Summary

An account of the species settling, with relative monthly abundance and periods of set are given for the more important species. Results parallel closely those obtained in other countries with non-toxic test units. Sedentary organisms were principally hydroids, polyzoa, ascidians, sponges, and algae. Hydroids, at first dominant, were replaced by sponges and algae. Mussels, an established species on the wharf piles, were only moderately common and of microscopic size on most blocks. The hydroids, and to a lesser degree the algae, grew rapidly and had a short life span. The majority of hydroids attained maximum growth and maturity within four weeks. Other species, notably the serpulids Spirorbis sp. and Galeolaria hystrix, the mussel Mytilus planulatus, and the two borers Limnoria quadripunctata and Bankia australis grew more slowly and spawned throughout the year. There were further species, chiefly among the polyzoa and ascidians, with a life span of several months but seasonal spawning. Tubularia attenuoides grew twice as fast in summer as in autumn, and in the late spring the population doubled in quantity.

The combined attack of the "shipworm" Bankia australis and the "gribble" Limnoria quadripunctata resulted in marked deterioration of the wooden test blocks. These species stand in contrast in respect to their method of attack. L. quadripunctata destroys the immediate surface layer of the timber, while B. australis works deeper in the wood. With L. quadripunctata, increasing numbers seemed to be largely the result of multiplying stock on the blocks and not the result of repeated invasion from outside sources. There was a sudden rise in population numbers in October and March, and these appeared directly related to increases in temperature. L. quadripunctata showed preference for the rougher surfaces, and greatest numbers were found in the vertical blocks. In contrast, B. australis vigorously attacked the horizontal blocks. Development of L. quadripunctata to an identifiable stage takes more than four weeks, as in general no invasion was recognized on the short-term blocks. In this respect, B. australis and L. quadripunctata are alike, as no juvenile of the former showed in the X-ray photographs of the short-term blocks. The growth of B. australis, based on measurement of burrow lengths is in general represented by an ordinary growth curve.

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The double series of blocks have been amply justified, as by this method some estimate can be made of the monthly set and invasions, and of the tune taken to reach maturity, etc. It also became evident in the course of the experiment that any future work could be more easily carried out with smaller test blocks and preferably with a smoother surface to facilitate removal and earlier recognition of species. As much fouling under natural conditions takes place on wooden or metal surfaces, we would retain wood as the principal material for future test units. Also, this medium enables a double study to be made—firstly, of attachment and growth of fouling organisms, and, secondly, of borer damage. Much of the value of the present study lies in the interesting problems it brings to light—e.g., the settling and growth of Mytilus planulatus and, speaking in more general terms, the history of the community itself—particularly its unstable and transient nature.