Evaluation of three methods for biomass estimation in small invertebrates, using three large disparate parasite species as model organisms

Abstract

Invertebrate biomass is considered one of the main factors driving processes in ecosystems. It can be measured directly, primarily by weighing individuals, but more often indirect estimators are used. We developed two indirect and non-destructive approaches to estimate biomass of small invertebrates in a simple manner. The first one was based on clay modelling and the second one was based on image analysis implemented with open-source software. Furthermore, we tested the accuracy of the widely used geometric approximation method (third method). We applied these three different methods to three morphologically disparate model species, an acanthocephalan worm, a crustacean and a flatworm. To validate our indirect estimations and to test their accuracy, we weighed specimens of the three species and calculated their tissue densities. Additionally, we propose an uncomplicated technique to estimate thickness of individuals under a microscope, a required measurement for two of the three indirect methods tested. The indirect methods proposed in this paper provided the best approximation to direct measurements. Despite its wide use, the geometric approximation method showed the lowest accuracy. The approaches developed herein are timely because the recently increasing number of studies requiring reliable biomass estimates for small invertebrates to explain crucial processes in ecosystems.

Small snails, high productivity? Larval output of parasites from an abundant host

“… Correction factor of 0.33 based on an overestimation of geometric volume approximation of flatworms (Llopis‐Belenguer et al., ). …”

“… a Correction factor of 0.33 based on an overestimation of geometric volume approximation of flatworms (Llopis‐Belenguer et al., ). b Productivity was converted to wet mass based on a wet‐to‐dry mass conversion factor of 0.1 (Benke, ; Preston et al., ). …”

“…Taking into consideration that indirect biomass estimates based on geometric approximations for trematodes are prone to a threefold overestimation (Llopis‐Belenguer et al., ; see Supporting Information Figure ), we corrected the biomass by a conversion factor of 0.33, leading to a summer production of cercariae estimated at 0.62 g m −2 5 month −1 . For the whole lake (0.96 ha), this adds up to a productivity of 5.92 kg of cercariae over the 5 months of summer.…”

“…However, these regular shapes (i.e., ellipsoids, cylinders and cones) do not accurately describe the flattened morphology of flukes, including their cercariae. As exemplified by adult flatworms, this indirect approach leads to a threefold overestimation of volumes, which has a severe impact on ecosystem‐wide biomass extrapolations (Llopis‐Belenguer et al., ). For this reason, we applied a correction factor of 0.33 when calculating the overall cercarial productivity for the whole lake system.…”

“…This method has been used previously in studies of cercarial productivity (Kuris et al., ; Soldánová et al., ; Thieltges et al., ). However, Llopis‐Belenguer, Blasco‐Costa, and Balbuena () recently showed that such geometric approximation methods lead to a roughly threefold overestimation of the actual volume of trematode flatworms, due to their flattened bodies (see Supporting Information Figure ). In order to allow comparison with previous studies and account for this, we provide both raw and corrected (one‐third of raw values) volume estimates.…”