The slippersnail (Crepidula fornicata), originally from the eastern coast of North America, has invaded European coastlines from Norway to the Mediterranean Sea . This species is capable of achieving incredibly high densities (up to several thousand individuals per square meter) and likely has major impacts on a variety of community- and ecosystem-level processes, including alteration of carbon and nitrogen fluxes and competition with native suspension feeders .
Given this potential for competition, it is important to understand the diet of C. fornicata and its potential overlap with native species. However, previous research on the diet of C. fornicata and related species suggests that the types of food consumed may change with age [3, 4]. This species has an unusual reproductive strategy. It is a sequential hermaphrodite, which begins life as a somewhat mobile male but eventually slows down to become sessile. Sessile individuals form stacks of up to 10 or more individuals, with larger individuals on the bottom of the stack, and decreasingly smaller individuals piled on top. Snails at the bottom of the stack are female, whereas snails at the top of the stack are male; when the females die, the largest males become female . Thus, understanding these potential ontogenetic dietary shifts has implications for both intraspecific (juvenile vs. male vs. female) and interspecific competition associated with an abundant, invasive species.
To this end, Androuin and colleagues evaluated the stable-isotope (d13C and d15N) and fatty-acid profiles of food sources and different life-history stages of C. fornicata . Based on previous work highlighting the potential for life-history changes in the diet of this species [3,4], they hypothesized that C. fornicata would shift its diet as it aged and predicted that this shift would be reflected in changes in its stable-isotope and fatty-acid profiles. The authors found that potential food sources (biofilm, suspended particulate organic matter, and superficial sedimentary organic matter) differed substantially in both stable-isotope and fatty-acid signatures. However, whereas fatty-acid profiles changed substantially with age, there was no shift in the stable-isotope signatures. Because stable-isotope differences between food sources were not reflected in differences between life-history stages, the authors conservatively concluded that there was insufficient evidence for a diet shift with age. The ontogenetic shifts in fatty-acid profiles were intriguing, but the authors suggested that these reflected age-related physiological changes rather than changes in diet.
The authors’ work highlights the need to consider potential changes in the roles of invasive species with age, especially when evaluating interactions with native species. In this case, C. fornicata consumed a variety of food sources, including both benthic and particulate organic matter, regardless of age. The carbon stable-isotope signature of C. fornicata overlaps with those of several native suspension- and deposit-feeding species in the region , suggesting the possibility of resource competition, especially given the high abundances of this invader. This contribution demonstrates the potential difficulty of characterizing the impacts of an abundant invasive species with a complex life-history strategy. Like many invasive species, C. fornicata appears to be a dietary generalist, which likely contributes to its success in establishing and thriving in a variety of locations .
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In this contribution, the authors use a combination of techniques, including fatty acid profiles and stable isotope analyses, to evaluate potential ontogenetic changes in the diet of invasive slipper snails, Crepidula fornicata. These molluscs change sex (from male to female), habit (from more mobile to more sessile), and diet (from some grazing to all suspension feeding) as they age, and the authors were interested in whether these changes would be reflected in the species' diet. In brief, the authors found little evidence for dietary shifts with ontogeny. However, they did reveal changes associated with season and age, independent of diet. The reviewers were split in their assessments: one was very positive, whereas the other had a number of concerns. However, even the more critical reviewer indicated that this work has the potential to make a contribution to the literature. I am therefore requesting a revision that addresses the many suggestions that both reviewers provide. I concur with the more critical reviewer that there is a lot of information here that is difficult to sort through. I suggest that, in addition to carefully addressing the reviewers' comments, the authors employ a hypothesis-testing framework to structure the manuscript: hypotheses and predictions in the introduction, structured methods and results to evaluate those predictions and assess support for the hypotheses, and a discussion that begins by specifically evaluating whether those predictions were borne out in the data and the hypotheses supported. If the authors choose to submit a revision, I request that they provide a description of their edits, including a careful comment-by-comment accounting of the reviewers' suggestions and the changes that they have made in response. **Additional requirements of the managing board**: As indicated in the 'How does it work?’ section and in the code of conduct, please make sure that: -Data are available to readers, either in the text or through an open data repository such as Zenodo (free), Dryad or some other institutional repository. Data must be reusable, thus metadata or accompanying text must carefully describe the data. -Details on quantitative analyses (e.g., data treatment and statistical scripts in R, bioinformatic pipeline scripts, etc.) and details concerning simulations (scripts, codes) are available to readers in the text, as appendices, or through an open data repository, such as Zenodo, Dryad or some other institutional repository. The scripts or codes must be carefully described so that they can be reused. -Details on experimental procedures are available to readers in the text or as appendices. -Authors have no financial conflict of interest relating to the article. The article must contain a "Conflict of interest disclosure" paragraph before the reference section containing this sentence: "The authors of this preprint declare that they have no financial conflict of interest with the content of this article." If appropriate, this disclosure may be completed by a sentence indicating that some of the authors are PCI recommenders: “XXX is one of the PCI XXX recommenders.”