290 



It will be noted that the tables show a slight but consistent ad\antage in number 

 of ridges of the shells from Goat Island over those from Crockett. This is correlated 

 with the generalization set forth above (p. 270) that the ridges are more numerous 

 and close-set on shells of teredos from the more saline waters. 



A few shells of known age were available from other localities, and these were 

 examined as a check on the accuracy and general applicability of the above figures. 

 From the Oakland Harbor Light Station, two miles across the channel from Goat 

 Island, 5 shells 3 months old averaged 16 ridges; 8 shells 4 months old averaged 18.7 

 ridges. From Dumbarton, near the southern end of the bay, a series of 10 shells 6 

 months old a\eraged 20.5 ridges. A series of 10 shells of similar age from Angel Island 

 averaged 20.6 ridges. The largest shell we have taken has 81 ridges. This is the shell 

 of the specimen from the Oakland Mole, mentioned on page 2,S3, which was 50.8 cm. 

 long and estimated to be two years old. 



The Annual Cycle 



The finding of a specimen of Teredo vavalis of an approximate age of two years 

 is very unusual. The death rate at all stages of the life history is extremely high. 

 Probably the great majority of the free swimming larvae become food for fishes, or 

 otherwise perish without finding settlement on wood. Of those which find attachment, 

 many die off during the early stages of boring, as testified by the many shallow bur- 

 rows which are found empty, and the numerous surface pits which have gone no farther. 

 In heavily attacked piling the surface punctures are always more close set than is 

 possible for the burrows as they expand within the pile; this means that only the earlier 

 arrivals, and those which bore most rapidly, are able to establish themselves and 

 survive. Late comers are crowded out, and soon die for lack of space and wood in 

 which to bore. 



The organisms which are comparatively successful and succeed in becoming 

 established in the wood and sur\i\ing for a period of months, in most cases become 

 victims at last of their own rapaciousness. All of the a\ailable wood is devoured. 

 Further, the substance of the piling is broken down to such an extent that it no longer 

 afTords adequate protection. Fragments are torn off by abrasion and wave action, 

 exposing the borers, which must of necessity die in such condition. When some have 

 died, the products of their decomposition affect others in adjoining burrows. Fungus 

 growth and bacterial action set in, and in a short time the pile becomes a focus of 

 death and decay, so that few of the borers are able to survive. 



In consequence of these conditions, the majority of specimens of Teredo which 

 enter a pile die within six months. It is safe to say that in most cases not more than 

 10 per cent of the specimens which find settlement on wood survive until the following 

 breeding season. Survival for more than one year is very imusual. In the instance 

 cited above, the organism apparently two years old was most favorably situated, 

 boring deep in a relatively sound pile, where it was not subjected to the unfavorable 

 effects of crowding. The pallets of this specimen were badly eroded, so that they 

 were of little or no use in closing the orifice of the burrow, and it is probable that 

 the animal would not have survi\ed much longer if it had been left undisturbed by man. 



Thus we may say in genera! that the cycle of birth, lar\-al life, growth in the wood, 

 reproduction, and death, is completed in a single year; and that in a majority of 

 cases the cycle is terminated by accident or disease. Only the immense numbers of 

 larvae produced by the relati\'ely few sur\i\-ors maintain the species at its destructive 

 level. 



