, Volume 87, Issue 1, pp 47-54

Predatory feeding of the hydromedusae Obelia geniculata and Phialla quadrata

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In 1982–1983 nauplii and adults of several co-occurring copepods and Artemia sp. nauplii (Lake Grassmere, New Zealand strain) were fed to the hydromedusae Obelia geniculata (L.) and Phialella quadrata (Forbes), two species which are very abundant in Wellington Harbour, New Zealand. Clearance rates of both hydromedusae were significantly correlated with predator diameter, but independent of water temperature and prey density. P. quadrata consumed all developmental stages of some copepod species, but O. geniculata consumed only nauplii. Clearance rates of O. geniculata were most strongly related with the strength of the prey escape response, and showed little relation with prey size or movement rates. Clearance rates of P. quadrata were equally influenced by the prey escape response and prey size, but a large amount of the variability in clearance rates could not be explained by any of the prey characteristics, or by a linear combination of the prey characteristics. Clearance rates of the hydromedusae are combined with measures of hydromedusa abundance in Wellington Harbour to calculate daily per capita death rates of the prey.

Communicated by J. M. Lawrence, Tampa