Publications

Keeping Coral Clean Seaweed overgrowth is a major problem for coral reefs and also seems to be a consequence of excessive harvesting of herbivorous fish. Dixson and Hay (p. 804 ) examined this effect on Fijian reefs. Species of small herbivorous gobies and coral-associated damselfish were compared for their effect on the toxic Chlorodesmis seaweed in experiments that required caging colonies of the branching coral Acropora nasuta and the associated fish species. Only the gobies actively removed algal fronds attached to the cages and only one species (itself toxic to predators) ate them; the damselfish simply defected from the arena when toxic algae were present. The hydrophobic toxins exuded in the algal mucus lysed coral polyps releasing cell constituents that, together with the algal toxin, attract the gobies, which then eat the algal fronds. Interestingly, the toxic goby became more toxic to predators after consumption of the seaweed, which may help to drive symbiosis with a coral colony.

Difficulties associated with manipulating plankton in situ have limited the ability of investigators to assess among-habitat variation in predation risk for plankton. We used plankton tethering units (PTUs) to tether zooplankton in a variety of reef and seagrass habitats, and used field and laboratory assays to test PTUs for tethering artifacts. Tethering did not affect the survivorship of 5 species of plankton (sizes <1 to 6 mm), indicating that the method works with a range of planktonic organisms. We then used the reef mysid Mysidium columbiae in additional assays and found that: (1) mysids remained on PTUs unless they were attacked by predators; (2) PTUs did not prevent planktivorous fishes from consuming tethered mysids; (3) untethered mysids commonly evaded predators, while mysids on PTUs did not; (4) the same types of predators consumed untethered and tethered mysids in the field; and (5) fishes were neither attracted to nor repelled from PTUs. We used PTUs and mysids to assess predation risk for plankton over various coral reef or seagrass habitats. Risk of attack varied among habitats and was correlated with abundance of planktivorous fishes. On the reef, attack rates were most intense over a topographically complex reef ledge, less intense over the less structurally complex center of the reef, and least intense over a structurally simple sand plain. Within the seagrass bed, attack rates were highest at the edge of the bed and less intense to the center of the seagrass bed and over an adjacent sand plain. Thus, attack rates at these sites varied tremendously over small spatial scales (meters).

Many seaweeds that occur in physically stressful habitats or habitats subject to moderate herbivory grow as colonial turfs rather than as spatially separated individuals.The turf growth form is energetically expensive (the net production per gram ash free dry mass of turfs being 33-61% lower than that of individuals), but turfs suffer less physiological damage during desiccating low tides and lose less biomass to herbivores.The upper portions of turf-forming species show significantly greater rates of apparent photosynthesi, and dark respiration than do the lower portions.This spatial partitioning of photosynthetic activity decreases the energetic cost of the turf arrangement and may allow basal portions to function as persistent resting stages during periods of adverse conditions when uprights cannot be maintained, Turf-forming species are specialized for areas that are subject to moderate grazing pressure and physical stresses, They are dependent upon these factors to prevent their competitive exclusion by more productive, but less resistant, seaweeds.Damage to apical portions causes increased branching that results in a more tightly compacted turf.Algae that regenerate in this way can adjust their growth form in accordance with varying levels of disturbance encountered in different habitats and thus incur the minimal cost consistent with survival in that area.For seaweeds occurring in stressful habitats, selection has favored characteristics that increase persistence in space and time even though these involve considerable losses in competitive ability and productivity,

Marine megafauna, the largest animals in the oceans, serve key roles in ecosystem functioning. Yet, one-third of these animals are at risk of extinction. To better understand the potential consequences of megafaunal loss, here we quantify their current functional diversity, predict future changes under different extinction scenarios, and introduce a new metric [functionally unique, specialized and endangered (FUSE)] that identifies threatened species of particular importance for functional diversity. Simulated extinction scenarios forecast marked declines in functional richness if current trajectories are maintained during the next century (11% globally; up to 24% regionally), with more marked reductions (48% globally; up to 70% at the poles) beyond random expectations if all threatened species eventually go extinct. Among the megafaunal groups, sharks will incur a disproportionate loss of functional richness. We identify top FUSE species and suggest a renewed focus on these species to preserve the ecosystem functions provided by marine megafauna.Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC). PMID: 32494601

Marine larvae are consumed by a wide variety of generalist fishes and particle—feeding invertebrates, but larvae of any particular species probably constitute a small and variable of the diet for these consumers. Because virtually all consumers can ingest small quantities of noxious compounds with minimal detrimental effects, it is uncertain that toxic chemicals in larvae could be consumed in quantities sufficient to select for predator recognition and avoidance. Despite this, chemically defended larvae do occur. We show that, at low doses, secondary metabolites (the didemnins) from adults and larvae of the Caribbean tunicate Trididemnum solidum induced vomiting in fish, resulting in rapid learned aversion of the didemnin—defended food. The particle—feeding anemone Aiptasia pallida did not learn to avoid the chemically defended food. When anemones ingested the chemical equivalent of 15 larvae/d, representing &lt;2% of the mass of their total daily diet, the didemnins in the "larvae" significantly reduced: (1) growth of adults by 82%, (2) combined growth of adults and daughter clones by 76%, (3) production of daughter clones by 44%, and (4) average mass of individual daughter clones by 41%. At higher water temperatures, anemones cloned more rapidly, but the negative effects of consuming didemnins also occurred more rapidly. Significant differences in the number of daughter clones produced between treatment and control anemones occurred after only 4 d at seawater temperatures of 27°—29°C vs. 32 d at seawater temperatures of 18°—21°C. Thus consumption of even very small quantities of secondary metabolites can decrease consumer fitness substantially and select for predators that recognize and avoid chemically defended larvae, as do many consumers that co—occur with Trididemnum solidum larvae. This is the first rigorous demonstration that consumption of marine secondary metabolites can decrease consumer fitness when ingested at ecologically realistic doses.