A review of: Mafra, L. L., Bricelj, V. M., Ouellette, C. and Bates, S. S. 2010. Feeding mechanisms as the basis for differential uptake of the neurotoxin domoic acid by oysters, Crassostrea virginica, and mussels, Mytilus edulis, Aquatic Toxicology, 97, 160-171.
Domoic acid is a neurotoxin produced by the diatom species, Pseudo nitzschia. The toxin can be transferred to humans via the consumption of contaminated bivalves, potentially causing Amnesic Shellfish Poisoning. During bloom events of Pseudo nitzschia, bivalve molluscs are able to accumulate high levels of domoic acid, while presenting only minor or no negative reactions. Oysters have been known to accumulate much lower domoic acid levels than other co-occurring bivalve species during the blooms, rarely ever reaching the regulatory limit for harvesting (20µg g-1). This study aims to investigate how feeding mechanisms may be responsible for the comparatively low domoic acid levels in oysters during toxic bloom periods of Pseudo nitzschia.
In this study, five toxic Pseudo nitzschia multiseries clones were used, along with other non-toxic algal species. Fully developed, but juvenile bivalves were selected; eastern oysters, Crassostrea virginica; mussels, Mytilus edulis. Several feeding mechanisms were tested in order to attempt to explain the low levels of domoic acid found in oysters, compared with other present bivalves. These mechanisms were compared for mussels and oysters and are as follows;
- Clearance rate – bivalves were fed varying algal suspensions. Samples collected over time.
- Temperature dependent clearance rate – As above, measured by various temperatures.
- Selective feeding – Bivalves exposed to mixed suspensions. Including Pseudo nitzschia multiseries clones of contrasting cell length, and Pseudo nitzschia multiseries clones mixed with non-toxic alga.
- Absorption efficiency – Bivalves subjected to varying cell densities (9000, 15000, 25000 cells ml-1). Bivalves were also exposed to varying ages of culture (11 or 41 days, representing the
exponential and late stationary phases respectively).
- Domoic acid accumulation – Two separate aquaria containing algal suspension for mussels and oysters. Fed continuously for 7-14 days. Cell density monitored every 30 minutes (except overnight) and maintained.
Oysters and mussels were sampled after 6 hours, and 1, 3, 7, 10 and 14 days of exposure. Domoic acid was extracted from animal tissues by sonication. Tissues were analysed using liquid chromatography with UV detection.
Results
- Oysters were found to accumulate 3-7.5x lower domoic acid levels than mussels when both exposed to short P. multiseries cells.
- Toxin levels in oysters were only slightly above the regulatory limit for harvesting after 14 days of exposure. However, mussels were significantly above this limit.
- When exposed to long P. multiseries cells, the differences found were much greater. Domoic acid levels were found to be up to 70x lower in oysters compared with mussels.
- No selective feeding was exhibited between short (34µm) and long (80µm) cells or between P. multiseries and non-toxic species of alga.
- No statistical difference in absorption efficiency was found between mussels and oysters.
- Temperature was found to effect clearance rate and domoic acid uptake in oysters. Clearance rate increased with temperature from 1-18°C. Domoic acid uptake increased with temperature from 4-18°C.
- Maximum domoic acid concentrations accumulated by juvenile oysters (44µg g-1) and mussels (320µg g-1) were highly significantly different.
No adverse effects on feeding or survival rates were observed during the experiments with mussels and oysters, as has also been found with scallops exposed to similar conditions. The results support field data on the comparatively low domoic acid levels found in oysters. Particulate domoic acid levels used in this study are also comparable to those found in some toxic Pseudo nitzschia spp. blooms on the Pacific coast of the USA. The results indicate that the difference in uptake of the toxin between species is strongly affected by differences in clearance rate, which can be exacerbated at low temperatures. The differences in domoic acid accumulation is also enhanced when longer-celled P.multiseries is offered (which are observed to be more dominant in nature), presenting the difference in ability to sort particles based on size between mussels and oysters. In conclusion, it was found that the feeding physiology in terms of clearance rate and selective feeding does play a significant role in the explanation of the relatively low capacity of oysters, Crassostrea virginica, to accumulate domoic acid during blooms, relative to co-occurring bivalves such as Mytilus edulis.
