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Zoology Publications from Victoria University of Wellington—Nos. 42 to 46

B. Annual Temperature and Growth Correlation — [Pls. I, II and III: Text-Figs. 6-9]

B. Annual Temperature and Growth Correlation
[Pls. I, II and III: Text-Figs. 6-9]

1961-1963 collection series

Plates I and II show the features of the erect stem for the four seasons. In winter (Pl. I, b: Pl. II, a) typical stems are close together, characteristically tall (15 to 20 mm), terminated by a hydranth bud, with 1 to 3 branches and up to 8 gonangia per stem. The stems also carry a fairly heavy epiphytic population of diatoms, and the chitin of the nodal annulations is medium to dark brown in colour. In summer (PL I, f: PL II, f) the stems are short (8.5 to 12 mm), wide apart, usually with a fully formed terminal hydranth, without branches, and with up to 2 gonangia per stem. They carry a heavy epiphytic population of diatoms. As could be anticipated, the spring and autumn stems (PL II, b and h) show features in between these two extremes. However, the spring stems frequently carry a greater epiphytic diatom population and more branches than the autumn stems (Pl III).

Statistical analysis

Figure 6 shows the pattern of increasing numbers of gonangia, terminal buds and branches with decreasing temperature, but does not indicate the well defined seasonal growth forms indicated for the erect stem in the visually selected examples of the stem described above. Figure 6 also demonstrates that gonangia and terminal buds may occur throughout the year. However, in late spring (November) or in the summer months (December, January, February), the percentage of stems with page 9
Plate II

Plate II

A: Tall, branched winter stems.

B and D: Short spring stems, with epiphytic diatoms.

C and E: Medium height spring stems.

F: Summer stems with epiphytic diatoms.

G and H: Autumn stems of O. geniculata. The epizooite is a species of Epistylus which is commonly attached to the erect stem at this season.

page 10 branches is very low. The percentage may even reach zero. Moreover, in the two late spring, summer seasons analysed, there is a well defined rise in the production of gonangia. This production rise of gonangia is more or less paralleled by increased numbers of medusae in the plankton.

Some of the data of the 1961-1963 collection series is shown statistically expressed using the method of least squares. The calculated percentage of variability explained for temperature and gonangia (Fig. 7) is 46.04%, where y = 3.06 — 0.15X. That is, at temperature 12°C, forty-six percent of the stems would have 1.2 gonangia per stem. For gonangia and branches (Fig. 8) the percentage of variability explained is 46.04% and Y = 0.15 — 7.63X. Also, the percentage of variability explained for temperature and branches and for temperature and feeding polyps was calculated, but is not figured. The percentage for temperature and branches is 27.5% and y = 0.176 — 0.0148X. In marked contrast is the percentage for the number of feeding hydranths on the hydrocaulus. The percentage explained is only 0.4% where y = 5.53 — 0.024X.

Remarks

Visual assessment (Pls. I and II) and graphic analysis (Fig. 6) demonstrates a changing pattern for growth of the erect stem during the year. The highest percentage of terminal buds, branches and gonangia occur in the autumn to winter season. All these stem characters are found with decreasing frequency on the stem as water temperatures rise in spring and summer. Branches rarely occur on stems when water temperatures approach the maximum. Gonangia in contrast rise in numbers with the approach of maximum temperature. All in all, these trends suggest a strong correlation between temperature and the seasonal form and maturity attained by the stem. However, when this correlation is tested utilizing the method of least squares, the assumption that temperature is the controlling factor in seasonal habit is less obvious. The percentage of variability explained ranges from non-significant for the feeding polyps (0.4%) to varying degrees of significance, for the other structures.

For the branches and terminal buds it is 27%.

For the gonangia it is nearly twice as high, namely 46%.

In brief, the analysis of the long-term series indicates that changes in stem form occur throughout the year. These are related to the four seasons and thus indirectly to temperature. The degree of correlation between temperature varies for the stem characters assessed. It is lowest and non-significant for the feeding polyps. It is highest and quite significant for the gonangia. Correlation for the terminal buds and branches is significant and in between the two extremes.