SEXUAL PHASES IN TEREDO

¹ OSBORN ZOÖLOGICAL LABORATORY, YALE UNIVERSITY

1. Teredo navalis morsei in New Haven Harbor and T. n. novangliae at Woods Hole are essentially protandric, nearly all females passing through a preliminary functional male phase before reaching the definitive phase of sexuality.

2. The primary gonad is bisexual in nature, having a cortical layer of potential ovogonia, with spermatogenic cells filling the lumen of each follicle.

3. The gonads of young individuals in early sexual maturity indicate three intergrading but more or less well demarcated types of functional males: (a) initial males, representing the protandric phase of the females; these have a complete cortical layer of ovocytes surrounding the relatively small amount of spermatogenic tissue in the lumens of the follicles; (b) hermaphroditic males; these have relatively few large ova scattered among the spermatogonia of the cortical layer; (c) true males; most of the ovocytes in the cortical layer remain small or degenerate.

3a. In the protandric females the cortical cells become differentiated into ovocytes and begin the deposition of yolk while spermatogenesis is in progress in the lumens. The functional male phase may be reached within six weeks after metamorphosis and is of brief duration. A few spermatozoa may be retained until the first ovulation, after which the animal seems to have exclusively female characteristics.

3b. In hermaphroditic males the functional male phase is of longer duration than in typical protandric females and the number of spermatozoa produced is much greater. A later transformation to the female phase is presumably of usual occurrence, but is not invariable.

3c. True males form a small proportion of the population. Growth continues during a long period of spermatogenesis. After the discharge of the first crop of spermatozoa most of the residual cells still retain their male characteristics and no evidence of a later change of sex has been found, although small ovocytes are usually scattered among the cortical cells.

4. The proportion of individuals in the two functional sexual phases changes with the season, there being a great excess of males at the beginning of the breeding season due to the protandric nature of the young animals in the early period of sexual maturity; there is a corresponding excess of females later in the season, due to the change of sexual phase after spermatogenesis in the initial male phase has been completed. After the end of July the proportion of males may again be greatly in excess if the new crop of young is included. Selection of the larger (and older) individuals will then yield mainly females, while the smaller (and younger) animals may all be in the male phase.

5. In the early part of the breeding season the populations from two localities each consisted of about 33 per cent functional males and 67 per cent mature females. Less than one-third of the functional males were classed as true males, the others indicating the potentiality of a later change to the female phase. By the middle of August only 9 males to 100 females were found in the older age group.

6. After the assumption of the definitive female phase there is no evidence of a second sex reversal.