Utlization and Processing of Freshwater Wetland Macrophytes by the Detritivore Asellus Forbesi

Abstract

Feeding experiments examined the use of freshwater wetland macrophytes as a food resource by the isopod Asellus forbesi. The effects of five factors on isopod consumption and growth rates were studies: (1) species of macrophyte being consumed, (2) length of vegetation conditioning period, (3) size of plant material (CPOM vs. FPOM), (4) isopod size, and (5) nutritional quality of the plant material. Consumption rates were low on unconditioned plants (x° = 0.07 mg.mg—1.d—1) but increased significantly following field conditioning (°x = 0.36 mg.mg—1.d—1). Isopod growth rates increased from no growth on unconditioned plant material to rapid growth on conditioned tissue (°x = 0.31 mg.mg—1.d—1). Consumption and growth rates varied according to the macrophyte species being consumed, with rates on conditioned macrophytes occurring in the order: Nuphar luteum > Peltandra virginica > Saururus cernuus > Pontederia cordata. Changes in the protein content of the plant material were critical in determining growth rates of the isopods. Protien content varied according to the species of plant, length of conditioning period, and particle size. Growth rates were positively correlated with the amount of protein ingested; consumption rates were not significantly affected by the protein content of the plant material except between unconditioned and 7—d conditioned tissue. A. forbesi thus did not alter its feeding rate in response to changes in nutritional quality of its food. An interaction between isopod size, plant particle size, and nutritional quality of the plant material was especially important in determining consumption and growth rates. Small isopods were more efficient at consuming the protein—enriched small particles, resulting in very high growth rates. large isopods wee less efficient at consuming small particles, but the higher protein content of these particles produced a growth rate similar to that found when they fed on the more efficiently consumed, but relatively protein—unenriched, larger particles. The isopods switch from being a FROM collector when you to being a CPOM shredder when older, corresponding with changes in their consumption efficiencies on different—sized particles and the seasonal changes in the relative amounts of CPOM and FPOM present in the wetlands.