(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.

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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.