It is still unclear if conditioning should take place before or after the leaves are cut
into discs, dried in the oven and weighed. Consequently, the authors recommend that
further investigations need to be undertaken to compare if drying the leaf discs in the
oven should be undertaken before or after the conditioning process and if either of
these methodologies alter the feeding rate of Gammarids. Organisms are usually fed
ad libitum during the acclimation period. To further reduce the inner variability and
strengthen the data, the authors recommend incorporating a starvation period in the
experimental design. This starvation period should take place before the feeding
experiment, and its purpose is to synchronise the organisms’ hunger levels. The
authors also recommend that organisms of a comparable size range should be used in
experiments as it has been proven that Gammarids of different sizes have a different
feeding rate. Juveniles are more sensitive to contaminants, but their feeding rate is
characterised by a higher variability over time, which makes them more suitable for
short-term studies. On the other hand, because of their greater sensitivity, juveniles
are better for ecotoxicological studies by providing ecologically relevant risk assessments for contaminants. Gammarus spp. has been widely adopted for ecotoxicological studies, but the genus contains many different species, and even though very
similar, there are still differences in their sensitivities, meaning that the choice of one
species over the other should be carefully considered, depending on the contaminant
tested.
This is of particular interest for in situ experiments since the adopted species
would be dependent on the site, but also dependent on the season, which could
determine the availability of particular organism sizes. So in order to further reduce
inner variability and allow better estimation of the feeding rate, organisms should be
measured at the start of an experiment, possibly by photography and length measurements, in order to have a pool of organisms of the same size and potentially the
same life stage. This is particularly noteworthy when growth is measured alongside
the organisms feeding rate.
The source of the organisms might also have an impact on the results. Organisms
collected in the wild could be better suited for in situ studies, as they could provide a
more realistic site-specific response. However, local site-related species may be
characterised by previous exposure histories that could ultimately influence their
feeding rate (e.g. they could potentially be acclimated to a certain level of pollution).
This is a problem that has to be taken into consideration both for in situ and ex situ
experiments. Perhaps laboratory-bred organisms should be used to reduce variability
even further, and it would provide a constant stock of Gammarids (Blockwell et al.
1996; Bloor and Banks 2006a, b; McCahon and Pascoe 1988). However, breeding
Gammarids is not always possible and it is highly species dependent. Long-term
culturing could also potentially lead to a higher or lower contaminant sensitivity and
a reduced genetic variability.
The last step of a feeding study involves the quantification of the feeding rate by
using an equation. As highlighted in this review (see Table 3), there are various
equations in the literature that are indiscriminately used to calculate the feeding rate.
However, some of these equations are mathematically different, and it raises the
question, are the equations and the feeding rates generated by them equal?
Feeding Behavioural Studies with Freshwater Gammarus spp.: The. . .
35
into discs, dried in the oven and weighed. Consequently, the authors recommend that
further investigations need to be undertaken to compare if drying the leaf discs in the
oven should be undertaken before or after the conditioning process and if either of
these methodologies alter the feeding rate of Gammarids. Organisms are usually fed
ad libitum during the acclimation period. To further reduce the inner variability and
strengthen the data, the authors recommend incorporating a starvation period in the
experimental design. This starvation period should take place before the feeding
experiment, and its purpose is to synchronise the organisms’ hunger levels. The
authors also recommend that organisms of a comparable size range should be used in
experiments as it has been proven that Gammarids of different sizes have a different
feeding rate. Juveniles are more sensitive to contaminants, but their feeding rate is
characterised by a higher variability over time, which makes them more suitable for
short-term studies. On the other hand, because of their greater sensitivity, juveniles
are better for ecotoxicological studies by providing ecologically relevant risk assessments for contaminants. Gammarus spp. has been widely adopted for ecotoxicological studies, but the genus contains many different species, and even though very
similar, there are still differences in their sensitivities, meaning that the choice of one
species over the other should be carefully considered, depending on the contaminant
tested.
This is of particular interest for in situ experiments since the adopted species
would be dependent on the site, but also dependent on the season, which could
determine the availability of particular organism sizes. So in order to further reduce
inner variability and allow better estimation of the feeding rate, organisms should be
measured at the start of an experiment, possibly by photography and length measurements, in order to have a pool of organisms of the same size and potentially the
same life stage. This is particularly noteworthy when growth is measured alongside
the organisms feeding rate.
The source of the organisms might also have an impact on the results. Organisms
collected in the wild could be better suited for in situ studies, as they could provide a
more realistic site-specific response. However, local site-related species may be
characterised by previous exposure histories that could ultimately influence their
feeding rate (e.g. they could potentially be acclimated to a certain level of pollution).
This is a problem that has to be taken into consideration both for in situ and ex situ
experiments. Perhaps laboratory-bred organisms should be used to reduce variability
even further, and it would provide a constant stock of Gammarids (Blockwell et al.
1996; Bloor and Banks 2006a, b; McCahon and Pascoe 1988). However, breeding
Gammarids is not always possible and it is highly species dependent. Long-term
culturing could also potentially lead to a higher or lower contaminant sensitivity and
a reduced genetic variability.
The last step of a feeding study involves the quantification of the feeding rate by
using an equation. As highlighted in this review (see Table 3), there are various
equations in the literature that are indiscriminately used to calculate the feeding rate.
However, some of these equations are mathematically different, and it raises the
question, are the equations and the feeding rates generated by them equal?
Feeding Behavioural Studies with Freshwater Gammarus spp.: The. . .
35