Genetic Evidence for Indo-Western Pacific Olive Ridley Sea Turtles in Mexican Waters
, Luis M. Enríquez-Paredes 3, David Petatán-Ramírez 4, Alejandra García-Gasca 5 and Sonia I. Quijano-Scheggia 6
Round 1
Reviewer 1 Report
It was very interesting to read the manuscript entitled Genetic evidence for Indo-Western Pacific olive ridley sea turtles in Mexican waters, and learn about the connectivity of olive ridleys in the Pacific. The authors conducted several surveys both closer and further offshore of Central Mexican Pacific and obtained a large sample size of 85 olive ridley turtles from different size-classes and sexes (sex identified in mature individuals) from which they characterized the mtDNA control region haplotypes to infer on their origin. They found 17 mtDNA control region haplotypes, one of which suggesting transoceanic transport from the Indo-Pacific to the East Pacific, adding to our understanding on the movements of this species, and three novel haplotypes, increasing the knowledge on the genetic diversity of olive ridleys. No structure was found between Pacific populations, supporting the panmixia reported in previous works. Additionally, this study assessed drifter buoys data to further contextualize the movements of olive ridleys, and the authors suggest that ocean currents play a key role in the movements of olive ridleys.
General comments:
This is a well-written (some minor revision of sentences is needed) and well-conducted study, very relevant by providing a robust sample along to the genetic characterization of important foraging aggregations and revealing novel inter-oceanic connections for a species understudied in its in-water habitats.
I think that authors should reinforce in the discussion the need for more in-water surveys and sampling.
Specific comments:
Introduction:
“Clearly, the pattern of oceanic currents plays a critical role for successful transoceanic transport; especially the NECC, since the NPC is too cold for green turtles [16,18].” – since the examples given in the preceding paragraph are of different species, and the study species is the olive Ridley, statement could be more general, not specific for green turtles. Several studies report on threshold temperatures for hard-shelled sea turtles, I suggest citing one to make the statement overarching. Also, this paragraph has a lot of details on oceanic currents that could be reduced.
“, the paucity of in-water genetic studies on olive ridley has a lack of knowledge of what happens to this species.” – revise the second half of this sentence. Perhaps best to rephrase to “the paucity of in-water genetic studies on olive ridley turtles greatly limits our knowledge on the connectivity of this species.”
Replace “but they were samplings from bycatch” with “but these used samples of bycatch turtles”
‘Spatiotemporal’ instead of spatial-temporal distribution’?
“…to infer the foraging ecology of the olive ridley turtle from the Central Mexican Pacific.” – olive ridley turtles should be plural here, as the authors are referring to turtles from a specific population. Singular only if referring to the species.
Suggestion: use “In this study” instead of “In this part of the project” for clarity.
Delete ‘from this species’, this is clear the start of the sentence.
Methods:
“Colima and Jalisco states where major olive ridley nesting take place (Figure 1).” – should be where nesting takes place.
Most turtles caught were those with a dry carapace which were easier to capture. – can the authors explain this better? I work with green and hawksbill turtles, I don’t really understand here what it means to have a dry carapace (turtles at the water surface?) and why this is an advantage for captures. And how were the turtles captured? Entanglement net? By hand? Rodeo? Please clarify.
A total of 85 turtles were captured and measured for CCL and TTL.
“Individuals with CCL <59 cm were assigned as immatures as it was impossible to infer sex from morphological characters.” – I understand that the authors are saying that immatures were not separated by sex as at that size males and females are not distinguishable, but as it is written it says that they were assigned as immatures because it was impossible to infer sex. But they were assigned to immatures due to their size. Edit to make clear, e.g., Individuals with CCL <59 cm were assigned as immatures and not separated by sex, as it was impossible to infer sex from morphological characters.
Please state that the biopsy was collected from each captured individual (if this was the case).
Discussion:
As expected, levels of genetic diversity from these aggregations closely resemble those previously reported for nesting beaches at the Eastern Pacific [7,9,38-39,43] (Supplementary File 2). –I would expect the foraging area to have higher genetic diversity than the rookeries, if it was a mixed stock, as is typical among sea turtles.
For olive ridleys, there is a marked structure on a larger geographic scale, differences between regions but not within regions [8]. – can the authors elaborate here, which regions? Ocean basins?
