The Settling and Growth of Wharf-pile Fauna in Port Nicholson, Wellington, New Zealand
Species Present on Long- and Short-Term Blocks
Species Present on Long- and Short-Term Blocks
The calcareous species Sycon ornatum Kirk was short lived and present in moderate numbers on the long-term blocks in October and November, and was replaced by the siliceous species Halichondria reticulata Brondsted for the remaining months of the investigation. This sponge showed greatest coverage and size in March and April, 1950. No evidence of sponges was found on the short-term blocks, indicating that the present two species of Porifera take more than a month to reach identifiable size. Also, the sponges were the last group to settle and become established. This is fairly well in accord with the finding of Allen and Ferguson Wood (1950), where no siliceous sponges appeared on the test plates at all, and the occurrence of calcareous species was mostly confined to Sycon-like juveniles up to 1·0 mm. in height on the monthly plates.
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.
Sixteen species of polychaetes were recorded from the test blocks. This is by far the greatest number for any one group in the present investigation, but as at least half of these were errant species, they can only be considered as casual members of the association. The most important species for the present investigation were the two serpulids, Spirorbis sp. and Galeolaria hystrix Morch., which showed clear evidence of spawning periods and were present for sufficient time to give adequate information for growth rates to be obtained. Both species were present almost constantly on the long-and short-term blocks, indicating that spawning takes place througout the year and that larval life is probably less than one month duration. Some months showed a heavier set than others. Spirorbis sp. set heavily in June and again in February, and G. hystrix in April, 1950 (Table I). Temperature does not appear to play any part in determining the density of the set, particularly in the case of Spirorbis sp., as one heavy set took place in winter and the other when temperatures were highest in the summer. In summary, it can be said of these serpulids that they were present practically throughout the year, but were never a dominant feature. The size range for G. hystrix at three-monthly intervals is as follows: one month, 3·0 mm. to 6·0 mm.; three months, 9·0 mm. to 16·0 mm.; six months, 13·0 mm. to 37·0 mm.; twelve months, 61·0 mm. to 80·0 mm.
The following species are errant and mud-dwelling species and appeared only in small numbers and for relatively short time periods on the blocks. The short-term blocks showed the errant Podarke sp. present in September and October, and Nereis kerguelensis McIntosh, Perinereis camiguinoides Augener, and Dorvillea oustraliensis (McIntosh) in January. D. australiensis appeared again in February. These four species were present on the long-term blocks earlier than on the short-term blocks—i.e., Podarke sp. in May, N. kerguelensis in June, D. australiensis in August, and P. camiguinoides in November. Next to Spirorbis sp. and G. hystrix, N. kerguelensis was the species present for the greatest length of time (seven months) on the long-term blocks. The long-term blocks also showed 10 species over and above those previously enumerated. These were present for varying periods, usually of short duration (Table II).
Only one species of barnacle was recorded for the experiment—namely, Elminius modestus Darwin. A very heavy set showed on the May short-term block. A count of this block showed 25 animals to the square inch and an estimated total number of 17,750. Less heavy sets occurred in June and April, 1950 (Table I). In all page 10cases, the specimens were large enough to be clearly recognized as E. modestus. The barnacles were smaller on the vertical and silted areas of the blocks. The species made only erratic appearances on the long-term blocks, apparently because of failure to survive. At no time was E. modestus a very prominent feature of these blocks.
The heavy set during May and the moderately heavy set of June and April, 1950, suggest that the main spawning period of this species is autumn. Work overseas (Corlett, 1948) indicates that settling periods are determined to some extent by temperature. Temperature may not have been so great a factor in determining set in the present instance. For example, in months (October and November) with a temperature range approximating those months when E. modestus set heavily, no real indication of the expected set showed on the test blocks. Allen and Ferguson Wood (1950) state that Balanus trigonus and two varieties of B. amphitrite set fairly heavily from November to March, with smaller settlings in October and April to July. No monthly temperatures are given to allow comparison with E. modestus. E. modestus, on the short-term blocks, i.e., up to four weeks' growth, showed a diameter ranging from 1·0 mm. to 2·0 mm., with an average of 1·5 mm. By the end of the experiment, specimens up to 6·0 mm. in diameter were present on the long-term blocks, but no estimate of the age of these specimens can be given, as no information as to the settling date is available.
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.
The most prominent polyzoan on both series was a species of Bugula. Bugula sp. was present on all the long-term Mocks except in April of both 1949 and 1950. It reached maturity and maximum growth and density in the spring months of August to November, and together with Cryptosula pallasiana (Moll.) and Bugula neritina (Linnaeus), attached in spring and early summer. C. pallasiana paralleled Bugula sp. for time range on the long-term blocks, but reached its peak for growth and development in the summer (December to February). Other species showing erratic appearance only on the long-term blocks were Beania bilaminata (Hincks) page 12 in May, Tubulipora sp. (June), and Idmonea sp. (June). Quantitatively, the polyzoan species, particularly Bugula sp. and Cryptosula pallasiana, formed a large proportion of the settling organisms. This was also the situation obtaining in Queensland and New South Wales (Allen and Ferguson Wood, 1950). In Australia, Bugula neritina was very prominent, with peaks in November and December. During maximum growth and development, Bugula showed a few colonies 9·8 cm. in height, and similarly the flat calcareous C. pallasiana reached an area 18·0 cm. by 13·0 cm.
