The Pervasiveness of Domoic Acid

Domoic Acid (DA), is a diatom-produced neurotoxin responsible for a severe neurologic and gastrointestinal illness called Amnesic Shellfish Poisoning (ASP). Planktivorous organisms have been identified as the vectors of the toxin and since 1987 several DA poisoning and mass mortalities of sea birds and sea lions have occurred worldwide. Although no confirmed DA toxicity events have been reported in whales, this study shows that humpback and blue whales can be exposed to the toxin through the consumption of DA contaminated prey such as krill and planktivorous fish.

In this study, anchovies and sardines viscera, as well as humpback (Megaptera novaeangliae) and blue whale (Balaenoptera musculus) fecal samples, have been analyzed with HPLC-UV methods for the presence of DA. All whale prey and fecal samples collected during August and September in Monterey Bay (CA), were found to contain DA at levels ranging from 75 to 444 μg/g in fish viscera and from 10 to 207 μg/g in whale feces. The collection dates corresponded to a bloom of the DA-producing diatom Pseudo-nitzschia australis and whale prey and fecal samples where found to contain DA only when diatoms were detected in surface waters at densities ≥103 cells/l. In addition, scanning electron microscopy (SEM), revealed the presence in the whale feces of numerous fragmented P. australis frustules, confirming in this way that whales were exposed to DA during the diatom bloom. Since krill break apart the diatoms on which they feed, the frustule's fragments were likely derived from ingested krill gut rather than directly from the water around the whales.

Using known DA levels in prey and the average feeding rate of humpbacks, the authors estimated a daily oral dose of 1.1 mg DA kg⁻¹ for an humpback whale feeding on contaminated fish. Blue whales instead, feed exclusively on krill (Euphausia pacifica) and thus the oral dose received by a whale feeding on contaminated krill was estimated by the authors to be 0.62 mg/kg. For these calculations, resting metabolic rate was used, but clearly metabolic demands are higher when growth, movement, and reproduction are considered. In addition, during particularly toxic or dense blooms, fish and krill could contain higher toxin levels resulting in higher oral doses. It is still not known whether these doses are sufficient to induce neurotoxicity in whales, but it has been suggested that during dives marine mammals may become more sensitive to toxins due to their ability to shunt blood to vital organs (heart and brain), while decreasing circulation to organs involved in detoxification (liver and kidneys). In this case, the estimated oral doses can become theoretically sufficient to induce neurotoxicity in whales.

In addition to whales, the authors found that also fish from both benthic and pelagic communities were noticeably exposed to DA. Fish as diverse as Pacific sanddab (Citharichthys sardidus), chub mackerel (Scomber japonicus), albacore (Thunnas alalunga), petrale sole (Eopsetta jordani), jack smelt (Atherinopsis californiensis) and walleye surfperch (Hyperprosopon argenteum) were found to contain DA at levels ranging from 1.4 to 275 μg/g viscera during the toxic Pseudo-nitzschia bloom. Sanddabs, which are bottom feeders, presumably consume toxin that has reached the sea-floor via the sinking of toxic microalgae, while albacore and mackerel, which are pelagic feeders, commonly feed on krill and anchovies, an obvious source of DA during toxic blooms.

Concluding, DA may be rapidly transferred throughout the whole marine food web, from algae to whales and to many other components of both pelagic and benthic ecosystems making them potential victims of DA toxicity. During toxic blooms, planktivorous krill, anchovies and sardines can accumulate DA providing a direct link between toxic algae and higher level consumers. As a result, whales may be negatively impacted by DA toxicity, if they can accumulate sufficient levels of toxin via consumption of toxic prey.  

Reference: 

Lefebvre KA, Bargu S, Kieckhefer T, Silver MW. (2002). From sanddabs to blue whales: the     pervasiveness of domoic acid. Toxicon 40:971–7.