“…and is a region potentially related to foraging for the species [54-55].” – I do not understand what the authors mean here. They captured foraging olive ridleys, so the region is definitely “related to foraging for the species”. I suggest re-writing for clarity.
Author Response
It was very interesting to read the manuscript entitled Genetic evidence for Indo-Western Pacific olive ridley sea turtles in Mexican waters, and learn about the connectivity of olive ridleys in the Pacific. The authors conducted several surveys both closer and further offshore of Central Mexican Pacific and obtained a large sample size of 85 olive ridley turtles from different size-classes and sexes (sex identified in mature individuals) from which they characterized the mtDNA control region haplotypes to infer on their origin. They found 17 mtDNA control region haplotypes, one of which suggesting transoceanic transport from the Indo-Pacific to the East Pacific, adding to our understanding on the movements of this species, and three novel haplotypes, increasing the knowledge on the genetic diversity of olive ridleys. No structure was found between Pacific populations, supporting the panmixia reported in previous works. Additionally, this study assessed drifter buoys data to further contextualize the movements of olive ridleys, and the authors suggest that ocean currents play a key role in the movements of olive ridleys.
We very much appreciate all the reviewer’s comments and suggestions to help improve this manuscript. Specific changes suggested by the reviewer are highlighted in yellow to facilitate the review, however the whole document (mostly the Discussion section) suffered extensive revision.
General comments:
This is a well-written (some minor revision of sentences is needed) and well-conducted study, very relevant by providing a robust sample along to the genetic characterization of important foraging aggregations and revealing novel inter-oceanic connections for a species understudied in its in-water habitats.
I think that authors should reinforce in the discussion the need for more in-water surveys and sampling.
Thank you for your suggestion. The need for more in-water surveys and increased sampling effort was included in Discussion:
“With such a scanty presence of a Western Pacific olive ridley in a sample size of 85 in the present study, it is statistically impossible to reach robust conclusions on the incidence and spatial distribution of this lineage in Eastern Pacific habitats, which remain critical, unanswered questions. Finding this turtle in the Central Mexican Pacific does highlight this zone as a potential migratory and foraging area. Nevertheless, considering the latitudinal range of the trans-oceanic corridor (the NECC), it is likely that the major zone where Western Pacific olive ridleys would be found is further South. The results presented here should encourage additional studies in offshore habitats and the use of molecular markers to verify our findings. Also, stored samples from previous studies pursuing other issues (e.g., foraging ecology) [54] could be re-assayed with molecular markers to evaluate the possible presence of Western Pacific olive ridleys.”
Introduction
“Clearly, the pattern of oceanic currents plays a critical role for successful transoceanic transport; especially the NECC, since the NPC is too cold for green turtles [16,18].” – since the examples given in the preceding paragraph are of different species, and the study species is the olive Ridley, statement could be more general, not specific for green turtles. Several studies report on threshold temperatures for hard-shelled sea turtles, I suggest citing one to make the statement overarching. Also, this paragraph has a lot of details on oceanic currents that could be reduced.
The first sentence of this paragraph was changed referring to sea turtles in general and not to green sea turtles according to the reviewer’s suggestion.
The study by Davenport, 1997 (18) has been cited referring to the threshold of 20°C sea surface isotherm for tropical and subtropical sea turtles (green, hawksbill, flatback and olive ridley sea turtles).
Oceanic current details were reduced.
the paucity of in-water genetic studies on olive ridley has a lack of knowledge of what happens to this species.” – revise the second half of this sentence. Perhaps best to rephrase to “the paucity of in-water genetic studies on olive ridley turtles greatly limits our knowledge on the connectivity of this species.”
The suggestion was attended.
Replace “but they were samplings from bycatch” with “but these used samples of bycatch turtles”
This sentence was restructured.
‘Spatiotemporal’ instead of spatial-temporal distribution’?
The suggestion was attended.
“…to infer the foraging ecology of the olive ridley turtle from the Central Mexican Pacific.” – olive ridley turtles should be plural here, as the authors are referring to turtles from a specific population. Singular only if referring to the species.
The suggestion was attended.
Suggestion: use “In this study” instead of “In this part of the project” for clarity.
