Feeding, growth, and fecundity of Abarenicola pacifica in relation to sediment organic concentration

For marine deposit-feeding invertebrates, the distribution of species with different life history strategics has long been known to be correlated with sediment organic concentration. Large populations of opportunistic species are found in sediments with enriched organic concentration, while equilibr ium species populate low organic concentration sediments. Differences in bo th behavioral (e.g. feeding rate) and physiological (e.g. growth rate, repr oductive output) adaptations determine the ability of species to establish populations in different environments. By systematically documenting differ ences in the way these factors vary as sediment organic concentration varie s for both opportunistic and equilibrium species, we can better understand the mechanisms underlying this correlation between sediment organic concent ration and species distributions. I-lt re, we present the results of experi ments examining the interactions among food concentration, Feeding rate, gr owth rate, and reproductive output (measured as egg number and size) for th e equilibrium species Abarenicola pacifica. A. pacifica is a large, long-li ved, iteroparous, sub-surface deposit-feeding polychaete. Individual worms were reared throughout most of one generation in sediments differing only i n the concentration of organic matter. Juveniles ( < 20 mg AFDW) had higher feeding rates and growth rates in sediments of higher organic concentratio n throughout the range tested. These results are consistent with the predic tions from optimal fr,raging theory. Rs worms grew, however, these patterns changed. Once worms reached a mean body size of <similar to>50 mg AFDW, fe eding rate was greater on sediments: of lower organic concentration (althou gh it rook worms in the sediments with lower organic concentration longer t o reach this size). Differences in growth rates among treatments decreased as worms grew. For worms > 100 mg AFDW, growth rates were uniformly low (ap proximate to 1%/day) on all sediments, but the early advantage obtained by worms in the high organic treatments resulted in much greater body sizes af ter 200 days. Worms had higher tissue triacylglyceride concentrations and p roduced mole eggs (independent of worm size) as sediment organic concentrat ion increased. We conclude that A. pacifica alters its feeding rate in resp onse to variations in food resources in such a way as to maximize its energ y intake and thereby maximize fitness. Future studies should investigate wh ether opportunistic species las well as other equilibrium species) also hav e this ability. (C) 2000 Elsevier Science B.V. All rights reserved.