FOULING OF SHIPS' BOTTOMS 



241 



It can be seen clearly then that there is a very definite relation between the color of the plates 

 and the kind and amounts of growths on each. The barnacles and hydroids apparently attach 

 only to dark-colored surfaces, while Bryozoa and worms attach to somewhat lighter surfaces as 

 well, but apparently prefer the red and yellow, respectively. 



Since the white barnacles were found most abundantly on the black plates, and since neither 

 the barnacles nor the worm tubes, both of which are conspicuously white, were found on the white 

 plates, it would seem that there is no evidence of protective coloration. 



The apparent selection of the darker surfaces can best be accounted for by a study of the behavior 

 of the larvae of these organisms. The newly hatched larvae of almost all sessile marine animals 

 (as well as many others) react positively to light; that is, they swim toward the source of light. 

 This period of positive reaction, however, is of only limited duration. (It appears to be only long 

 enough to carry the young organism to the surface of the water, there to be carried about and dis- 

 tributed by the ocean currents.) Most of these larvae then become negative to light. It is in this 

 period that they attach and molt into sessile organisms with characters similar to those found in 

 the adult. This fact has been demonstrated experimentally for certain hydroids, annelids, and 

 tunicates. It is also known that lights of different wave lengths have different effects on various 

 organisms. Some go toward red, others toward blue, green, etc., depending on their relative 

 stimulating efficiency on the specific organism. 



On this hypothesis one can readily explain the results found on the plates described above. It 

 would seem (from the limited evidence at hand) that barnacles are strongly negative to light at the 

 time of attachment. Hydroids (grass) are likewise so. The Bryozoa, although negative to white 

 light, are apparently "attracted" especially by the red, and the worms (Hydroides) apparently 

 by the yellow-red light waves. It would appear that this selection or "tropism," holds good for 

 the animal forms only, as the algae were found extensively on the white plates. 



As this hypothesis is in accord with observations made on ships' bottoms, where one finds the 

 densest growths in regions least exposed to light, it seems safe to conclude that most organisms 

 commonly found attached to the bottoms of ships become attached there because of a relative 

 decrease in the amount of light given off by such areas. 



It was realized that these notes and tentative conclusions were based on very 

 limited evidence, and it was hoped that this problem might be investigated more 

 thoroughly by experiments in which many of the unknown factors would be more 

 definitely controlled. Sources of error in the above experiments were numerous, 

 although probably more or less equal for all. The relative amount of light, the 

 amount reflected from other plates of different colors and composition in the im- 

 mediate vicinity, are all unknown factors that should be eliminated in future tests. 

 The behavior of pelagic larvse of different ages was not known for any of the species 

 commonly found on ships' bottoms. It was believed, accordingly, that such studies, 

 with controlled factors, would be of value both from an economic and a purely scientific 

 viewpoint, and a few were carried out subsequently, as described in the following 

 pages. 



Although several successive series of panels were submerged, not all presented as 

 clear-cut results as did the series recorded. This lack of differentation was especially 

 noticeable after the plates had been exposed for several months (if in spring or summer 

 months), which, no doubt, can be explained by the fact that once the plate is heavily 

 coated, colors lose their influence, and, consequently, within a relatively short period 

 during the season of the year when fouling is most severe, all of the plates become 

 very heavily fouled, regardless of color. However, as less than 10 per cent of all 

 active vessels become heavily fouled, and those that become moderately foul do so, as 

 a rule, only after a considerable period out of dry dock, it will be realized that under 

 practical conditions the relative influence of colors will be greatly prolonged. 



