Interspecific Aggression and Food Competition between the Global Invader Palaemon macrodactylus and the Native Palaemon elegans

Round 1

Reviewer 1 Report

The article is interesting and provides a good insight into the competitive advantages that this species may have when invading new areas or potential impacts on native species.  Generally it is well written and researched.

As a general comment, the study seems to suggest that the two Palaemon species may not compete too much when food is not limiting but they may compete when food resources do become limiting.  A similar conclusion was reached by another study investigating dietary overlap between P. macrodactylus and P. longirostris (Ashelby et al., 2016). It may be relevant to explore this concept a bit more in the discussion and cite this study, which is currently missing from this article.

Some specific points to address are:

Line 51: Palaemonetes varians is now Palaemon varians

Line 64: it is not necessary to include (Decapoda, Palaemonidae) here

Line 71: please state where the individuals were obtained (rough location is fine rather than exact positions)

Line 74: bucks, should this be buckets?

Line 136 (Figure 1 title): Italicise Latin name

Line 162: Italicise Latin name

Line 190: individuals without capital i

Line 203: Italicise Latin name

Line 205: Italicise Latin name

Line 217: I would remove the word Worldwide. The cited study only shows salinity tolerance in Argentina, not elsewhere in the World

Lin 248 (Acknowledgments): The word 'the' appears to be in bold but should not be

Reference 8: in the title for this paper there should be a hyphen between SE and Portugal (SE-Portugal)

Author Response

Review Report Form 1

The article is interesting and provides a good insight into the competitive advantages that this species may have when invading new areas or potential impacts on native species.  Generally it is well written and researched.

We thank the reviewer for the comments and suggestions

As a general comment, the study seems to suggest that the two Palaemon species may not compete too much when food is not limiting but they may compete when food resources do become limiting.  A similar conclusion was reached by another study investigating dietary overlap between P. macrodactylus and P. longirostris (Ashelby et al., 2016). It may be relevant to explore this concept a bit more in the discussion and cite this study, which is currently missing from this article.  We thank the reviewer for the valuable suggestion. The citation was added in introduction and considerations were done in discussion

Some specific points to address are:

Line 51: Palaemonetes varians is now Palaemon varians    Done

Line 64: it is not necessary to include (Decapoda, Palaemonidae) here Done

Line 71: please state where the individuals were obtained (rough location is fine rather than exact positions) Done in materials and methods

Line 74: bucks, should this be buckets? Corrected

Line 136 (Figure 1 title): Italicise Latin name Done

Line 162: Italicise Latin name Done

Line 190: individuals without capital I Done

Line 203: Italicise Latin name Done

Line 205: Italicise Latin name Done

Line 217: I would remove the word Worldwide. The cited study only shows salinity tolerance in Argentina, not elsewhere in the World Done

Lin 248 (Acknowledgments): The word 'the' appears to be in bold but should not be Done

Reference 8: in the title for this paper there should be a hyphen between SE and Portugal (SE-Portugal) Done

Reviewer 2 Report

Please find my comments below. I am lacking to know whether food is a limiting resource for these species in nature. Methodology section needs to be edited and described in more detail

INTRODUCTION

 Ln 27: what is ecological unbalance? Please describe.

Ln 27: is this in the context of aquatic environments in general or marine in particular?

Ln 33: this is quite outdated timeframe for biological invasions (2001). Please provide more recent data.

Ln 42: please explain the relations hip between lower oxygen consumption and eurythermy of P. macrodactylus and it threat to P. elegans

Ln 42: do these species already co occur and where?

Ln36-48: several English phrases in the paragraph require editing

Ln 67: higher performance should be termed resource holding potential which is actually related to dominance at least in decapod crustaceans. Basically, dominance translates to higher access to food. Also – please explain whether food is a limiting resource in nature for these species or not?

Ln104: if shrimps were held together for 24h before the feeding test, this means that dominance hierarchy was already established before testing food competition, right?

 

 

METHODS

Ln80: state the chosen photoperiod please.

Ln93: on decapod agonistic interactions the differences below 5% in weight and length is also taken into account, as 0.1. g difference in weight might in some case be well below 5% and in some cases (smaller individuals) well above. Did you take this into account? Please state clearly and add references.

Ln 96: please provide size ranges and standard deviations for all used individuals and their numbers.

Ln99: only one shelter or multiple shelters?

Ln106: what is MC-HC software? Please provide reference.

Ln 106: How many pairs/individuals were analysed?

