ESTIMATION OF SIZE CLASSES IN LAEONEREIS ACUTA (NEREIDIDAE) BASED ON JAW LENGTH AND BODY WIDTH AS A PRACTICAL TOOL IN TROPHIC STUDIES
Ieno, E.1,2,3; Martin, J. P. 1,2,3 & Bastida, R. 1,2,3
1 CONICET, 2 Fundación Mundo Marino, 3 Universidad Nacional de Mar del Plata. Dean Funes 3350 7600- Mar del Plata, ArgentinaTelephone: +54 23 752426 Fax: +54 23 753150
Polychaetes are the most important prey for the main wading species and estuarine coastal fishes occurring along intertidal and subtidal areas of the southernmost part of Samborombón Bay. This extensive food source is used by high numbers of knots, plovers and gotwits, wintering in the bay or using it as a stop-over during their migrations. Common demersal and benthic juvenile fishes are also active predators of worms during the breeding season. Field observations on prey consumption showed that Laeonereis acuta is one of the dominant prey items in the area. Quantifying the selection of different size classes of Laeonereis is complex. However, analyses of feces and stomach contents, containing fragments and jaws, provide a more relevant measure of prey size selection. Our first goal was to search for any anatomical structure that could supply more accurate information about size class distributions. Two benthic sampling programs were carried out in May and December 1997. Sixty sediment grab samples were taken at San Clemente creek. The samples were washed through a 500 µm sieve and worms were stored in 70 % ethanol. Sorting was carried out with a binocular microscope. Prey were divided into 1 mm size classes, and the density in each class was determinated. The length of incomplete worms was estimated from the relation between the width of the tenth segment and the length of intact worms. Jaw length was measured and the relationship between jaw and body length was determined. Benthic samples collected in May contained a total of 1,238 L. acuta. The regression analysis of body length as a function of jaw length is highly significant (R = 0.93, p,0,05, n = 330). Results derived from the relationship between body width and length in undamaged worms is also significant (R = 0.91, p,0,05, n = 330). We conclude that jaws and incomplete worms can be used to determine size classes of L. acuta taken by avian predators and coastal fishes.