General Settling and Growth on the Surface of the Test Blocks

As stated above, the present system of making use of "short" and "long" term blocks gives an indication of monthly larval sets and also how these are modified by animals already established. Where species of a single class—for example, the Hydroids or the Polyzoa—are involved, the long-term blocks also provided interesting successional studies for that particular group. All the major elements of the well-established association on the wharf piles were at some time present on the test blocks. Prominent species of the complex wharf-pile community at the commencement of the experiment were the gymnoblastic hydroid Tubularia ottenuoides, the polychaetes Spirorbis sp. and Galeolaria hystrix, and the barnacle Elminius modestus. The three latter species were conspicuous on the shell valves page 4 of the mussel Mytilus planulatus in the vicinity. Other species in some numbers were a polyzoan Bugula sp., the ascidians Botryllus schlosseri, Asterocarpa Cerea, and Corella eumyota, and the sponges Sycon ornatum and Halichondria reticulata.

Two points were clear almost from the commencement of the experiment. Firstly, the effect the already established species on the long-term units had on the species attempting to settle during the month. Secondly, the very definite if not spectacular change in variety and number of species forming one community after another in fairly rapid succession; although the blocks did not show a climax association even after thirteen months of submergence as evidenced in the fact that mussels, which were an established part of the upper zone of the wharf-pile community at the commencement of the experiment, were scarcely represented on either the short- or long-term blocks. A mussel 2 cm. in length was taken from the last block in April, 1950. This animal had settled and grown on the test block. Other specimens much larger in size (6·0 or 7·0 cm.) were taken from test blocks fairly early in the experiment, but these had obviously fallen from the wharf stringer above. A recent examination (November, 1951) of the wharf piles revealed the interesting fact that practically none of the hydroid, polyzoan, ascidian, or sponge species that were prominent at the commencement of the experiment are now present on the piles—the pile fauna consisting almost entirely of mussels. This indicates in this particular case that mussels dominate the climax association, which takes about three years to come into being.

Any description of settling and growth on the surface of the test blocks falls fairly readily into three sections—viz., a review in general terms demonstrating the salient features of group succession, a month-by-month survey of all the species present on both long- and short-term blocks, and, lastly, notes on the individual species. In general, then (Table 2), the first long-term block showed a dominant hydroid settling. The following month (May), the hydroid element shared equal rank with polyzoan and ascidian elements. This was followed in June with ascidian and dendritic polyzoan elements gaining ascendancy, but in July the hydroid element again appeared and retained equal rank with ascidians and polyzoans till September. In October, the colonial ascidians declined in numbers and importance and two new elements replaced them—namely, algae and sponges. These two groups and a flat crustose polyzoan remained the dominant features of the units till February, when the crustose polyzoans became fewer in number, until in April, 1950 (the last month of the experiment), the algae and sponges were present as the dominant groups. Had the experiment been continued, present indications are that both these groups would have been replaced by mussels.