Ln 115: please provide references for measurement of this behaviour

 

RESULTS

Ln151: spent more time in contact with food than….The difference waa significant at salinity 30….but not significant at salinity 20….

Line 159: replace ha with displayed this behavior

Ln 171: with…displaying ….

 

DISCUSSION

Ln180: is food limited resource for these species. Please state this.  

Ln187: I do not see how you determined that P. macrodactylus is more aggressive than P. elegans? Be higher number of chasings and lower number of tailflips? Aggression is usually determined by determining more behavioural parameters, and this should be elaborated in the methods section -i.e. why you think that these two factors are indicators of aggression

Ln194: ok here you mention that food is a limiting resource for these species but you do not provide any references for this. Please provide references to back up your statements.

Ln 196: 24 hours without feeding is far from starvation! This is not a reason to consider food a limiting resource. More importantly – is food a limiting resource in nature?

Ln201: I do not understand this sentence. Please rephrase.

Ln207: escape is the most frequent strategy for what? please explain. In decapoda it is a part of dominance hierarchy formation, not a specific strategy.

 

Author Response

Review Report Form 2

Please find my comments below. I am lacking to know whether food is a limiting resource for these species in nature. Methodology section needs to be edited and described in more detail.

We thank the reviewer for the comments and suggestions. The aspects related to dietary were added in introduction and discussion (see details below).

INTRODUCTION

 Ln 27: what is ecological unbalance? Please describe. We thank the reviewer to highlight the misuse of the verb instead of the noun. The verb UNBALANCE was changed with the noun  IMBALANCE and sentence was added to briefly illustrated it.

Ln 27: is this in the context of aquatic environments in general or marine in particular? The term marine was used instead of aquatic.

Ln 33: this is quite outdated timeframe for biological invasions (2001). Please provide more recent data. A bibliographic review was carried out, but most of recent papers refer to local introductions. However, the database AquaNIS allowed to update the overall number of invasive decapods (added in the text).

Ln 42: please explain the relations hip between lower oxygen consumption and eurythermy of P. macrodactylus and it threat to P. elegans. See line 47-49 with the explanation

Ln 42: do these species already co occur and where? Yes, they do. The co-occurrence was added in the section MATERIALS AND METHODS.

Ln36-48: several English phrases in the paragraph require editing. Done

Ln 67: higher performance should be termed resource holding potential which is actually related to dominance at least in decapod crustaceans. Basically, dominance translates to higher access to food. Also – please explain whether food is a limiting resource in nature for these species or not?

The suggested definition was included at line 75, as follows “The main goal was to assess whether one of the two species showed a higher performance resource-holding potential in possessing and maintaining the food resource through aggressive interactions, comparing two salinity conditions”.

Moreover, a reference was added (lines 50-53) regarding the dietary overlap and potential food competition. Ashelby et al., 2016 suggest that, seasonally, the preys can be limited. Further considerations were added in discussion

Ln104: if shrimps were held together for 24h before the feeding test, this means that dominance hierarchy was already established before testing food competition, right?

A dominant hierarchy could have been established between the two individuals leading to the control of a shelter and a territory, although these were not limiting since two shelters and two territories were available. This occurred without any food competition but our preliminary observations showed that the access to food increased the intensity of inter-specific as well as of intraspecific aggression. Interactions are very rare without food competition. So, the experiments were designed to establish a space partitioning between the two individuals and to elicit food competition after a period of fasting that could increase the motivation to feed.

 

 

METHODS

Ln80: state the chosen photoperiod please. Done

Ln93: on decapod agonistic interactions the differences below 5% in weight and length is also taken into account, as 0.1. g difference in weight might in some case be well below 5% and in some cases (smaller individuals) well above. Did you take this into account? Please state clearly and add references. With regard to size differences between the tested shrimp pairs, 0.1 g is an arbitrary threshold value, chosen on the basis of our experience. As the referee rightly points out, this can result in a difference of more or less than 5% (which is still an arbitrary threshold value). We were only concerned that (1) overall, there was no difference between the average sizes of the two species at the two salinities, (2) at each salinity, in 4 cases P. elegans was bigger the P. macrodactylus and in the other 4 cases P. elegans was smaller than P. macrodactylus. In the text, lines 106-108, the importance to reduce the competitive advantage due to difference in size was better written.

