(i) Collection and associated techniques

Colonies of Obelia geniculata were collected from two localities, namely Kau Bay (a weekly sample from September 1961 to July 1963) and from Point Haswell (a biweekly (= twice per week) sample from March to July 1967). The 1961-1963 samples, 48 in all, were subsidiary to a wider programme of harbour plankton collection (Fig. 1). The Obelia medusae from these collections were described by Wear (1965). The data given here for medusae are based on that of Wear.

Kau Bay is a relatively sheltered, wide, horse-shoe shaped bay, with the opening facing north-east. A narrow shingle beach is exposed at low tide. The kelp, Macrocystis pyrifera (L), on which the Obelia grows, occurs commonly along the rocky shoreline in Wellington Harbour at depths of 0.5 to 1.5 fathoms (Fig. 1). At Point Haswell a causeway has been built across the rocky foreshore to the small lighthouse beacon at the seaward extremity.

Fig. 1 Location map to show areas sampled in Wellington Harbour. (After Wear, 1966).

During the 1961-1963 period the Obelia was collected by a grapple thrown at random into the belt of kelp, from the deck of the Department's 16-foot launch. The surface temperature was recorded from water collected in a dip-bucket, using a centigrade thermometer graded to 0.2°C. The water, tide, and general weather conditions were also noted. One lamina, occasionally two, of Macrocystis was placed in a plastic one gallon bucket of fresh sea water for transportation to the laboratory. A random segment of approximately 2 cm square was cut from the lamina. The erect stems of Obelia were killed and fixed by placing the cut sample in 250 ml of sea water, and adding 40% formalin drop by drop from a pipette until the concentration of the formol-saline reached approximately five percent. The range of stem height; the number of feeding polyps per stem; the number of stems with terminal buds; the number of branches per stem; the number of gonothecae; the range of internode length, and the range of the distance apart of the stems on the hydrorhiza were recorded for 100 stems from each side of the lamina.

The colonies of Obelia from Point Haswell were collected from a Macrocystis lamina situated about the mid-portion of the stipe. Laminae below this area were usually damaged. Those near the growing point showed little or no colonization by Obelia geniculata. A lamina was cut from the stipe using a three-pronged wire rake attached to a long pole, and transported to the laboratory as in the previous collection series in a clean, plastic bucket of sea water. The sample was cut, killed, fixed and preserved in the manner noted above.

The numbers of Obelia medusae in the plankton catches from Kau Bay were estimated as described by Wear (1965, p. 2), and converted to a percentage of the total volume of the catch.

Remarks

On several occasions weather, and other factors affecting the running of the launch made collection impracticable in the Kau Bay area. Another feature of the original methodology that seemed unsatisfactory was collection by grapple from a boat deck. A piece of lamina could not be removed with any degree of accuracy. Therefore Point Haswell was chosen for the 1967 collection site because the kelp could easily be obtained in quantity under most weather conditions. On two occasions only in this locality, no samples were taken as high tides and stormy conditions made it impossible to reach the mid-sections of the stipes of the kelp. It should be noted however that combined analyses of both the short and long-term data indicated finally that the original "lucky-throw" method was the better collection method.

Temperature was the only external factor measured systematically. It is possible that the temperature of the water covering the surface laminae could be in the order of degrees warmer, or cooler, than that sampled in the bucket. The greatest degree of difference between the temperature of the water sample and that at the lamina surface is most likely to occur in calm weather, when the kelp floats virtually undisturbed. The well defined fluctuations that appear in the biweekly statistical analyses page 5

Plate I

A: Habitat of O. geniculata in Wellington Harbour, on the laminae of Macrocystis pyrifera.

B: Side view of Macrocystis lamina to show colonization by O. geniculata in winter.

C: An atypical erect stem taken in April showing a circlet of basal gonangia arising directly from the stolons.

D and E: Gonangia arising directly from the stolons. Sample collected in February.

F: Widely spaced summer stems showing decreasing order of vertical height.

page 6 (Figs. 4 and 5) possibly reflect sudden and short-lived temperature changes in the microclimate of surface laminae. However, calm conditions do not occur very often in Wellington Harbour. This is evidenced by meteorological records, and by the fact that of the 77 samples taken, only one sample had colonies of O. geniculata on one side of the lamina, and not on the other. No significant difference between colonization of the two surfaces of the lamina was found in any other sample. From this it seems reasonable to conclude that laminae have a fairly constant turnover by wind and water. Each side has an equal opportunity for colonization by settling planulae. It also indicates that with rare exceptions the water temperatures measured in the dip-bucket would closely approximate that at the lamina surfaces.

Fresh water influence, and pollution from the Ngauranga Freezing Works were found by Wear (1965) to have no marked effect upon the zoo-plankton in the collection area. Wear also found that the peak of the planktonic cycle occurs within the winter—May to August—period. Plankton is probably a major component of the food taken by Obelia. Moreover, it was anticipated that if these and other factors such as pH played a major role in influencing the growth rate of the hydroid, they would be indicated in the statistical analyses of the data.

Features of the life cycle of O. geniculata that bear on the interpretation of seasonal growth changes given here, but are masked in statistical analyses, are noted later in the text.