Gelatinous versus non-gelatinous zooplankton: their value as food for planktivorous coral reef fishes

James Gahan, David Bellwood, Orpha Bellwood & Jodie Schlaefer

Abstract:

Coral reefs are highly productive ecosystems, in part due to the productivity of planktivorous fishes. The planktivorous fish community contains two distinct groups which target either the gelatinous or the non-gelatinous fractions of the incoming zooplankton. However, the nutritional value of these prey fractions and, consequently, their potential contribution to planktivorous fish productivity are poorly understood. We explored the zooplankton fractions potential contribution to planktivorous fish productivity (our function of interest), by quantifying the nutritional content a key trait of the gelatinous and non-gelatinous prey fractions which are accessible to reef-associated diurnal planktivores. By combining vertical plankton tows with stoichiometric analyses, we found that the three zooplankton community fractions—gelatinous, > 2 mm non-gelatinous and < 2 mm non-gelatinous—were all relatively good sources of nitrogen, with median C:N values of 4.81:1, 4.22:1 and 4.62:1, respectively. The delivery of gN m−2 to our study reef over a flood tide was then estimated. The abundant < 2 mm non-gelatinous community delivered the largest gN m−2 (median: 0.88 gN m−2 6 h−1), which was estimated to be one to two orders of magnitude greater than the delivery from the > 2 mm non-gelatinous (0.06 gN) and gelatinous (0.03 gN) communities, respectively. Overall, our results highlight the quality of both gelatinous and non-gelatinous zooplankton as prey for planktivorous fishes, emphasizing the potential importance of the often-overlooked gelatinous fraction.