238 DISCOVERY REPORTS 



shortly be seized upon by Cryptoniscids. Another haul of large Grimothea taken later in 

 the year yielded an even lower percentage of infected specimens (1-9 per cent). 



There seems to be no evidence that great inconvenience is caused to the host by the 

 presence of the parasite. If a certain proportion of mortality w^ere caused by harbouring 

 the parasite, the percentage of infection in early life should exceed that for the whole 

 population. 



DISTRIBUTION 



The geographical distributions of M. stibrugosa and M. gregaria have been briefly 

 dealt with by Matthews (1932). He has given their general distribution, so far as it is 

 known, and it remains for us to examine the more detailed distribution in the area 

 covered by the trawling surveys, especially in the light of the large numbers obtained 

 during the period October 193 1 to April 1932. 



MUNIDA SUBRUGOSA 



The area covered by the trawling surveys does not extend beyond the known limits of 

 the occurrence of this species. No station was made either as far north as 35° S, which 

 is given as the northern limit by Matthews, or as far south as Cape Horn where the 

 R.R.S. ' Discovery ' took large numbers in 1927. The species is distributed generally 

 over most of this area, but regions of concentration and absence can be mapped. Depth 

 is one of the factors appearing to limit its distribution, and the increasing depth at the 

 edge of the continental shelf forms the eastern boundary. Few hauls of the trawl were 

 made in waters of any great depth, but M. subrugosa has been taken at several stations 

 where the haul has commenced or passed into moderate depths from shallower depths, 

 as, for example, at WS 246 and WS 772 where the hauls rose from 267 to 192 m. and 

 309 to 162 m. respectively. The catches at these stations were always small and the 

 limiting depth was no doubt in the neighbourhood. Only two or three stations of a 

 depth greater than 200 m. and with a level bottom yielded records. It seems evident 

 that M. subrugosa is passing beyond its optimum depth when descending below the 

 200-m. contour. Henderson (1888) records this species at a depth of 600 fathoms off 

 Monte Video ; but the record is of a single specimen in a "very imperfect state of pre- 

 servation ", and too much importance should not be attached to it. 



Certain areas of the continental shelf show very few, or an absence of, M. subrugosa in 

 such a manner as to suggest some definite cause (Fig. 17). The distribution shows a 

 heavier concentration near the coasts, especially off the north and north-west coasts of 

 the Falkland Islands, off north-east Tierra del Fuego, and in the neighbourhood of the 

 mouths of the Rio Gallegos and Rio Coig. Lying outside these two last-named regions 

 comes a long strip, roughly parallel to the coast, of diminished catches and then beyond 

 this an area of absence. Similarly, moving west and north-west from the area of con- 

 centration to the north-west of the Falkland Islands an area of scanty numbers is 

 passed through before the same region of absence is reached and records cease. Thus a 

 region, running roughly north and south, in which conditions may be unfavourable to 

 the species, is seen to separate a Patagonian population and a Falkland Islands popula- 



