Morphological, calorific and nutritive characteristics of 656 freshwater invertebrates taxa

Abstract Background The Freshwater Animal Diversity Assessment (FADA) project estimated that freshwater animal species represent 9.5% of the 1.2 million species described. Knowing that freshwater represents only 0.01% of the earth's surface, these wetlands are suitable habitats for a great part of the world's total biodiversity. However, it has been shown that there is a lack of knowledge on these species, including freshwater invertebrates. Nevertheless, they play a key role in the majority of freshwater ecosystems and in their foodweb networks. Freshwater invertebrates are the food resource of many species, such as fish and birds. The knowledge of their morphological, energetic and nutritive characteristics allows a better understanding of their selection by predators (size, energy intake etc.), but also leads to the improvement of wetland management. Although information about freshwater invertebrates exists in literature, they are generally heterogeneous, dispersed and difficult to collect. To facilitate the accessibility of these data and, thus, optimise and accelerate research projects including freshwater invertebrates, we propose a literature review describing 14 morphological and nutritive characteristics (size, dry weight, gross energy, crude protein etc.) for 656 taxa of freshwater invertebrates. New information This dataset is a review from 104 publications from 1935 to 2020, compiling 14 characteristics when available (size, dry weight, gross energy, crude protein etc.) for 656 taxa of freshwater invertebrates.


Introduction
In twenty years, the estimated world's total number of species has increased from 3 to 30 million (May 1990) to between 3 and 100 million (May 2010). Currently, 1.2 million species have been listed, but it was estimated that about 86% of terrestrial species and 91% of oceanic species are still to be described (Mora et al. 2011). Invertebrates would represent 60% of known species and three quarters of new species identified each year (Chapman 2009). Occupying the majority of habitats, they could potentially represent a significant part of the global biomass (Ellwood and Foster 2004). They play a key role in most trophic webs (Vanni 2002, Hornung and Foote 2006, McCarthy et al. 2009) and some are even good indicators of the environmental quality (Stork andEggleton 1992, Hodkinson andJackson 2005).
Wetlands are amongst the most impacted environments by climate change (Dawson et al. 2003, Erwin 2009. They have particularly declined worldwide, losing at least 50% of their surface since the beginning of the 19th century (Finlayson et al. 1999, Davidson 2014, Gardner et al. 2015. However, some species depend directly on these habitats, such as fish (Gilinsky 1984, Diehl 1992, Garvey et al. 1994, Bouffard and Hanson 2006, McCarthy et al. 2009 or birds (e.g. Bolduc and Afton 2004, Taft and Haig 2005, Ma et al. 2010, particularly because of the available food resources, such as freshwater invertebrates (Covich et al. 1999). The links between freshwater invertebrates and waterbirds were examined (Goss-Custard 1977, Colwell and Landrum 1993, Lillie and Evrard 1994, Weber and Haig 1997, Brodmann and Reyer 1999, Halse et al. 2000, Arzel et al. 2009, Guareschi et al. 2015, but remain insufficiently explored (Sanders 2000, Prather et al. 2013. The lack of knowledge about freshwater invertebrate abundance and diversity is, therefore, an important issue for the conservation of the species living in these environments, but also for their habitats management (Vanni 2002). Knowing the species that contribute to this diversity, as well as their morphological characteristics, energetic value or nutritional composition (Fredrickson and Reid 1988, Zwarts and Blomert 1992, Davis and Smith 2001, would improve the understanding of the prey-predator interactions (Nudds andBowlby 1984, Anderson andSmith 2000). For example, waterfowl feed on aquatic invertebrates and, consequently, the abundance, accessibility, size or even the energetic values of these foods affect the use of foraging habitats by waterfowl (Ma et al. 2010).
Invertebrates are very sensitive to environmental variations, included climate change (Lawrence and Soame 2004, Prather et al. 2013, Khamis et al. 2014) and affect finally ecosystem functions and associated ecosystem services (Lavelle et al. 2006, Prather et al. 2013, Khamis et al. 2014. Some of them are also very sensitive to the water physicochemical variations and are indicators of its quality, especially since beginning of anthropogenic pollution (Gaufin and Tarzwell 1952, Gaufin and Tarzwell 1956, Hellawell 1986, Dallinger 1994. Obtaining knowledge on freshwater invertebrates is, therefore, essential for the habitat and species conservation (Vicente 2010), but also to answer the global change (Strayer 2010, Collier et al. 2016. Studies in literature were more specifically focused on knowledge of the orders and families, in particular, according to their stages of development (larvae, nymphs, adults) and their sizes, but it appears as still incomplete and insufficient (Vicente 2010, Strayer and Dudgeon 2010, Collier et al. 2016). If information exists, it is generally heterogeneous, scattered or restricted to local journals and, therefore, not widely distributed (Strayer 2006, Balian et al. 2008b, Strayer and Dudgeon 2010, Appeltans et al. 2012, especially when the interest is on other factors, such as the individuals weight, the energetic value or the nutritional composition (proteins, lipids etc.). Even if some books or publications have a high level of details (Cumminns and Wuycheck 1971, Nudds and Bowlby 1984, Anderson and Smith 2000, James et al. 2012, none includes all information about these physical, nutritive or energetic characteristics, placing limits to the progress of some studies. Indeed, the skills of researchers or managers do not always include know-how in taxa or species identification (Krieger 1992).
Thus, the available information in literature is generally uncommon (Strayer 2006, James et al. 2012) and/or difficult to collect (Krieger 1992). The solution for acquiring these data may be to characterise all the taxa detected and collected. However, this option raises problems related to the availability of technological tools, time (taxa collection, sorting and identification, when this skill is available, are long) (James et al. 2012) or financial (Krieger 1992, Strayer 2006 because of the human resources and the material to be mobilised. These points constitute obstacles to the progress of some studies or even points of renunciation.
In order to facilitate the accessibility of these data and, thus, promote research projects on the importance of freshwater invertebrates in wetland ecosystems, we propose a literature review of the main biological characteristics of all freshwater invertebretates available in literature up to 2020.

