Interpretation of Results

Our interpretation of the overall growth sequence for Obelia geniculata from the settling of the planula, is nutrition, reproduction and regression. We regard this sequence as genetically determined, and not alterable by exogenous influences. But the rate at which the sequences proceed is materially affected by the season at which the planula settles and thus indirectly by temperature. Consequent on this conclusion, is the fact that the developmental growth form finally attained by the erect stem is variable from season to season and within the season, according to the length of time the stem has been growing within a particular range of temperature. This hypothesis is shown diagramatically in figures 10 and 11.

Support for the idea that exogenous factors such as temperature affect the growth processes of hydroid colonies comes from the work of Berrill (1948). Berrill found that growth in colonies of Obelia and other hydroids fluctuated greatly with temperature. They disappeared and reappeared respectively as the temperature rose and fell significantly above and below 20°C. One of the three species of Obelia studied by Berrill was O. geniculata. It is probable therefore in latitudes where the mean annual sea temperature is approximately 20°C that O. geniculata will be seasonal in its growth. Temperature is thus not a limiting factor in Obelia growth in Wellington page 18

Fig. 10 Diagrammatic presentation of the hypothesis that seasonal changes in the exogenous factor temperature, is related to the metabolic rate and growth of the erect stem.

Harbour, except perhaps in February, when the microclimate of the surface laminae may exceed 20°C in days of calm hot weather. Also, growth is unlikely to be seasonal in latitudes in the New Zealand area higher than 40°S.

It is also concluded from the present study, that increasing temperature produces an increase in the metabolic rate of the colony, and that the endogenous sequences, nutrition, reproduction and regression, follow one another in rapid succession. Of significance for this conclusion is the decreasing order of stem height illustrated in Plate I, f., because as Manton (1940, p. 248) notes, budding in colonial hydroids takes place when growth is complete. This growth pattern of decreasing height along the stolon, also suggests that the rate of metabolism may be sufficiently rapid when temperatures approach the upper lethal limit for the two growth sequences, nutrition and reproduction, to be virtually telescoped one into the other. The "erect stem" then consists of a gonangium arising directly from the stolon (Pl. I, d and e). This type of "stem", is most likely to occur in Wellington Harbour if the planula settles in a period of high (18°-20°C) temperature. Nutrition for the developing gonangium would be provided by the hydranth-bearing stems already present on the stolon.

Fig. 11 Diagrammatic presentation of the hypothesis that the seasonal stem form attained, is related to the season at which the planula settles and the time available for growth within the seasonal temperature range.

If the planula settles earlier in the summer season, it will have a longer growing time in cooler water, and the stem will be able to produce feeding polyps in varying number, the number attained being dependent on the temperature when the planula settled and the subsequent growing time available before lethal temperature is reached.

In winter the metabolic rate slows with a decrease in temperature. The time period for the sequence nutrition, reproduction, regression, is lengthened. Before gonangia are produced the stem attains a vertical height approximately twice that attained by a stem in summer. If the planulae settle in a month of minimum temperature range the stems will give expression to growth, not only in vertical height, but also in the production of branches. Further evidence for this conclusion is given by the stem form of colonies collected in February from Kerguelen Island (Ralph, 1956). Mean annual sea temperature for Kerguelen Island is approximately 2°C below minimum winter temperature of 9°G in Wellington Harbour. No gonangia are present on these sub-antarctic colonies, but 80% of the stems have long branches. The erect stems without branches are very short, with terminal growing buds suggesting that page 20 they were young stems. The length of some branches approximated that of the main stem, and we regard this growth pattern as an indication that absolute vertical height is genetically governed. Moreover, it is possible that this vertical height may be attained in advance of the endogenous control mechanism initiating reproduction when long slow growth is possible. The nutritive growth phase is expressed then in the form of an axial bud which forms a branch and not a gonangium. The correlation (46% of variability explained) between gonangium and branches further supports this conclusion. Slow growth in cold waters has been postulated for other hydroid coelenterates, for example Myriothela penola from the Argentine Islands area of Graham Land (Manton, 1940).

Other marine invertebrates in New Zealand waters are known in which there is a significant size difference in breeding animals in warm and cool water areas of the distribution range. For example, the females of the spiny lobster Jasus edwardsii (Hutton, 1875), come into berry in warm east coast, North Island areas at a smaller size than the females in the south-eastern and southern regions of the South Island. (Dr. R. B. Pike, Zoology Dept., V.U.W.—personal communication).

If it is assumed that the above hypothesis concerning fast growth in summer and slow growth in winter is correct, then it follows that there will be a critical temperature in autumn and spring at which temperatures below will slow down the overall stem metabolism. Such temperatures are indicated in the statistical analysis of the autumn winter data for 1967, i.e. approximately 13°C for terminal buds and gonangia and 11.5°C for branches.

We do not know the length of the stem life cycle of O. geniculata, but have concluded from the present study that the length is temperature dependent in large measure. Therefore it would be "short" in summer and "long" in winter. It is also probable that it is longer still in latitudes significantly higher than Wellington Harbour. For example, Kerguelen Island. It could be indefinite in this latitude. The feeding hydranths may be able to survive much longer before regression sets in. They may not have a cycle of regression followed by regeneration at all. Once fully formed and functional they may remain as such until the whole erect stem regresses. It is not inconceivable that regeneration of the blastostyle also occurs in such latitudes. Furthermore, it is possible that regression of the feeding hydranth in winter temperatures in Wellington Harbour is not followed by regeneration to any great extent, and that the reverse is the case in summer temperatures. For example in summer, the number of feeding polyps during the life of the stem could be the same number as in winter, if there was a very rapid succession of feeding polyps regenerating in already existing hydrothecae.

As is so often the case with studies of the present nature, while some questions may be answered, many more arise and cannot be answered from the data obtained in the originating experiment. Future laboratory studies could provide some answers to the questions posed by the present field work.