sensitive to contaminants than adult organisms (Adam et al. 2010), and their feeding
rate varies over time, making them more suitable for short-term feeding studies
(Agatz and Brown 2014). Agatz and Brown (2014) and Nilsson (1974) identified
that smaller specimens of Gammarus pulex have a higher feeding rate but higher
variability over time, in comparison to larger organisms.
However, other studies have reported that the feeding rate increases with organism size (Coulaud et al. 2011), but the adoption of different units of measurement
and a small size range might be contributing factors for those findings. It has been
suggested that using organisms with a specific body mass (given in dry weight)
could reduce experimental variability. For feeding studies, up to a 57% reduction in
variability has been documented for specific body mass studies compared to mixed
body mass studies (Agatz and Brown 2014). There is also a recommendation that
body length should be used as an indicator of dry weight and the correlation for
organisms between 2 and 16 mm (Graça et al. 1993b).
Alternatively, organisms might be divided into size groupings by applying
passive underwater separation techniques (Bundschuh et al. 2009, 2017; Zubrod
et al. 2017), by measuring the dorsal length of the Gammarids’ first thoracic segment
after the organisms are photographed (De Castro-Català et al. 2017), by considering
their wet weight (Blockwell et al. 1996; Danger et al. 2012; Weber et al. 2018) or by
using their dry weight at the end of an experiment (Agatz et al. 2014). There is no
agreed standard method on how to separate or select specimens of Gammarids for
this experimental technique, but the chosen method will ultimately determine the
unit of measurement for calculating the feeding rate, e.g. if wet weight is used, the
unit of measurement will be wet weight. The use of either dry or wet weight seems
straightforward, but it is only an estimate, and it lacks accuracy, as the dry weight
range is only known at the end of the study. Furthermore, wet weight does not
provide an accurate measurement due to the unknown volume of liquid in each
sample. Blotting the sample dry before weighing could help to remove a proportion
of the moisture, but it could potentially stress the organisms and consequently affect
the results; therefore, the authors recommend the use of dorsal length as the authors
believe it to be a more accurate way to measure the organisms. In in situ experiments,
the organisms are often divided by size before the start, but the weight is not taken
into consideration. This means that the amount of consumed food is usually related
to the number of living organisms at the end of the experiment (e.g. Coulaud et al.
2011; Dedourge-Geffard et al. 2009) (see Table 3).
Same-sex tests with organisms (female-only, Geffard et al. 2010, or male-only,
Crane and Maltby 1991; De Castro-Català et al. 2017; Forrow and Maltby 2000;
Kelly et al. 2002; Maltby et al. 2002; Naylor et al. 1989; Zubrod et al. 2015) (see
Table 1) of a specific size are often undertaken, although sex is not always specified,
which leads to female and male organisms being used indiscriminately (Agatz et al.
2014; Agatz and Brown 2014; Alonso et al. 2009; Arsuffi and Suberkropp 1989;
Bärlocher and Kendrick 1973a; Blarer and Burkhardt-Holm 2016; Blockwell et al.
1998; Bundschuh et al. 2009, 2011b, 2013, 2017; Dedourge-Geffard et al. 2009;
10
G. Consolandi et al.
rate varies over time, making them more suitable for short-term feeding studies
(Agatz and Brown 2014). Agatz and Brown (2014) and Nilsson (1974) identified
that smaller specimens of Gammarus pulex have a higher feeding rate but higher
variability over time, in comparison to larger organisms.
However, other studies have reported that the feeding rate increases with organism size (Coulaud et al. 2011), but the adoption of different units of measurement
and a small size range might be contributing factors for those findings. It has been
suggested that using organisms with a specific body mass (given in dry weight)
could reduce experimental variability. For feeding studies, up to a 57% reduction in
variability has been documented for specific body mass studies compared to mixed
body mass studies (Agatz and Brown 2014). There is also a recommendation that
body length should be used as an indicator of dry weight and the correlation for
organisms between 2 and 16 mm (Graça et al. 1993b).
Alternatively, organisms might be divided into size groupings by applying
passive underwater separation techniques (Bundschuh et al. 2009, 2017; Zubrod
et al. 2017), by measuring the dorsal length of the Gammarids’ first thoracic segment
after the organisms are photographed (De Castro-Català et al. 2017), by considering
their wet weight (Blockwell et al. 1996; Danger et al. 2012; Weber et al. 2018) or by
using their dry weight at the end of an experiment (Agatz et al. 2014). There is no
agreed standard method on how to separate or select specimens of Gammarids for
this experimental technique, but the chosen method will ultimately determine the
unit of measurement for calculating the feeding rate, e.g. if wet weight is used, the
unit of measurement will be wet weight. The use of either dry or wet weight seems
straightforward, but it is only an estimate, and it lacks accuracy, as the dry weight
range is only known at the end of the study. Furthermore, wet weight does not
provide an accurate measurement due to the unknown volume of liquid in each
sample. Blotting the sample dry before weighing could help to remove a proportion
of the moisture, but it could potentially stress the organisms and consequently affect
the results; therefore, the authors recommend the use of dorsal length as the authors
believe it to be a more accurate way to measure the organisms. In in situ experiments,
the organisms are often divided by size before the start, but the weight is not taken
into consideration. This means that the amount of consumed food is usually related
to the number of living organisms at the end of the experiment (e.g. Coulaud et al.
2011; Dedourge-Geffard et al. 2009) (see Table 3).
Same-sex tests with organisms (female-only, Geffard et al. 2010, or male-only,
Crane and Maltby 1991; De Castro-Català et al. 2017; Forrow and Maltby 2000;
Kelly et al. 2002; Maltby et al. 2002; Naylor et al. 1989; Zubrod et al. 2015) (see
Table 1) of a specific size are often undertaken, although sex is not always specified,
which leads to female and male organisms being used indiscriminately (Agatz et al.
2014; Agatz and Brown 2014; Alonso et al. 2009; Arsuffi and Suberkropp 1989;
Bärlocher and Kendrick 1973a; Blarer and Burkhardt-Holm 2016; Blockwell et al.
1998; Bundschuh et al. 2009, 2011b, 2013, 2017; Dedourge-Geffard et al. 2009;
10
G. Consolandi et al.