Hydroida

The hydroids were short lived but quick growing, and showed more clearly evidence of species succession than any of the attaching organisms. The gymnoblast Tubularia attenuoides Coughtrey settled very heavily in April, 1949, and again, though less densely, from November to March, but no great bloom comparable to that of April, 1949, was observed the following April. Pyefinch and Downing (1949) found that the presence of a mature colony of Tubularia larynx contributed substantially to the amount of successful settling of the species that takes place in the immediate vicinity, and also that Tubularia does not settle on a surface covered by long tufts of filamentous algae. We have no information as to the nearness or otherwise of mature Tubularia colonies to the April test block, but the alga Myriogramme denticulata was dense on the long-term block and of sufficient height (up to 7·3 cm.) to at least limit the set of Tubularia. Thus, T. attenuoides is an example of a species dominant in the pioneer community (April, 1949), but through changing conditions suppressed in a later community (April, 1950) at a time when a heavy set was anticipated. Of all species, T. attenuoides was the most spectacular as far as rapid growth was concerned. The average monthly height was 3·0 cm., but in the warm summer months many colonies grew to page 6

Table I.

page 7

Table II.

page 8 6·0 cm. Some of the polyps of the colony showed mature gonophores after four weeks' growth. In accordance with Corlett's findings for T. indivisa, temperature had its effect on maturity in T. attenuoides. When temperatures were highest, a greater number of polyps showed ripe gonophores at the end of a month than was the case when temperatures were lower. The life span of T. attenuoides is, under local conditions, approximately two months.

Campanulina repens Allman replaced T. attenuoides on both series of blocks, and settled in June (long-term blocks) and again in December-January, March, and April, as shown by the short-term blocks. As with T. attenuoides, Campanulina repens attains maturity and full height (8·0 mm.) within four weeks. Mainly dead stalks are found on the long-term blocks by the end of October. The reappearance of the species on the short-term blocks in December and January and the long-term block in February suggests that the medusae are sexually mature in three to four months. C. repens, in common with other hydroids attempting to become established on the long-term blocks from February onwards, showed little growth compared with previous settlings.

Obelia australis von Lendenfeld replaces C. repens as the dominant hydroid element of the long-term block in October and November, and had attained its greatest height (2·5 cm.) within the month when medusae were being shed from the gonophores. The reappearance of the species on the long-term blocks in March suggests that the medusae are sexually mature in about two months. No gonophores were present in March. O. australis, like T. attenuoides, has, under local conditions, a life span of two months. Halecium sp. appeared on the long-term blocks in November, and showed stems 1·5 mm. to 3·0 mm. in height. Those with a stem height of 2·5 mm. to 3·0 mm. had well-developed hydrophores. Smaller stems were without reproductive structures. The spawning period is about one month, as the species appeared on the short-term blocks in December and January. Halecium was usually found growing on the polyzoan Bugula or on the dead stems of other hydroids.

Syncorne tenella (Farquhar) was recorded on the first block when stems 16 mm. in height were found and medusae buds were being freely produced. The species seemed seasonal in occurrence and did not appear again until the following autumn on both the short- and long-term blocks. These specimens were much smaller in size (6·0 mm.) than those of the previous autumn and without medusae buds. They may have settled late in the month or found conditions unfavourable for rapid growth and development. Turritopsis nutricula McCrady was recorded only once, in July, on the long-term blocks, when stems up to 15·0 mm. were found. Mature medusae and planulae of this species were fairly common in the water in the vicinity of the test blocks at the commencement of the experiment. Four months passed before the species appeared on the blocks, which suggests that few page 9 planulae from these medusae survived, as it seems unlikely that planulae would take this length of time to settle and show on the blocks.