Mollusca

Apart from the molluscan borer Bankia australis Caiman, the only other mollusc giving data significant for growth and attachment was the mussel Mytilus planulatus Lamarck. As stated previously, this species was a well-established member of the upper zone of animals on the wharf piles, but by the time the experiment was ended only one specimen of any appreciable size (2·2 cm.) was found which had set and grown on the test blocks. This was taken from the last block of the long-term series. The long-term block shows that M. planulatus spawns throughout the year, but growth appears slow, and in the majority of cases individuals fail M. planulatus settled each month on the short-term blocks from December to April. The size range of these individuals was 0·25 mm. to 0·4 mm., with an average of 0·3 mm. A possible explanation of the non-appearance of the species on the short-term blocks before summer is the absence of suitable other species for enmeshing larvae. These were not present until spring. When the rapidly growing hydroids and branching polyzoa appear, mussels were found on the short-term blocks. On the long-term blocks, the size range of the juveniles (0·25 mm. to 0·4 mm.) for June, and September to April, 1950, indicated that they had been present on the units for not more than a month. The greatest size attained by animals surviving for more than a month was 1·75 mm. It would appear that, although sets occur practically all the year round, the species was unable to firmly page 11 establish itself on the test blocks, even after they had been in the water thirteen months. Some major factor necessary for the proper growth and development of the mussel must have been lacking. Normally, one would expect a much greater increase in size than was shown over such a time period.

As mentioned above, M. planulatus was found entangled in the upright stems of various hydroids and polyzoans. These species are well known for their efficiency in enmeshing settling larvae, but both also are capable of preying on larvae. As the hydroids are in turn food for aeolid nudibranchs (specimens of which were variously found on the blocks) and as each hydroid species flourishes for only a short period, then dies, and is replaced by another, it seems possible that these fairly rapid environmental changes may cause an upset in the normal growth and development of M. planulatus. That is, the juveniles do not become sufficiently well established before new factors change the ecological balance, and the young mussels die, or those that survive show little or no growth. It is perhaps noteworthy that those test units from which M. planulatus showed evidence of continuous growth for a period exceeding one month were also those blocks where conditions could be said to be fairly stable—i.e., Bugula sp. was at its peak for size and coverage, and also there was no replacement of one hydroid species by another, as C. repens was present during the whole period. From observations of the surrounding wharf piles, it appears that a full succession of species producing a complex but temporarily stable community must take place before M. planulatus can establish to become a dominant member of the wharf-pile community. The latter condition can be seen on the wharf piles at present, three years after the commencement of the experiment. Mussels were but a part of a larger and more varied neighbouring community when the experiment began. At the present time, even their shell valves are almost free of encrusting organisms. Other factors besides rapidly changing conditions no doubt affect the settling and development of M. planulatus, but as yet we are unable to offer any hypothesis than that of rapidly changing local environmental conditions causing marked retardation in normal growth and development of the species or even death to fairly large numbers of juvenile individuals.