Coprophagy in Caribbean parrotfishes

Parrotfishes are widely considered to be important grazers on coral reefs
that remove autotrophic biomass from the reef substrate and create bare
space that is conducive to larval coral settlement and recruitment.
Because of the top-down effects associated with their benthic foraging,
this has been a major focus of parrotfish research. Another aspect of
parrotfish foraging and trophic ecology that has received very little
attention is coprophagy, the consumption of fecal matter. The feces of
planktivorous fishes, including Chromis spp., have been identified as
important sources of nutrients and trace elements to tropical and
temperate reef ecosystems. Their feces are readily consumed by a variety
of fishes, including parrotfishes. Although parrotfish coprophagy has been
observed in prior studies, its frequency has not yet been quantified. In
this study, we observed foraging in five parrotfishes on the fringing
reefs of Bonaire, Netherlands: Scarus iseri, Scarus taeniopterus, Scarus
vetula, Sparisoma aurofrenatum, and Sparisoma viride. For three of these
species, we observed individuals of both ontogenetic phases (terminal and
initial phase) to investigate ontogenetic differences in foraging. We
found that coprophagy was common in four of these species
(Sc. iseri, Sc. taeniopterus, Sc. vetula,
and Sp. aurofrenatum), occurring in 46-90%
of individuals (Sc. vetula and Sc. taeniopterus,
respectively). Though we did not identify the origin of every
fecal pellet consumed, we directly observed focal fishes targeting fecal
pellets produced by planktivorous Chromis spp. that were often seen
schooling above the reef during this feeding behavior. Additionally, most
of the fecal pellets consumed by the parrotfishes were similar in
appearance (i.e., relative size, shape, coloration, and consistency) to
the feces produced by Chromis spp., predominantly Chromis multilineata,
suggesting this common origin. However, bites on fecal matter were a
relatively small proportion of the total bites taken by these
species (< 5%). In contrast, a majority of bites
taken by these species were taken on substrates classified as
eplithic algal matrix (EAM) or crustose coralline algae (68.5-90.6% of
total bites across all five species). Despite being an infrequent
target of parrotfish foraging, we estimated that daily fecal C consumption
is equivalent to approximately 27% of the daily algal C intake by
parrotfishes targeting the major benthic foraging targets of parrotfishes
(large turfs, small turfs on endolithic algae or crustose coralline algae,
and crustose coralline algae) in Bonaire. The feces of
plantivorous reef fishes like Chromis spp. are also
likely a valuable source of nutrients to reef fishes, because the fecese
of Chromis spp. has higher protein and lipid content and lower
C:N and C:P than many benthic marine algae and cyanobacteria, including
from the tropics. The absence of coprophagy in Sp. viride and
reduced rates of coprophagy in Sc. vetula relative to the other
coprophagic species could be the result of increased access to
protein-rich endolithic components of the benthos. Access to endolithic
components of the benthos increases with body size and the
ability to excavate benthic substrate while foraging. Sparisoma viride is
an important excavating parrotfish on Caribbean coral reefs, and
Sc. vetula is generally larger than the other coprophagic species in our
study. Future work should attempt to further quantify the
contribution of fecal matter to the nutrition of parrotfishes relative to
benthic foraging targets in order to provide a more complete understanding
of parrotfish nutritional ecology and to elucidate the importance of
coprophagy in nutrient recycling and retention on coral reefs.