Comparative Account of the Autumn-Winter and Annual Growth Trends

The autumn-winter seasons of 1967 and the annual analyses of the 1961-1963 seasons give a correlation between temperature and growth expression for the branches, and terminal buds as 21% and 27% respectively. While this is not a high degree of correlation it is acceptable statistically as significant for biological material. Nonetheless, it indicates that factors other than temperature are operative in determining the changing pattern of these stem structures, and that these factors operate at varying levels throughout the year.

In contrast is the correlation between temperature and gonangia for the autumn-winter seasons of 1967 and for the similar seasons of the 1961-1963 series. The percentage of variability explained for the autumn-winter seasons is 88% as against 46% for the whole year. In the autumn-winter seasons, with r² −0.94, the probability of there being no correlation is less than 1%. Temperature then accounts for page 11

Plate III

Photomicrograph of part of a late spring stem to show the epiphytic diatoms.

page 12 88% of all factors influencing the growth of the gonangia in autumn and winter. This is highly significant. In a biological system it is recognised that it is impossible to make a 100% correlation with any factor or factors. It is unlikely therefore that data on other external factors, e.g. pH, salinity, etc., would increase the coefficient by a significant amount. The margin of 0.006 left to be explained is very small, and could be an error in methodology.

The much lower correlation with temperature for the four season range, in comparison with the two seasons, is a reflection of two facts. Firstly, gonangia occur in small numbers throughout the year. Secondly, the numbers of gonangia produced rise sharply when the temperature approaches the annual maximum. Neither of these occurrences were predictable from the short-term results. The short-term data give a straight line by the method of least squares and suggest that gonangia numbers will drop to zero for temperatures above 17°G (Fig. 2). If the autumn-winter series had been the only series analysed, the result, as already noted, would have been strongly biased in favour of an exogenous factor being the controlling agent of the stem form attained by O. geniculata. Sampling over a whole year clearly shows that a sigmoid curve is a better representation for the seasonal trends in gonangia formation. Nonetheless, the results for the terminal buds, the branches and the gonangia demonstrate that temperature is significant in the overall growth pattern of the hydroid. It is particularly significant in the formation of the gonangia.

The results obtained from study of gonangial growth are recognised here as a more reliable guide than other stem features to the factors governing growth in O. geniculata. The reason is as follows. Where a series of samples are being utilized over a period of time, rather than observations on one colony, or one erect stem, it is best to have colonies at approximately the same stage of maturity. The presence of gonangia usually indicates that a colony is well established. Therefore their presence will also give a better indication than the number of hydranths per stem or the presence of terminal buds, etc. that a colony is mature. Moreover, hydranth development from bud initiation to senility is completed within 72 hours (Hammett, 1943, p. 350). Gonangial development including the initiation and development of medusae is not reached within 72 hours (Hammett, 1943) and may take several weeks. As the blastostyle in the gonangium reaches senility, it is shed and not replaced. In contrast, a hydranth may be produced from a site where a polyp had previously emerged, matured and regressed, and recurrent growth is integrated into the growth of the colony as a whole (Hammett, 1943). This growth cycle, characteristic of the feeding hydranth, undoubtedly contributes to the very low non-significant correlation between temperature and the number of feeding polyps present on the stem. Regeneration of hydranths in already present hydrothecae could mask almost completely the true relationship of temperature to this stem growth feature in long-term field samples. The gonangia therefore with their longer time of development and lack of regenerative powers are better stem structures for statistical analysis in the present instance. Coupled with the longer development time of the gonangia is the possibility that they do not respond to the same extent to minor day to day temperature fluctuations as do the hydranths and terminal buds. This is substantiated in the statistical analysis of the 1967 autumn-winter data where the time interval beween collections was the shortest for the present study. The fluctuations in successive biweekly samples (Figs. 4 and 5) are more clearly defined for terminal buds and branches than for gonangia (Fig. 3).

