April i, 1922] 



NA TURE 



427 



many large industries, the water-front structures 



of which had been built on untreated piles because it 



was believed that the discharge into the bay of fresh 



water by the rivers would prevent invasion by salt 



Aater and therefore the advent of marine borers. 



: lie attack of 1914 appeared to be sporadic, like 



irlier ones which had been reported as far back as 



S70, but in 1917, at Mare Island, attacks by the same 



ship-worm " (Teredo) again occurred, and during 



le following years spread rapidly and increased in 



verity. By the latter part of 19 19 the attacks had 



uogressed to such an extent that parts of water-front 



structures, and, in some cases, whole docks, began to 



fail, and the report of the committee appointed to 



investigate the problem is now before us. 



The marine borers at work in the area comprise the 

 most widely known and most destructive representa- 

 tives of the groups to which they belong, in addition 

 to one other species which seems to be, as yet, purely 

 Californian. Of the boring molluscs, the first con- 

 sidered is Xylotrya setacea, which, when full grown, 

 reaches a length of two feet or more. Details are 

 given of its external features and internal structure. 

 Reference is made to the capacious pouch opening out 

 of the stomach, which serves as a receptacle for the 

 particles of wood rasped off the burrow by the shell. 

 The mechanism of burrowing has been carefully 

 studied, and it is shown that the contraction of the 

 stout, reddish posterior adductor spreads the shell- 

 valves apart and causes their edges to rasp away the 

 wood ; the return of the shell- valves to the initial 

 position, ready for the next thrust, is accomplished 

 by the small, weaker, anterior adductor muscle. The 

 shell-valves are not attached to each other by a con- 

 tinuous dorsal hinge (as in the fresh- water mussel), 

 but are widely separated except at two knob-shaped 

 projections which meet in the middle line and serve 

 as fulcra for the rocking movements of the two valves. 

 The cutting action is due to the contact of the anterior 

 face of the shell with the wood and the scraping of 

 this finely ridged edge over the wood by the outward 

 thrust of the shell as the powerful posterior adductor 

 muscle contracts. The shell is held in contact with 

 the bottom of the burrow by the sucker action of the 

 foot. The chips cut away are not more than 001 mm. 

 wide, and are several times as long as wide. Prof. 

 Kofoid states that when the borers are active it is 

 possible to hear the rasping of their tools on the wood 

 by placing the ear against the top of the pile. 



A short account is given of the life-history of 

 Xylotrya. The eggs are expelled, and fertilisation 

 takes place in the water ; the larva forms a velum, 

 develops a pair of shell-valves and a tongue-like pro- 

 trusible foot. After swimming for about a month 

 the larva, now about 0-75 mm. long, settles down, 

 preferably on wood, and usually near the mud-line. 

 It fastens itself to the wood by a sticky byssus thread, 



■- secreted by a gland in the base of the foot, the velum 

 is absorbed, and the foot is transformed into a sucker ; 

 the adult type of shell is produced, the elongate 

 siphons are formed, and the animal begins its boring 

 operations. Thirty days after attachment it is about 

 63 mm. long, and begins breeding, and by the fol- 

 lowing summer it attains the length of two feet. 

 The largest burrows of this species met with were 



t I in. in diameter in their lower portions, and more 

 than 30 in. long. 



t A short notice is given of Teredo diegensis, the 



• smallest of the molluscan borers in this area, which 

 was distinguished from other species in 1916. It is 

 only known from the Californian coast, and is of least 

 importance from the economic standpoint. 



The well-known " ship-worm," Teredo navalis, is 

 considered more fully, as befits its greater importance. 

 This species seems to reach sexual maturity in the 



NO. 2735, VOL. 109] 



first year of its growth. Prof. Kofoid records a heavy 

 death-rate in the autumn, associated with falling 

 salinity of the water, and there is also a heavy death- 

 rate in the crowded territory near the mud-line. The 

 eggs are reported to be retained in the female until 

 they develop into larvae, which, on liberation, become 

 at once free-swimming. The larva may be carried 

 by tidal currents for long distances, but Prof. Kofoid 

 remarks that, so far as records are available, no 

 evidence exists to show that T. navalis has hereto- 

 fore been found on the Pacific coast ; it is, however, 

 only a question of time before this pest will appear 

 in other ports on the Pacific coast. The number of 

 larvae which may settle on a given pile seems to be 

 limited only by the surface ; 437 were counted on a 

 square inch. 



Of the three well-known crustacean borers — Lim- 

 noria, Sphaeroma, and Chelura — which attack marine 

 structures, only the first two have been found in San 

 Francisco Bay. Limnoria (the gribble) produces few 

 young, but these are not free-swimming, and are 

 ready at once to dig in for themselves. The colony of 

 Limnoria in timber extends peripherally and the 

 burrows constantly deepen. A square inch of Douglas 

 fir heavily attacked by Limnoria was found to con- 

 tain 79 females, 82 males, and 221 young. Limnoria 

 works at all levels in harbour waters, from near the 

 mud-line to the uppermost tidal level, but is most 

 active between tide-marks, often whittling away the 

 piles to an hour-glass shape. It may be found at work 

 even in the creosoted zone of a pile. Whether it has 

 become slowly acclimatised to the repellent substance 

 or whether creosoting was defective is not known, but 

 Prof. Kofoid states that it is possible, by gradually 

 increasing the strength of the solution, to acclimatise 

 Limnoria experimentally to live and thrive in solutions 

 which originally would have been deadly. Sphaeroma 

 pentodon is of small economic importance. 



In a general account of the biological indicators of 

 Teredo, Prof. Kofoid points out that, as this mollusc 

 enters the wood as a minute larva and there is only 

 a pin-hole to mark its entry, a close inspection with 

 a lens is necessary to reveal these small openings. 

 But there are other marine animals easily identified 

 and quickly recognised, which may be taken as 

 indicators of conditions suitable for the occurrence of 

 Teredo, and among these he names the barnacle and 

 hydroids, which, from the results of the Californian 

 survey, precede Teredo as part of an invading marine 

 fauna. Their presence, therefore, indicates the possi- 

 bility of invasion of the area by Teredo, but not the 

 certainty that the latter has arrived. The occurrence 

 of young mussels (Mytilus) on piling is also another 

 danger-sign warning the engineer to look out for 

 Teredo. As the result of the biological inquiry Prof. 

 Kofoid suggests that it is necessary to restrict the 

 uncontrolled use of untreated or unprotected timbers 

 in marine structures, that harbours should not be 

 used as a dumping-ground for waste wood, and that 

 unused infected timber structures should be removed. 



In a concluding summary the committee directs 

 attention to certain other practical matters. In those 

 parts of San Francisco Bay where the attack is severe 

 the borers destroy untreated piling in six to eight 

 months, but in other places untreated piling may last 

 two to four years. A life of five to eight years may 

 be expected from paint and batten protections if the 

 work is well done and the covering not damaged by 

 careless handling. Properly creosoted Douglas-fir 

 piling, if carefully handed so that there is no injury 

 extending through the " shell " of treated wood, may 

 last twenty-five to thirty years, but, on account of the 

 damage liable to occur during repeated handling, 

 storage, and rafting, the average length of life has 

 been considerably less. Most of the attacks on 



