New Zealand Journal of Marine and Freshwater Research abstracts

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Summer distribution patterns of the backswimmer, Anisops wakefieldi (Hemiptera: Notonectidae), in a New Zealand pond

JOHN J. GILBERT

Department of Biological Sciences
Dartmouth College
Hanover, New Hampshire 03755
United States

CAROLYN W. BURNS⁺

Department of Zoology
University of Otago
P. O. Box 56, Dunedin
New Zealand
email: carolyn.burns@stonebow.otago.ac.nz

CAROLINE C. GILBERT

6 Hovey Lane
Hanover, New Hampshire 03755
United States

Abstract  The relative abundance of different-sized Anisops wakefieldi in a small, fishless pond was determined from sweep-net samples taken along one open-water and two shoreline transects at noon on two dates, and at noon and midnight on a third date. During the day, large individuals occurred only along the shore, whereas smaller ones occurred both along the shore and in open water. At night, large individuals also occurred in the open water. The diurnal size-class segregation suggests that young individuals migrate into less preferred, open water to avoid interference and cannibalism from larger individuals. The shoreline may be preferred during the day because it provides greater food resources or a refuge from visually feeding avian predators. Laboratory experiments in containers with floating aquatic plants at one end complemented the field data. When alone, both small and large A. wakefieldi sought cover during the day and moved into open water at night. During the day, the presence of large individuals induced small ones to move into open water. The results of this study on a notonectid population are consistent with some earlier ones in demonstrating that interactions between large and small size classes may lead to pronounced spatial segregation of size classes, and show for the first time that spatial distributional patterns during the day and night may be very different.

Keywords  aggregation; Anisops; backswimmers; diel movements; distribution; habitat choice; notonectids; predator avoidance; size-class segregation

⁺ Corresponding author.
M99003
Received 22 January 1999; accepted 4 June 1999

PDF file of entire paper: medium quality (884K); (scanned from paper original: notes about this process)

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