not to mention the massive rain 

 of organic debris they must produce in death, no doubt (Wolff, i960) provide an important food 

 source for benthic animals living at abyssal depths. 



REACTION OF LARVAL, ADOLESCENT AND ADULT KRILL 



TO SHIP AND NETS 



In studying the distribution, relative abundance and absolute density of plankton animals, we have 

 to rely mainly on townets which sample them for the most part at random and at more or less widely 

 scattered points. And since, as Hardy (1936a and 1936^) has shown, the plankton as a whole is 

 inclined to be patchy, or very uneven in its distribution, in any large-scale investigation such as this 

 a very large number of samples are required before any reliable estimate of the distribution, relative 

 abundance and absolute density of some selected species can be formed. This is obvious and applies 

 to all plankton work of the kind in which we have been engaged. It applies, however, with particular 

 emphasis to E. superba, an exceedingly patchy species, more patchy perhaps than any other, forming 

 in the surface zone dense swarms of exceedingly active animals, swarms separated from one another 

 by considerable 'voids' in which the krill are very scarce or absent. It is of manifest importance, 

 therefore, that being now about to consider the distribution and relative abundance of these animals 

 as revealed by the random sampling of nets, we should consider in some detail how and under what 

 conditions our nets may be sampling the swarms and how and under what conditions the swarms 

 themselves, or the individuals of them, behave in relation to, or react to the intrusion of, both nets and 

 ship. These questions will be considered under four headings, (i) the powers of the krill to evade as 

 individuals, (2) the capacity of the individuals of a swarm to act in unison, (3) the mechanical effect 

 of scattering by the ship, and (4) the inferences that may be drawn from (i), (2) and (3). 



Powers of the individuals to evade 

 We have already seen (pp. 154-7) that in daylight, especially in bright sunlight, the krill, both young 

 and old, are exceedingly sensitive to outside stimuli, reacting immediately to sudden disturbances 

 and taking effective measures to avoid them. The first and most obvious thing to consider, therefore, 

 is to what extent and under what conditions the younger and older stages of the surface population 

 may take similar action when confronted with the warp and bridles of an approaching net. That they 

 take such action is certain, the comparative ease with which they have been seen, with a sudden 

 backward kick, to avoid the vastly more rapid approach of a fish for example, suggesting it must be 

 extremely effective. How effective, in daylight at any rate, may be gathered from the following 

 passage from Mackintosh (1934). 'Active avoidance of the net must sometimes take place, especially 

 by such species as Eiiphausia superba. I have been able from the ship's side to watch a net being 

 towed a few feet below the surface and passing through a shoal of this species. The Eiiphausians 

 could clearly be seen to leap backtoards out of the way of the approaching net.'^ Clearly this represents 

 an instance of 'evasion by sight', combined to some extent perhaps with evasion stimulated by 

 touch or vibration. 



Our observations repeatedly show that the measure of sight evasion achieved can be correlated, 



^ The italics are mine. 



