Introduction

The present work was commenced some six years ago by one of the authors (P.M.R.) as a follow-up study arising from an hypothesis (Ralph, 1956) that the temperature range of the three major water masses round the New Zealand coast affected the growth habit of Obelia geniculata. In the subtropical water mass of northern New Zealand, O. geniculata has short stems (5-6 mm) that lack branches, while in the intermediate zone of mixed waters the stems are longer (10-20 mm) and 8%-50% are branched. In the cold waters of the subantarctic zone the colonies are tall (up to 40 mm) and 80%-100% of the stems have branches.

The temperature in Wellington Harbour ranges from approximately 9°C in winter to approximately 20°C in summer. This range seemed sufficiently wide to further test the hypothesis that temperature was a major factor in the growth form attained by the erect stem. The first series of samples for the experiment were collected from Kau Bay (Fig. 1) in September 1961. Sampling continued through 1962 and terminatd in July 1963 as the senior author went on overseas leave. A partial assessment of the data made at this time was of sufficient interest to encourage further sampling if another opportunity arose for collection—particularly for collection in the autumn and winter seasons—as the records for this seasonal range were incomplete. Also, page 2as the original hypothesis formulated that the number of branches increased with decreasing temperature, it was very desirable to have as many samples from this temperature range as possible. The opportunity came to obtain more autumn-winter samples in 1967 and was undertaken by the junior author (H.G.T.) from March to July.

Hammett & Hammett (1945) recorded seasonal changes in colony composition for Obelia geniculata from Provincetown Harbour, Massachusetts, latitude 42.1°N. This is almost the same latitude in the northern hemisphere as the present study area is south of the Equator, namely, 41.2°S. The present study differs however from that of Hammett & Hammett in several aspects.

Firstly, the habitat of the hydroid colonies. The substrate in Wellington Harbour for Obelia is the broad, often longitudinally crinkled lamina of the southern kelp Macrocystis pyrifera (L.). The stolons are most frequently found in the grooves of the lamina, and the erect stems grow singly in rows along the stolons, Many of the laminae float at, or just below the surface of the water. Rarely is O. geniculata found growing on substrates other than Macrocystis pyrifera in New Zealand waters. In Provincetown Harbour, growing stems of O. geniculata were collected either from the shells of barnacles, or, when this habitat was depleted, from Fucus. The colonies grew in dense clusters on the barnacles, but on the Fucus singly "like trees in a grove".

Secondly, analysis of the Wellington Harbour material was made from formalin-preserved random samples. Hammett & Hamett selected living colonies (? the equivalent of our erect stems), "for desirable qualities"; this selection commenced at the time of collection and subsequently through to the setting up of the colonies for growth study in laboratory vessels. Sampling in Wellingon Harbour covered a full seasonal range for two and a half years, and a short "cold-water", 5 month, autumn-winter season. Sampling in Provincetown Harbour covered the "warm-water", 6 month, spring-summer season for seven consecutive years.

Thirdly, Hammett & Hammett regard endogenous chemical factors as solely responsible for seasonal changes in growth in O. geniculata. Our conclusion is that both endogenous and exogenous factors influence growth in this hydroid. The present study gives evidence that assessment of data for part of the seasonal range leads to a biased conclusion. Because of this conclusion, we have described the results of our short-term, autumn through winter sampling before that of the longer term annual data.