THE REPRODUCTION OF LIMACINA RETROVERSA (FLEM.)

¹ From the Biological Laboratories, Harvard University, and the Woods Hole Oceanographic Institution, Woods Hole, Massachusetts

1. In the winter months sexual differentiation commences when the animals are between 0.6 and 0.85 mm. in diameter. In the summer sexual differentiation starts when the animals are smaller.

2. The early proliferation of the male reproductive cells leads to a protandric phase. The "pure males" give place to hermaphroditic males and later to hermaphroditic females as the size of the animals becomes greater than 1 mm. in diameter. After the specimens are 1.5 mm. in diameter the female reproductive cells overtake the male in development, bringing about a decrease in the proportion of hermaphroditic males and an increase of the opposite sexual type. This change continues until the two types are equal, when the animals are 2 mm. or more in diameter.

3. Functionally the male phase starts among smaller Limacina than do the functional female and hermaphroditic phases. The highest proportion of actively functioning males is found among animals 1.2 to 1.8 mm. in diameter. Among larger animals the functional hermaphroditic and functional female phases replace the male phase. Hermaphroditic females begin the functional phase after they are greater than 1 mm. in diameter.

4. Members of a population which entered the Gulf of Maine in the winter were all sexually differentiated by the beginning of March. Nearly all of these individuals developed into protandric males, about half of which were "pure males." The number of such "pure males" decreased in March and disappeared in April, being replaced by animals with greater predominance of feminine tissue in the gonads. By April about 40 per cent of the specimens were hermaphroditic females and in May the two sexual types were equal.

5. The male functional phase began in winter and continued into spring and summer. Hermaphroditic females matured in March and their proportion increased in April and May. Spawning of ova commenced by the end of April. The largest proportion of functional individuals of all types was seen in May.

6. Reasons are advanced for the belief that the change in sexual constitution of the populations is due to a reversal of sex predominance of the individuals.

7. The sequence of the sexual phases of Limacina retroversa agrees in a general way with those found for other members of the gastropod group of mollusks, particularly that of Valvata.