Potent marine neurotoxins known as brevetoxins are produced by the 'red tide' dinoflagellate Karenia brevis. They kill large numbers of fish and cause illness in humans who ingest toxic filter-feeding shellfish or inhale toxic aerosols 1 . The toxins are also suspected of having been involved in events in which many manatees and dolphins died, but this has usually not been verified owing to limited confirmation of toxin exposure, unexplained intoxication mechanisms and complicating pathologies 2-4 . Here we show that fish and seagrass can accumulate high concentrations of brevetoxins and that these have acted as toxin vectors during recent deaths of dolphins and manatees, respectively. Our results challenge claims that the deleterious effects of a brevetoxin on fish (ichthyotoxicity) preclude its accumulation in live fish, and they reveal a new vector mechanism for brevetoxin spread through food webs that poses a threat to upper trophic levels.In the spring of 2002, 34 endangered Florida manatees (Trichechus manatus latirostris) died in southwest Florida, and 107 bottlenose dolphins (Tursiops truncatus) died in waters off the Florida panhandle in the spring of 2004. In both of these unusual mortality events, extensive water surveys revealed that only low concentrations of K. brevis were present.We tested for the presence of brevetoxin in the fluids and tissues of 63 of these animals (27 manatees, 36 dolphins) and found very high concentrations in the tissues of all of them (see Supplementary information accompanies this communication on Nature's website.

Symptoms consistent with inhalation toxicity have long been associated with Florida red tides, and various causal agents have been proposed. Research since 1981 has centered on a group of naturally occurring trans-fused cyclic polyether compounds called brevetoxins that are produced by a marine dinoflagellate known as Karenia brevis. Numerous individual brevetoxins have been identified from cultures as well as from natural bloom events. A spectrum of brevetoxin derivatives produced by chemical modification of the natural toxins has been prepared to examine the effects of functional group modification on physiologic activity. Certain structural features of natural and synthetic derivatives of brevetoxin appear to ascribe specific physiologic consequences to each toxin. Differential physiologic effects have been documented with many of the natural toxins and derivatives, reinforcing the hypothesis that metabolism or modification of toxin structures modulates both the specific toxicity (lethality on a per milligram basis) and potentially the molecular mechanism(s) of action. A series of naturally occurring fused-ring polyether compounds with fewer rings than brevetoxin, known as brevenals, exhibit antagonistic properties and counteract the effects of the brevetoxins in neuronal and pulmonary model systems. Taken together, the inhalation toxicity of Florida red tides would appear to depend on the amount of each toxin present, as well as on the spectrum of molecular activities elicited by each toxin. Toxicity in a bloom is diminished by the amount brevenal present.

Summary1. Florida red tides produce profound neurotoxicity that is evidenced by massive fish kills, neurotoxic shellfish poisoning, and respiratory distress. Red tides vary in potency, potency that is not totally governed by toxin concentration. The purpose of the study was to understand the variable potency of red tides by evaluating the potential for other natural pharmacological agents which could modulate or otherwise reduce the potency of these lethal environmental events.2. A synaptosome binding preparation with 3-fold higher specific brevetoxin binding was developed to detect small changes in toxin binding in the presence of potential antagonists. Rodent brain labeled in vitro with tritiated brevetoxin shows high specific binding in the cerebellum as evidenced by autoradiography. Synaptosome binding assays employing cerebellum-derived synaptosomes illustrate 3-fold increased specific binding. 3.A new polyether natural product from Florida's red tide dinoflagellate Karenia brevis, has been isolated and characterized. Brevenal, as the nontoxic natural product is known, competes with tritiated brevetoxin for site 5 associated with the voltage-sensitive sodium channel (VSSC). Brevenal displacement of specific brevetoxin binding is purely competitive in nature.4. Brevenal, obtained from either laboratory cultures or field collections during a red tide, protects fish from the neurotoxic effects of brevetoxin exposure.5. Brevenal may serve as a model compound for the development of therapeutics to prevent or reverse intoxication in red tide exposures.