The suggestion was attended.
Delete ‘from this species’, this is clear the start of the sentence.
The suggestion was attended.
Methods
“Colima and Jalisco states where major olive ridley nesting take place (Figure 1).” – should be where nesting takes place.
It was attended, thank you.
Most turtles caught were those with a dry carapace which were easier to capture. – can the authors explain this better? I work with green and hawksbill turtles, I don’t really understand here what it means to have a dry carapace (turtles at the water surface?) and why this is an advantage for captures. And how were the turtles captured? Entanglement net? By hand? Rodeo? Please clarify.
Information was clarified, thank you for the observation.
A total of 85 turtles were captured and measured for CCL and TTL.
“Individuals with CCL <59 cm were assigned as immatures as it was impossible to infer sex from morphological characters.” – I understand that the authors are saying that immatures were not separated by sex as at that size males and females are not distinguishable, but as it is written it says that they were assigned as immatures because it was impossible to infer sex. But they were assigned to immatures due to their size. Edit to make clear, e.g., Individuals with CCL <59 cm were assigned as immatures and not separated by sex, as it was impossible to infer sex from morphological characters.
The suggestion was attended.
Please state that the biopsy was collected from each captured individual (if this was the case).
Information was clarified, thank you.
Discussion
As expected, levels of genetic diversity from these aggregations closely resemble those previously reported for nesting beaches at the Eastern Pacific [7,9,38-39,43] (Supplementary File 2). –I would expect the foraging area to have higher genetic diversity than the rookeries, if it was a mixed stock, as is typical among sea turtles.
Please notice that the Discussion suffered several modifications.
The reviewer is right; in our study we found the highest number of haplotypes with respect to other genetic studies (discussed later). However, the genetic diversity index (haplotypic) was very similar to those reported on nesting beaches, since both in our study and on nesting beaches, there is a dominant haplotype and the rest are found at low frequencies, which is attributed to panmixia of this species in the Eastern Pacific.
A higher number of haplotypes in the foraging areas compared to rookeries is indeed evidence of a mixed stock.
For olive ridleys, there is a marked structure on a larger geographic scale, differences between regions but not within regions [8]. – can the authors elaborate here, which regions? Ocean basins?
The information was included as suggested by the reviewer.
“…and is a region potentially related to foraging for the species [54-55].” – I do not understand what the authors mean here. They captured foraging olive ridleys, so the region is definitely “related to foraging for the species”. I suggest re-writing for clarity.
The sentence was rewritten as suggested.
Due to the peculiar migratory behavior of the olive ridley turtle, without defined patterns or migratory corridors, our study area could be a foraging area or a simple migratory corridor where they feed if there is food availability.
Reviewer 2 Report
The paper has improved even if I remain of the idea (but this is only my opinion) that it is a little difficult to be able to numerically quantify the number of individuals present in an open environment. Finally, I don't want to open a discussion on statistical analysis which, although sufficient, I don't consider particularly adequate (but this is only my opinion).
I believe, in general, that the goal of a reviewer is to make suggestions for improvement of the paper and not to assert one's beliefs. For this reason the paper can be accepted.
Author Response
The paper has improved even if I remain of the idea (but this is only my opinion) that it is a little difficult to be able to numerically quantify the number of individuals present in an open environment. Finally, I don't want to open a discussion on statistical analysis which, although sufficient, I don't consider particularly adequate (but this is only my opinion).
I believe, in general, that the goal of a reviewer is to make suggestions for improvement of the paper and not to assert one's beliefs. For this reason the paper can be accepted.
We thank the reviewer for their observations of the document. Specific changes suggested by the reviewer are highlighted in yellow to facilitate the review, however the whole document (mostly the Discussion section) suffered extensive revision.
The reviewer is right. We cannot apply a more rigorous analysis based on the appearance of one event in the whole sample. A larger sampling effort is necessary to study olive ridley aggregations from Mexico to South America. We added this information to the Discussion:
“With such a scanty presence of a Western Pacific olive ridley in a sample size of 85 in the present study, it is statistically impossible to reach robust conclusions on the occurrence of this lineage in Eastern Pacific habitats, which will remain as a critical unanswered question. Finding this turtle in the Central Mexican Pacific does highlight this zone as a potential migratory and foraging area [54]. Nevertheless, considering the latitudinal range of the potential trans-oceanic corridor (the NECC), it is most likely that the main zone where Western Pacific olive ridleys would be found is further south. The results presented here should encourage future surveys in offshore habitats (e.g., [55]) and the use of additional molecular markers to verify our findings. Also, stored samples from previous studies pursuing other issues (e.g., foraging ecology) [56] could be re-assayed with molecular markers to evaluate the possible presence of Western Pacific olive ridleys.”
