IO , DISCOVERY REPORTS 



(iii) Sexual succession 

 In the colder waters of the North Atlantic according to Hsiao, sexual differentiation in L. retroversa 

 commences in the winter months and sexually differentiated individuals appear as in ' pure males ' 

 by the beginning of March. The predominance of pure males decreases towards the end of 

 March and disappears in May, being replaced by a predominance of female animals. In April the 

 percentage of females was forty and early in May the two sexual types were in equal numbers. 

 Spawning had commenced by the end of April, which may be compared with Lebour's statement that 

 in British waters the eggs are ripe for the greater part of the year, with a maximum in summer when 

 each individual may apparently have had several broods. In retroversa from the Gulf of Maine the 

 period of egg-laying thus reached its height at the end of the spring. The Benguela bulimoides were 

 from autumn material. There were few cases of individuals having been spent in spawning and a 

 relatively small number of very young individuals. The immature males may well belong to a com- 

 ponent of the population resulting from spawning earlier in the spring of the same year. All sexual 

 stages of the population were represented in the autumn material of the first survey and there is a very 

 uniform correlation of successive sexual stages with increasing shell size throughout all the material 

 examined. There is no indication, in the form of variable sex-size relations that this population was 

 heterogeneous, as was demonstrated by Redfield (1939) in North Atlantic retroversa. 



Several questions arise regarding L. bulimoides which probably can be answered after examining 

 material from the second survey, with a spring population available for comparison. First, Is there 

 a differentiated breeding season in L. bulimoides, or does reproduction, as in the English retroversa, 

 (as distinct from cold water Gulf of Maine material) take place all the year round? In Hsiao's retroversa 

 a spring breeding season commences by the end of April, with a fairly rapid maturation of oocytes and 

 a change in sex predominance during the preceding month. In L. bulimoides, where an autumn 

 breeding season has been shown to occur, it would seem likely that reproductive activity in March 

 may have been preceded by breeding in spring, possibly continuing without interruption during the 

 summer months. This leads to the query, Are single individuals of Limacina able to undertake a second 

 breeding season during the same year, or does the breeding life of a single Limacina extend beyond the first 

 breeding year? From the sex-size analysis of the autumn population of bulimoides, there seems to be a 

 single peak within a definite size group for each successive sexual stage. As yet there is no evidence for 

 this species that a restitution of the male phase is possible in individuals in which the oocytes are spent. 

 Such a form of sexual alternation occurs, as shown by Orton (1926) in the oyster ; and the present writer 

 obtained evidence from a colony of the primitive pulmonate Carychium, which had laid eggs in June, that 

 a restitution of sperm-producing tissue took place during the same year in the gonad of spent females. 



Further, Do seasonal differences occur in Limacina bulimoides, in the proportions of the various 

 sexual stages at different sizes of the animal? Hsiao (1939b) suggested that small individuals in the 

 hermaphrodite female condition in his 'population B' did not pass through a protandric phase of 

 noticeable length, though other specimens of the same population had a distinct 'young male phase' 

 preceding the female. He concludes that ' during the warmer part of the year sexual differentiation 

 takes place sooner than in colder times and that the sexual phases may be telescoped together when 

 development is speeded up in the summer'. This is in agreement with observations made by Coe 

 (193 1 ) and Orton (1909; and other papers) on Ostraea. Such a trend towards the 'telescoping' of 

 sexual stages in the warmer parts of the year points the way to the simultaneous hermaphroditism of 

 higher opisthobranchs and pulmonates, where the female phase is pushed back until it is contem- 

 poraneous with the male and the two types of sexual product reach maturity at the same time in the 

 common ovotestis. 



