Investigators working in both terrestrial and marine systems hypothesize that prey defenses are better developed in tropical than in temperate habitats because tropical consumers are more active and diverse. To investigate the hypothesis that chemical defenses are more potent in tropical than in temperate seaweeds, we conducted 38 feeding assays involving seven genera of algae. We offered tropical and temperate sea urchins the choice of a temperate alga from North Carolina or a closely related tropical alga from the Bahamas. Plants were freeze-dried, ground to a fine powder, embedded in agar, and presented in congeneric pairs (North Carolina species vs. Bahamian species), or same-species pairs, to the temperate urchin Arbacia punctulata and the tropical urchin Lytechinus variegatus. For the genera we tested (the red alga Amphiroa, the green alga Udotea, and the brown algae Dictyopteris, Dictyota, Lobophora, Padina, and Sargassum), we documented a general pattern in which the mean amount of North Carolina seaweeds eaten was approximately twice that of their Bahamian congeners. Dictyota was an exception to the overall pattern, in that mean consumption of temperate and tropical species was equivalent. In addition to showing a latitudinal difference in seaweed palatability, we found that some species exhibited significant, between-population variation in herbivore resistance on a local scale. The most notable example was Dictyota menstrualis from North Carolina. When collected from a shallow inshore site, this species was significantly more palatable than tropical species of Dictyota in four of six contrasts. When collected from a deeper offshore site, it was significantly less palatable than the tropical species in four of six contrasts. Thus, for this species, the between-population variance in palatability within a geographic region (North Carolina) bracketed the variance in palatability that we found among species and between regions. Feeding assays with both temperate and tropical urchins produced similar results, indicating that choices were made based on fundamental characteristics of the algae rather than on greater familiarity with local prey species. To see if chemical defenses could explain the differences in herbivore resistance that we noted using freeze-dried algae, we tested the effects of lipid-soluble and water-soluble extracts from each alga on urchin feeding by incorporating extracts into a standard palatable food. We also measured protein content and percent ash-free dry mass of the seaweeds in order to correlate urchin feeding preferences with these measures of food quality. Lipophilic chemical extracts explained urchin preferences in 9 of 15 (60%) North Carolina-Bahamas contrasts. Water-soluble extracts, protein content, and percent ash-free dry mass generally did not account for feeding patterns seen in the original assays. Our results support the general hypothesis that tropical seaweeds have stronger chemical defenses than temperate seaweeds. For some genera, however, smaller-scale local variability in herbivore resistance within a species can be as striking as overall latitudinal differences.
Corals and reef fish choose nice homes Young animals tend to disperse into new habitats. Can we use populations in protected areas to colonize nearby recovering or overused habitats? It seems that for corals and reef fish, the answer may be no. Dixson et al. show that dispersing juvenile corals and reef fish were overwhelmingly attracted to healthy reefs but were repelled by seaweeds that colonize degraded reefs (see the Perspective by Bruno). Thus, even species that appear passive in their choice of habitat may have stronger preferences than we thought. Science , this issue p. 892 ; see also p. 879
The tropical green seaweed Avrainvillea longicaulis is a low preference food for coral‐reef fishes, and it produces a brominated diphenylmethane derivative called avrainvilleol that significantly deters feeding by reef fishes in field bioassays. In contrast to the pattern for fishes, the ascoglossan gastropod Costasiella ocellifera and the crab Thersandrus compressus live on and eat only Avrainvillea. The gastropod sequesters avrainvilleol from its algal host and uses this compound as an effective defense against predatory fish. The crab does not sequester chemical defenses; however, it is camouflaged when on Avrainvillea and thus also experiences less predation when associated with this alga. Specialization on this chemically defended seaweed allows Costasiella to deter and Thersandrus to avoid predation. When coupled with other recent studies of specialist marine herbivores, these findings suggest that predator avoidance and deterrence are major advantages associated with the evolution of feeding specialization among herbivorous marine invertebrates.
Chemical defenses against consumers have been hypothesized to be common among marine macro‐holoplankton, but few studies have assessed macro‐holoplankton susceptibility to predators or the traits affecting palatability. We used generalist fishes to determine the palatability of fresh tissues, freeze‐dried homogenates, and chemical extracts from 19 species of macro‐holoplankton. Fishes rejected fresh tissues of all the cnidarians, ctenophores, and cyanobacteria we examined but consumed salp and chaetognath tissues. In contrast, fishes consumed homogenates and chemical extracts of all macro‐holoplankton except for the cyanobacterium Trichodesmium sp. We examined nematocysts and low nutritional quality as mechanisms causing rejection of fresh tissues. Once nematocysts were deactivated, fishes consumed cnidarian tentacles, indicating that nematocysts served as defenses. The nutritional quality of macro‐holoplankton varied almost 500‐fold among species and was strongly bimodal, with most macroholoplankton species having ≪0.7 mg soluble protein ml -1 or ≫7 mg ml -1 . In laboratory assays, there was a significant positive relationship between the nutritional quality of artificial foods and their acceptability to fishes. In field assays, reef fishes avoided experimental foods that had a protein content similar to low‐quality macroholoplankton but fed rapidly on higher quality foods. Furthermore, macro‐holoplankton that were high in protein content possessed defensive traits that low‐protein species lacked. Although fresh tissues of most macro‐holoplankton were rejected by generalist fishes, we found evidence of chemical defense only in a cyanobacterium. Thus, chemical defenses were rare among macro‐holoplankton, and rejection for ≫90% of the species we assessed was due to nematocysts or low nutritional quality.
Abstract Seaweeds produce a variety of secondary metabolites that deter feeding by herbivores. The defensive value of a compound is, however, a specific function of the compound’s structure and the species of herbivore attacking the plant; previous generalizations about the effectiveness and mode of action of different classes of compounds (e.g., qualitative vs quantitative chemical defences) are unfounded.
Inundations of pelagic Sargassum plague the tropical Atlantic, with size and impacts steadily increasing to surpass 30 million tons in 2025. Understanding the drivers of Sargassum growth in the so-called Great Atlantic Sargassum Belt is fundamental to developing effective mitigation strategies for affected nations. We present a nonlinear regression model that both explains the seasonal and interannual variability observed between 2011 and 2022 and predicts Sargassum concentrations in 2023 and 2024. The growth of Sargassum, initiated by a prolonged negative phase of the North Atlantic Oscillation, is initially enhanced through winter mixed layer deepening in response to stronger winds. An additional overlooked driver is the recycling of nutrients within the mixed layer, carried out by the community of organisms associated with Sargassum and aging Sargassum mats. This contribution increases over time to become dominant in recent years, offsetting the increase in stratification in 2023 and 2024.
An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
