THE CERION BREEDING EXPERIMENTS AT 

 THE TORTUGAS 



Bv PAUL BARTSCH, 



Ciiralor, Di7'isio]i of MoIIiisks. U. S. A'alioiial Mitscniu 



For the past i6 years I have conducted experiments in heredity 

 through the cross-lireeding of mollusks of the genus Cerion at the 

 Marine Biological Lahoratory of the Carnegie Institution at the 

 Tortugas, Florida. My visit to the station this year was a short one ; 

 I arrived on August 17, 1928. and left with the Dohrn on the closing 

 of the laboratory August 20. However, sufficient time was granted me 

 in this period to make the examination of the Cerion colonies. A 

 careful examination of the contents of the little artificial islands, on 

 each of which a specimen of two species are segregated for possible 

 cross-breeding, showed the presence of some voung individuals, but 

 no adidt hybrids. 



An examination of the cut-down cages, however, revealed one 

 adult of the much looked- for cross between Cerion vlarcgis and 

 C. incaiiiiiii. It will be remembered that considerable criticism was 

 expressed because, in my original crossing- experiments, I had em- 

 ployed large groups (500 individuals) of each of the two species, and 

 doubt was expressed as to whether the organisms which I claimed to 

 be crosses were really crosses. Some of my critics held that they might 

 be mutations of one of the two species involved. To settle this, the 

 individual cage and island method was resorted to. These restricted 

 areas were stocked with a virgin individual of each of the two species 

 mentioned above. The results were what I expected, a perfect vindica- 

 tion of the mass experiment, for the adult hyl)rid obtained in the cage 

 is in every way identical with those secured from the mass experiment 

 on New Found Harbor Key. This of course was to be expected, for 

 in no colony of C. viarcgis or C. incanum has any form appeared 

 comparable in appearance to the hybrids in question. Thousands upon 

 thousands of specimens were examined in all these colonies, and never 

 once has an individual turned up wdiich might have given one the 

 slightest doubt as to the specific identity of the individual in question. 



While at the Tortugas I had sufficient time to spend part of a day 

 under water with the diving hood and the undersea camera, going- over 

 the fields photographed in the years gone by, in order to have a con- 

 tinuous record of what is happening on the reefs impressed upon 

 photographic film. 



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