SEQUENCE OF FUNCTIONAL SEXUAL PHASES IN TEREDO

Abstract

Examination of the gonads of 2,987 specimens of Teredo navalis of all ages, at all seasons, and during three successive years supplements the conclusions previously published regarding the sequence of sexual phases in this species. The functional phase, if any, in which any given individual is found will, in general, depend upon: a, age; b, size; c, season; d, season when hatched; e, its particular combination of genetic factors. Each normal individual shows a tendency toward an alternating series of functional sexual phases. There is good evidence that at least four of these phases may sometimes be completed in a lifetime of less than two years, although the average individual experiences hardly more than one or two. Among the variable sequences of phases which different individuals may experience the following may be considered typical: 1. Male … female. 2. Male … female … male … female. 3. Male … female … male. 4. Male … female … female. 5. Male ("true male") … male … female. 6. Male ("true male") … male. 7. Female (exceptional) … male … female. Because the population consists of a vastly greater number of individuals of the younger ages than of the older age groups, it is obvious that death by suffocation, starvation, parasitization or some other cause will usually terminate the individual's existence before the normal cycle is completed. Protandric bisexuality is generally characteristic of the early sexual phases, with a tendency toward alternating unisexuality in later life. Of the latter, the female phase is usually of longer duration than the alternate male phase. The proportion of individuals in each sexual phase in the general population or in selected groups will obviously be correlated with a combination of the conditions mentioned above, but limited by associated environmental influences. The total population of teredos. living in a particular block of wood may thus consist of upwards of 90 per cent of individuals in either functional sexual phase or of about equal percentages of each, according to the age group or groups represented at the time of the examination. Attention has been previously (Coe, 1934a) called to the similarity of the sexual rhythm in Teredo to that of the larviparous oysters, "although the usually shorter life of the former limits the number of the alternating sexual phases." There is some resemblance also to the sexual changes in the oviparous oysters (Coe, 1934b) and in the hard-shell clam Venus (Loosanoff, 1936), which are predominantly, but not exclusively, protandric, with a later change to the seasonally unisexual condition of the older individuals, in which there is an approximate equality in the sex ratio.