Freshwater hydrozoan blooms alter activity and behaviour of territorial cichlids in Lake Tanganyika

Blooms of gelatinous zooplankton can represent dramatic environmental perturbations for aquatic ecosystems. Yet, we still know little about how blooms impact fitness-related behaviours of fish caught within their areas of effect, especially for freshwater systems. Here, we documented the behavioural impacts of freshwater hydrozoan (Limnocnida tanganjicae) blooms on a territorial cichlid (Variabilichromis moorii), as well as on the wider community of cichlids in a shallow-water rocky habitat of Lake Tanganyika. Compared with non-bloom conditions, V. moorii individuals in the midst of blooms reduced their swimming and territory defence activities (each by approx. 50%) but not their foraging or affiliative behaviours. Despite this reduction in activity, V. moorii could not entirely avoid being stung and preferred to remain closer to the rocky substrata as opposed to the more open demersal zone. Many other fishes similarly hid among the benthic substrata, changing the composition of the fish community in the demersal zone during bloom conditions. Reductions in activity could have multiple fitness-related implications for individual fish. Establishing the consequences of these behavioural changes is important for understanding the effects of gelatinous zooplankton blooms in freshwater systems.


Are the interpretations and conclusions justified by the results? Yes
Is the language acceptable? Yes

Do you have any ethical concerns with this paper? Yes
Have you any concerns about statistical analyses in this paper? No

Recommendation?
Major revision is needed (please make suggestions in comments)

Comments to the Author(s)
This is a fascinating study on the impact of a poorly-known jellyfish species on fish behavior. Overall the manuscript is well-written, but reflects several outdated views on jellyfish blooms which, even if accurate, are not comparable to this system. Unlike highly impacted coastal marine areas, the author's study area is an ancient freshwater lake in which the jelly species in question is a native member of the ecosystem. While it is true that jellyfish blooms impact fish behavior in marine systems, framing this study in the context of 'rising jellyfish numbers' and the 'global rise of jellyfish' (claims that are now being questioned), detracts from the fascinating work being done here on an remarkable and unique system. Because the human-associated causes and consequences of marine blooms likely do not impact this system, and because the field of marine blooms is itself rapidly changing, the authors should re-frame their introduction and discussion to highlight their system's fascinating qualities. The fact that so little is known about freshwater jellyfish ecology, and this species in particular, should be celebrated. Further, I see no reason their findings are not applicable to marine systems, but the language and literature currently cited reflect a "jellyfish as negative" perspective when really these are all native species interacting in a fascinating way.
15: Jellyfish are also beneficial to many fish species, so the relationship is complicated and should not be reduced to one of purely threat, but of impact more generally: Unravelling the macroevolutionary ecology of fish-jellyfish associations. Caution is especially advised for poorlyunderstood freshwater jellyfish like Limnocnida tanganjicae. Resource: life in the 'gingerbread house' Donal C. Griffin  The editors assigned to your paper ("Jellyfish blooms alter activity and behaviour of territorial fish") have now received comments from reviewers. We would like you to revise your paper in accordance with the referee and Associate Editor suggestions which can be found below (not including confidential reports to the Editor). Please note this decision does not guarantee eventual acceptance.
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Once again, thank you for submitting your manuscript to Royal Society Open Science and I look forward to receiving your revision. If you have any questions at all, please do not hesitate to get in touch. This paper reports very interesting observations about the interactions between freshwater jellyfish and the behavior of fish in the African Great Lakes. The manuscript was assessed by two expert reviewers. Both reviewers were excited about the study. However, both felt (among other things) that the authors relied too much on the marine literature. I agree the elegance and importance of this nascent study, is its uniqueness in terms of study system and behavioral/ecological interactions. I felt the reviews were clear, fair, and constructive. With much gratitude to the expert reviewers, I invite the authors to address these comments.
Reviewers' Comments to Author: Reviewer: 1 This is a good, well written paper dealing with the behavioural impacts of a freshwater hydrozoan on a territorial cichlid and on the composition of cichlids assemblages. The main issue with the manuscript is that authors have chosen a highly debatable or wrong context to justify their study, which in my view, they don't need: the study is interesting and valid without establishing a parallel with fish-jellyfish interactions in coastal marine systems. Despite the diversity of studies suggesting that anthropogenic stressors facilitate jellyfish blooms and a global trend of increasing jellyfish blooms, two recent studies (Pitt et al. 2018;Sanz-Martín et al. 2016) have clearly shown that the evidence supporting those claims is weak and mainly based in a couple of cosmopolitan species. These species do not represent the huge diversity life forms included in the term "jellyfish", which depending on the authors may comprise between two (Cnidarians, Ctenophores) and five phyla (Cnidarians, Ctenophores, Molluscs, Polychaetes and Chordata-Salps and Larvaceans). Therefore, I strongly recommend not to use this parallel to justify the study or to discuss the results. Specific comments Title: It is too general; it does not reflect the scope of the study. The manuscript deals with the effect of a freshwater hydrozoan on the behaviour of cichlids in the Tanganyika Lake. Introduction: -I suggest either avoid the use of the term "jellyfish" or clearly define it in the context of this study -I suggest, if available, to include information on the abundance seasonality of the L. taganjicae L34-63. See general comments and reconsider the need of this section. L88. Please define -here or in material and methods-how a bloom and non-bloom condition was defined. It was not clear for me if "normal" or "non-bloom" conditions mean no medusas at all or a low density. Results: L243-244. This is an interesting result, suggesting that the medusae population was mainly comprised of juveniles -this, by the way was not discussed in the next section. I am wondering if there is differences between juvenile and adult medusa in prey patterns and interspecific competitive interactions. Discussion: Generally I suggest avoiding stating again the results in order to discuss about them. The discussion is a bit too long. This is a fascinating study on the impact of a poorly-known jellyfish species on fish behavior. Overall the manuscript is well-written, but reflects several outdated views on jellyfish blooms which, even if accurate, are not comparable to this system. Unlike highly impacted coastal marine areas, the author's study area is an ancient freshwater lake in which the jelly species in question is a native member of the ecosystem. While it is true that jellyfish blooms impact fish behavior in marine systems, framing this study in the context of 'rising jellyfish numbers' and the 'global rise of jellyfish' (claims that are now being questioned), detracts from the fascinating work being done here on an remarkable and unique system. Because the human-associated causes and consequences of marine blooms likely do not impact this system, and because the field of marine blooms is itself rapidly changing, the authors should re-frame their introduction and discussion to highlight their system's fascinating qualities. The fact that so little is known about freshwater jellyfish ecology, and this species in particular, should be celebrated. Further, I see no reason their findings are not applicable to marine systems, but the language and literature currently cited reflect a "jellyfish as negative" perspective when really these are all native species interacting in a fascinating way.
15: Jellyfish are also beneficial to many fish species, so the relationship is complicated and should not be reduced to one of purely threat, but of impact more generally: Unravelling the macroevolutionary ecology of fish-jellyfish associations. Caution is especially advised for poorly-