Geographic coverage
Description: This literature review concerns the worldwide freshwater wetlands (lakes, rivers, marshes, temporary and permanent ponds etc.).

Taxonomic coverage
Description: This dataset describes 656 taxa of freshwater invertebrates (Table 1). . Headers corresponding to variable names are included as the first row in the data file. Each characteristic is subdivided into two categories, "value" corresponding to the value of the variable with sometimes a note corresponding to the comment associated with the value ("WS" for "with shell" and "SR" for "shell removed") and "reference" corresponding to the literature reference (Suppl. material 2). The datasets are deposited in Dryad (doi: 10.5061/dryad.j3tx95xfg).

Additional information
Steps of database building

1.
Classification and characteristics of freshwater invertebrates: A literature review of the different morphological, calorific and nutritive characteristics of freshwater invertebrates was established. They were classified from phylum to species from three manuals of references (Balian et al. 2008a, Covich 2009, Thorp andRogers 2011). In this review, 104 scientific publications were used and 14 criteria were described: total length, head width or shell width, body length or shell length, wet weight, dry weight, gross energy of wet weight, gross energy of dry weight, gross energy of ash free dry weight, crude protein, crude lipid, crude fibre, nitrogen free extract, proportion of ash and water.

2.
Referencing and retranscription of literature values: Publications presenting original data or non-published data from another study were cited. Within publications, the values of the different criteria were represented heterogeneously (e.g. different measure units, taxonomic level). Due to this heterogeneity, all values were transformed to obtain a homogeneous set of measurement units and taxonomic level (i.e. μm in mm, µg in mg, kcal/g in cal/g, J/g in cal/g (1 J = 0.239006 cal), kJ/g in cal/g).

Prospects for use
Scientific literature shows that abundance of food resource is one of the first things that the animal ecologists measure when they want to understand the species they are studying, whether it is individual behaviour, reproduction or even population dynamics (Newton 1998). Thus, estimates of biomass or the calorific equivalent of freshwater organisms are necessary in the study of the food ecology of fish, amphibians or even birds. For example, the availability of food is often considered to be a fundamental factor affecting the migration and reproduction of animals, especially birds. For example, Anatidae conventionally have a diet dominated by seeds and plant matter in winter, but aquatic invertebrates dominate in spring and summer (Krapu and Reinecke 1994). Thus, the annual migration of Anatidae between the wintering and breeding site is often explained by the exploitation of abundant food resources at higher latitudes to increase breeding success (Berthold et al. 2003). In addition, many scientists agree that migration is programmed according to local peaks of food abundance at successive stopover sites in order to feed during migration and prepare for reproduction (Ankney et al. 1991, Drent andDaan 2002). After prenuptial migration, the majority of surface ducks must efficiently replenish fat and protein stores because egg formation and incubation are expensive processes (Alisauskas and Ankney 1992). Consequently, the abundance of food appears to be a decisive factor in the choice of breeding habitat . For all these reasons, the abundance, biomass and nutritional value of invertebrates are essential in studying habitat choice, reproductive success and annual cycles of dabbling ducks (Batt et al. 1992). In addition, prey size is essential to understand the food ecology of the organisms studied. For example, the Northern Shoveler has a spatula-shaped beak made up of many lamellae. This physical characteristic allows it to select only small prey, giving it a specific food niche. Thus, it is important to know the size of the prey available in order to understand the use of a site by this species.
The abundance, biomass and nutritional value of freshwater invertebrates are, therefore, essential in the study of predatory of freshwater invertebrates (Towers et al. 1994). The most common method for determining the wet or dry weight of aquatic invertebrates is to directly weigh individual specimens (Smock 1980). However, this approach often takes time and is prone to error if individuals were fixed (Downing 1984). Indeed, preservative often modifies the fresh mass of conserved invertebrates (Johnston and Cunjak 1999). The dry mass measurement has the disadvantage of rendering the sample unusable for further examination following the drying process (Towers et al. 1994). In this case, a table synthesising the biometric and calorific data of freshwater invertebrates is a real benefit for the ecologist. Indeed, this table will allow freshwater ecologists to estimate biomass, energy and size class of freshwater invertebrates at survey sites quickly and economically.
However, it is important to note that there is variation in calorific values due to the collection season, the diet of the organisms and the sex of the individuals (Arzel et al. 2009). In addition, there is variation in individual values because the measurements taken almost always include the intestinal contents of the organisms or because females carrying eggs should have the highest values for a given species (Cumminns 1967). Given the errors that environmentalists face, it might be more realistic to use a median or overall calorific value. For the taxon studied, this value is, thus, easily obtainable thanks to this table. Indeed, the comparison of values for the same species obtained in different laboratories should allow problems due to seasonal differences, habitat and diet to be overcome (Johnston and Cunjak 1999).