A new ladder-frame polyether compound containing five fused ether rings was isolated from laboratory cultures of the marine dinoflagellate Karenia brevis. This compound, named brevenal, and its dimethyl acetal derivative both competitively displace brevetoxin from its binding site in rat brain synaptosomes. Significantly, these compounds are also nontoxic to fish and antagonize the toxic effects of brevetoxins in fish. The structure and biological activity of brevenal, as well as the dimethyl acetal derivative, are described in this paper.A number of bioactive polyether compounds have been isolated from the marine dinoflagellate Karenia brevis, the organism responsible for toxic red tides along Florida's Gulf Coast. The most well-known bioactive compounds isolated from K. brevis are a family of neurotoxins called the brevetoxins (Figure 1), which consist of nine different toxins with two different structural backbones: brevetoxin-A (containing 10 fused cyclic ether rings, 5,8,6,7,9,8,8,6,6,6) 1-3 and brevetoxin-B (containing 11 fused cyclic ether rings, 6,6,6,7,7,6,6,8,6,6,6). 3-8Brevetoxins bind with high affinity to site 5 of voltage sensitive sodium channels (VSSC) in neurons. 9,10 Binding of brevetoxins to tissues containing VSSC results in membrane depolarization, repetitive firing, and increased sodium currents. 11-14 Investigation of the effect of brevetoxins on excitable membranes using voltage clamp experiments indicates that brevetoxins activate VSSC by prolonging mean open time, inhibiting channel inactivation, and shifting the channel activation potential to more negative values. 12-14 During K. brevis red tides humans are most commonly affected by brevetoxins that have been aerosolized in sea spray or bioaccumulated in shellfish. Inhaled brevetoxins cause respiratory irritation and breathing difficulties in sensitive populations. 15-17 At sufficiently high concentrations ingested brevetoxins lead to a collection of symptoms commonly referred to as neurotoxic shellfish poisoning (NSP). 18,19 NSP in humans is characterized by sensory abnormalities, cranial nerve dysfunction, gastrointestinal symptoms, and sometimes respiratory failure. 19In 1989 Prasad and Shimizu 20 isolated and described another polyether ladder compound from K. brevis cultures that contained a different structural backbone and named it hemibrevetoxin-B. Hemibrevetoxin-B contains structural features similar to brevetoxin but is about half of the size and contains only four fused cyclic ether rings (6,6,7,7). Hemibrevetoxin-B (Figure 2) showed cytotoxicity in mouse neuroblastoma cells at concentrations of 5 μM, but no fish or In this report we describe the isolation, structural characterization, and biological activity of a new ladder-frame polyether aldehyde named brevenal (Figure 3a), isolated from cultures of K. brevis. Brevenal as well as its dimethyl acetal derivative ( Figure 3b) contain five fused cyclic ether rings, have low toxicity to fish, and have a structural backbone different from both brevetoxins and hemibre...

Brevetoxins and ciguatoxins are closely related potent marine neurotoxins. Although ciguatoxins accumulate in fish to levels that are dangerous for human consumption, live fish have not been considered as potential sources of brevetoxin exposure in humans. Here we show that, analogous to ciguatoxins, brevetoxins can accumulate in live fish by dietary transfer. We experimentally identify two pathways leading to brevetoxin-contaminated omnivorous and planktivorous fish. Fish fed with toxic shellfish and Karenia brevis cultures remained healthy and accumulated high brevetoxin levels in their tissues (up to 2675 ng g −1 in viscera and 1540 ng g −1 in muscle).Repeated collections of fish from St. Joseph Bay in the Florida panhandle reveal that accumulation of brevetoxins in healthy fish occurs in the wild. We observed that levels of brevetoxins in the muscle of fish at all trophic levels rise significantly, but not to dangerous levels, during a K. brevis bloom. Concentrations were highest in fish liver and stomach contents, and increased during and immediately following the bloom. The persistence of brevetoxins in the fish food web was followed for 1 year after the K. brevis bloom.