DOI: 10.5061/dryad.g1jwstqq1
title: A global meta-analysis of temperature effects on marine fishes’ digestion across trophic groups

description: Aim: The temperature constraint hypothesis proposes that marine
 herbivorous fishes are rare at high latitudes relative to carnivorous
 fishes because low temperatures impair the digestion of plant material. To
 test this hypothesis, we compared the effects of temperature on the
 digestive performance and investment of marine fishes across trophic
 groups. Location: Global marine ecosystems. Major Taxa
 Studied: Marine fishes. Methods: We analyzed data from
 304 species consuming a range of diets to quantify the effects of
 temperature on three indicators of digestive performance and investment:
 gut passage time, absorption efficiency, and gut length.
 Results: Decreasing temperatures increase gut passage time in
 fishes consuming macroalgae more than fishes consuming other fish or
 invertebrates. Low temperatures do not impair absorption efficiency in
 fishes regardless of diet, but herbivores have lower absorption
 efficiencies than carnivores overall. Gut length decreases with decreasing
 temperature in all trophic groups. Main Conclusions: Our analyses
 reveal limited evidence to support the temperature constraint
 hypothesis.  Low temperatures slow digestion more in fishes
 consuming macroalgae than those consuming animal prey; however, this may
 not reflect a meaningful disadvantage for herbivores but rather could be
 explained by greater representation of fishes relying on microbial
 fermentation at high latitudes.  Herbivorous fishes absorb
 nutrients and energy from their food in similar proportions regardless of
 temperature, contrary to the expectations of the temperature constraint
 hypothesis.  Decreased gut length was associated with decreasing
 temperature across all trophic groups, likely due to improved food quality
 at high latitudes, which should benefit all trophic groups by reducing
 their required investment in gut tissues. Altogether, our findings run
 counter to the general hypothesis that low temperatures disadvantage the
 digestion of plant material and suppress the diversity and abundance of
 herbivorous fishes at high latitudes.
description: <b>Literature search
 and data collection</b> <i>Eligibility
 criteria</i> For an estimate of gut passage
 time, absorption efficiency or gut length to be included in our
 meta-analysis it had to have the following characteristics:
 Taken from marine or estuarine fish
 species Taken from non-larval individuals
 Not taken from species that are strictly detritivorous or
 corallivorous Not taken from individuals fed an
 artificial diet with optimized composition for nutrient uptake/digestion
 (i.e., as found in many aquaculture studies) Not
 taken from individuals force-fed food items not typically consumed in the
 wild and In
 the case of gut length, reported some metric of body length or
 size. We note that there exists a large
 body of literature on the “gastric evacuation rates” (the rate at which
 food leaves the stomach and enters the hindgut) of carnivores, which was
 developed as a method for indirectly estimating feeding rates by
 quantifying the minimum amount of time required for stomach clearance
 between meals (Bromley, 1994).  We chose not to include this data in our
 analysis because: a) almost no data on gastric evacuation rates exists for
 herbivores so we could not make valid comparisons between trophic groups,
 and b) gastric evacuation rates have been estimated using a wide range of
 methods and models, and it is unclear under what circumstances comparisons
 between such models is meaningful (Bromley, 1994).  Gut passage time has
 been quantified for both herbivores and carnivores and is a more easily
 comparable metric. <i>Information sources and
 search</i> We searched Web of Science,
 JSTOR, and Google Scholar.  We also included data that were found in
 relevant review papers and in publications that cited, or were cited in,
 the papers discovered in our database search, as well as relevant papers
 found incidentally.  Altogether, we included papers published between 1958
 and March 2020.  All search term combinations included marine and fish, as
 well as the following combinations of additional search terms (but not all
 search term combinations were used for all databases): assim*,
 assimilation eff*, absorption, absorption eff*, “absorption efficiency”,
 “assimilation efficiency”, excret*, egest*, conversion, “gut length”, “gut
 morphology”, gut passage, gut transit, “gut passage time”, “gut transit
 time”, digest*,  digest* + temperature.  We initially excluded studies if
 their titles or abstracts were not relevant, then checked the remaining
 studies for data that met the eligibility criteria described above.  By
 the end of this process, 99 studies were eligible for inclusion in the
 meta-analysis.               <i>Data collection
 </i> Data
 were extracted from text, tables, and figures in the accepted studies.  We
 collected temperature and diet as predictor variables, as well as body
 length, since many biological traits scale with size (Brown et al.,
 2004).  However, metrics of body size in reported in collected studies
 varied widely.  Metrics included standard length, total length, and fork
 length, these were further divided into average values, median values,
 maximum/minimum values, or no reported values.  Whenever possible, we used
 the average body size; if this was not provided, we used the mid-range
 (maximum – minimum / 2) body size provided.  We did not distinguish
 between standard, total and fork length.  If fish mass but not length was
 reported, we used published length-weight relationships for the species in
 question to calculate fish length.  These instances are noted in the raw
 data. Ambient
 temperature was likewise not always included in all the studies included
 in our dataset, particularly for estimates of gut length. We supplemented
 these datasets by extracting the date and location of data collection from
 the study, and used that data to find the local sea surface temperature
 (SST) at the time of sampling from the COBE SST data provided by
 NOAA/OAR/ESRL PSD via http://www.esrl.noaa.gov/psd/. On the rare occasion that the specific collection period was not included in the publication, we took the average temperature of the location for five years preceding publication of the data. These instances are noted in the data.
description: See README.txt.