Ln 96: please provide size ranges and standard deviations for all used individuals and their numbers.  Please find here below the weights of individuals used in each trial. We prefer to leave in the text the graph A in Fig. 2, to be more synthetic. Actually, our most important goal was to avoid size disadvantages. Moreover, even if P. elegans was slightly bigger than P. macrodactylus, the latter tended to be the resource-holding.

 

Trial	Salinity 20		Salinity 30
	P. elegans	P. macrodactylus	P. elegans	P. macrodactylus
1	0.646	0.704	0.367	0.448
2	0.997	0.993	0.349	0.392
3	0.413	0.434	0.576	0.456
4	0.956	1.014	0.865	0.953
5	0.907	0.923	0.753	0.602
6	1.08	1.016	0.856	0.731
7	0.865	0.785	0.995	0.913
8	0.683	0.599	0.684	0.835
Weight (g) of the animal used in the 16 trials (8 trials at 2 salinities)

 

Ln99: only one shelter or multiple shelters? Two shelters, one for each individual (added in the text)

Ln106: what is MC-HC software? Please provide reference. Done

Ln 106: How many pairs/individuals were analysed? Done

Ln 115: please provide references for measurement of this behaviour Done

 

 

RESULTS

Ln151: spent more time in contact with food than….The difference waa significant at salinity 30….but not significant at salinity 20…. Done

Line 159: replace ha with displayed this behavior Done

Ln 171: with…displaying …. Done

 

DISCUSSION

Ln180: is food limited resource for these species. Please state this.  A paragraph was added with a new reference

Ln187: I do not see how you determined that P. macrodactylus is more aggressive than P. elegans? Be higher number of chasings and lower number of tailflips? Aggression is usually determined by determining more behavioural parameters, and this should be elaborated in the methods section -i.e. why you think that these two factors are indicators of aggression

We replaced the term “more aggressive” with “more successful in aggressive encounters”, that is what the data indicated. The species has a higher resource-holding potential, that is mediated by aggressive interactions. Higher frequency of chasing and lower of tail tips means that this species is more successful in the aggressive outcome. A reference describing the assessment of aggressive behaviour was added:  Eglé and Ulrika (2021) describe aggression as the number of attacks or rushes towards another individual.

Ln194: ok here you mention that food is a limiting resource for these species but you do not provide any references for this. Please provide references to back up your statements. The reference was added at the beginning of the Discussion

Ln 196: 24 hours without feeding is far from starvation! This is not a reason to consider food a limiting resource. More importantly – is food a limiting resource in nature? The term “starvation” was substituted in the text with “fasting”. The topic of food as limiting sources is discussed in the text and responses were given to the previous comment.

Ln201: I do not understand this sentence. Please rephrase. Done

Ln207: escape is the most frequent strategy for what? please explain. In decapoda it is a part of dominance hierarchy formation, not a specific strategy. The term strategy was changed in behaviour as described in the paper here below cited, where such mechanism was described

Correa, C.; Thiel, M. Mating systems in caridean shrimp (Decapoda: Caridea) and their evolutionary consequences for sexual dimorphism and reproductive biology. Rev Chil Hist Nat 2003, 76, 187–203.

 

 

Round 2

Reviewer 2 Report

INTRODUCTION

lN 90: Should this be acclimatization period?

 

RESULTS

Please correct your statistical reporting - all reports should include sample size, mean value and SD for t-tests and provide all required data for reporting GLM models. I would also ask that by having in mind the Open science framework you publish your data on the sizes of individuals in pairs (provided in the review), as well as detailed tables describing the results of your anaylses, including means, SDs and confidence intervals.

Author Response

Dear Editor,

We are thankful to you and to the reviewers for the job done on the manuscript and  comments and suggestions. The manuscript was improved accordingly and please see below our reply in bold.

Review Report Form 2 – Round 2

INTRODUCTION

lN 90: Should this be acclimatization period? Correction done, thank you

RESULTS

Please correct your statistical reporting - all reports should include sample size, mean value and SD for t-tests and provide all required data for reporting GLM models. I would also ask that by having in mind the Open science framework you publish your data on the sizes of individuals in pairs (provided in the review), as well as detailed tables describing the results of your anaylses, including means, SDs and confidence intervals.

Two Tables were added as supplementary material to complete the statistical reporting. Table 1S contains data on the sizes of individuals in pairs with mean+standard deviation and the t-test outputs. Table 2S contains mean+standard deviation for each parameter, the sample size and the t-test output.

The main text was changed citing the new tables. GLM model outputs are included in the main text.