09-Oct-2019
Dear Dr Sefc, I am pleased to inform you that your manuscript entitled "Freshwater hydrozoan blooms alter activity and behaviour of territorial cichlids in Lake Tanganyika" is now accepted for publication in Royal Society Open Science.
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Follow Royal Society Publishing on Twitter: @RSocPublishing Follow Royal Society Publishing on Facebook: https://www.facebook.com/RoyalSocietyPublishing.FanPage/ Read Royal Society Publishing's blog: https://blogs.royalsociety.org/publishing/ Reviewer: 1 This is a good, well written paper dealing with the behavioural impacts of a freshwater hydrozoan on a territorial cichlid and on the composition of cichlids assemblages. The main issue with the manuscript is that authors have chosen a highly debatable or wrong context to justify their study, which in my view, they don't need: the study is interesting and valid without establishing a parallel with fish-jellyfish interactions in coastal marine systems. Despite the diversity of studies suggesting that anthropogenic stressors facilitate jellyfish blooms and a global trend of increasing jellyfish blooms, two recent studies (Pitt et al. 2018;Sanz-Martín et al. 2016) have clearly shown that the evidence supporting those claims is weak and mainly based in a couple of cosmopolitan species. These species do not represent the huge diversity life forms included in the term "jellyfish", which depending on the authors may comprise between two (Cnidarians, Ctenophores) and five phyla (Cnidarians, Ctenophores, Molluscs, Polychaetes and Chordata-Salps and Larvaceans). Therefore, I strongly recommend not to use this parallel to justify the study or to discuss the results. > Thank you for the positive thoughts and constructive comments on our manuscript. We hope that we have addressed all of your concerns below.

Specific comments
Title: It is too general; it does not reflect the scope of the study. The manuscript deals with the effect of a freshwater hydrozoan on the behaviour of cichlids in the Tanganyika Lake.

