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Seasonal Occurrence of Obelia Medusae in Wellington Harbour

So far, description has been of the colonial phase of the life cycle. Assessment of the seasonal occurrence of the medusa in the plankton has also a place in the final interpretation of stem growth. Obelia medusae are found all the year round in Wellington Harbour. Continuous occurrence is also recorded for the British Isles (Russell, 1953). Around British coasts, Obelia medusae are most abundant from spring to late autumn. In Wellington Harbour, they are most abundant from late autumn to winter. Numbers of medusae in the latter harbour do however also rise on occasion during summer (Fig. 6). But there is good evidence that this is due to aberrant growth causing the formation of gonangia on the hydrorhiza in place of the normal hydrocaulus. Thus, it is not a comparable rise in numbers to that found each winter. This aberrant summer rise is reflected in the annual low percentage of variability explained for the medusae per volume of catch when plotted against temperature. The percentage explained is 34.4%, where Y = 100.26 − 4.59X. (Fig. 8.)

Further evidence that there is a reversal in the southern hemisphere of the seasonal peak recorded for the northern hemisphere comes from records of Obelia from Perseverance Harbour, Campbell Island (latitude 52.3°S). Medusae are first recorded in January, when the sea-surface temperatures average approximately 10.0°C. A peak is reached in March (temperature average, 9.0°C). The medusae number is then between five and six thousand per 15 minute tow. By May, temperatures have dropped to about 7.0°C and the numbers of medusae per tow decrease, until by June no medusae were recorded (P. Roberts, Zoology Dept., V.U.W.—personal communication). The lowest temperature in this subantarctic area is approximately 5.0°C recorded in July-August.

The temperatures at which medusae have been recorded from Perseverance Harbour are comparable with those from Wellington Harbour in winter, where the lowest temperature is about 9.0°C. Wear (1965) found the peak in planktonic medusae in the latter area to occur in July-August. From both plankton studies it appears then that the largest numbers of medusae are obtained in the New Zealand region when the sea surface temperatures are around 9.0°C. It is also probable that there is a lower lethal temperature. This may occur in areas where the sea surface temperatures fall markedly below 9.0°C for any length of time.