From the evidence of the long-term blocks, it would seem that these species take more than a month to reach identifiable size. The main spawning period for Cryptosula and Bugula, as indicated from sets on the short-term blocks, was late spring and summer. These blocks were probably principally seeded from animals on the long-term blocks, where they were at maturity and maximum growth at that period. Cryptosula and Bugula sp. have a life span of approximately nine months. Bcania, Tubulipora, and Idmonea appear on the long-term blocks for a month only. Either these species have a short life span or else for some reason failed to survive.
The colonial ascidians Diplosoma macdonaldi Herdman and Botryllus schlosseri (Pallas) were prominent on both long-and short-term blocks. On the long-term blocks, D. macdonaldi reached maximum development covering large areas of both horizontal and vertical blocks in the winter months of June and July. Some colonies were 9·5 cm. by 12·0 cm. across at this time. A second heavy set was shown by the short-term blocks to occur in the summer and early autumn. B. schlosseri was present to a greater or lesser degree during the whole of the experiment except for the first month. Its peak for surface coverage was early spring, when it replaced D. macdonaldi. Colonies up to 7·0 cm. by 5·5 cm. were present at this time. B. schlosseri did not develop to an identifiable size on the short-term blocks. Three species of solitary ascidians—namely, Corella eumyota Traustedt, Cnemido-carpa nisiotis (Sluiter), and Asterocarpa cerea (Sluiter) appeared during the experiment. On the short-term blocks, C. eumyota was present during October and December-January, and Cn. nisiotis in December-January and March. A. cerea did not appear on the short-term blocks, but was present in some numbers in late spring and early summer, when it replaced C. eumyota as the simple ascidian element on the long-term blocks. By April, 1950, the largest specimen was 6·5 cm. in length. C. eumyota was common on the long-term blocks during winter, and a few small animals were present in October, December, and January, reflecting the sets shown for the species on the short-term blocks. Cn. nisiotis was at its peak for size and numbers in May, and was present also in June, July, and October, but in much smaller numbers. The evidence indicates that the ascidians, as also the page 13 polyzoa, spawn over many months, with a heavy set in spring and summer, and that the species with the longest life span within the group takes approximately four months to attain maturity and maximum growth.
Three species of algae—namely, Enteromorpha procera Ahln., Bangia vermicularis Harv., and Ceramium apiculatum J. Ag., showed on the short-term blocks. E. procera settled in spring (September to November), B. vermicularis appeared in very small numbers in March, 1950, and C. apiculatum in the summer (December–January). The majority of the algal species on the long-term blocks were short lived, more so even than the hydroids—e.g., five species that reached identifiable form appeared on the long-term blocks for one month only. These were Ectocarpus sp. (May), Antithamnion sp. (September), Letterstedtia petiolata J. Ag. (November), E. procera Ahln. (December–January), and Derbesia novae-zelandiae Chapman (April, 1950). Bryopsis plumosa (Huds.) Ag. appeared for two consecutive months. Myriogramme denticulate Kylin and C. apiculatum are the only species with a life span longer than two months under local conditions, appearing on the blocks for three and more months consecutively.
M. denticulata is first recorded as moderately common in July on the long-term block, but does not appear at all on the short-term block, suggesting that the tetraspores take more than a month to reach identifiable size. This species continues to be present in increasing numbers to the end of the experiment. On the last block of the long-term series, this alga, with the sponge Halichondria reticulata, were together the most prominent features of the block. M. denticulata seemed one of the few species that was capable of vigorous growth on a silted surface. At the end of the experiment, some specimens had reached a length of 7·3 cm. C. apiculatum, the other algal species common from July to April, 1950, showed a second set in December and January on the short-term blocks, which probably signifies a life-cycle of about five months. The length of appearance of the species on the long-term block substantiates this.
Other Species Associated with the Sedentary Forms on the Test Blocks
Species of two other groups of mollusc—viz., the tectibranchiate Pteurobranchus sp. and the nudibranch Acolidia gracilis (T. W. Kirk) were found frequently associated with the fixed species of the test blocks. There is little doubt that A. gracilis was feeding on the hydroid polyps as sea slugs of this type are known to feed extensively on simple ascidians, and it was probably the presence in some numbers of these animals that caused the appearance of this species on the blocks. The small blennie Tripterygion varium (Forster) was twice found browsing among the algae and polyzoans.