> The title has been changed to better reflect the scope of our study: Freshwater hydrozoan blooms alter activity and behaviour of territorial cichlids in Lake Tanganyika
Introduction: -I suggest either avoid the use of the term "jellyfish" or clearly define it in the context of this study > We now use the term "gelatinous zooplankton" when discussing the literature in general, and we use the term "jellyfish" when specifically referring to L. tanganjicae.
-I suggest, if available, to include information on the abundance seasonality of the L. taganjicae > Unfortunately, there are no data pertaining to the seasonality of L. tanganjicae blooms. Overall, this species has received very little research attention. We now emphasize this point in the text.
L34-63. See general comments and reconsider the need of this section. > Thank you, we have extensively rewritten our introduction and in-so-doing changed our justification and motivation for our study. This version is a more accurate representation of the current state of the field.
L88. Please define -here or in material and methods-how a bloom and non-bloom condition was defined. It was not clear for me if "normal" or "non-bloom" conditions mean no medusas at all or a low density. > We have now added more information on how we differentiated the two conditions. Lines 74-76.

Results:
L243-244. This is an interesting result, suggesting that the medusae population was mainly comprised of juveniles -this, by the way was not discussed in the next section. I am wondering if there is differences between juvenile and adult medusa in prey patterns and interspecific competitive interactions. > We have avoided attempting to classify L. tanganjicae into different life stages (e.g. juvenile vs. adult). However, we raise the interesting point that L. tanganjicae were apparently ignored as a food source despite being of the appropriate prey size for our study fish. Lines 282-283. It would certainly be interesting to look at how fish-jellyfish interactions might differ across size classes of both species, but we were unable to gather these data here for this question.

Discussion:
Generally I suggest avoiding stating again the results in order to discuss about them. The discussion is a bit too long. > We have reduced the overall length of the discussion, while keeping all relevant discussion points.
L 274-280: Please see my general comments. L280. In my view this section should start here > We have removed this first portion of the discussion as it was no longer relevant to our study's justification and motivation. As suggested, we start the discussion at the indicated line. Figures: Figure 2. Please revise the term "affiative" in the X-label > Thank you for catching this typo. It has been corrected. Reviewer: 2 Comments to the Author(s) This is a fascinating study on the impact of a poorly-known jellyfish species on fish behavior. Overall the manuscript is well-written, but reflects several outdated views on jellyfish blooms which, even if accurate, are not comparable to this system. Unlike highly impacted coastal marine areas, the author's study area is an ancient freshwater lake in which the jelly species in question is a native member of the ecosystem. While it is true that jellyfish blooms impact fish behavior in marine systems, framing this study in the context of 'rising jellyfish numbers' and the 'global rise of jellyfish' (claims that are now being questioned), detracts from the fascinating work being done here on an remarkable and unique system. Because the humanassociated causes and consequences of marine blooms likely do not impact this system, and because the field of marine blooms is itself rapidly changing, the authors should re-frame their introduction and discussion to highlight their system's fascinating qualities. The fact that so little is known about freshwater jellyfish ecology, and this species in particular, should be celebrated. Further, I see no reason their findings are not applicable to marine systems, but the language and literature currently cited reflect a "jellyfish as negative" perspective when really these are all native species interacting in a fascinating way. > Thank you for the encouraging review. We have addressed the points that you raise below and think that the changes have made significant improvements to the manuscript.

References
15: Jellyfish are also beneficial to many fish species, so the relationship is complicated and should not be reduced to one of purely threat, but of impact more generally: Unravelling the macro-evolutionary ecology of fish-jellyfish associations. Caution is especially advised for poorly-understood freshwater jellyfish like Limnocnida tanganjicae. Resource: life in the 'gingerbread house' Donal C. Griffin , Chris Harrod , Jonathan D. R. Houghton and Isabella Capellini Published:20 March 2019https://doi.org/10.1098/rspb.2018.2325 > We have rewritten a great deal of our introduction and discussion to downplay the threats of jellyfish blooms and better convey the current state of the field. 23: What is your evidence that Limnocnida tanganjicae have a noticeable sting to fish? > Our evidence is behavioural. We noticed the fish making sudden, quivering or jolting movements during the jellyfish bloom conditions, which we did not observe during the non-bloom conditions. We have now emphasized this in the text (lines 68, 268) and in Table 1. The supplementary video shows an example of a fish jolting in reaction to contact with a jellyfish.