TAXONOMIC CHARACTERS 



According to the view shared today by all 

 specialists on pelecypod taxonomy, the genus 

 Ostrea (in a broad sense), as characterized by 

 Lamarck, comprises several groups of oysters of 

 the family Ostreidae sufficiently different to be 

 considered as separate genera or subgenera 

 (Lamarck, 1819; Thiele, 1935). There is, how- 

 ever, no general agreement about the validity 

 of various genera and species. A uniform system 

 of classification of oysters is lacking, and for the 

 separation of genera and species various authors 

 use the characters of different categories: namely, 

 shape and structure of shell, anatomy, sex and 

 spawning, habitat, and structure of the larval 

 shell (prodissoconch). 



SHELL 



In spite of great variability, certain shell charac- 

 ters are generally constant although they may 

 be obscured in grossly distorted specimens. Two 

 characters of this category are important: the 

 cavity of the valves and the structure of the shell. 



The lower valve is usually deep and cup-shaped 

 with a depression or recess of greater or lesser 

 extent near the hinge area. The upper valve 

 may be flat or curved and slightly bulging near 

 the hinge. 



The oyster shell consists of extremely thin 

 outer periostracum, the median prismatic layer, 

 which is well developed on the flat (right) valve, 

 the inner calcitic ostracum that constitutes the 

 major part of the shell thickness, and hypostracum , 

 a very thin layer of aragonite (orthorhombic 

 CaCOs) pad under the place of attachment of the 

 adductor muscle.' The prismatic layer of Ostrea 

 edulis is confined to the intricate brown scales 

 of the flat valve, while among the Australian 

 oysters only one species, 0. angasi Sowerby, has 

 a well-developed prismatic layer (Thomson, 1954). 

 In the genus Pycnodonte the shell is peculiarly 

 vacuolated (Ranson, 1941). The white patches 

 or so-called "chall^y deposits" in the shells of 

 many oysters are not significant as taxonomic 

 characters. 



Size, shape, curvature, and proportion of the 

 beak, i.e., the pointed (dorsal) end of an oyster 

 shell, are useful generic characters, but Wke other 

 parts of the shell they are variable and cannot be 

 entirely depended upon for identification. 



The sculpture of the shell may be useful for 



' The author is prateful to H. B. Stenzel for the information on aragonite 

 in oyster shells. 



recognizing some species (0. (Alectryonia) mego- 

 don, fig. 3) with valves reinforced by a number of 

 prominences or folds (also called ribs, ridges, or 

 flutings by various authors) which end in the 

 crenulations at the edge of the shell. In American 

 oysters this character varies greatly depending on 

 local conditions but is rather constant in 0. 

 eqtiestrin (Galtsoff and MerrUl, 1962). 



The position of the muscle scar and its outline 

 differs in various species and, therefore, is used 

 as a taxonomic character. 



ANATOMY 



Anatomical characters are of limited usefulness 

 to malacologists who have to base their identifica- 

 tion primarily on shells alone. Consequently the 

 anatomical characteristics have been ignored by 

 the majority of taxonomists. Some of the ana- 

 tomical differences are, however, important for 

 the separation of the genera. Thus, the presence 

 of the promyal chamber separates the Ostrea, in 

 which this feature is absent in sufficiently studied 

 species, 0. edulis, 0. lurida, 0. equestris, from 

 Crassostrea in which the chamber is well developed. 



Size of the gill ostia, large in the larviparous 

 species and relatively small in oviparous ones, is 

 of generic significance. The relation of the rectum 

 and the heart is of importance since in the genus 

 Pycnodonte the ventricle is penetrated by the 

 rectum, a unique feature not found in other 

 Ostreidae. 



Convolution of the edge of the mantle with 

 three folds in the majority of the species and only 

 two in some Japanese species (Hirase, 1930) has 

 been mentioned by some investigators as a 

 specific character. The existence of two or 

 three folds may be significant, but other characters 

 such as ridges of the mantle, pigmentation of the 

 tentacles and their size and spacing are variable 

 and, in my opinion, have no taxonomic value. 



SEX AND SPAWNING 



On the basis of sexual habits, oysters fall into 

 two distinct categories of ncniincubatory (or ovi- 

 parous) species, {Crassostrea spp.) i.e., those in 

 which tlie eggs discharged into the water are 

 fertilized outside the organism ; and the incubatory 

 or larviparous species {Ostrea spp.) in which 

 fertilization takes place in the gill cavity, and 

 the larvae are incubated and discliarged after 

 having reached an advanced stage of develop- 

 ment. Incubatory oysters, as for example 0. 

 edulis, 0. lurida, and 0. equestris, are bisexual 



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