Reviewer 3 Report
The manuscript Genetic evidence for Indo-Western Pacific olive ridley sea tur-
tles in Mexican waters by Rodolfo Martín-del-Campo et al. analyzes the genetic structure of an assemblage of olive ridley turtles caught in the Pacific Mexican waters.
This paper is interesting because olive ridleys, although being the most numerous marine turtle species, is poorly known at sea.
The main finding is that haplotype Lo37 showed close evolutionary relationships with the lineage of the East Coast of India. It proves a connection between East and West Pacific, probably as a result of passive drift. It should be noted however that only one individual exhibits the East Coast of India haplotype. It indicates that when the sampling effort is large (n=85 here), new pattern will emerge. Our current knowledge is generally based on a too small sample size to be certain about the conclusions.
It is true for at sea distribution but also for the “migratory corridors” (page 2) that are often based on a so small sample size that they should not be called corridor! Perhaps the authors could discuss that point: before to generalize a pattern, it is important to have good sample size (true for also for migration)!
The individual sharing the haplotype Lo37 should be described more precisely: sex? Size.
Minor changes:
First line of paragraph 2.1: km2 must be on the same line than the value.
Paragraph 3.2: What about sex difference?
Author Response
The manuscript Genetic evidence for Indo-Western Pacific olive ridley sea turtles in Mexican waters by Rodolfo Martín-del-Campo et al. analyzes the genetic structure of an assemblage of olive ridley turtles caught in the Pacific Mexican waters.
This paper is interesting because olive ridleys, although being the most numerous marine turtle species, is poorly known at sea.
The main finding is that haplotype Lo37 showed close evolutionary relationships with the lineage of the East Coast of India. It proves a connection between East and West Pacific, probably as a result of passive drift. It should be noted however that only one individual exhibits the East Coast of India haplotype. It indicates that when the sampling effort is large (n=85 here), new pattern will emerge. Our current knowledge is generally based on a too small sample size to be certain about the conclusions.
It is true for at sea distribution but also for the “migratory corridors” (page 2) that are often based on a so small sample size that they should not be called corridor!
We very much appreciate all the reviewer’s comments and suggestions to help improve this manuscript. Specific changes suggested by the reviewer are highlighted in yellow to facilitate the review, however the whole document (mostly the Discussion section) suffered extensive revision.
The reviewer is right. The need for more in-water surveys and increased sampling effort was included in Discussion:
“With such a scanty presence of a Western Pacific olive ridley in a sample size of 85 in the present study, it is statistically impossible to reach robust conclusions on the occurrence of this lineage in Eastern Pacific habitats, which will remain as a critical unanswered question. Finding this turtle in the Central Mexican Pacific does highlight this zone as a potential migratory and foraging area [54]. Nevertheless, considering the latitudinal range of the potential trans-oceanic corridor (the NECC), it is most likely that the main zone where Western Pacific olive ridleys would be found is further south. The results presented here should encourage future surveys in offshore habitats (e.g., [55]) and the use of additional molecular markers to verify our findings. Also, stored samples from previous studies pursuing other issues (e.g., foraging ecology) [56] could be re-assayed with molecular markers to evaluate the possible presence of Western Pacific olive ridleys.”
Perhaps the authors could discuss that point: before to generalize a pattern, it is important to have good sample size (true for also for migration)!
Thank you for the suggestion. The point was included in Discussion. Please notice that the Discussion suffered several modifications.
The individual sharing the haplotype Lo37 should be described more precisely: sex? Size.
The description was included.
Minor changes:
First line of paragraph 2.1: km2 must be on the same line than the value.
The suggestion was attended. Thank you.
Paragraph 3.2: What about sex difference?
It was clarified, maturity stages include adult males and females, and immature.
