Abundance, biomass, and sedimentary relationships of Americonuphis reesei (Polychaeta: Onuphidae) in the Gulf of Nicoya, Costa Rica. The main population of Americonuphis reesei known in Costa Rica is located along the intertidal muddy sand sediments at the middle part of the Eastern bank of the Gulf of Nicoya. This is a tubicolous, viviparous, hermaphrodite worm, and one of the world’s largest polychaetes. In the studied area it typically measures 0.6 to 1.2m, or more, and weights 50 to 70g. Granulometric curves of core sediment samples were correlated to the spatial density of individuals at the corresponding sampling site. The preferred substrate by this species is a mixture of poorly to moderately sorted sands, usually where there are over 80% of sand particles (75 to 2 000μm), mostly medium sands (250 to 425μm), and with little contents of silt and clays. The particle size frequencies distribution curve is positive asymmetric and strongly leptokurtic around the median. By means of area measurements on a density distribution map, the population was assessed as around 10.24x106 individuals, and fresh biomass as approximately 599.9 metric tons. The population is divided into four main aggregations, that all together cover some 110 ha, but 85% of worms are concentrated in less than 50 ha. The density of individuals per square meter is 9.3 (=545.8g·m-² FW) in average, but its observed maximum density of 134 individuals equals a biomass concentration of 7 847.4g·m-² FW (=1 306.4g·m-² DW), which might be the highest density of biomass ever reported for a polychaete. Some hypothesis proposed as explanations of this high biomass are: the worm’s specificity to a substrate generally not accepted by other animals and a tube-dwelling habit that protects the worm against predation and inhibits competition from other benthic megainvertebrates, the high ecosystem’s primary production that sustains trophic webs that include this species, and the contribution to these webs made by algae and epibiotic fauna that use A. reesei’s tubes as substrate. Rev. Biol. Trop. 56 (Suppl. 4): 59-82. Epub 2009 June 30.
Keywords: Costa Rica, Gulf of Nicoya, tropical benthos, intertidal flat, Polychaeta, Onuphidae, Americonuphis reesei, benthic biomass, grain size