of the Fishery Board for Scotland. 



175 



III. — ON THE REARING OF THE LARVAL AND POST-LARVAL 

 STAGES OF THE PLAICE AND OTHER FLAT-FISHES. By 

 Harald Dannevig (Plate IV.). 



The development of the eggs of the common food fishes is now rela- 

 tively well known, only few species being yet undescribed. It has also 

 been comparatively easy to obtain abundant materials for such studies, as 

 the eggs are easily developed in glass jars or tanks in laboratories. All 

 that is necessary for the maintenance of life in the majority of one or two 

 hundred fish eggs, is to place them in a jar holding from one to three gallons of 

 sea water. When the temperature is kept within reasonable limits, and the 

 water changed perhaps twice a day, the development generally proceeds 

 without difficulty. Within the last twenty or twenty-five years, such 

 studies have been taken up systematically, and many valuable and impor- 

 tant papers dealing with the early stages of development have more 

 recently been published by a number of naturalists such as Prof. Mcintosh, 

 Mr Holt, Mr J. T. Cunningham, Dr Canu, Dr Pettersen, and others. 

 And not only has the development of the eggs been closely followed, but 

 detailed descriptions and drawings have also been given of the various 

 stages of the young larvae from the moment of hatching till the absorp- 

 tion of the yolk has been completed. The larvae, during the time they 

 are nourished from the yolk, merely require for their well-being protection 

 and change of water, both of which are easily provided. 



As the whole of the development from the impregnated egg till the end 

 of the larval stage, when the yolk is all absorbed, has been studied from 

 specimens kept in confinement, it has also been possible to ascertain the 

 rate of development, always knowing the age of the egg or larva in ques- 

 tion.* 



It has, therefore, naturally been much desired by naturalists to be able to 

 retain in confinement such young larval fish also after the absorption of 

 the yolk in order to follow up the post-larval stages, so that the later part 

 of the development could be studied as closely and accurately as the pre- 

 vious part. This has been so much the more desirable, as it is often very 

 difficult to distinguish definitely between closely related species that occa- 

 sionally are captured in tow-nets ; and even, when the distinction has been 

 possible, the age is unknown, and can only be given approximately. 



But, as far as I have been able to ascertain, no experiment of this kind 

 has hitherto succeeded, though most investigators have probably made one 

 or more attempts. Dr C. G. Joh. Petersen, indeed, has stated that "At 

 the pelagic stage, after the yolk has been absorbed (what I have called the 

 post-larval stage), it (the plaice) cannot be kept in aquariums. "f It may, 

 therefore, be of interest to describe in detail the arrangements by which I 

 succeeded in rearing plaice during the summer of 1896. J 



* It has been fouud*that the larger fish eggs take longer time in hatching, and 

 their larva? a longer time in the absorption of the yolk than eggs and larvre of a 

 smaller size ; and it has also been shown experimentally, that the rate of develop- 

 ment of egg and larva is much influenced by the temperature. See my paper on 

 " The Influence of the Temperature on the Development of Fish eggs." Thirteenth 

 Annual Report. Part III., page 147. 1895. 



t Report of the Danish Biological Station, Part IV., 1893, page 14. 



X I am greatly indebted to Dr Fulton, the Scientific Superintendent of the Board 

 for procuring for me the required literature, and for valuable advice relating to this 

 paper, and I have also got to thank Mr Scott, Naturalist to the Board, who kindly 

 identified the various forms of molluscs and crustaceans that I had drawn or pre- 

 served of the food materials ; the diatoms were identified by Prof. Cleve of Upsala. 



