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.
Highlights
Invertebrate biomass is considered one of the main factors driving processes in ecosystems
For C. oblonga, the average individual mass estimated by indirect methods 1 and 2 were very similar to and not significantly different from that estimated with the direct method (t = −1.86, P = 0.08; t = 0.74, P = 0.47;)
For B. capitatum and Ca. elongatus, the mean values obtained with methods 1 and 2 were very close to and not significantly different from those of the direct method (Table 1)
Summary
Invertebrate biomass is considered one of the main factors driving processes in ecosystems. Indirect estimators have been proposed, such as using body surface areas or volumes as proxies of individual mass based on linear measurements[17,18,28,29,30,31,32,33,34,35], linear lengths of different features converted into biomass through generalised regression equations[20,21,22,23,24,36], displacement of water volume in a graduated cylinder[8,13,28,32,37], or biovolume estimated using confocal microscopy and image analysis[38] Most of these methods are taxon- or age-specific, destructive, laborious and time consuming or overlook the contribution of appendages to the total individual mass. The three model species analysed here, each have a parasitic mode of life, they are good representatives of morphological diversity of small invertebrates in general, because they represent three different phyla, cover both soft and hard-body species, with different transversal sections and levels of ornamentation (Fig. 1)
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