There are other featuŕes of the long-term series that bear on our interpretation of seasonal growth that have not yet been discussed. For example, gonangia and medusae occur throughout the year. It is probable therefore, that planulae settle and new colonies arise throughout the year. Thus, the monthly mean for any expression page 13

Fig. 4 1967 autumn-winter seasons. Correlation between the average number of branches per stem, and temperature.

Fig. 5 1967 autumn-winter seasons. Correlation between the percentage of terminal buds present and temperature.

page 14 of growth for the erect stem does not necessary represent the true average growth pattern of stems growing for one month at the mean monthly temperature. Individual samples for the month might represent a mixture of tall, mature stems; short immature stems, and stems of medium height and maturity depending on the time the stems have been growing from planula settlement. Alternately, the monthly average may represent individual samples in which one of the three stem types is dominant. The average percentage of gonangia shown in Figure 6 for November 1962 illustrates this situation. In one sample, 96% of the stems had gonangia present, were tall stems, with branches and with nodal rings of dark brown chitin. The stems and hydrothecae

Fig. 6 1961-1963 seasonal changes in the percentage of gonangia, medusae, branches and terminal buds relative to changes in temperature.

were thick with diatoms. All the characters of these stems were indicative of approaching senescence following a long growth period in cold water. The second sample had small stems, 100% of which lacked gonangia, and overall the stems were indicative of a short period of growth. The third sample, was a mixture of the above growth forms, namely the taller stems in the main had gonangia, while the majority of short stems lacked gonangia.

It has already been noted that there are two peaks in the annual production of gonangia, one in the winter and one in summer. But the stems that carry gonangia in summer are very different from those bearing gonangia in winter. This is another example where the bare statistic does not fully represent the seasonal morphology of the animal. Direct observation of the samples for February shows that most stems at this time of year are very short, seldom have more than 4 hydranths, and that the distal end is terminated not by a bud, but by a fully formed hydranth. There may also be one or two gonangia present. However, other patterns of stem growth along the stolon have been observed. Plate I, f, shows a decreasing order of vertical height for stems along the stolon, and the "erect stem" may be represented only by a page 15gonangium arising directly from the stolon (Pl. I, d and e). The latter "stems" however were not included in the statistical analyses of the data.

A variation of this pattern is seen in Pl. I, c., where the erect stem growth expression is shown by a single stem surrounded by a circlet of gonangia arising directly from short, radially arranged stolons. This example is of interest as it comes from an autumn collection. The pattern of numerous gonangia on the stolons in this sample suggests a sharp upward fluctuation in temperature at a critical level, combined with calm weather, resulting in near lethal temperature conditions at the lamina surface. That is, summer conditions in autumn.

Figure 6 shows what we regard as another example of the effect of unseasonable climate. In the March-April period for 1962, temperature fell from a mean maximum of 19.5°C in February, to a mean of 12.8°C in April. The lowest temperature recorded for April was 10.4°C. The latter temperature approaches the annual minimum for Wellington Harbour. A temperature range of approximately 10°C to 13°C usually occurs from June to August not March to April. It is probable therefore, that this rapid, unseasonable temperature drop is responsible for the very high percentage of stems with terminal buds in March and April instead of later in the year as is suggested by analysis of other data obtained during the experiment. The more gradual fall in temperature from the February maximum in the 1963 March-April period shows an appreciably lower percentage of stems with terminal buds.

In brief, the above paragraphs indicate firstly, that assessment of part only of the seasonal data gives a biased result in favour of temperature as the prime factor in the seasonal growth form attained by the erect stem. Secondly, that some facets of stem growth are also obscured in statistical analyses of the annual data. But a combined assessment strongly suggests the same seasonal trend in changing growth form for the stem as that noted in direct visual observation of the two collection series.

Fig. 7 1961-1963 sample series. Gonangia plotted against temperature using the statistical method of least squares.

Fig. 8 1961-1963 sample series. The percentage of medusae per total volume of catch, plotted against temperature using the statistical method of least squares.

Fig. 9 1961-1963 sample series. Average number of gonothecae plotted against the average number of branches per stem using the statistical method of least squares.