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Zoology Publications from Victoria University of Wellington—Nos. 42 to 46

Abstract

Abstract

Seasonal growth changes shown by the erect stem of Obelia are described, based on the results of two series of field samples from Wellington Harbour, latitude 41.2°S, New Zealand. Endogenous and exogenous factors are involved in the form of growth expressed by the stem. Endogenous factors govern the sequential order in which the stem structures are formed. The feeding hydranths form first, then the reproductive gonangia. Exogenous factors, in the main temperature, affect the rate at which these growth sequences proceed. Warm temperatures (approximately 15°-20°C) give rapid metabolism and cold temperatures (approximately 9°-12.5°C) slow metabolism. Rapid metabolism shortens the time between sequences so that growth expressed in terms of gonangia production is attained when feeding hydranths are few (4-5) in number. Slow growth lengthens the time between the sequences. Maturity is then not attained until 8-10 feeding hydranths have been formed in comparison with rapid metabolic growth.

Seasonal growth forms can be distinguished within the annual temperature range. In winter, the stem is tall (20 mm), has up to 8 gonangia per stem, and from 1-4 branches on the hydrocaulus; in summer it is short, approximately half its winter height, with 1-2 gonangia per stem and no branches. Stems intermediate in growth form between these two extremes are found in spring and autumn. Production peak for gonangia is in the 9°-10°C winter temperature range in Wellington Harbour.