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GELATINOUS ZOOPLANKTON

Weddell seals cruise by underwater but gelatinous zooplankton -- medusae (jellyfish), siphonophores, salps, ctenophores (comb jellies), and some molluscs -- are the most prominent midwater organisms seen by divers under the ice near McMurdo Station. Gelatinous zooplankton refers to a general grouping of animals that are well adapted to life in open water and that lack rigid skeletal structures [6]. Gelatinous zooplankton can be quite small and so transparent that a diver's underwater light is needed to see and highlight their bodies; others can be longer than a diver. Gelatinous zooplankton are more numerous near the ice edge or in open water than under the McMurdo Sound sea ice but there are plenty to see while diving [1]


Large gelatinous carnivores, like ctenophores and the medusa Desmonema glaciale shown here, are a predominant and sometimes the main component of the macroplankton and nekton community in the Southern Ocean surrounding Antarctica [3,4]. Gelatinous carnivores are important components of the food web because they are a control mechanism for its structure [6].


Here, the diver shoots underwater video of the medusa Desmonema glaciale, drifting along with the prevailing current. Desmonema glaciale occurs near the surface in Antarctic continental shelf waters and its bell can be over one meter in diameter [5].

The larger medusae are active swimmers and more properly classified in the nekton than the plankton.



The sea butterfly Clione antarctica is a free-swimming shell-less pteropod mollusc up to 4.2 centimeters long [7]. Clione antarctica is commonly found in McMurdo Sound near the undersurface of the sea ice and is sparse in water deeper than twenty meters [7].

Clione antarctica preys on a planktonic shelled pteropod mollusc Limacina helicina [7]. Clione antarctica is also part of a unique relationship with the hyperiid amphipod Hyperiella dilatata; this amphipod holds onto Clione antarctica, using it to chemically defend itself from predators [10].



The ctenophore or comb jelly Beroe cucumis completely engulfs prey as large as itself; larger prey are bitten into pieces with bundles of fused cilia lining the inner lips [8].

In the open ocean, there is no place to hide from predators so gelatinous zooplankon appear to use their transparency to hide in a transparent environment [6].



Some gelatinous zooplankon are carnivores while others are particle feeders or herbivores, like this shelled pteropod mollusc Limacina helicina which filter feeds on phytoplankon [6,9].



Medusae brush the bottom in shallow water and are captured by the tentacles of anemones like Urticinopsis antarcticus or, as shown here, Isotealia antarctica, devouring the medusa Periphylla periphylla [2]. The struggle continues for quite awhile; the medusa pulses its bell as it tries to swim away while the anemone slowly pulls the medusa into its mouth.

Oftentimes, two adjacent anemones pull in different directions while devouring the same medusa -- a slow-motion tug of war. It may be that the initial capture and struggle with one anemone leads to the medusa bumping into an neighboring anemone and getting captured by that neighboring anemone as well.

1: Antarctic Journal of the United States 23(5):135-136, 1988; 2: Antarctic Ecology, Volume 1. MW Holdgate, ed. NY: Academic Press, 1970. pp. 244-258; 3: Marine Ecology Progress Series 141(1-3):139-147, 1996; 4: Annales de l'Institut Oceanographique 73(2):139-158, 1997; 5: Pelagic Scyphomedusae (Scyphozoa: Coronatae and Semaeostomeae) of the Southern Ocean. RJ Larson. Washington, DC: American Geophysical Union, 1986; 6: Annales de l'Institut Oceanographique 73(2):123-124, 1997; 7: American Malacological Bulletin 8(1):67-75, 1990; 8: Guide to the Ctenophores of the Southern Ocean and Adjacent Waters. D O'Sullivan. ANARE Research Notes No.36. Kingston, Tasmania : Australian National Antarctic Research Expeditions, 1986; 9: Polar Biology 8(1):41-48, 1987; 10: Nature 346:462-464, 1990

Text ©Peter Brueggeman. Photographs ©Norbert Wu, M Dale Stokes, Jim Mastro, & Canadian Museum of Nature (Kathleen Conlan). Photographs may not be used in any form without the express written permission of Norbert Wu (for himself and M Dale Stokes), Jim Mastro, & Canadian Museum of Nature (Kathleen Conlan). Norbert Wu no longer grants permission for uncompensated use of his photos under any circumstances whatsoever; want more info?