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 strategies 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 equilibrium species populate low organic concentration sediments. Differences in both behavioral (e.g. feeding rate) and physiological (e.g. growth rate, reproductive output) adaptations determine the ability of species to establish populations in different environments. By systematically documenting differences in the way these factors vary as sediment organic concentration varies for both opportunistic and equilibrium species, we can better understand the mechanisms underlying this correlation between sediment organic concentration and species distributions. Here, we present the results of experiments examining the interactions among food concentration, feeding rate, growth rate, and reproductive output (measured as egg number and size) for the equilibrium species Abarenicola pacifica. A. pacifica is a large, long-lived, iteroparous, sub-surface deposit-feeding polychaete. Individual worms were reared throughout most of one generation in sediments differing only in the concentration of organic matter. Juveniles (<20 mg AFDW) had higher feeding rates and growth rates in sediments of higher organic concentration throughout the range tested. These results are consistent with the predictions from optimal foraging theory. As worms grew, however, these patterns changed. Once worms reached a mean body size of approximately 50 mg AFDW, feeding rate was greater on sediments of lower organic concentration (although it took worms in the sediments with lower organic concentration longer to reach this size). Differences in growth rates among treatments decreased as worms grew. For worms >100 mg AFDW, growth rates were uniformly low ( approximately 1%/day) on all sediments, but the early advantage obtained by worms in the high organic treatments resulted in much greater body sizes after 200 days. Worms had higher tissue triacylglyceride concentrations and produced more eggs (independent of worm size) as sediment organic concentration increased. We conclude that A. pacifica alters its feeding rate in response to variations in food resources in such a way as to maximize its energy intake and thereby maximize fitness. Future studies should investigate whether opportunistic species (as well as other equilibrium species) also have this ability.