[Reprinted from THE AMERICAN NATURALIST, Vol. LI., June, 1917.] 



ON THE PERIODIC SHOREWARD MIGRATIONS OF 

 TROPICAL NUDIBRANCHS 1 



MANY northern gastropods, including nudibranchs, are well 

 known to exhibit the habit of congregating in shallow water 

 along the shore at their time of breeding. This has been com- 

 monly interpreted as the result of migration from deeper water 

 at the approach of the egg-laying season. Certain species, at any 

 event, are from time to time found in great quantity at shore 

 stations which they do not frequent at other periods, and field 

 observations have apparently established beyond a doubt that 

 this inshore appearance is closely connected with mating and ovi- 

 position. The migration into shallow water, or other means 

 which accomplishes the shallow-water flocking in these cases, 

 may be regarded as a device which insures the concentration of 

 individuals within a relatively small area, thus tending to make 

 more certain the chances of pairing in a large number of in- 

 stances, as well as a method of determining favorable conditions 

 for larval development. 



Collectors of nudibranchs who have worked in tropical waters 

 have also reported cases which at first sight seem to afford addi- 

 tional examples of the coincidence of the spawning period with 

 appearance in great numbers in the littoral zone (e. g., Cross- 

 land, quoted by Eliot. 1904, p. 87). While engaged in working 

 along the shore during a period of some days or weeks, it is 



1 Contributions from the Bermuda Biological Station for Keseareh, No. 59. 



