LARVAL SHELL (PRODI SSOCONCH) 



The difficulty in identifying a species by its 

 shell led Ranson (1948b) to base the classification 

 of oysters entirely on the features of a "definite" 

 prodissoconch, i.e., the shell of a fully developed 

 larva. He claims that distinctive crenulations of 

 prodissoconchs are sufficient for the separation of 

 species and that these specifically larval charac- 

 ters can be detected in well-preserved adult shells 

 and even in fossils. In a brief paper comprising 

 only 6 incomplete pages of text and 35 pages of 

 drawings of 34 species of oyster larvae Ranson 

 (1960) summarizes the basic idea of his classi- 

 fication. He states that a lamellibranch larval 

 shell passes through two distinct stages, the first 

 one is a "primitive" prodissoconch with un- 

 differentiated hinge and the second phase, which 

 he calls "definite" prodissoconch, is characterized 

 by the development of hinge teeth. At the first 

 phase all lamellibranchs have similar prodisso- 

 conchs, but at the second phase the hinge becomes 

 dift'erentiated. This makes it possible to dis- 

 tinguish the families and the genera. He main- 

 tains, without giving substantiating evidence, 

 that the general shape of a definite prodissoconch 

 is absolutely constant even if the size of the 

 larva varies and that each species of oysters can 

 be recognized by the shape of its larval shell 

 and its structural characteristics. 



Ranson's system of classification recognizes the 

 following three genera of oysters: Pycnodonte 

 {Pycnodonta in Ranson's spelling), Crassostrea, 

 and Osirea. His diagnoses of the prodissoconchs 

 of these genera are given in the following section 

 of this chapter. Unfortunately, the diagnoses 

 of the larvae of 34 species of oysters studied by 

 Ranson are lacking, and the prodissoconchs are 

 shown only by diagrammatic drawings, some of 

 which are reproduced in chapter XVI of the book. 

 My attempts to locate the prodissoconchs on the 

 shells of fully grown C. inrginica (from 3 to 8 

 years old) were not successful. On a few occasions 

 the prodissoconchs were faintly visible, but the 

 structure of the hinge, and the number and 

 location of hinge teeth could not be detected. 

 Final decision regarding the possibility of identi- 

 fj'ing adult oysters by their larval characters must 

 wait, however, until Ranson's system is given a 

 fair trial by malacologists. His suggestion that 

 identification can be made by observing spat 

 attached to the shell of the adult is not valid 

 because in many places several species of oysters 



live together in the same locality and the larvae 

 settle indiscriminately on any shell or other object 

 available at the time of setting. 



THE GENERA OF LIVING OYSTERS 



There is an obvious need for a complete taxo- 

 nomic revision of the family Ostreidae. This 

 revision should cover all the principal species of 

 living oysters and must be supplemented by 

 morphological, anatomical, and ecological observa- 

 tions which at present are available only for a 

 few commercially utilized species. In the absence 

 of these data for the large majority of species of 

 living oysters, it is at present impossible to propose 

 a logical taxonomical system for the family. 



Opinions vary regarding the number of genera of 

 living oysters of the family Ostreidae. Stenzel 

 (1947) recognizes 12 valid generic names, some 

 of which, as was shown by Gunter (1950), are 

 synonymous. The latter author admits the 

 existence of three definite genera {Ostrea, Cras- 

 sostrea and Pycnodonte) and three others {Den- 

 dostrea Swainson, Aledryonia Fisher de Wald- 

 heim, and Striostrea Vialov) of doubtful validity. 



Ranson (1941, 1948b, 19G0) merges the three 

 doubtful genera of Gunter in Ostrea and recognizes 

 only the three definite genera listed above. This 

 opinion, based primarily on structure of pro- 

 dissoconchs, is shared by Thomson (1954) and is 

 supported by the evidence accumulating from 

 morphological and biological data. It can be, 

 therefore, stated with a certain degree of assurance 

 that on the basis of present knowledge, the living 

 Ostreidae comprise three genera: namely, Ostrea 

 Linnaeus; Crassostrea Sacco; and Pycnodonte 

 Fisher de Waldheim. The genus Lopha, named 

 by Bolton in 1798, without definition and described 

 as a subgenus only in 1898 by Dall (1898), is 

 undoubtedly a synonym of Ostrea. The dis- 

 tinctive feature upon which this genus was founded 

 was the sharply crenulated nature of the shell 

 margin, a very poor distinguishing character. 



The three genera of oysters can be defined as 

 follows: 



Genus Ostrea Linne, 1758. Genotype: O. edulis L. 



Shell subcircular; lower valve shallow, not 

 recessed under the hinge; upper valve flat, opercu- 

 lar, sometimes domed; muscle scar subcentral. 

 Promyal chamber absent. Gill ostia relatively 

 large. Incubatory. 



6 



FISH AND WILDLIFE SERVICE 



