Exposure to moderately elevated temperatures changes food preferences in the tropical marine herbivore Haliotis squamata

Ocean warming is affecting marine ectothermic herbivores as well as the macroalgal species they consume and this has the potential to alter their trophic interaction. However, it is currently still unknown how these two important components of benthic food webs will react to a warming environment. Consumption rates of grazers change with increasing temperatures, but it is unclear whether this is also true for feeding preferences. In this study, multiple-choice feeding assays with the tropical abalone Haliotis squamata from Western Indonesia were conducted in August 2018. After brief acclimation of either the grazer or the macroalgae to moderately elevated water temperatures (maximum 2 °C above the long-term average) in the laboratory, three species of living macroalgae were simultaneously offered to the abalone in feeding assays. Consumption rates of H. squamata were lower under elevated water temperatures, while its feeding preference switched: At 27 °C (2 °C below long-term average), abalone preferred non-acclimated Gracilaria salicornia, but switched to non-acclimated Amphiroa spp. at 31 °C. Interestingly, no such switch in preference occurred when the macroalgae, but not the grazers were acclimated. This indicates that the grazer will presumably be the driver of this potential change in interactions between H. squamata and its macroalgal food. Ocean warming may result in changes in the structure of benthic communities, mediated by changes in the feeding behaviour of herbivorous invertebrates.

Intraspecific competition in size-structured populations: ontogenetic shift in the importance of interference competition in a key marine herbivore

Individuals rarely have equal competitive abilities, with body size being one of the most important attributes affecting the mechanism (i.e. exploitative and interference) and consequences of competition. Competitive interactions within size-structured populations are complex and can have major implications for population dynamics, community structure and evolutionary processes. Destructive grazing of kelp beds by the green urchin Strongylocentrotus droebachiensis creates barrens where high-quality food is scarce and intraspecific competition may have an important role in structuring populations. In this study, we experimentally identified the mechanisms underlying size-asymmetric competition between small, medium, and large size classes of the green urchin. A field-based mesocosm experiment showed that small and medium sea urchins grew less and produced smaller gonads when competing for food with large conspecifics. Surprisingly, when food was provided ad libitum but large urchins were present, small individuals’ growth and foraging behavior were reduced, providing strong evidence for interference competition between small and large sea urchins. Interactions between medium and large sea urchins were, however, more influenced by exploitative competition, suggesting that sea urchins shift ontogenetically from a situation of intense interference competition to one dominated by exploitative competition. The size structure of the population can thus determine the relative importance of interference and exploitative competition. In turn, the importance of interference competition may influence size structure by inhibiting the growth of smaller urchins, a pattern consistent with the prediction of theoretical models. The consideration of size-asymmetric competitive interactions can lead to a better understanding of population size structure and dynamics.

Macroalgal resource use differences across age and size classes in the dominant temperate herbivorous fish Aplodactylus lophodon (Aplodactylidae)

Herbivorous fishes comprise a substantial proportion of temperate fish communities, although there is little understanding of their trophic resource use and whether this changes throughout post-settlement ontogeny. With increasing loss of macroalgal forests, understanding how temperate fishes use macroalgae will be vital in predicting future effects on temperate fish biodiversity. The Australian rock cale (Aplodactylus lophodon) is one of the most abundant herbivorous fish inhabiting shallow temperate south-eastern Australian reefs. We examined gastrointestinal contents throughout ontogeny and demonstrated that this species maintains a herbivorous diet through all life stages. Differences in algal taxa consumed were apparent through ontogeny, with the juvenile diet dominated by filamentous red and green algae and the adult diet dominated by brown and calcareous red algae. Relative gut length increased through ontogeny, potentially facilitating dietary transition to less digestible algae, but no concurrent increase in jaw power was observed. The results highlight the diversity of trophic resource use in a temperate marine herbivore, but the near-complete dominance of dietary algae throughout ontogeny indicates the reliance on primary producers across all life stages. Given the importance of fucoid resources in the adult diet, any loss of macroalgal forests within south-eastern Australia may affect foraging success and persistence.

Nutrition of marine mesograzers: integrating feeding behavior, nutrient intake and performance of an herbivorous amphipod

Consumers can regulate the acquisition and use of nutrients through behavioral and physiological mechanisms. Here, we present an experimental approach that simultaneously integrates multiple nutritional traits, feeding assays, and juvenile performance to assess whether a marine herbivore (the amphipod Ampithoe valida) regulates the intake of elements (carbon and nitrogen), macronutrients (protein and non-protein) or both when offered freeze-dried tissues of seaweeds varying in nutritional content. We assessed behavioral regulation of nutrients in three ways. First, during no-choice assays, we found that amphipods ingested similar amounts of carbon, but not nitrogen, non-protein and protein, across algal diets. Second, herbivore intake rates of carbon, protein and non-protein components across no-choice assays was similar to intake rates when offered a choice of foods. Third, variation in intake rates of carbon and non-protein components among algal diets was significantly greater than was tissue content of these components, while variation in intake rates of nitrogen was significantly lower; differences in protein intake variation was equivocal. While these analytical approaches are not uniformly consistent, carbon and nitrogen seem to emerge as the nutrient components that are more strongly regulated by A. valida. Juveniles reared on single diets shown patterns of survivorship, growth and reproduction that could not be predicted by these feeding preferences, nor nutrient content. We conclude that an integrative approach that considers the intake of multiple nutrients potentially yields insights into feeding behavior and its performance consequences.