Domoic acid is a neurotoxin produced by the diatom species, Pseudo nitzschia. The toxin can be transferred to humans via the consumption of contaminated bivalves, potentially causing Amnesic Shellfish Poisoning. During bloom events of Pseudo nitzschia, bivalve molluscs are able to accumulate high levels of domoic acid, while presenting only minor or no negative reactions. Oysters have been known to accumulate much lower domoic acid levels than other co-occurring bivalve species during the blooms, rarely ever reaching the regulatory limit for harvesting (20µg g-1). This study aims to investigate how feeding mechanisms may be responsible for the comparatively low domoic acid levels in oysters during toxic bloom periods of Pseudo nitzschia.
In this study, five toxic Pseudo nitzschia multiseries clones were used, along with other non-toxic algal species. Fully developed, but juvenile bivalves were selected; eastern oysters, Crassostrea virginica; mussels, Mytilus edulis. Several feeding mechanisms were tested in order to attempt to explain the low levels of domoic acid found in oysters, compared with other present bivalves. These mechanisms were compared for mussels and oysters and are as follows;
- Clearance rate – bivalves were fed varying algal suspensions. Samples collected over time.
- Temperature dependent clearance rate – As above, measured by various temperatures.
- Selective feeding – Bivalves exposed to mixed suspensions. Including Pseudo nitzschia multiseries clones of contrasting cell length, and Pseudo nitzschia multiseries clones mixed with non-toxic alga.
- Absorption efficiency – Bivalves subjected to varying cell densities (9000, 15000, 25000 cells ml-1). Bivalves were also exposed to varying ages of culture (11 or 41 days, representing the
exponential and late stationary phases respectively).
- Domoic acid accumulation – Two separate aquaria containing algal suspension for mussels and oysters. Fed continuously for 7-14 days. Cell density monitored every 30 minutes (except overnight) and maintained.
Oysters and mussels were sampled after 6 hours, and 1, 3, 7, 10 and 14 days of exposure. Domoic acid was extracted from animal tissues by sonication. Tissues were analysed using liquid chromatography with UV detection.
Results
- Oysters were found to accumulate 3-7.5x lower domoic acid levels than mussels when both exposed to short P. multiseries cells.
- Toxin levels in oysters were only slightly above the regulatory limit for harvesting after 14 days of exposure. However, mussels were significantly above this limit.
- When exposed to long P. multiseries cells, the differences found were much greater. Domoic acid levels were found to be up to 70x lower in oysters compared with mussels.
- No selective feeding was exhibited between short (34µm) and long (80µm) cells or between P. multiseries and non-toxic species of alga.
- No statistical difference in absorption efficiency was found between mussels and oysters.
- Temperature was found to effect clearance rate and domoic acid uptake in oysters. Clearance rate increased with temperature from 1-18°C. Domoic acid uptake increased with temperature from 4-18°C.
- Maximum domoic acid concentrations accumulated by juvenile oysters (44µg g-1) and mussels (320µg g-1) were highly significantly different.
No adverse effects on feeding or survival rates were observed during the experiments with mussels and oysters, as has also been found with scallops exposed to similar conditions. The results support field data on the comparatively low domoic acid levels found in oysters. Particulate domoic acid levels used in this study are also comparable to those found in some toxic Pseudo nitzschia spp. blooms on the Pacific coast of the USA. The results indicate that the difference in uptake of the toxin between species is strongly affected by differences in clearance rate, which can be exacerbated at low temperatures. The differences in domoic acid accumulation is also enhanced when longer-celled P.multiseries is offered (which are observed to be more dominant in nature), presenting the difference in ability to sort particles based on size between mussels and oysters. In conclusion, it was found that the feeding physiology in terms of clearance rate and selective feeding does play a significant role in the explanation of the relatively low capacity of oysters, Crassostrea virginica, to accumulate domoic acid during blooms, relative to co-occurring bivalves such as Mytilus edulis.
2 comments:
Hi Samantha
Interesting post. However I just wonder if this study had any wider aims, as in how this study would contribute to perhaps preventing human poisoning or did they just investigated it for the sake of oysters and mussels ?
Hi Eva (Sorry if this isn't Eva! Guessing :) )
The studies aims were not wider than the effects on mussels and oysters, however this understanding could help us in the future to better understand how domoic acid affects humans through the consumption of such contaminated organisms.
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